AVIATION ELECTRONICS TECHNICIAN 3

CHAPTER 2 INFRARED, LASERS, AND FIBER OPTICS In this chapter, you will learn about infrared, lasers, cryogenics, and fiber optics. The basic operations of these systems are also discussed. For information about the safety precautions you must follow, look at chapter 9 of this TRAMAN. INFRARED Learning Objective: Identify infrared advantages and remote sensing types. Infrared radiation (IR) is important in missile guidance, target detection, fire control, com- munications, and mapping. Like radar, IR equipment was developed and used by the military during World War II. In some military appli- cations, IR has advantages over radar. When used for communications, IR is usually less susceptible to detection and interference than visible light. Also, infrared equipment is usually less complex than radar equipment used for similar tasks. Another advantage of infrared equipment is remote sensing, which is the process of detecting or sensing infrared radiation from a target without being in physical contact with that target. While IR detection systems are passive, both active and passive systems are used for remote sensing. Active systems send a signal to the target and receive a return signal. Radar sets are examples of active systems. Passive systems detect a signal or disturbance starting at the target. The signal may be either target emission or another source. Photography, using natural light, is an example of a passive system. Now, with an idea of some advantages of using infrared, lets get into some of the basics. To help you understand infrared, lasers, and fiber optics, the electromagnetic spectrum and infrared radiation are covered in the next section of the TRAMAN. ELECTROMAGNETIC SPECTRUM Learning Objective: Recognize the charac- teristics of the electromagnetic spectrum to include the characteristics of the infrared frequency range. The term infrared is a Latin word meaning beyond the red. Humans only see a small part of the entire electromagnetic spectrum. However, other parts of the spectrum contain useful information. The infrared spectrum is a small portion of the entire electromagnetic spectrum. IR radiation is a form of electromagnetic energy. IR waves have certain characteristics similar to those of light and RF waves. These characteristics include reflection, refraction, absorption, and speed of transmission. IR waves differ from light, RF, and other electromagnetic waves only in wavelength and frequency of oscillation. The IR frequency range is from about 300 gigahertz (10 9 Hz) to 400 tetrahertz (10 12 Hz). Its place in the electromagnetic spectrum (fig. 2-1) is between visible light and the microwave region used for high-definition radars. The IR region of the electromagnetic spectrum lies between wavelengths of 0.72 and 1,000 micrometers (approximately). Discussion of the IR region is usually in terms of wavelength rather than frequency. NOTE: Formerly, the micron (10-6 meter) symbol µ expressed measurements of wavelength in the electromagnetic spectrum. In 1967, the 13th General Conference of Weights and Measures abolished the micron and its symbol. This unit is now called the micrometer, symbol µm. The IR portion of the electromagnetic spectrum is frequently divided into three bands. 1. Near infrared (NIR), which extends from the visible region out to around 1.5 µm 2-1

Figure 2-1.-Electromagnetic spectrum. 2-2

2. Intermediate infrared (IIR), which extends from 1.5 to 5.6 µm 3. Far infrared (FIR), which extends from 5.6 µm to the microwave frequencies Some confusion exists because the infrared range of wavelengths is so close to the visible range in the electromagnetic spectrum. Thus, it is not uncommon to hear references to infrared light. Infrared radiation is also known as thermal or heat radiation. All materials emit radiation in the IR region of the electromagnetic spectrum. In addition to emitting this radiation, a solid object subjected to IR radiation undergoes an increase in temperature, absorbs heat, and then reradiates it. For example, when an aircraft is parked in the sun on a runway, it gets hotter and hotter. It also radiates more and more IR radiation. The aircraft retains heat after the sun sets and continues to radiate that heat. Infrared systems detect the presence of an aircraft on a runway even after the aircraft is moved. This happens because the area of the runway that was directly below the aircraft is cooler than the surrounding runway. You can see how the military might use IR radiation. Heat differs from IR waves in much the same way that electricity differs from radio waves. Q1. Q2. Q3. Q4. Q5. List some of the advantages of IR over radar. Define remote sensing. List the similar characteristics of infrared and light. What frequencies of the electromagnetic spectrum are considered to be in the IR frequency range? Name the three IR bands of the electro- magnetic spectrum. INFRARED RADIATION Learning Objectives: Identify the advan- tages of IR detection systems. Identify the characteristics of emissivity and the effects of atmospheric attenuation. Identify the types of optical devices used in IR systems. All objects above absolute zero (0 K or –273 °C or –460 °F) emit infrared radiation. Radiation emits from any given object over a wide 2-3 range of wavelengths, but it reaches a peak at one particular wavelength. This wavelength has military applications. Detection of IR energy depends on the contrast between the IR radiation from the source under consideration and IR radiation emitted by the background. A cold object with a warm background has as good a target definition as does a warm object with a cold background. There are several advantages in using IR for target detection. Some of these are as follows: IR systems are passive. Complete jamming is difficult. (Although IR systems are sometimes confused.) Military targets are usually good sources of IR. IR systems are smaller, lighter, less complex, and less expensive than other comparable systems. IR systems have a high target resolution. EMISSIVITY One useful concept about IR is the blackbody concept. A blackbody is an object that absorbs all radiation incident on it. Conversely, the radiation emitted by a blackbody is the maximum for any given temperature. Therefore, a black- body is a perfect absorber and radiator of IR at all temperatures and wavelengths. All matter whose temperature is above –273°C (absolute zero) emits IR radiation, The amount of the IR radiation emitted is a function of heat. Theoretically, a perfect emitter is a blackbody with an emissivity of 1. Realistically, the best emissivity is somewhere around .98. The emissivity of various objects is measured on a scale of 0 to 1. The total energy emitted by an object at all wavelengths directly depends on its temperature, If the temperature of a body increases 10 times, the IR radiation emitted by the body increases 10,000 times. If you plot the energy and its wavelengths emitted by a blackbody on a graph,

Figure 2-2.-Blackbody radiation. shill-shaped curve results (fig. 2-2). By looking at this graph, you can see that the energy emitted by short wavelengths is low. As the wavelengths get longer, the amount of energy increases up to a peak amount. After reaching the peak, the energy emitted by the body drops off sharply with a further increase in wavelength. Emissivity is the ratio of the total radiation emitted by any object at any temperature (T) to the total radiation emitted by an ideal blackbody at the same temperature. Emissivity is used to compare the radiation emitted by an actual radiator (source) with that of a perfect radiator. The emissivity of any object depends on the amount of energy its surface can absorb. If the surface absorbs most of the IR striking it, it emits a relatively high amount of radiation, and the emissivity of the object is comparatively large. If the surface reflects most of the incident radiation, the object has a relatively small emissivity. By definition, a blackbody has an emissivity of unity. Therefore, any other body (surface) has an emissivity of less than 1. Table 2-1 shows the emissivity of various surfaces. Table 2-1.-Emissivities of Various Surfaces 2-4

The basic laws that describe the characteristics of IR were first developed for blackbody radiation (the ideal case). Then they were modified to describe radiation from any source. Temperature is the most important parameter in determining the IR characteristics of any body. As the temperature of an object changes, two specific changes in the IR characteristics take place: 1. the wavelength where peak radiation occurs shifts, and 2. the total energy radiated varies with the fourth power of the temperature. There are two laws that describe the relation- ship between these IR characteristics. 1. Wein’s displacement law. This law states that “‘the wavelength at which maximum radiation occurs (Am) is inversely proportional to the absolute temperature of the body.” This law can be expressed by the formula Figure 2-3.-The wavelength of the peak radiation from a blackbody in relation to its temperature. where wavelength is in micrometers, and the con- stant (K) has a value (for a blackbody) of about 2,900. For example, a block of ice emits peak energy at about 10 µm and a jet aircraft engine emits peak energy at about 3.5 µm (fig. 2-3). 2. Stefan-Boltzmann law. This law states that “radiation intensity (E) is directly proportional to the fourth power of the absolute temperature. ” The law can be expressed by the formula where E has dimensions of power per unit areas, and (sigma) is the proportionality constant. Thus, if the temperature of an object is doubled, radiation from the object will be 16 times as much. The Stefan-Boltzmann law can be modified to include the emissivity factor, and total radiation can be computed from the formula where (epsilon) is the emissivity factor of the radiating surface. Figure 2-4 shows the distribution of energy radiated from a blackbody at various tempera- tures. A blackbody at a temperature of 300K (81°F) (not shown) radiates 46 milliwatts of power per square centimeter of its surface. A painted surface, such as the skin of a commercial airliner, at the same absolute temperature radiates 41 milliwatts per square centimeter. If the aluminum aircraft skin weren’t painted, the emissivity factor would be considerably smaller, and the radiation would be less than 4 milliwatts of power per square centimeter. Figure 2-4.-IR distribution curves for a blackbody at various temperatures. 2-5

IR from a source covers a good part of the spectrum, but the maximum radiation occurs at some specific wavelength. For example, IR from jet and rocket engine exhaust plumes is primarily due to molecular excitation of water vapor and carbon dioxide, which are characteristic by-products of combustion. This molecular radiation peaks at 2.77pm (due to carbon dioxide alone). However, in a practical situation it is easier to get more radiation from the hot tail pipe and other heated surfaces. ATMOSPHERIC ATTENUATION In military applications, the IR transmitting medium is often the atmosphere. The effect of atmospheric attenuation on transmission is a very important factor in considering the overall effectiveness of the systems. There are two primary causes of atmospheric attenuation: 1. scattering by suspended particles (solids), and 2. absorption by free molecules in the atmosphere. These two attenuations are additive, but absorp- tion is the more important. The amount of scattering caused by particles depends on the relationship between the wave- length of the radiated energy and the size of the particles. When the wavelength is considerably shorter than the dimensions of the particles, scattering is essentially independent of wave- length. Usually this relationship is the case in the IR spectrum. Therefore, attenuation caused by scattering can be measured at one wavelength and applied over a relatively wide band of wavelengths. However, this technique does not work with attenuation caused by molecular absorption. The amount of molecular absorption is closely associated with wavelength. The two substances in the atmosphere that absorb the most radiation are water vapor and carbon dioxide. In both substances, there are several wavelength bands in which absorption is relatively high. Molecular resonance causes this condition. (Each molecule has a natural frequency of vibration, or resonant frequency.) The resonant frequencies of these molecules are in the infrared region. Their structure is such that this natural vibration creates an oscillation of the electric charge in the molecules, increasing the absorption. At low Table 2-2.-Wavelength Limits of IR Transmission Windows in the Atmosphere altitudes, this absorption is so great in some wavelength bands that the percentage of radiation transmitted drops rapidly to zero. This is due to denser atmosphere at low altitudes. Between these absorption bands are transmission bands in which the atmospheric attenuation is not as great. These transmission bands, known as windows, contain wavelengths as shown in table 2-2. The atmosphere is not a very good transmitter of infrared radiation because of the absorption properties of C0 2, H 2O, and O 3. Figure 2-5 shows the transmission spectrum characteristics of the atmosphere. You can see that the best transmission is between 3 µm and 5 µm and between 8 µm and 14 µm. The range between these frequencies is a window, Infrared imaging devices operate in one of the two windows, usually the 8 µm and 14 µm. The absorption bands are much narrower at high altitudes because of the thinner atmosphere. Therefore, the absorption bands are of lesser consideration in the design of high- altitude IR systems. Figure 2-5.-Transmission spectrum of the atmosphere. 2-6

OPTICAL DEVICES Optical devices are used in front-end optics to gather and focus the infrared radiation upon the detector. They can be used because of the similarity between infrared and visible light. Figure 2-6 shows a simple optical system for gathering and focusing IR radiation. The entire system lies within a protective housing to protect the detector and the optical system from the weather. The dome is a continuation of the protective housing and must be able to pass IR radiation easily. Many of the materials commonly used in visible light optics can’t be used in IR imaging systems because these materials are opaque at IR frequencies. The optical materials used in IR imaging systems should have most of the following qualities: Be transparent at the wavelengths on which the system is operating. Be opaque to other wavelengths. Have a zero coefficient of thermal expansion to prevent deformation and stress problems in optical components (parts). Have high surface hardness to prevent scratching the optical surfaces. Figure 2-6.-Simple IR optical arrangement. Have high mechanical strength to allow the use of thin lenses (high-ratio diameter to thickness). Have low volubility with water to pre- vent damage to optical components by atmospheric moisture. Be compatible with antireflection coatings to prevent separation of the coating from the optical component. Although none of the materials now used for IR optics have all of these qualities, silicon, germanium, zinc selenide, zinc sulfide, and IRTRAN have many of them. The actual material used for IR optics depends on the material’s best characteristics and their application. Typical materials for making domes include glass, quartz, synthetic sapphires, germanium, and silicon. The transmission coefficient of the optical material is an important factor in the design of IR equipment. Glass and quartz are satisfactory material for NIR, and generally for IIR, Figure 2-7 shows that glass, quartz, and synthetic sapphires have excellent transmission characteristics in the visible and near infrared regions. They cut off sharply in the intermediate infrared region. Optical glass is completely opaque to wavelengths longer than 3 µm, quartz cuts off at 4 µm, and synthetic sapphire loses its Figure 2-7.-Wavelength versus transmission coefficient. 2-7

transparency at wavelengths greater than 6 pm. Germanium and silicon are semiconductor materials that are opaque to visible light and transparent to IR throughout most of the near and intermediate infrared regions. FIR requires a completely different type of optics. Single crystals of silver chloride, rolled flat, are satisfactory windows for the transmission of FIR. Single crystals of sodium chloride (rock salt), cut and ground into a lens or window, is excellent for FIR. However, rock salt is highly soluble in water; therefore, it must be protected from atmospheric moisture. This characteristic makes rock salt impractical for use as an IR dome material. There are some problems involved in designing IR optical systems. The material used must match the wavelength to which the detector will respond. Optical materials are physically weak, and many damage easily by high temperature and thermal shock. Pressure and chemical reactions will change the properties of some optical materials. Heat is another problem. When any part of the IR optical system becomes heated by the energy it absorbs, the energy reradiates at wavelengths other than those of the original radiation. If the detector is sensitive to these new wavelengths, this closer source will obscure the target or cause ghost images. Surface reflections and attenuation by the material cause attenuation in optical materials. Surface reflections may be overcome by anti- reflection coatings. Attenuation by the material is the more serious problem. IR systems often have a chopping reticle (chopper) in the principal focal plane. The chopper generally is a rotating disc with some clear and some opaque areas. Although a chopper is not absolutely necessary in a search system, it has several useful properties. The chopping rate furnishes a conveniently high carrier frequency for the electronic amplifiers, and the reticle pattern can operate as a discriminator or filter. Manufacturers can design this filter for the types of background expected to provide better differentiation between target and background. Optical filters in IR instruments isolate certain wavelength regions of interest, such as atmospheric windows, and screen out undesired wavelengths. There are three general types of filters: 1. Those that pass short waves. 2. Those that pass a particular band of waves. 3. Those that pass long waves. Q6. List the advantages in using IR for target detection. Q7. What is the blackbody concept? Q8. Of all the parameters in determining IR characteristics, which one is the most important, and why? Q9. What is the primary factor that affects the IR transmitting medium and its primary cause? Q10. Absorption is the major cause of attenuation in IR system design. What happens at higher altitudes? Q11. List the problems involved when designing IR optical systems. Q12. Optical filters isolate certain wavelengths and screen out undesired wavelengths. What are the three general types of filters? DETECTORS Learning Objective: Identify the charac- teristics of detectors to include thermal detectors. The most critical component of any IR system is the detector (or sensor), which detects and converts IR into an electrical signal. The characteristics of the atmosphere and of the source (if it is a military target) cannot change. Optical materials are somewhat standard, as are display devices and control circuits. Research and development have resulted in some very good all-around detectors, but selecting the proper detector for a particular application must be done carefully. Many variables confront the selection process. These variables and the characteristics of the radiation involved determine the selection of the detector. DETECTOR CHARACTERISTICS The detector is the most important component of the IR imaging system. There are many types of detectors, each having a distinct set of operating characteristics. Bolometers, Golay cells, mercury-doped germanium, lead sulfide, and phototubes are the most commonly used types of detectors. Two ways to characterize detectors is 2-8

by their optical configuration or by the energy- matter interaction process. Two classes of detectors include the photoelectric and thermal. There are two types of optical configurations— elemental and imaging. 1. Elemental detectors. Elemental detectors average the portion of the image of the outside scene falling on the detector into a single signal. To detect the existence of a signal in the field of view, the detector builds up the picture by sequentially scanning the scene. The elemental detector requires time to develop the image because the entire scene requires scanning. 2. Imaging detectors. Imaging detectors yield the image directly. An imaging detector is like a myriad of point detectors. Each of the detectors respond to a discrete point on the image. Therefore, the imaging detector produces the entire image instantaneously. A good example of an imaging detector is photographic film. To compare the relative merits of different detectors in different situations, you must know several parameters of detector operation. These parameters make it possible to discuss the characteristics of a particular detector in terms applicable to any detector. Responsivity When IR strikes either the photoelectric or thermal detector, a change takes place in the detector material, causing an electrical output signal. The responsivity (R) of the detector is the amount of output signal that each unit of input radiation intensity produces. Responsivity is expressed by the following ratio: where R is generally given in volts per watt. Many factors influence responsivity such as detector and source temperatures, detector area, detector time constant, and spectral distribution of the radiation. Spectral Response One important influence on the responsivity of a detector is the change in detector sensitivity with the change in the wavelength of received radiation. The spectral limit of responsivity is the wavelength, where the value of responsivity is half that of its maximum value. Spectral response is a nonlinear characteristic. Therefore, you must know its value for each wavelength considered. Any discussion of values must include details of the conditions involved. Time Constant In any IR scanning system, the time constant of the detector must be such that the detector can fully respond before the radiation intensity changes. The time constant is the time required for the detector to develop 63 percent of its maximum output signal. The maximum scanning rate depends on this time constant. Noise Equivalent Power (NEP) Noise exists in any circuit that carries current. Most outside noises can be reduced or eliminated by shielding and proper design. However, thermal noise is an ever-present problem. Power supplies used with IR detectors require extremely good filtering. Since the IR radiation received by the detector is very small, noise of any appreciable amount could be enough to generate weak IR signals or cause false targets. IR systems generate many different types of noise. The most important of these are— current noise, caused by bias currents within the detector, and Johnson (thermal) noise, caused by thermal fluctuations in the detector material. At low bias voltages, current noise is negligible, and the output noise consists almost entirely of Johnson noise. The current noise increases linearly with bias voltage and may eventually become the primary source of noise. NOTE: In modern IR systems, cryogenic cooling of the detector reduces much of the Johnson noise. Another useful and important detector parameter is the noise equivalent power (NEP) of a detector. NEP is the radiation power (in watts) that must strike a detector to produce a signal response equal to the noise output over a reference bandwidth. Thus, a signal-to-noise ratio is equal to 1. 2-9

When comparing two different IR detectors, the one with the lower NEP has the higher useful sensitivity. Since this use of NEP may be confusing, another parameter, defectivity may be easier to use. Detectivity is simply the reciprocal of the given NEP of a detector. Thus, the higher defectivity a cell has, the higher its useful output. For example, a detector with an NEP of 4.0 x 10 -9 has a defectivity of The best IR detector would have the greatest possible spectral response within the frequency band of interest, and the lowest possible NEP (or highest possible defectivity). A properly chosen detector might have a maximum range of 90 miles, with a signal-to-noise ratio of 5, from a 1-square- meter target at 300K. This range is equivalent to an ability to detect IR emitted by a cubic inch of ice at 3 miles. Energy-Matter Interaction There are two basic types of energy-matter interaction. They are the photon effect (photoelectric effect) and the thermal effect. 2-10

PHOTON EFFECT.— In the photon effect energy-matter interaction, the photons of the radiant energy interact directly with the electrons in the detector material. Usually, detectors using the photon effect use semiconductor material. There are three specific types of photon effect detection. The three major types of photodetectors are the photoconductive, photovoltaic, and photo- emissive types. The signal-to-noise ratio of each of these detectors is the limiting factor in determining its effectiveness. 1. Photoconductive. Photoconductivity is the most widely used photon effect. It is also known as the internal photoelectric effect. (See fig. 2-8.) Radiant energy changes the electrical conductivity of the detector material. An electrical circuit measures the change in the conductivity. The photoconductor contains a semiconductor crystal that absorbs the photon energy from the radiation, which strikes the surface of the crystal. This changes the crystal’s resistance or conductivity. Several different materials are used for this type of detector, including lead sulfide, lead telluride, lead selenide, and cadmium sulfide. Gold-doped germanium is a good detector material. However, there are some difficulties such as long time constants. 2. Photovoltaic effect. In the photovoltaic effect (fig. 2-9), the radiant signal causes a potential difference across a PN junction. The Figure 2-8.-Photoconductive detector circuit and graphic symbols. Figure 2-9.-Photovoltaic effect and graphic symbol. photocurrent (current generated by light) adds to the dark current (current that flows with no radiant input). The total current is proportional to the amount of light that falls on the detector. The photovoltaic effect uses a photovoltaic cell similar to a solar cell. This is a semiconductor with a high-resistance, photosensitive barrier between two layers. When exposed to IR, a potential difference builds up across the two layers of the cell. 3. Photoemissive. The photoemissive effect (fig. 2-10) is also the external photoelectric effect. The action of the radiation causes the emission of an electron from the surface of the photo- cathode in the surrounding space. Figure 2-10.-Photoemissive effect and graphic symbol. 2-11

The photoemissive cell’s cathode is exposed to IR and causes electronic emission. The number of emitted electrons depends on the intensity of the IR striking the cathode. THERMAL EFFECT. —The thermal effect type of energy-matter interaction involves the absorption of radiant energy in the detector. This results in a temperature increase in the detector element. You detect the radiation by monitoring the temperature increase in the detector. Both the elemental and imaging forms of detectors use the thermal effect. THERMAL DETECTORS Thermal detection is the sensing of the change in temperature of the detector material as a result of IR striking its surface. There are three different types of sensing elements employed in modern thermal detectors. 1. The thermopile, a series combination of several thermocouples 2. The bolometer, which senses changes in resistance of the detector material 3. The pneumatic cell, which uses the expansion of a gas as an indicator Thermocouple One of the basic heat detectors is the thermocouple. When applying heat to the junction of two dissimilar metals such as iron and copper, a measurable voltage is generated between them. Figure 2-11 shows a basic thermocouple. The voltage difference across the thermo- couple is small. However, you can increase the sensitivity to a point where the thermocouple becomes useful as an IR detector. You can obtain an increase in sensitivity by connecting or stacking several thermocouples in series, forming a thermopile. The complete thermopile action is like connecting several flashlight cells in series; the output of each thermocouple adds to the output of the others. For example, 10 thermocouples, with individual outputs of 0.001 volt, have a total output of 0.01 volt when connected in series. The effective sensitivity increases further by mounting a thermopile at the focal point of a parabolic reflector. When using this method, the reflector focuses the IR from the target onto the thermopile. Bolometer A bolometer is a very sensitive device whose resistance will vary, depending on the IR exposure. There are two main classes of bolometers—the barretter and the thermistor. A barretter is a variable resistor made of a short length of very fine wire (usually platinum) that has a positive temperature coefficient of resistance. (A substance has a positive temperature coefficient if its resistance increases with an Figure 2-11.-Thermocouple. 2-12 Figure 2-12.-Various thermistors.

Figure 2-13.-Comparison of thermistor and barretter sensitivity. increase in temperature. It has a negative coefficient if its resistance decreases with an increase in temperature.) A thermistor is a variable resistor made of semiconductor material, such as an oxide of manganese, nickel, cobalt, selenium, or copper. The thermistor has a negative temperature coefficient of resistance. A thermistor is usually in the form of a bead, disc, rod, or flake, as shown in figure 2-12. The mixing of various proportions of the heat-sensitive materials provide specific characteristics of resistance versus temperature necessary for target detection. Figure 2-13 shows changes in resistance that a typical thermistor can produce compared to those in a barretter. Note the thermistor has the steeper temperature coefficient of resistance curve. There- fore, it is the more sensitive of the two sensors. One simple type of infrared detector consists of two thin strips of platinum that form two arms of a Wheatstone bridge. To increase the thermal Figure 2-14.-Infrared detecting device. sensitivity of the strips, one strip is black on one side. The blackened surface absorbs the IR. As the strip absorbs heat, its resistance changes and unbalances the bridge. The imbalance causes a change in current produced by an external voltage applied to the input terminals of the bridge. The infrared detecting device (fig. 2-14) is like the one discussed in the previous paragraph. It consists of four nickel strips supported by mounting bars that have electrical leads attached to them. A silvered parabolic reflector (mirror) focuses the IR on the nickel strips. The change of resistance in the strips causes an unbalanced condition in the bridge circuit, producing an output signal. Pneumatic Cell Another unique infrared detector is the Golay detector (pneumatic cell), shown in figure 2-15. Figure 2-15.-Golay detector. 2-13

This detector is actually a miniature heat engine. IR energy entering the window causes expansion of a volume of gas located between the reflecting diaphragm and the window. The lamp at the after end of the detector emits a light beam. The lens focuses the beam that passes through the grid and onto the reflecting diaphragm. Changes in the amount of infrared energy entering the window cause changes in the shape of the diaphragm. This causes its light-reflective properties to vary accordingly, modulating its light output. The light reflected from the diaphragm passes back through the grid, which intensifies the variations of the reflected light. After passing through the grid, some of the light (reflected by the diaphragm) strikes the mirror. This light reflects to a photocell of high sensitivity (not shown in the figure). The modulated output of the photocell is a voltage proportional to the intensity of the IR entering the window. The Golay detector has the most rapid response of any infrared detector, but it can operate only when intermittently receiving radiant heat. An optical chopper can interrupt the flow of IR to the cell periodically. Another advantage of the Golay detector is its extremely wide bandwidth, making it a good choice for use in IR spectrum analysis. Q13. Q14. Q15. Q16. Q17. Q18. Q19. Q20. What is the most critical component of any IR system? List the most common types of detectors. Define responsivity as it relates to the detector. What are two of the most important types of noise generated by an IR system? Name the two basic types of energy-matter interaction. What are the three major types of photo- detectors? Three different types of sensing elements are used in modern thermal detectors. What are they? The Golay detector has the most rapid response of any infrared detector, but it requires an optical chopper. Why does it need the optical chopper? APPLICATIONS Learning Objective: Identfy military appli- cations to include homing techniques, imaging system component operation, and configurations. The number of military and industrial appli- cations of IR has grown in recent years. A complete discussion of all applications is beyond the scope of this manual, but some IR systems and concepts applicable to military situations are discussed in the following paragraphs. During World War II, infrared found its first military use in a snooperscope device. This device worked in total darkness, and outlined enemy troops by the heat radiated from their bodies. A rifle with a sniperscope made it possible to see a target in total darkness and to fire with normal accuracy at a target. Since IR is invisible but behaves much like visible light (that is, it can be reflected and controlled in a beam pattern), it served as a means of communication for specific wartime purposes. Development of equipment to receive the invisible light was the base for design and successful use of some important weapons. Infrared used for short-range communication between sea-level stations, such as ships, affords excellent security. Line-of-sight limitations of IR rays and their rapid attenuation at sea level provides security for short- range communications. Military use of infrared for communications requires a powerful source and a sensitive receiver for detecting the modulated source. Such sources and receivers are available for near infrared energy. Photography uses infrared because it is effective against camouflaged targets. Night photography, using infrared, can produce a better visual presentation of terrain than the best mapping radar. Navigation also uses infrared. Ground speed indicators are available that can compete with Doppler radar. Anticollision circuits using IR are undergoing experiments. Image-forming devices, thermal or ship detection devices, and infrared radar are also using IR. The portable infrared detector (PID) is a passive far infrared (FIR) equipment for detecting personnel, vehicles, tanks, small boats, and ships. It detects the difference in temperature between a body and its immediate background and provides an audible signal output. A larger FIR system for ship detection is the stabilized ship detector (SSD). This system provides a permanent 2-14

record of the true bearing of all targets detected within the angle scanned. Infrared radar uses a pulsed IR source and receives reflected IR energy as in microwave radar. Infrared has excellent application to guided missiles. Military targets, such as ships, factories, and aircraft, are normally warmer than their surroundings. Detection of these targets is from the heat they radiate. Heat radiated at lower temperatures is particularly important in passive detection of surface targets. There is no economical way for the enemy to camouflage self-radiated heat. The Felix bomb was the first guided missile to use IR. Its automatic guidance system was an infrared homing device in the nose of the bomb. The Felix bomb was reliable and adequate for operational use. World War II ended before it could be used under combat conditions. However, this bomb opened the way to a new and different method of guidance, infrared homing. A homing guidance system controls the flight path of a missile by a device in the missile that reacts to some distinguishing feature of the target. Homing guidance systems are the most accurate of all guidance systems. There are three types of homing systems; they are subdivided by the source of target radiation. 1. 2. 3. Active homing—Both the source that illuminates the target and the receiver that detects the echoes are within the missile. Semiactive homing—The target illumina- tion is from some source outside the missile, and the missile receiver uses the target reflections. Passive homing—The missile receiver detects the natural radiation of the target. Active and semiactive types of homing systems typically use radar or lasers. Passive types use heat, light, or in some cases, a radio or radar signal for homing. The Sidewinder is probably the most simple and economical guided missile. It contains an infrared homing system and can destroy high- performance aircraft flying at any altitude from sea level to about 50,000 feet. While it is unlikely that IR will ever entirely replace radar, IR has certain advantages over radar. You can expect that radar and IR will be used together in fire control, guidance, and search applications. INFRARED IMAGING SYSTEMS An infrared imaging system consists of detectors, a scene disection system, front-end optics, a refrigeration system (if required), and an image processing system. Detectors You have learned about imaging detectors. Now, you will learn how imaging detectors are used in IR imaging systems. Detectors convert the IR radiation signal into an electrical signal for processing into information used by an operator. Detectors have many different configurations for their use in IR imaging systems. DETECTOR ARRAY.— The detector (ele- ment) needs only a small portion of the image scene to achieve maximum resolution. You can form an array by grouping several detector elements (fig. 2-16, view A). This array has closely packed elements in a regular pattern. Thus, the image of the scene spreads across the array like a picture or a mosaic with no scanning. Each detector element views a small portion of the total scene. The disadvantage of this type of system is that each detector element requires a supporting Figure 2-16.-Detector arrays. 2-15

electronic circuit to process the information that it provides. Also, each detector element requires a preamplifier to boost the signal to a useful level. SINGLE DETECTOR.— Another method that provides the operator with information is the single scanning detector (fig. 2-16, view B). Here, there is one detector requiring one set of supporting circuitry. In this type of system, the scanning of the image is across the detector so that the detector can see the whole image. An optical system supplies the scanning. This type of system is adequate if real-time information is not important, or if the object of interest is stationary or not moving quickly. Scene Disection System The scene disection system scans the scene image. There are many types of scanning—one associated with each type of detector array. A single detector with one fast scan axis and one slow scan can scan the scene rapidly in the horizontal direction and slowly in the vertical direction. A vertical linear array is scanned rapidly in the horizontal direction. One detector element scans one line of the image. In the linear array, there is a space one element wide between each element. The scan is one axis with an interlace. After each horizontal scan, the mechanism shifts the image upward or downward one detector element width. This allows the next scan to cover any of the missed lines. Each system has an optimum configuration of detector array and image disection. If the number of elements in the detector array are increased, the system becomes more complicated. The cost of the system increases, and the reliability of the system decreases. If you decrease the number of detectors, you reduce the amount of information that you can process. A compromise between increasing the number of elements (increased cost) and decreasing the number of elements (reduced information) is to use a linear array scanned in one direction only. Each detector 2-16

scans one line of the scene image. This reduces the complexity of the electronics and increases the amount of information you can process. Thus, the viewing size of the scene and the detail of the scene increase. There are many types of mechanisms you can use to scan the scene. When scanning using two axes, you must synchronize the two scanning motions. The electronic signal that controls the sampling of the detectors must also synchronize with the scanning motions. Front End Optics The front end optics collect the incoming radiant energy and focus the image at the detec- tors. The optics may be reflective or refractive, or a combination of both. Many systems offer a zoom capability, allowing a continuous change in amplification of the image without changing the focus. Spectral filters restrict the wavelength of light entering the system. This prevents unwanted wavelengths of light from reaching the detector and interfering with the imaging process. Refrigeration System Many types of infrared detectors require low temperatures to operate properly. A refrigeration system in imaging systems provide the necessary operating temperatures. The two types of detector cooling are the open cycle and closed cycle types. The open cycle type of cooling provides a reservoir of liquified cryogenic gas. The liquid travels to the detector, where it reverts to a gas. As it changes from a liquid to a gas, it absorbs a lot of heat from the surrounding area and the detector. The closed cycle type of cooling compresses the gas, and the heat generated by the compression is radiated away by the use of a heat exchanger. The gas then returns to the compressor, and the cycle repeats itself. Image Processing Systems The image processing system converts the data collected by the detectors into a video display. Multiplexing of the data from the detectors allows handling by one set of electronics. Then further processing ensures the information coming from the detectors is in the correct order of serial transference to the video display. At this point, the addition of any other display information takes place. Other image processing systems amplify the sig- nals from the detectors and send them to an LED display. Others optically amplify by photomuhi- plier tubes and project on a phosphorescent screen. INFRARED IMAGING SYSTEM CONFIGURATIONS Presently, the Navy uses several IR imaging system configurations. They are the direct view parallel scan linear system, the serial scan parallel video two-dimensional array system, and the serial scan standard video system. Direct View Parallel Scan Linear System The direct view parallel scan linear system (fig. 2-17) is the simplest type of infrared imaging Figure 2-17.-Direct view parallel scan linear system. 2-17

system. The scene image enters the system through Serial Scan Parallel Video the infrared lens. Then, it strikes a double-sided Two-Dimensional Array System scan mirror. The image scans across a linear detector array. Preamplifiers amplify the signals Figure 2-18, view A, shows a serial scan from the detectors. Then, the signals are sent to parallel video two-dimensional array system. A the LED drivers, which lie in a linear array. Light two-dimensional array of detectors is coupled one from the LED array scans across the field of view for one to a similar array of LED. The scan mirror of an ordinary eyepiece directly from the second operates in two dimensions. This system offers side of the scan mirror, or it is viewed on a the same options of direct viewing or CRT viewing cathode-ray tube (CRT). as found in the one-dimensional array. Figure 2-18.-Serial scan video systems. 2-18

Serial Scan Standard Video System Figure 2-18, view B, shows a serial scan standard video system. Scanning of the incoming image is done in two dimensions by a scan mirror and an interlace mirror. The interlace mirror shifts the image one detector element width. This is using a linear detector array. Preamplifiers amplify the information from each detector. Then, it is sent to the delay circuitry for changing into serial form. This circuitry samples each detector at the appropriate time for correct length of time, resulting in a serial output to the video processor. ELEMENTS OF A SCANNING INFRARED IMAGING SYSTEM Refer to figure 2-19 while you read about the elements of a scanning infrared imaging system. The observer views the system output and interprets the information while operating the controls. The system control interfaces between the operator and system, allowing the operator to control the system. The stabilization and pointing gimbals provide a stabilized platform from which the imaging system operates. It isolates the system from vibration and sudden motions of the aircraft. Also, it provides a pointing capability for the imaging system. The collecting optics and filters collect the light (thermal radiation) originating from the target. Special filters or optical components that transmit only the desired wavelengths filter any unwanted wavelengths of radiation. The optical components focus the scene image on the detector array. The optomechanical scanner scans the scene image across the detector array in a process called scene disection. The optomechanical scanner includes a mirror(s) or prism(s) with the mechani- cal drive controlled by a scan synchronizer. The scan encoders convert mechanical information about the motion of the scanner to electronic signals. These encoders synchronize the scanner motion with the image generation of the video monitor. This information then goes to the scan synchronizer. The scan synchronizer controls the motion of the scanner. It interacts with the video process to synchronize the scanner with the display image generation. The detector assembly contains the detector array that converts the optical signal from the target to an electrical signal. The detector cooler provides cooling for the detector assembly, if required. The detector bias and preamplifier circuits supply voltage or current for operating the detectors. They scan the detectors at the appropriate times, and they amplify the signal Figure 2-19.-Forward looking infrared (FLIR) set 2-19 block diagram.

from the detectors before further processing. They also convert the output of the detectors into a serial form. The video processor converts the detector information into the format necessary for the video monitor. It adds any additional information for the observer, if needed. The video monitor, usually a CRT, provides information to the operator. The built-in test (BIT) locates and reports the nature of failures in the system. Q21. Q22. Q23. Q24. Q25. Q26. There are many uses for infrared, including short-range communication, navigation, and anticollision circuit experimentation. Why is infrared used in photography? Name the three types of homing systems classified by their source of radiation . List some of the components of a typical infrared imaging system. What is the purpose of front end optics? Why do some infrared imaging systems need refrigeration systems? Name some of the elements and components of a scanning infrared imaging system. INTRODUCTION TO CRYOGENICS Learning Objective: Identify cryogenic characteristics. Cryogenics is the science that involves the study of very low temperatures. The word cryogenic comes from the Greek root cryo or kyros, which means icy cold or relating to the cold . Cryogenic temperatures extend from – 150°C downward to –273°C (absolute zero). Under such extreme temperatures, many metals become brittle and shatter, atmospheric gases turn into liquids, electrical resistance disappears in some materials, and current flows indefinitely without loss (super conductivity). Heat is a form of energy, and cold is the absence of heat. When a system cools, heat flows out of the system. Therefore, you might say cold is physical manifestation of a lack of energy. The temperatures of a system are an internal feature of the system; therefore, cold can relate to a low internal energy of a system. Many modern systems require cryogenic temperatures to operate properly, imaging systems being one of these. The detectors of the imaging system require cooling for maximum efficiency. Therefore, you need some sort of refrigeration system to provide these low temperatures. If you are to understand the operating principles of a refrigeration system, you must understand thermodynamics. Before you begin the following section, refer to figure 2-20. This figure illustrates a numerical scale that you can use to measure degrees of hot and cold. When bodies at different temperatures meet in thermal contact, heat flows from the body at the higher temperature to the body at the lower temperature. The flow of heat stops when the two bodies are at the same temperature (thermal equilibrium). IDEAL GAS LAW Nearly all thermodynamic systems have a working fluid of some type. To explain the ideal gas law, we use a theoretical fluid (gas, depending upon temperature), and this fluid is the ideal gas. The assumptions about the nature of this ideal gas are as follows: The molecules that make up ideal gas are very hard, small spheres whose volume you may disregard when compared volume of the gas as a whole. Figure 2-20.-Absolute temperature scale. to the 2-20

The molecules do not interact with each PHASE CHANGES other, only with the walls of the container; they do so by elastic collision (a molecule leaves the wall at the same speed it was traveling before the collision). Real gas behaves like ideal gas, especially at low pressures. However, real gas differs from ideal gas in the following ways. The molecules of a real gas are large enough that their volume does matter when calculating gas volumes, and the molecules do collide with each other. Matter exists in three states: gas, liquid, and solid. For matter to change from a solid to a liquid or from a liquid to a gas, it must absorb a large amount of energy. The reverse is also true. Fusion is the process by which a solid changes to a liquid. Vaporization is the process by which a liquid changes to a gas. This process is a good vehicle for heat transfer. It is the basic theory behind refrigeration. LAWS OF THERMODYNAMICS ENERGY Energy is the driving force of the universe. You can make the following assumptions about energy: Energy is the fuel required to make things happen. No system can operate without a transfer of energy. Heat is a form of energy. A system has an internal energy (which includes all potential and kinetic energies of the system or molecules of a gas). A closed system conserves energy, although it may change energy states (potential to kinetic). ENERGY AND THE IDEAL GAS In the ideal gas, energy is in the form of kinetic energy of the molecules. When the internal energy of the gas increases, the molecules move faster; therefore, they have a higher kinetic energy. If the mass of the molecules is low, the molecules move faster. Therefore, the higher the tempera- ture of the ideal gas, the higher its internal energy and the faster the molecules move. Molecular motion (movement of molecules within a mass) also produces the phenomenon of pressure. As the molecules move about a container, they collide with the walls, exerting a force on the walls. The hotter the gas, the faster the molecules collide with the walls; thus, the higher the pressure. The four laws contained in this section deal with thermodynamics, They are basic to the theory of refrigeration and cryogenic systems. 1. The Zeorth law of thermodynamics states that “when two systems of the same temperature are in thermal contact, no heat will flow.” Heat will flow between two systems when one system is at a higher temperature than the other. In this case, heat will flow away from the higher temperature. There are three types of heat flow: convection, conduction, and radiation (fig. 2-21). Convection is the transfer of heat through macroscopic movement of material. (Macroscopic meaning large or visible as opposed to microscopic [small or invisible].) Conduction is the transfer of heat through materials when there is no macroscopic motion, as in the heat flow in metals. The rate Figure 2-21.- Heat flow, conduction, convection, and radiation. 2-21

of heat flow depends upon the following physical situations: a. The higher the temperature gradient, the greater the rate of heat flow (the temperature gradient is equal to the difference in temperatures divided by the distance over which the heat must flow). b. The larger the area across which the heat is flowing, the higher the rate of heat flow. c. The shorter the distance the heat must flow, the higher the rate of heat flow. Radiation is the transfer of energy by electro- magnetic radiation. All bodies that have a temperature greater than 0 K give off electro- magnetic radiation. The higher the temperature, the greater the amount of radiation emitted. 2. The first law of thermodynamics states that “the change in the internal energy of a system is equal to the heat introduced into the system minus the energy expended by the system when it does work on the environ merit.” 3. The second law of thermodynamics states that “a cyclic process must transfer heat from a hot reservoir if it is to convert heat into energy.” Also, work must be done to transfer heat from 4. The third law of thermodynamics states that “it is not possible by any procedure, no matter how idealized, to reduce the temperature of any system to absolute zero in a finite number of steps.” Absolute zero is a limit that you can only approach and never achieve. The lowest tempera- ture that has ever been attained is .00002 K. The closer that a system gets to 0 K, the harder it is to get heat from the system. PRINCIPLES OF REFRIGERATION Refer to figure 2-22 during the following discussion. The working fluid used in the system is (Freon). The compressor (A) delivers gas at high temperature and pressure to the coils (B). Water or air cooling removes the heat from the gas in (B), resulting in condensation of the gas into a liquid. The liquid flows by force through a small orifice (C) and expands as it leaves the orifice. It leaves the valve as a mixture of liquid and vapor at a lower temperature. The mixture of liquid and vapor now enters the coil (D), and heat from the surrounding area supplied to the working fluid converts the remaining liquid to a gas. The gas enters the compressor, and the a cold reservoir to a hot reservoir. cycle-repeats. 2-22

Q27. Q28. Q29. Q30. Q31. Figure 2-22.-Common refrigeration cycle. Define cyrogenics and identify its tempera- ture range. What happens when bodies of different temperatures meet in thermal contact? Energy is the driving force of the universe. What assumptions can you make about energy? Name the three types of heat flow. How does heat flow through radiation? LASERS Learning Objectives: Identify the principles of optics and lasers to include terms, theory, and the partical theory of light. Recognize the purpose of Q-switching and identify solid-state laser types. A laser is a device that produces or amplifies ultraviolet, visible, or infrared radiation. This is done by a process of controlled stimulated emission. The word laser is an acronym for light amplification by stimulated emission of radiation. The first lasers were used for surveying because they accurately measured distance. Later, lasers were used by the military. The initial military application of the laser was for fire control. To direct gunfire, the range to and the direction of the target must be determined. This is done by the laser system. Then, the data gathered by the laser system is used to direct the weapon system. Currently, the technology exists for laser designation of the target for laser-guided munitions. Military laser systems have both a range-finding capability for conventional munitions and a designation for laser-guided munitions. TERMS There are several terms that you may find useful when dealing with lasers. These are watts, irradiance, joules, and radiant exposure. Watts. A watt is a unit of power associated with light energy. Irradiance. Irradiance is the amount of power per unit area, watt/cm 2 . Energy cannot be created or destroyed. In a vacuum, the amount of energy that is available at the output of the laser is the same amount of energy contained within the beam at some point downrange. However, since lasers are not normally used in a vacuum, some energy is lost downrange. Figure 2-23 shows a typical laser beam. The amount of energy available within Figure 2-23.-Irradiance. 2-23

the sampling area is considerably less than the amount of energy available in the beam. For example, a 0.1-watt laser output might have 0.04 watt measured within a 1-square-centimeter (cm 2 ) sampling area. In this example, the irradiance is 0.04 watt/cm 2 . Joule. A joule is a unit of energy. It is the number of watts being delivered during a short period of time (1 watt per second). NOTE: The output of a continuous-wave (CW) laser is normally given in watts while the output of a pulsed laser is normally given in joules. Radiant exposure. Radiant exposure is the amount of energy per unit area, J/cm 2 . PRINCIPALS OF OPTICS AND LASERS NOTE: Before reading this section, you should review the information on light found in chapter 1. The theory of lasers was published around 1956. Along with the theory, a study was reviewed. In the study, methods of extending the range of lasers were looked at using various solids and gases as the method of range extension. It was from this study that laser theory evolved. The first laser was built in 1960 by Hughes Research Laboratories. A simplified solid-state laser currently used by the military is shown in figure 2-24. The elements of the laser are the material, pump source, optical Figure 2-24.-Typical solid-state laser. 2-24

cavity (amplifying and modifying the emission), and the output radiation. The electrons in the atoms of the laser material normally reside in a steady-state lower energy level. When energy is added to the atom, the electrons are raised to a higher energy level. The flash lamp (fig. 2-24) is the device used in the solid-state laser to add energy to the atoms. When energy is added to the electrons, they are in an unstable condition. They stay in this condition for a short time and then spontaneously return to their steady-state lower energy level. The transition of the electrons from the higher energy level to the lower energy level releases energy in the form of photons of light. The emitted light rays travel back and forth in the optical cavity through the lasing material between the 100-percent reflecting mirror and the 99-percent reflecting mirror. The photons collide with other excited electrons in the laser material, thereby stimulating the emission of other photons of light. The light energy is amplified in this manner until sufficient energy is built up to be transmitted through the 99-percent reflecting mirror. This action is termed lasing. The equipment that accomplishes lasing is the laser. Find the Q-switch shown in figure 2-24. It is used to provide pulses of extremely short duration. One type of Q-switching is provided by a rotating prism. Only at the point of rotation where there is a clear optical path is light energy allowed to pass. Another type of Q-switching device is a normally opaque electro-optical device such as a Pockels cell. At the time of voltage application, the Pockels cell becomes transparent to light. A complete optical path is formed that allows the transmission of light. The construction of the gas laser is slightly different from that of the solid-state laser. A glass tube filled with gas is placed in the optical path. This tube replaces the lasing material and flash lamp in the solid-state laser. A voltage (the external energy source) is applied to the tube. The light emitted from this type of laser is normally continuous wave rather than pulsed. Light from a conventional light source is extremely broadband. It emits several wavelengths across the electromagnetic spectrum. But, if you place a filter that allows only a very narrow band of wavelengths (such as a red filter) in front of a broadband light source, only red light exits the filter. An analogy can be made between the light from the filter and the light from the laser, with one exception—there is only a single wavelength emitted from the laser. The wavelength (or color) of light emitted from the laser depends on the type of material used in the laser. For example, if a Nd:YAG crystal is used as the material in the laser, the laser emits light with a wavelength of 1.064 micrometers. Look at figure 2-25. It shows you some of the types of material that are used for lasing and the wavelengths that are emitted by lasers using these materials. Note that some materials and gases emit more than one wave- length. In these cases, the wavelength of the light emitted depends on the optical configuration of the laser. Light from a conventional light source diverges or spreads quite rapidly. If you hold a sheet of paper near a 100-watt light bulb, the entire sheet is illuminated. Figure 2-26 shows the divergence (amount of beam spread) from a conventional light source. On the other hand, laser light has a very narrow beam divergence. If a sheet of paper is held the same distance from the laser as it was from the conventional light source, the laser light has a very narrow beam divergence; it shows a very small point of light (fig. 2-27). The laser light beam has a very narrow beam divergence. For example, if the paper were placed double the distance from the original point, the spot would be twice the size of the one first described. If a paper were held three times the distance, a spot three times the original size would be seen. Materials reflect, absorb, or transmit light rays. Reflection of light can be shown by using a mirror. If light rays strike a mirror, almost all of the energy incident on the mirror is reflected. Refer to figure 2-28. This figure shows how a plastic or glass surface acts on an incident light ray, The amount of energy transmitted, absorbed, and reflected equals the amount of energy incident on the surface of the material. A surface is termed specular when the sizes of surface imperfections and variations are much smaller than the wavelength of the incident optical radiation. A surface is termed diffuse when surface irregularities are randomly oriented and much larger than the incident optical radiation. In the intermediate region of the laser section of the electromagnetic spectrum, it is sometimes necessary to regard the diffuse and specular components separately. If light is incident upon an interface that separates two transmitting media (such as an air-glass interface), some light is transmitted while some is reflected, and no energy is absorbed at the interface. Since no energy is absorbed at the 2-25

Figure 2-25.-Laser electromagnetic spectrum. 2-26

Figure 2-26.-Divergence of a conventional light source. interface, T + R = 1.00; where T and R are the fractions of the incident beam intensity that are transmitted and reflected. T and R are the transmission and reflection coefficients, respectively. These coefficients depend not only upon the wavelength of the radiation, but they also depend upon the angle of incidence of the beam. The amount of the incident light beam that is reflected and the amount that passes through the material (transmitted) also depends upon the polarization (aligning the light to certain directions) of the light beam. The angle that an incident ray of radiation formed with the normal to the surface determines the angle of refraction and the angle of reflection (the angle of reflection equals the angle of Figure 2-27.-Divergence of a laser source. incidence). The relationship between the angle of incidence and the angle of refraction is n sine = n´ sine

where n and n´ are the incidence of refraction of the media that the incident and transmitted rays move through, respectively. A flat specular surface does not change the divergence of the incident light beam significantly. However, a curved surface may change the divergence, The amount of change in the divergence depends upon the curvature of the surface and the beam size incident to the surface. Figure 2-28.-Light ray incident on a glass surface. 2-27

Figure 2-29.-Specular reflectors. Figure 2-30.-Diffused reflectors. In figure 2-29, the reflection of an incident laser beam is shown on the two surfaces. (The divergence and curvature of the reflector have been exagger- ated.) You should note that the value of irradiance measured at a specific range from the reflector is less after reflection from the curved surface than when a beam is reflected from a flat surface. A diffuse surface is a surface that reflects the incident laser beam in all directions. The beampath is not maintained when the laser beam strikes it. Whether a surface is a diffuse reflector or a specular reflector depends upon the wave- length of the incident laser beam. A surface would be a diffuse reflector for a visible laser beam, while it might be a specular reflector for an infrared laser beam, such as CO 2. Look at figure 2-30. It shows the effect of different curva- tures of diffuse reflectors on incident laser beams. Q32. Q33. Q34. Q35. Describe the basic principle of a laser. What determines the wavelength (or color) of light emitted by a laser? Some terms are useful in dealing with lasers. These include watts, joules, and irradiance. What is meant by irradiance? What is meant by a diffuse surface? LASER THEORY To understand laser and infrared operation, you must understand wave propagation, the component parts of waves, and wave interaction. Wave Propagation Wave propagation is the travel of a wave through a medium. Refer to figure 2-31. Here a Figure 2-31.-Parts of waves. 2-28

plain wave is shown, and you can see that the propagation (direction of travel) is perpendicular to the lines of the crest. Another type of wave is a spherical wave that propagates outward like that which a pebble causes when it is thrown into a pond. Wave Optics When light strikes an object or a medium, it is either reflected or absorbed. Wave optics involve the reflection or absorption of waves. REFLECTION.— Refer to figure 2-32. This figure illustrates light reflection and refraction. As an incident wave strikes a reflective surface, it is reflected from the surface. If the reflective surface is smooth, the angle of reflection equals the angle of incidence. REFRACTION.— Again, refer to figure 2-32. When light passes through a transparent medium, it is bent or refracted. The term index of refraction refers to the amount that the light is bent or the angle of refraction. The higher the index of refraction, the more the light is bent. The index of refraction is a function of wavelength of the incident light. Since different colors have different wavelengths, they have a different index of refraction. DIFFUSION.— Earlier, you saw how light is reflected when it strikes a smooth surface. When the same type of beam strikes a rough surface, the light is scattered. The term used to describe this scattering is diffusion. Diffusion allows you to see nonluminous objects. Lens Optics Lenses are used extensively in laser and infrared system operation. Therefore, you need to understand lens optics before you can under- stand the system. A lens is defined as a piece of transparent material with two opposite refracting surfaces. Converging and diverging lenses are the two categories of lenses. Within these categories, there are three basic types of lenses—convex, concave, and meniscus (fig. 2-33), The converging lenses are thin at the edge and thick in the middle, while the diverging lenses are thick at the edges and thin in the middle. Figure 2-32.-Reflection and refraction. Figure 2-33.-Types of lenses. 2-29

THIN CONVERGING LENS.— A thin converging lens is shown in figure 2-34. Light rays traveling parallel to the axis of a thin convex lens are refracted so that they converge at a point called the focal point of the lens. The distance from the center of the lens to the focal point is the focal length of the lens. THIN DIVERGING LENS.— A thin diverg- ing lens is shown in figure 2-35. In the case of a thin diverging lens, light rays that travel parallel to the axis of the concave lens are refracted so that they diverge at a point known as the focus. The distance from the center of the lens to the focus is known as the focal length. Since the focus is on the viewing side of the lens, it is considered negative. Particle Theory of Light Light, and all other forms of electromagnetic radiation, is energy. Light is composed of particles called photons, which are bundles of massless energy. PHOTOELECTRIC EFFECT.— In 1887, Heinrich Hertz discovered that metals eject electrons when illuminated. This discovery gave rise to the particle theory of light. The photoelectric effect is shown in figure 2-36. The following conclusions can be drawn about the nature of light: The number of photoelectrons ejected is proportional to the intensity of light; that is, the more intense the light, the greater the number of photoelectrons ejected. Figure 2-34.-Thin converging lenses. 2-30

Figure 2-35.-Thin diverging lenses. Figure 2-36.-Photoelectric effect. 2-31

Maximum kinetic energy (Kmax) is a function of the frequency of incident light. Photoelectrons are ejected instantaneously, regardless of the intensity of the incident light. The surface of the specific metal has a threshold frequency; that is, the threshold is the minimum frequency of light that causes photoelectrons to be ejected. PHOTON THEORY OF LIGHT.— The photon theory of light was announced by Einstein in 1905. This theory explains the photoelectric effect and adds to the understanding of the photoelectric effect in the following ways: A beam of light is a stream of photons. The intensity of the beam is proportional to the number of photons in the beam. If one photon knocks out one electron, the photoelectrons will be proportional to the intensity of the beam. The energy created in the collision of the photons is transferred instantaneously. Stimulated Emissions Lasers operate by stimulated emission. Refer to figure 2-37 while you read this section. An excited atom is struck by a photon. The energy of the incident photon is equal to the transition energy of the excited atom, and the excited atom triggers or stimulates an emission from atom number two. The output produced by the stimulation is emitted instantaneously upon impact, and it is considered an amplified output. Refer to figure 2-38. The laser rod and the flash lamp are placed at the foci of the elliptical mirror (fig. 2-38, view A). The elliptical mirror can be focused on the laser rod and also the flash lamp. The flash lamp is fixed (fig. 2-38, view B). The photons from the lamp enter the laser tube, causing the tube to go to a high state (excited). The input light signal hits the excited atoms of the laser rod, causing stimulated emissions (fig. 2-38, view C). Finally, the amplified signal leaves the laser tube (fig. 2-38, view D). Q-Switching As you can see by looking at figure 2-39, uncontrolled laser output consists of a series of Figure 2-37.-Stimulated emission. 2-32

Figure 2-38.-Light amplification. Figure 2-39.-Typical laser output. 2-33

Figure 2-40.-—Pockels cell. Figure 2-41.-Laser pulse comparison. 2-34

sharp spikes with random heights and random intervals. Normally, this type of output is unusable. Some method of control is needed to regulate or change this output into a single pulse of demand, and quality switching (Q-switching) meets this need. There are many ways to provide Q-switching, from simple mechanical methods to more elabo- rate electronic methods. The type of Q-switching discussed in this chapter is the Pockels cell. POCKELS CELL.— The Pockels cell is a type of electro-optic Q-switch (fig. 2-40). The Pockels cell is placed between the laser rod and the mirror (fig. 2-40). This cell is composed of lithium niobium POCKELS CELL WITH ZERO VOLTAGE APPLIED.— When light from the laser strikes the first calcite prism, the calcite prism splits the light into ordinary o and extraordinary e beams, which are diverged slightly. These beams strike the second prism where they are bent or diverged again. They leave this prism in parallel and strike a mirror, which reflects them 100 percent. The light stays inside the Pockels cell; thus, there is zero output. POCKELS CELL WITH 5 KV APPLIED.— Look at figure 2-40, views A and B. Once again, the light from the laser strikes the first calcite prism. Again, it splits and becomes the o beam and e beam. These two beams strike the lithium niobium with the voltage applied, and it becomes birefringent. (Birefringent means to refract the light in different directions.) The outputs from the lithium are the e beam and the o beam, rotated by 90°. This causes the beams to interchange or become each other. The new o beam strikes the second prism where it is refracted sharply to hit the Porro prism, which reflects it sharply back into the optical path to provide the feedback that causes sustained optical oscillations (power buildup). The Pockels cell is the device that allows these oscillations to build until a threshold is reached. Then the laser fires (fig. 2-41). Solid-State Lasers The demand for lasers with diverse appli- cations caused the development of many types of lasers. Most lasers are grouped into five categories—solid state, gas, ion, chemical, and dye. Solid-state lasers were developed first and were most widely used for military applications. For this reason, solid-state lasers are the type discussed in this chapter. CRYSTALLINE LASERS.— Crystalline lasers are widely used. Two materials are used in these lasers: the matrix substance (host) and an impurity (dopant). The host is an inert, optically transparent crystalline substance. The main purpose of the host is to lattice sites (honeycomb arrangement) occupied by the dopant. The substances commonly used as the host include sapphires, yttrium aluminum garnet (YAG), fluorite, glass, calcium tungstate, and calcium molybdate. Dopants are ions of rare earth metals, with the exception of chromium. The most commonly used dopants are chromium, neodymium, holmium, erbium, uranium, and samarium. RUBY LASERS.— The host material for ruby lasers is sapphire crystalline alumina. The dopant is triply ionized chromium, which gives a charac- teristic red color. Although natural rubies could be used in lasers, their use is rare because large natural rubies with uniform color are rare. Synthetic crystals can be grown to a desired size with no flaws and uniform color. NEODYMIUM YAG LASER.— Normally, the YAG laser is used as a continuous-wave (CW) laser. The YAG is the host for the trivalent neodymium ion dopant. The neodymium gives the YAG a pale, reddish-purple color. The laser rod is produced synthetically, as is the ruby laser. The major difference between the ruby and YAG laser is the output wavelength. SEMICONDUCTOR DIODE LASERS.— A semiconductor functions somewhere in between a metal (conductor) and a nonmetal (insulator). At high temperatures, the semiconductor has low resistance; while at very low temperatures (near absolute zero), it has extremely high resistance. An example of a semiconductor diode laser is shown in figure 2-42. The semiconductor diode Figure 2-42.-Diode laser. 2-35

is made by sandwiching a diode between two metal conductors that are polished to provide feedback. The semiconductor diode laser has several advantages when compared to other types of lasers. They are more efficient. They have a wider bandwidth. They are faster and do not require Q-switching. Military Applications Target designation and range-finding are two of the military applications of lasers. TARGET DESIGNATION.— Target desig- nation is provided by a laser fixed on a target. A beam is reflected from the target and produces a small, bright spot. Then, a laser-guided bomb, shell, or missile can home in on the spot. To prevent the enemy from jamming the signal, a coded pulse repetition rate is added. RANGE-FINDING.— When used for range- finding, a laser fires a pulse of light that is pointed at a target. When the pulse is fired, a clock starts. The pulse strikes that target and is reflected. When the returning pulse is detected, the clock stops. Because the speed of light is known, this system is accurate to within 1 foot at a range of 2 miles. Q36. Q37. Q38. Q39. Q40. Explain wave optics. Name two categories of lenses. What is the particle theory of light? What is meant by stimulated emission (fig. 2-37)? List the five categories in which most lasers are grouped. DETECTING-RANGING SET (DRS) AN/AAS-33A Learning Objectives: Identify major com- ponents and functions of the AN/AAS-33A. Identify the system shop replaceable assem- blies (SRAs) and recognize their functions. The Detecting-Ranging Set (DRS) AN/ AAS-33A is part of the A-6E integrated weapons system. The DRS provides three electro- optical sensors and associated controls and indicators to enhance the all-weather capability of the weapons system to detect, recognize, and identify targets accurately. NOTE: While reading this section, you should refer to table 2-3 for a listing of the components and associated assemblies of the AN/AAS-33A. The physical location of the AN/AAS-33A within the aircraft can be seen by referring to figures 2-43 through 2-48. The three sensors are housed in a 20-inch, fully gimballed turret and are collectively known as the receiver group (RG). This group is installed in the aircraft underneath the radome, forward of the nosewheel. The three sensors are the laser range finder/designator (LRD), forward air controller (FAC) receiver, and forward looking infrared (FLIR) receiver. The LRD is also known as the laser receiver-transmitter. The LRD functions as a range finder and target designator. It provides range-to-target data to the ballistic computer set and designated targets for laser-guided bombs (LGBs). The FAC receiver is used as an aid for the bombardier/navigator (B/N) in locating a target designated by an external laser source from a ground observer or another aircraft. A laser source can serve as the offset aimpoint in the solution of a computer-controlled bombing attack. The FLIR receiver is a passive sensor that is used to detect targets of interest by their emitted infrared radiation. The infrared radiation signals are processed and a real-time, television-like image is displayed on the FLIR indicator. The SRAs consist of turret-stabilized platform, FLIR receiver, laser range finder designator, forward air controller receiver, reciprocating compressor, electronic control amplifier, generator processor, signal processor, infrared indicator, detecting ranging set control, power supply, and cable assembly. TURRET STABILIZED PLATFORM (TSP) The turret stabilized platform (TSP) consists of a two-axis turret and a vernier two-axis gimbal that provides azimuth coverage of –195° and elevation coverage of +20° to +180°. A turret stow position of 0° azimuth and –210° elevation 2-36

Table 2-3.-AN/AAS-33A Components I I REF NOMENCLATURE PLACARD OR DES COMMON NAME Components 89A1 89A2 89A3 89A4 89A5 89A6 89A7 89A8 89A9 89A10 02A2 02A3 02A11 03A2 14A10 23A1 Receiver Group 0R-203/AAS-33A or OR-203A/AAS-33A Major SRAs: 1. Forward Looking Infrared Receiver 2. Laser rangefinder/Designator or Laser Receiver-Transmitter (LRT) 3. Forward Air Controller Receiver 4. Turret Stabilized Platform Reciprocating Compressor HD-1032/ AAS-33A Power Supply PP-7417/AAS-33A Generator Processor 0-1761/AAS-33A Signal Processor CV-3460/AAS-33A Electronic Control Amplifier AM-6959A/ AAS-33A Infrared Indicator IP-1301/AAS-33A Detecting-ranging Set Control C-10301/ AAS-33A Temperature Control C-10358/AAS-33A Cable Assembly W1 of AN/AAS-33A 3-Way, 2-Position, DRS Solenoid Selector Valve Receiver group (RG) FLIR receiver Laser rangefinder designator (LRD) or laser receiver-transmitter (LRT) FAC receiver Turret stabilized platform (TSP) Compressor Low voltage power supply (LVPS) Laser transceiver electronics (LTE) Laser receiver electronics (LRE) Electronic control amplifier (ECA) Forward looking infrared indicator (FLIR) DRS control panel — Pulse forming network cable (PFN cable) Solenoid selector valve Associated Assemblies Nosewheel Well Circuit Breaker Box (Forward) Bombardier/Navigator Circuit Breaker Panel Nosewheel Well Circuit Breaker Panel (Aft) Top Deck Relay Box Temperature Control Box Caution Dim and Test Light Assembly Nosewheel well circuit breaker panel (FWD) CB panel (NWW) (Aft) — Caution lights panel 2-37

Table 2-3.—AN/AAS-33A Components—Continued REF NOMENCLATURE PLACARD OR DES COMMON NAME Associated Assemblies—Continued 50A1 Ballistics Computer CP-985/ASQ-133 or Ballistics computer CP-1391/ASQ-155A 50A3 Computer Control C-9535/ASQ-155 Pedestal control unit (PCU) 50A10 Analog-to-Digital/Digital-to-Analog A/D converter Converter CV-3163/ASQ-155 61A1 Mission Recorder Electronics Unit Electronics unit MX-9276/USH-17(V) 61A3 Mission Recorder Control Panel C-9071/ MISSION RECORDER control panel USH-17(V) 75A4 Power Supply PP-6574/APQ-148 Low-voltage power supply (LVPS) 75A12 Analog Display Indicator IP-722D/ ADI AVA-1 or IP-722F/AVA-1 75A15 F ault Locating Indicator ID-1933/APQ-156 BIT panel 75A16 Pilot’s Control Box PCB S67 Nose Gear Down and Locked Switch — S6030 Right Main Gear Weight-on-Wheels — Switch ANSWERS FOR REVIEW QUESTIONS Q36. THROUGH Q40. A36. Wave optics involve the reflection or absorption of waves. Light strikes an object or medium and is either reflected or absorbed. A37. Converging and diverging. A38. The particle theory of light states that “light is composed of particles called photons, which are bundles of massless energy.” A39. The energy of the incident photon in figure 2-37 is equal to the transition energy of the excited atom; the excited atom triggers or stimulates an emission from atom two. A40. Solid state, gas, ion, chemical, and dye. 2-38

Figure 2-43 .-Outside view A-6E. protects the three optical windows of the lower ball when the receiver group is not in the on-target mode of operation. A hydraulic motor connected to the aircraft hydraulic system provides power for turret outer azimuth drive. The elevation axis and inner gimbal drives are powered elec- trically. FLIR RECEIVER The FLIR receiver provides infrared target detection and recognition capability. It has a continuous optical zoom ratio capability of 5 to 1 (5x). A counterbalance weight moves in an opposing motion to the zoom to maintain a balance when the FLIR is installed in the TSP. LASER RANGE FINDER DESIGNATOR (LRD) The LRD provides target ranging and desig- nating capability. It contains separate telescopes for its transmitter and receiver, which view through a common window on the TSP. Com- puter control of range-finding and target desig- nation modes is provided. FORWARD AIR CONTROLLER (FAC) RECEIVER The FAC receiver provides position infor- mation of acquired targets that are illuminated by remotely operated ground or airborne laser designators. It receives the laser energy through a separate window on the TSP. A four-quadrant 2-39

Figure 2-44.-Aft view with pallets extended and radome raised. detector generates the position signals, which are processed to locate the position of a target symbol displayed on the FLIR indicator. RECIPROCATING COMPRESSOR HD-1032/AAS-33A The HD-1032/AAS-33A compressor is a piston device that is driven by a 115-volt ac, 400-Hz, three-phase induction motor that is an integral part of the compressor assembly. The compressor provides helium pressure pulses for the required cooling for the detectors. ELECTRONIC CONTROL AMPLIFIER (ECA) AM-6959/AAS-33A The ECA contains the electronics circuits that provide the capability to accurately position or show the receiver group up to 1 radian/sec in response to input signals from the ballistic computer. GENERATOR PROCESSOR 1761/AAS-33A The generator processor is also known as the laser transceiver electronics (LTE). It provides precise timing signals and a high-voltage firing pulse to the LRD. All mode commands and power for the laser subsystem interface with the rest of the DRS through LTE. SIGNAL PROCESSOR CU-3460/AAS-33A The signal processor is also known as the laser receiver electronics (LRE). It processes four video 2-40

Figure 2-45.-View looking inboard and aft with pallets stowed. signals from the FAC receiver, which are propor- tional to the position of a designated target in the FAC receiver field of view. INFRARED INDICATOR IP-130/AAS-33A The infrared indicator (fig. 2-48, view A) presents a high-resolution video display of the infrared scene in real-time on an 8-inch diagonal CRT. In-flight video tape recordings can be made and played back on the infrared indicator. Six status lights on the front panel provide the B/N with the operating status of the DRS subsystem. DETECTING-RANGING SET (DRS) CONTROL C-10301/AAS-33A The DRS control panel (fig. 2-48, view B) provides on/off power and mode command control logic for FLIR, stabilization, laser, and FAC subsystem operation. It also has controls for the FLIR indicator and FLIR subsystem. The DRS control panel also houses the BIT interface circuits between the aircraft BIT panel and the DRS WRAs. POWER SUPPLY PP-7417/AAS-33A The low-voltage power supply (LVPS) generates the low voltage necessary to operate the entire DRS system. CABLE ASSEMBLY WI (PFN CABLE) The PFN cable conducts the pulse-forming network voltage from the LTE to the receiver group. 2-41

FIBER OPTICS Learning Objectives: Describe fiber optics to include a basic system, advantages, and fiber construction. Describe light trans- mission, fiber types, cables, and coupling. Fiber optics has revolutionized the telephone industry and will become the preferred norm of aviation and electronics technology. You won’t see the cumbersome myriad of wires, connections, and cabling we have today. Weight will be reduced, and capabilities will be increased. As an Aviation Electronics Technician, you should see fiber optic technology in the near future. Fiber optics is not new. In the mid 1800s, William Wheeler patented a device for piping light from room to room, Alexander Graham Bells’ photophone could reproduce voices through detection of the amount of light received from a modulated light source. In the last decade, a practical means of sending light has evolved—in Figure 2-46.-Receiver group. the form of glass fibers. Figure 2-47.-Cockpit. 2-42

BASIC SYSTEM Figure 2-48.-FLIR indicator and control panel. The principles of fiber optics follow the basic properties of light, as discussed in chapter 1, and include refraction and reflection. Light traveling within a fiber obeys the laws of propagation. Fiber optics is the technique of sending data in the form of light through long, thin, flexible fibers of glass, plastic, or other transparent materials. A basic fiber optic system (fig. 2-49) consists of a transmitter, a fiber medium, and a receiver. The transmitter converts electrical signals into current to drive a light source for injection into a fiber. The fiber or fibers guide(s) the light to a light detector that converts the light back into an electrical signal. The receiver is a low-noise and large-voltage gain receiver that provides further processing. ADVANTAGES OF FIBER OPTIC SYSTEMS There are many advantages of using fiber optics over systems in use today. Some of these advantages are shown below: Fiber optics can be used in flammable areas because light, not an electrical pulse, is the energy sent. Figure 2-49.-Basic fiber optic system block. 2-43

Fiber optic systems are immune to radio frequency interference (RFI), electro- motive interference (EMI), and noise caused by lightning and cross talk. Fiber optic systems are immune to electromagnetic pulse effects induced by nuclear explosions. Fiber optics aren’t affected by moisture or temperature changes. Fiber optic systems are easy to repair. Fiber optic systems have very high data transmission rates. Fiber optic devices are small and lightweight. OPTICAL FIBER CONSTRUCTION A typical fiber is a transparent, dielectric cylinder (core) enclosed within a second trans- parent dielectric cylinder (cladding). The core and cladding are enclosed by insulation (fig 2-50). The dielectric cylinders consist of various optical glasses and plastics. The cladding, which has a relatively low index of refraction, encloses the core, which has a very high index of refraction. The cladding contains most of the transmitted light within the core. This low index prevents light leakage and increases efficiency. The insulation protects a single fiber or several fibers from stress and the environment. LIGHT TRANSMISSION The light injected into a fiber travels in a series of reflections from wall to wall between the core and cladding. The reflections depend on the cone of acceptance and resulting angles of refraction and reflection propagation (fig 2-50). The cone of acceptance is the area in front of the fiber that determines the angle of light waves it will accept. The acceptance angle is the half-angle of the cone of acceptance. The light enters the core and refracts to the interface of the core and cladding. The light reflects at the same angle of impact. The light, reflecting from wall to wall, continues at the same angle to the end of the fiber at the detector. Like the physics of light, the maximum critical angle is that angle that, when surpassed, won’t reflect; in this case, it is lost in the cladding of the fiber. As long as the light wave is at a lesser angle than the maximum critical angle of the fiber (as determined by the function of the fibers’ core and cladding indexes of refraction), light will travel to the receiver. TYPES OF OPTICAL FIBERS There are two types of optical fibers. The step-index type has large differences in the core Figure 2-50.-Transmission of light in a fiber. 2-44

and cladding indexes of refraction. When held constant, these differences cause light to reflect from the interface back through the core to its opposite wall. The graded-index type has a decreasing core refractive index as the radial distance from the core increases. This causes the light rays to continuously refocus as they travel down the fiber. These types operate in either single-mode or multi- mode operation. Single-mode operation accepts a specific wavelength, otherwise large attenuation will result. The multi-mode type operates over a range of wavelengths with minimum signal loss. (See fig. 2-51.) PROPERTIES OF OPTICAL CABLES Optical cables are affected by many physical properties, Some of these are discussed in the following section. Numerical Index The numerical index of optical cables deals with the sine of the angle of acceptance. The numerical aperature (NA) or numerical index can be found using the formula shown below: where i = acceptance angle, n1 = Core Index of Refraction, and n2 = Cladding Index of Refraction. The acceptance angle is a measure of the numerical aperature (NA) or numerical index of a fiber. This lets the manufacturer select the proper fiber for the desired specific light waves and for optimum power coupling. NA is a measure of the light capture angle (half- acceptance angle). It describes the max core angle of light rays that will be reflected down the fiber by total reflection. The refractive index (Index of Refraction) of a material is the ratio of the speed of light in a vacuum to the speed of light in the material. Review chapter 1 for more information on refraction if you don’t understand this section. The higher the refractive index of a material, the lower the velocity of light through the material. Also, there will be more refraction or bending of the light when it enters the material. If NA increases, angle i must have increased, and the fiber sees more light. NA can never be greater than 1.0; normal values are low (0.2 and 0.6). Dispersion Dispersion is the spreading or widening of light waves due to the refractive index of the material and the wavelength of the light traveling in the fiber. There are two types of dispersion— intermodal and intramodal. Intermodal (multi-mode) dispersion. Inter- modal dispersion is the propagation (travel) of rays of the same wavelength along different paths through the fiber. These wavelength rays arrive at the receiving end at different times. Figure 2-51.-Types of optical fibers. 2-45

Intramodal dispersion. is due to variations of the the core and cladding. Attenuation Intramodal dispersion index of refraction of Attenuation is the loss or reduction in amplitude of the energy transmitted. These losses are due to differences of refractive indexes and imperfections in fiber materials. Also, man-made scratches or dirt and light scattering within the fiber cause unwanted losses. Efforts to reduce these losses include the forming of the following standard parameters: Bandwidth parameters. Bandwidth param- eters include attenuation curves, which provide all designers the ability to chose the best fiber. These parameters are plotted in decibels per kilometer (dB/km). They measure the efficiency of the fiber as a comparison of light transmission to light loss through a fiber. Rise time parameters. These parameters set speed requirements for operation. Fiber strength parameters. These parameters set tensile strength standards to help reduce flaws and microcracks in the fiber. FIBER COUPLING One important aspect of a fiber system is the connection between the fiber and the other parts. The coupling efficiency is the ratio of power accepted by the fiber to the power emitted by the source Coupling efficiency increases with the square of the NA (numerical aperature) and decreases with source and fiber mismatches. Optical power coupled into the fiber is a function of the radiance of the source and the NA. Q41. Q42. Q43. Q44. Q45. A basic fiber optic system consists of a transmitter, a fiber medium, and a receiver. Describe the basic technique of fiber optics. List the advantages of fiber optic systems. By what means does light travel through a fiber optic? What is the difference between single-mode and multi-mode operation? Attenuation is the loss or reduction of energy transmitted. Efforts to reduce these losses include the forming of standard parameters. What are these parameters? 2-46

(THIS PAGE IS INTENTIONALLY LEFT BLANK.) 2-47

ANSWERS FOR REVIEW QUESTIONS Q41. THROUGH Q45. A41. Fiber optics is the technique of sending data, in the form of light, through long, thin, flexible fibers of glass, plastic, or other transparent materials. A42. (a) Usable in flammable areas (b) Immune to noises generated by RFI, EMI, lightning, and cross talk (c) Immune to electromagnetic pulse effects (d) Not affected by moisture or temperature changes (e) Easy to repair (f) Very high transmission rates (g) Small size and lightweight A43. The light injected into a fiber travels in a series of reflections from wall to wall between core and cladding. The reflections depend on the cone of acceptance and resulting angles of refraction and reflection propagation. A44. Single-mode types accept a specific wavelength, otherwise, large attenuation results. Multi-mode types operate over a range of wavelengths, with minimum signal loss. A45. (a) Bandwidth parameters provide designers the ability to choose the best fiber. (b) Rise time parameters set the speed requirements for fiber operation. (c) Fiber strength parameters set tensile strength requirements to help reduce flaws and microcracks in the fiber. 2-48

CHAPTER 5 AVIATION SYSTEMS FUNDAMENTALS AND SUPPORT EQUIPMENT Aviation Electronics Technicians (ATs) operate and maintain complex electronic installa- tions in modern naval aircraft. To do this, the AT must know aircraft systems and support equipment (SE). Therefore, you, as an AT, must also understand the systems and SE of a typical aircraft, such as the power generation equipment, the conversion units, the power control, regu- lation, and protection devices, and the general power distribution systems. AVIATION SYSTEMS FUNDAMENTALS Learning Objective: Identify systems char- acteristics for communications, navigation, radar, ECM, and ASW systems. In this chapter, you are introduced to a few equipments you may be responsible for main- taining. It includes coverage of displays, radar, IFF, air navigation, communications and data link, ECM, ESM, weapons control, and ASW acoustic and recorder systems. The Aviation Electronics Technician 2 (Organizational), NAVEDTRA 1 40 30, and Aviation Electronics Technician 2 (Intermediate), NAVEDTRA 1 4029, contain a more in-depth coverage of these subjects. The specific maintenance instructions manuals (MIMs) contain in-depth information on specific systems and equipment. DISPLAYS Learning Objective: Identify various types of displays used in aviation systems. To understand the basic fundamentals of any display system, you need to know the operation of cathode-ray tubes (CRTs), amplifiers, power supplies, and other solid-state devices. For more information about CRTs and related circuitry, you should refer to Navy Electricity and Electronics Training Series (NEETS), modules 6, 7, 8, 16, 18, and 21 for help in understanding electronics and troubleshooting many different types of display systems. Display systems can range from a simple monitor to a highly sophisticated head-up display (HUD). They include radar and loran indicators as well as most systems that use a CRT or visual display. Most display systems contain a CRT and associated circuitry to present information using a PPI-scan, A-scan and/or graphics, alphanumerics, and conies generation. The next section of this chapter contains information about some typical radar indicators. The various types and operational principles of radar indicators, such as the A-scope, B-scope, and PPI-scope, are discussed in NEETS, module 18. A-Scope Figure 5-1 shows a simplified block diagram and scan presentation of a typical A-scope. The A-scope is only included to show you how scopes work. Then, the more advanced types are discussed. In the operation of the A-scope, an initial trigger pulse from the timer is applied to both the radar transmitter and the one-shot (monostable) multivibrator. The one-shot multivibrator generates the following: A negative gate pulse that is fed to the range marker generator and the range sweep generators A positive gate pulse that is fed to the control grid of the CRT The gate pulse to the range marker generator causes a series of equally spaced range marks to be generated. These range marks are added to the receiver output signal in the video mixer. The output of the video mixer is applied between ground and one vertical-deflection plate of the 5-1

Figure 5-1.-Typical A-scope block diagram and scan presentation. CRT. The other vertical-deflection plate is connected to the vertical-centering control. The negative gate pulse fed to the range sweep generator causes a nearly linear sawtooth sweep voltage to be generated. The different timing capacitors in the one-shot multivibrator and in the range sweep generator are connected to a common range switch. Therefore, when the operating range is changed, the RC time constants of both circuits are simultaneously changed. When the duration of the negative gate pulse is changed, the duration of the sawtooth sweep voltage is changed; but, the amplitude of the sweep voltage is unchanged. Therefore, at different operating ranges, the scanning spot travels about the same distance across the A-scope screen. However, the speed of the scanning spot increases as the range setting is decreased. The sawtooth output of the range sweep generator is amplified by the range sweep amplifier. Then, it is applied to the paraphase amplifier (phase splitter). The paraphase amplifier outputs the sawtooth sweep voltage in push-pull fashion to the horizontal-deflection plates of the CRT. This reduces defocusing of the electron beam. The positive gate pulse applied to the control grid of the CRT intensifies the electron beam during the sweep time, displaying the output of the video mixer on the A-scope screen. When the positive gate pulse is removed, blanking results (the electron beam is cut off). 5-2

Clamping circuits are frequently used with A- scopes. They keep the display properly positioned despite changes in the average (de) value of the sweep or signal voltages. Remember, clampers hold one part of the signal waveform at a constant voltage level. In some A-scopes, expanded sweep circuits are used. These circuits let a small section of the sweep expand to cover the A-scope screen. Thus, more accurate range measurements are made. B-Scan The B-scan represents a compromise between the extremes of simple and complex circuitry. When radar requirements call for simple circuitry and construction, the B-scan is used. In the B-scan, three variables are possible: 1. Range (a function of time)

2. Azimuth (a function of antenna rotation)

3. Intelligence received by the radar or associated equipment B-scan circuitry involves the simplest circuitry construct ion of any two-dimensional presentation, yet it presents information as a reasonably faithful replica of the area scanned by the antenna (fig. 5-2). It works best under conditions where the antenna scans a sector of less than 180 degrees. However, it can be used in a situation where a 360-degree area is scanned. Range is usually presented vertically by the use of a conventional sweep circuit. Azimuth is Presented horizontally by the use of a potentiometer mechanically connected to the antenna. The intelligence is presented on the indicator by intensity- modulating the sweep. The antenna scanning speed is approximately one scan per second, and the sweep speed is at the PRF rate; therefore, the intelligence has range and bearing. C-Scope C-scopes (fig. 5-3) present data on the bearing and elevation of targets. C-type indicators may Figure 5-3.-C-scope presentation. Figure 5-2.-B-scan presentation. 5-3

Figure 5-4.—PPI presentation. 5-4

sometimes be used in aircraft interception. Like B-scopes, C-scopes provide a rectangular display on their screens. However, in C-scopes, the vertical axis represents elevation and the hori- zontal axis represents bearing. Thus, in aviation fire control radar, targets may appear on either side of both the horizontal and vertical axes. To get a rectangular display on the screen of a C-scope, both horizontal and vertical-sweep generators are used. Since the sweep frequencies are relatively low, potentiometers (like the azimuth sweep potentiometer of the B-scope) are generally used. These potentiometers are connected to the radar antenna, When the antenna turns sideways, the scanning spot on the C-scope screen is deflected horizontally. When the antenna is tilted up or down, the scanning spot is deflected vertically. Echo signals, applied to the control grid (or cathode) of the CRT during the sweep period, cause the brightness of portions of the horizontal trace to be increased. The position of a bright spot indicates the elevation and bearing of a target. Targets at different ranges, but with the same bearing and elevation, appear as a single spot on a C-scope. Targets of this kind cannot be distinguished individually on the C-scope. For this reason, an indicator that presents range data is generally used along with a C-scope. Once the range of a particular target is determined, a range gate pulse (rectangular pulse) is applied to the C-scope. This intensifies the electron beam only for the duration of the range gate pulse. Thus, only the desired target echo appears on the C-scope; all other signals are blanked out. By this means, the bearing and elevation of a particular target at a specific range is determined. PPI-Scope P-type indicators, known as plan-position indicators (PPI or PPI-scopes), are used to present the range and bearing data of targets. Like B- and C-scopes, PPI-scopes generally use CRTs with long-persistence screens. The PPI presentation is practically an exact replica of the region scanned by the radar antenna. Distance along the radial sweep line represents target range. Rotation of the radial sweep line, synchronized with the antenna’s rotation, produces a circular display. When echo signals are applied to the control grid (or cathode) of the PPI CRT during the sweep period, the brightness of portions of the radial sweep line is increased, Like the B-scope, an increase in the brightness of portions of the PPI radial sweep line results in a maplike picture. Figure 5-4 shows a typical PPI presentation. E-Scan (RHI) The range-height indicator (RHI) (fig. 5-5) is another type of scan used to present range and height information. The RHI is also known as an E-scan. The E-scan is a modification of the B-scan on which an echo appears as a bright spot. The range is indicated by the horizontal coordinate and the elevation (height) by the vertical coordinate. This type of scan is used in directing aircraft during ground- and carrier- controlled approaches and in fire-control systems for terrain clearance. Miscellaneous Presentations Many other types of radar indicators are used. Often, more than one type of presentation is incorporated into one indicator. Most indicators in aviation fire control radar use two or more electron guns—one gun is used to develop a B-type presentation, and the other to develop the various elements of an attack presentation. These elements may consist of an elevation strobe, artificial horizon, steering information, acquisition circle, and range circle. Some of the systems and equipment that use displays include radar, IFF, and fire control. RADAR Learning Objectives: Identify the characteris- tics of radar to include range, resolution, azimuth, and accuracy. Recognize the factors that affect radar performance . Identify the components of a pulse-modulated radar, and recognize the functions of the components within the system. Figure 5-5.-E-scan presentation. 5-5

The word radar applies to electronic equipment used to detect the presence of objects. Radar determines an object’s direction, altitude, and range by using reflected radio waves. Characteristics of Radar The characteristics of radar discussed in this section include the range, azimuth, resolution, and accuracy. Also, some of the factors that affect radar performance are discussed. RANGE.– Radar measurement of range, or distance, is possible because radiated radio- frequency (RF) energy travels through space in a straight line at a constant speed. However, the straight path and constant speed are altered slightly by varying atmospheric and weather conditions. Velocity. – RF energy travels at the speed of light, about 186,000 statute miles per second, 162,000 nautical miles per second, or 300 million meters per second. Radar timing is expressed in microseconds; the speed of radar waves is given as 328 yards or 984 feet per microsecond. One nautical mile is equal to about 6,080 feet. This means that it takes RF energy about 6.18 microseconds to travel 1 nautical mile. Range Measurement.– The pulse-type radar set determines range by measuring the time it takes for the emitted pulse to travel to the target and return. (This is known as the elapsed time.) Since two-way travel is used in range measurement, the elapsed time for the pulse to leave the antenna, travel to the target, and return takes a total time of 12.36 microseconds per nautical mile. The range, in nautical miles, of an object is found— 1. 2. by measuring the time that elapses during a round trip of the radar pulse (in microseconds), and then dividing this quantity by 12.36. Mathematically, The minimum range of a pulse radar is determined by adding the time of the transmitted pulse, or pulsewidth (PW), to the recovery time of the duplexer and the receiver. Recovery time is the time required for the receiver to become operative after the transmitter has fired. To find the minimum range (in yards) at which a target is detected— 1. add the PW (in microseconds) to the recovery time, 2. divide the result of step 1 by 2, and 3. multiply the result of step 2 by 328 yards. Mathematically, = (PW + recovery time) x 164 yd. Targets closer than this range are not seen. The receiver is inoperative for the time necessary for a signal to travel this distance. The maximum range of any pulse radar depends upon the transmitted power, PRF, and receiver sensitivity. The peak power of the transmitted pulse determines the maximum range that the pulse can travel to a target and return in usable echo strength. There must be enough time allowed between transmitted pulses for an echo to return from a target located at the maximum range of the system. AZIMUTH.– The azimuth (bearing) of a target is its clockwise angular displacement in the horizontal plane with respect to true north. This angle is measured with respect to the aircraft heading. In this case, it is relative bearing. The angle is measured from true north, giving true bearing, if the installation contains azimuth stabilization equipment. The angle is measured by using the directional characteristics of a unidirectional antenna. Then the position of the antenna is determined when the strongest echo returns from the target. RESOLUTION.– The range resolution of a pulse radar is the minimum resolvable separation, in range, of two targets on the same bearing, Range resolution is a function of the width of the transmitted pulse. The type and size of the targets and the characteristics of the receiver and indicator also affect resolution. With a well- designed radar, sharply defined targets on the same bearing are easy to resolve. Their ranges differ by the distance the pulse travels in one-half of the time of the pulsewidth (164 yards per microsecond of PW). If a radar set has a pulsewidth of 5 microseconds, the targets must 5-6

be separated by more than 820 yards before they could appear as two pips on the scope. The formulas for range resolution and minimum target separation are given below: range resolution = PW x 328 yd minimum target separation = PW x 164 yd Azimuth resolution is the ability to separate targets at the same range but on different bearings. Azimuth resolution is a function of the antenna beamwidth and the range of the targets. The antenna beamwidth is the angular distance between the half-power points of an antenna’s radiation pattern. Two targets at the same range appear as one target instead of two. They must be separated by at least one beamwidth to distinguish between them. Strong multiple targets appearing as one target are resolved in azimuth (bearing) by reducing the gain of the receiver. ACCURACY.– The accuracy of a radar is a measure of its ability to determine the correct range and bearing of a target. To determine the degree of accuracy in azimuth, the effective beamwidth is narrowed. On a PPI scope, the echo begins to appear when energy in the edge of the beam first strikes the target. The echo is strongest as the axis of the beam crosses the target. The echo continues to appear on the scope as long as any part of the beam strikes the target. The target appears wider on the PPI than it actually is. The relative accuracy of the presentation depends on the width of the radar beam and range of the target. The true range of a target is the actual distance between the target and the radar set (fig. 5-6). In airborne radar, the true range is called slant range. The term slant range indicates that the range measurement includes the effect of a difference in altitude. The horizontal range of a target is a straight-line distance (fig. 5-6) along an imaginary line parallel to the earth’s surface. This concept is important. An airborne target, or the observer’s aircraft, only needs to travel the distance represented by its horizontal range to reach a position directly over its target. For example, an aircraft at a slant range of 10 miles at an altitude of 36,000 feet above the radar observer’s aircraft has a horizontal range of 8 miles. The timing sequence of a radar range- indicating device starts at the same instant that the transmitter starts operation. Therefore, with airborne surface-search radar, the first targets seen are those directly beneath the aircraft. However, Figure 5-6.-Slant range versus horizontal range. 5-7

on the PPI scope, there is a hole in the middle of the picture (fig. 5-7), with a minimum radius corresponding to the altitude of the aircraft. The hole is known as the altitude ring. Objects directly beneath the aircraft appear on the scope at a distance equal to the distance between the aircraft and ground. Factors Affecting Radar Many factors affect radar performance; the principal one is maintenance. Keeping the equipment operating at peak efficiency affects the overall capabilities and limitations of the radar. A second factor is the radar operator’s knowledge of the equipment. This knowledge must include the maximum and minimum ranges at which the operator can expect to pick up various targets, the range and bearing accuracy of the gear, and the range and bearing resolution. If the radar is a height finder, the operator must know the altitude determination accuracy and the altitude resolution. Some of the factors that affect radar are covered below. For more detailed information, you should refer to the maintenance instruction manual (MIM) for each radar. PEAK POWER.— The peak power of a radar is its useful power. The range capabilities of the radar increase with an increase in peak power. Figure 5-7.-Effect of altitude on radar. (A) Radar tilted down; (B) radar with zero tilt. 5-8

Doubling the peak power increases the range capabilities by about 25 percent. PULSEWIDTH.– The longer the pulsewidth, the greater the range capabilities of the radar because of the greater amount of RF energy sent out in each pulse. In addition, because narrow bandpass receivers are used, the noise level is reduced. Remember though, an increase in pulse- width increases the minimum range and reduces the range resolution capabilities of the system. BEAMWIDTH.– The beamwidth is in degrees between the half-power points in the radiation pattern. The effective beamwidth of a radar is not a constant quantity, The receiver gain (sensitivity) and the size and range of the target affect it. The narrower the beamwidth, the greater the concentration of energy. The more concentrated the beam, the greater the range capabilities for a given amount of transmitted power. RECEIVER SENSITIVITY.– The sensitivity of a receiver is a measure of the ability of the receiver to amplify a very weak signal. Increasing the receiver sensitivity increases both the detection range of the radar and the radar’s ability to detect smaller targets. However, sensitive receivers are easier to jam, and interference shows on the scope more easily. INDICATORS.– The choice of the type of scope used to display weak pips adds to the capabilities of the radar. A deflection-modulated A-scope would be more sensitive to weak echoes than the intensity-modulated PPI. A weak target is seen on the A-scope before it can be detected on the PPI. ANTENNA ROTATION.– The more slowly the antenna rotates, the greater the detection range of the radar. Therefore, an antenna that is not rotating has the greatest range in the direction it is pointing. For tactical reasons, antennas are rotated. Pointing the antenna beam at the target momentarily allows you to gain information about the composition of a target. Q1. Q2. The A-scope’s positive gate pulse goes to the control grid of the CRT, causing the electron beam to What type of display works best under conditions where the antenna scans a sector of less than 180 degrees? Q3. Q4. Q5. Q6. Q7. The PPI scope provides what type of presentation? List the factors that affect the maximum range of pulse radars. What are the characteristics of radar? Define azimuth resolution. Why does a long pulse width increase or decrease the range capabilities of a radar? Functional Components of Pulse-Modulated Radar The functional breakdown of a pulse- modulated radar can be divided into six essential parts 1. 2. 3. 4. (fig, 5-8). The synchronizer (also known as the timer or keyer) supplies the synchronizing signals that time the transmitted pulses and the indicator. It also coordinates other associated circuits. The transmitter generates the RF energy in the form of short, powerful pulses. The antenna system takes the RF energy from the transmitter, radiates it in a highly directional beam, receives any returning echoes, and passes these echoes to the receiver. The receiver amplifies the weak RF pulses returned by the target and reproduces them as video pulses, which are applied to the indicator. Figure 5-8.-Functional block diagram of a funda- mental radar system. 5-9

5. 6. The indicator produces a visual indication of the echo pulses in a manner that furnishes the required information. The power supply provides the electrical power for the radar set. The physical configuration of radar systems differ. However, the fundamental characteristics remain the same. Radar also works with the identification friend or foe (IFF) system. Normally, the IFF antenna is mounted on and shares the radar antenna, and its information is displayed on the same radar scope. IDENTIFICATION FRIEND OR FOE (IFF) Learning Objective: Recognize IFF theory of operation to include interrogation and transponder functions. Identification friend or foe (IFF) was developed because of the destructive power of modern weapon systems and the speed of their delivery. You cannot wait to identify a detected radar target. Figure 5-9 shows a typical IFF system. It consists of an interrogator unit, a coder synchronizer unit, a search radar unit, and a Figure 5-9.-IFF system block diagram. 5-10

transponder unit. The interrogator, synchronizer, and radar units make up the challenging station. The transponder unit is the responder station. By looking at figure 5-9, you can see that the challenging station can be a ground station, a ship, or another aircraft. The responder station is normally an aircraft. There are five modes of IFF operation used by the air traffic control radar beacon system (ATCRBS) and naval aircraft–mode 1, mode 2, mode 3/A, mode C, and mode 4. In addition, there is a test mode used only by the aircraft transponder as a self-check of the transponder equipment. Modes 1 and 2 are used exclusively by the military as tactical modes for target identification. Mode 3/A is used at military and civilian air traffic control stations. Mode C is used with an external pressure altitude digitizer to report the aircraft’s altitude to an ATCRBS. Mode 4 is a military encrypted mode, which is controlled by an external computer. The operation of mode 4 is classified. Only interrogators and transponders using the same encrypted codes can respond. NAVIGATION Learning Objectives: Recognize the naviga- tion-related terms and definitions basic to inertial navigation system operation. Recognize the operating principles and characteristics of the inertial navigation system, to include Schuler loops and tuning. Recognize components and operat- ing principles and features of airborne navigation systems used by the Navy. Navigation is the procedure by which you move from one point to another point. Air navigation is the process of directing the movement of an aircraft from one point to another. The function of air navigation is to locate positions and measure distance and time along the intended direction of flight. Terms As you read about air navigation, you must understand the terms that are being used. In this part of the TRAMAN, you will learn about some of these terms. Position. Position is a point defined by stated or implied coordinates. One basic problem of navigation is to fix a position. If navigators do not know where they are, they can’t direct the movement of the aircraft to its intended destination. Direction. Direction is the position of one point in space relative to another, without reference to the distance between them. Direction may be either three-dimensional or two- dimensional. For example, the direction of San Francisco from New York is approximately west (two-dimensional). However, the direction of an aircraft from an observer on the ground may be west and 20° above the horizontal (three- dimensional). Direction is not itself an angle, but it is often measured in terms of its angular distance from a reference direction. Course. Course is the intended horizontal direction of travel. For example, the direction of NAS Jacksonville from NAS Pensacola is east. This should be the intended direction of flight. Heading. Heading is the horizontal direction in which an aircraft is pointing. Heading is the actual orientation of the aircraft’s longitudinal axis at any instant. The term heading includes the following: True heading uses the direction of the geographic North Pole as the reference. Magnetic heading uses the direction of the earth’s magnetic field at that location as the reference. Compass heading differs from magnetic heading by the amount of magnetic deviation. Magnetic heading differs from true heading by the amount of magnetic variation at that location. Compass heading differs from true heading by the amount of compass error (deviation ± variation). Bearing. Bearing is the horizontal direction of one terrestrial point from another. Bearings can be expressed by two terms—true north or the direction in which the aircraft is pointing. If true north is the reference direction, the bearing is a true bearing. If the reference direction is the heading of the aircraft, the bearing is a relative bearing. If you get a bearing by radio, it is a radio bearing; if visual, it is a visual bearing. You can accurately describe the direction between two 5-11

objects on or near the surface of the earth by saying: THE (RADIO, VISUAL) BEARING OF A FROM B IS X ± (RELATIVE/TRUE). Distance. Distance is the separation between two points. To measure distance, you measure the length of a line joining the two points. This seems understandable enough. However, suppose that the two points are on opposite sides of a baseball. How do you draw the line? Does it run through the center of the ball or around the surface? If around the surface, what path does the line follow? You must qualify the term distance used in navigation to show how to measure the distance. The shortest distance on the earth’s surface from NAS San Diego to Sydney, Australia, is 6,530 miles. If you travel via Honolulu and Guam, a frequently used route, it is 8,602 miles. You can express the length of a chosen line in various units, such as miles, kilometers, or yards. Time. Time has many definitions. The two definitions used with navigation are— 1. the hour of the day, and 2. an elapsed interval. The first appoints a definite instant, as takeoff time is 0215. The second definition appoints an interval, such as time of flight, 2 hours 15 minutes. Poles. The earth’s geographic poles are the extremities of the earth’s axis of rotation. As the earth rotates, a man on the surface facing the direction of rotation has the North Pole on his left. East is in front of him, the South Pole is on his right, and west is behind him. The earth has some of the properties of a bar magnet. The magnetic poles are the regions near the ends of the magnet. This is where the highest concentration of magnetic lines of force exist. However, the earth’s magnetic poles are not at the geographic poles, nor are they opposite each other. Great circles and small circles. The intersection of a sphere and a plane is a circle. The intersection is a great circle if the plane passes through the center of the sphere. It is a small circle if it does not. Parallels and meridians. Look at figure 5-10. Here, the earth’s equator is a great circle. If a Figure 5-10.-The equator is a great circle whose plane is perpendicular to the polar axis. second plane (fig. 5-11) passes through the earth parallel to the equator, its intersection is a small circle. If the small circles are perpendicular, then all points on the small circle are equidistant from the equator; that is, the circles are parallel to the equator. Such small circles, together with the equator, are parallels. Parallels are one component of a system of geographical coordi- nates, Planes that pass through the earth’s poles (fig. 5-12) form great circles. Great circles through the poles of the earth are meridians. All meridians are perpendicular to the equator. Meridians form the second part of a system of geographical coordinates. These coordinates are commonly used by navigators. Latitude and longitude. Look at figure 5-13 . You can identify any point on earth by the intersection of a parallel and a meridian. It is the same as an address at the corner of Fourteenth Street and Seventh Avenue. Figure 5-11.-The plane of a parallel is parallel to the equator. 5-12

Figure 5-12.-Great circle through the poles form meridians. You just use different names for identifying the parallels and meridians. Latitude is the north- south geographical coordinate and longitude is the east-west geographical coor dinate. Longitude is described as being east or west of Greenwich, England. This longitude at Greenwich is the Prime Meridian of 0°, the starting point. Longitude extends 180° east and west of the Prime Meridian, and it is broken down into degrees, minutes, and seconds. A degree is divided into smaller units. However the common method of subdividing the degrees is by— 1. degrees—60 minutes (60'), and 2. minutes—60 seconds (60"). To convert minutes or seconds into decimals of degrees, divide by 6. Thus, 15°30' = 15.5°, and 15°30'24" = 15°30.4'. Variation. The earth’s true (geographic) poles and its magnetic poles are not at the same locations. Lines of magnetic force are not generally straight because of irregu-lar iron deposits near the earth’s surface. Since a compass needle aligns to the lines of force at its location, it may not point to true or magnetic north. When connected together, lines connecting the locations on the earth where the compass does point to true north form an irregular line. This is the agonic line. At other locations, the angle between the direction of true north and the direction of the earth’s magnetic field is the location’s variation. Lines connecting locations having the same variation are known as isogonic lines. The earth’s field direction may not be the same as the direction of the magnetic poles. This same angle is also often called the angle of declina- tion. You label variation (or declination) east or west as the magnetic field direction Figure 5-13.-Longitude and latitude. 5-13

Figure 5-14.-Easterly magnetic variation. is east or west, respectively, of true north. (See figures 5-14 and 5-15.) Deviation. Deviation is the error in a magnetic compass caused by nearby magnetic influences. These influences may relate to magnetic material in the structure of the aircraft and to electrical (electronic) circuits. They deflect a compass needle from its normal alignment with the earth’s magnetic field. These deflections are expressed as degrees. The deflection is east or west as the compass points east or west, respectively, of the earth’s magnetic lines of force. Deviation varies with the heading of the aircraft. Figure 5-16 shows one reason for this deviation. Compass error. The net result of both variation and deviation is the compass error. If Figure 5-15.-Westerly magnetic variation. Figure 5-16.-Deviation changes with heading. variation and deviation have the same name (east or west), you add to get compass error. If they have different names, subtract the smaller from the larger. Give the difference given as the name of the larger. (See fig. 5-17.) Label variation and deviation plus (+) if west, and minus (–) if east. Example 1. Given: Required: Solution: Variation 7° west (W), deviation 2° west (W). Compass error. 7°W + 2°W = 9°W. To fly a true course of 135°, this aircraft over this spot on the earth would fly a compass heading of 144°. Example 2. Given: Variation (–)2°, deviation (+)5°. Required: Compass error. Solution: (–)2° + 5° = (+)3°. Magnetic dip. At the magnetic poles, the direction of the earth’s magnetic field is vertical (perpendicular to the earth’s surface). Along the aclinic line (sometimes called the magnetic 5-14

Figure 5-17.-Effect of compass error. equator) roughly half way between the poles, the field’s direction is parallel to the earth’s surface (horizontal). The difference between the direction of the earth’s field and the horizontal at any location is the magnetic dip. The magnetic dip varies from very small angles near the equator to very large angles near the poles. You can measure the angles with a dip needle, which is a magnetic needle free to turn about a horizontal axis. A line connecting all locations having equal dip angles is an isoclinic line. Dead reckoning. Dead reckoning is the process of determining a position from the record of a previously known position, course, speed, and time traveled. To be accurate, every change of course and speed during the flight is considered. It does not matter whether the pilot or the air mass (wind) through which the aircraft is flying makes the changes. Radar navigation. Modern radar is a valuable aid to navigation. Some radars present a maplike display of the terrain around the aircraft on the screen of a CRT. This lets the pilot go beyond some of the limitations of visual observations. Radar transponders are devices that do not operate until interrogated or triggered into action by a signal from another radar transmitter. Then, they transmit their own signal, which the interrogating radar receives. These are used both as fixed navigational aids, such as radar beacon stations, and as airborne identification friend or foe (IFF) systems. Doppler radar detects and shows actual ground speed and drift of an aircraft, regardless of wind speed or direction. Radar altimeters give the actual distance from the aircraft to the surface below, The surface below can be a body of water or land mass far above sea level. Radio navigation. Radio navigational aids vary from a fairly simple direction-finding receiver to complex systems using special transmitting stations. These special stations make it possible to fix the position of an aircraft with considerable accuracy. The usable range varies according to its intended use, and also with weather and ionospheric conditions. Beacon stations associated with an instrument landing system (ILS) are usually of low power. Long-range air navigation (loran) stations have a range extending to 1,400 miles under favorable conditions. Aviation Electronics Technicians (ATs) maintain the airborne portions of radio and radar systems. Celestial navigation. Celestial navigation is the method of fixing the position of the aircraft relative to celestial bodies. Since the earth is constantly revolving, an accurate time device is necessary. In celestial navigation, three references are needed. The navigator tries, whenever possible, to select three bodies about 120 degrees apart in azimuth. This results in lines of position that cross cleanly and minimizes the effects of a constant error in the observations. Inertial navigation. An inertial navigation system (INS) is a dead-reckoning device that is completely self-contained. It is independent of its operating environment, such as wind, visibility, or aircraft attitude. It does not radiate or receive RF energy; therefore, it is not affected by countermeasures. An INS makes use of the physical laws of motion that Newton described three centuries ago. Air Navigation Air navigation is the process of determining the geographical position and maintaining the 5-15

desired direction of an aircraft relative to the earth’s surface. Certain conditions are unique to air navigation and have a special impact on the navigator. Continued motion. If necessary, a ship or land vehicle can stop and resolve any uncertainty of motion or wait for more favorable conditions. Most aircraft must keep going. Limited endurance. Most aircraft can remain aloft for only a relatively short time, usually a matter of hours. Greater speed. Navigation of high-speed aircraft requires detailed flight planning, navi- gation methods, and procedures that are quick and accurate. Effect of weather. Visibility affects the availability of landmarks. Wind directly affects the position of aircraft. Changes of atmospheric pressure and temperature affect the height measurement of aircraft using barometric altimeters. The primary problem in air navigation is to determine the direction necessary to accomplish the intended flight, to locate positions, and to measure distance and time as means to that end. The following equipments are used in airborne navigation. HORIZONTAL SITUATION INDICATOR (HSI).— Aircraft, such as the P-3, use the horizontal situation indicator to provide the pilot with a visual indication of the navigational situation of the aircraft. BEARING-DISTANCE-HEADING INDI- CATOR (BDHI).— The BDHI is used with various navigation systems and provides information according to the mode selected. Some aircraft have more than one BDHI (fig. 5-18), wit h separate select switches for each instrument. The distance counter numerals may be in a vertical row or horizontal. The lubber index is a fixed reference mark at the top of the instrument face. The compass card (read under the lubber index) shows the aircraft heading (either true or magnetic, depending on Figure 5-18.-Bearing-distance-heading indicator. the mode used). Two pointers, a single bar and a double bar, can indicate the following: Bearing to a ground electronic station Bearing to destination Aircraft ground track Aircraft drift angle Heading error The BDHI select switch selects the available combinations of these indications in a given aircraft configuration. ATTITUDE HEADING REFERENCE SYSTEM (AHRS).— The AN/ASN-50 attitude heading reference system (fig. 5-19) generates and provides continuous roll, pitch, and heading signals. These signals go to the aircraft attitude indicator and other avionics equipment. Error signals develop in the displacement gyroscope as a result of displacement of synchro sensing devices from their null position. A remote compass transmitter supplies additional heading informa- tion to the system. For detailed information on the AN/ASN-50 system, you should refer to Reference Altitude Heading, NAVAIR 05-35LAA-1. 5-16

Q8. Q9. Q10. Q11. Q12. Q13. Figure 5-19.-Attitude heading reference system. List the units in an IFF that make up the Q14. challenging station. A point that is defined by stated or implied Q15. coordinates is known as a . Q16. The intended horizontal direction of travel is known as . Q17. In what two reference directions can you express bearings? Q18. The east/west geographical coordinate is known as . Q19. You measure longitude 180° east or west from what point? Q20. 5-17 The angle between true north and the direction of the earth’s magnetic field is known as . How do you label variation? Magnetic influences cause what type of error in magnetic compasses? The net result of both variation and deviation is known as . You can determine a position from the record of a previously known position, course, speed, and time traveled by what process? What navigation system makes use of the physical laws of motion that Newton described three centuries ago? Describe navigation.

Inertial Navigation System The inertial navigation system (INS) is sometimes maintained by personnel in the Aviation Electronics Technician (AT) rating. Some squadrons have an integrated weapons team (IWT). It is composed of the three avionics/armament division (work center 200) ratings—AT, AO, and AE. Navigation is defined as the process of directing a vehicle from one point to another. Navigation can be divided into two basic categories—position fixing and dead reckoning. In position fixing, you determine position relative to positions of known objects such as stars and landmarks. The most common example of navigation by position fixing is celestial navi- gation. Loran is another example of navigation by periodic position fixes. Except for INS, navigation systems rely on some information that is external to the vehicle to solve its navigational problem. Dead reckoning, the second category, is the process of estimating your position from the following known information: Previous position Course Speed Time elapsed Two examples of navigation by dead reckoning are Doppler radar and inertial navigation systems. BASIC PRINCIPLES.– The operating principle of the inertial navigation system (INS) ANSWERS FOR REVIEW QUESTIONS Q8. THROUGH Q20. A8. The interrogator, synchronizer, and radar. A9. Position. A10. Course. A11. True north or the direction the aircraft is pointing. A12. Longitude. A13. Prime Meridian, 0 degree in Greenwich, England. A14. Variation. A15. You label variation east or west as the magnetic field direction is east or west, respectively, of true north. A16. Deviation. A17. Compass error. A18. Dead reckoning. A19. Inertial navigation. A20. Air navigation is the process of determining the geographical position and maintaining the desired direction of an aircraft relative to the earth’s surface. 5-18

is Newton’s first law of motion. This law states “Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it.” In laymans terms, this law says that a body at rest t ends to remain at rest. It also says a body in motion tends to remain in motion, unless acted upon by an outside force. The full meaning of Newton’s first law is not easy to visualize in the earth’s reference frame. An inertial reference system can be defined as a nonrotating coordinate frame. It can be either stationary or moving linearly at a uniform speed in which there are no inherent forces such as gravity. A true inertial system can exist only in empty space, far from any mass. A reference system attached to the earth can closely approximate an inertial system. For this system to work, you must balance the gravitational force on a body by a second force. For example, an object sliding on a flat, frictionless plane on the earth’s surface moves in a nearly straight line. The object will have a nearly constant speed. Newton’s second law of motion is as important as his first law in an inertial navigation system because the inertial navigation system works on Newton’s second law. Newton’s second law of motion states “Acceleration is proportional to the resultant force and is in the same direction as this force.” Written mathematically— where, F = force m = mass a = acceleration The physical quality in the equation that pertains to the inertial navigation system is acceleration. You can derive velocity and displace- ment from acceleration. Differentiation is the process of investigating or comparing how one physical property varies with respect to another. Integration, the reverse of differentiation, is the process of summing all rate of changes that occur within the limits under investigation. The inertial navigation system is an integrating system. Yet, before integration can be done, it must first have a rate of change. Therefore, the inertial navigation system is a detector and an integrator. It first detects changes of motion. It then integrates these changes of motion with time to arrive at velocity, and again with time to arrive at displacement. DOPPLER RADAR PRINCIPLES.– Doppler radar uses continuous-wave (CW) radio- frequency (RF) transmission along with the Doppler effect. Pulse-type radar determines the distance to the target by measuring the period between transmission of a pulse and receipt of the reflected pulse. The CW Doppler radar senses velocity by measuring a proportional shift in frequency of the reflected signal. This frequency shift is the Doppler effect. Airborne Navigation Systems The airborne navigation systems now in use are classified as either self-contained or ground- referenced. A self-contained system is complete in itself. It does not depend on the transmission of data from a ground installation. Some self-contained systems, such as search radar and Doppler radar, do require transmission of energy from the aircraft. Other self-contained systems, such as the inertial system and celestial-referenced aids, are completely passive in operation; they do not radiate energy from the aircraft. Ground-referenced aids include all aids that depend on transmission of energy from the ground. THE IDEAL SYSTEM.– Every navigation system has certain advantages and disadvantages. An ideal system would not have to contend with advantages of one system over another. Such an ideal system would have the following characteristics: Ground information. The system indicates the ground position of the aircraft. Global coverage. The system positions and steers the aircraft accurately and reliably any place in the world, Self-contained. The system does not rely on ground transmissions of any kind. 5-19

Passive operation. The system does not betray the position of the parent aircraft by transmitting signals of any kind. Immune to countermeasures. The system is not susceptible to countermeasures of any type. Useless to enemy. The system does not provide navigational aid or intelligence of any kind to enemy forces. Flexible. The system is flexible. The system tracks the aircraft, even when unplanned devia- tions are made from the preflight course. The system also operates at any altitude and at any speed within the capability of the aircraft. ADF .– Radio beacons transmit a nondirectional signal that is easily identified as a specific station. If an aircraft has automatic direction finding (ADF) equipment, the direction of the beacon from the aircraft can be determined. Most low-frequency, direction-finding equipment receives any frequency between 100 and 1750 kHz. UHF/DF.– Some aircraft are equipped with automatic direction finders in the UHF frequency range (225.00 to 399.95 megahertz), which use loop and sensing (antennas) to give bearing information. Operation of the direction finder is controlled from the UHF radio panel. It is used to obtain a bearing to other aircraft and to emergency locator beacons that operate on 243.0 MHz and 282.0 MHz. VOR/ILS.– The VHF omnidirectional range (VOR) is a radio aid that has practically eliminated interference due to atmospheric conditions. VOR stations operate between 108.00 and 117.95 MHz. Station identifiers for VOR navaids are given in code or voice or by alternating code and voice transmission. The VOR provides an infinite number of courses or radials from the station. The VOR also provides instrument landing system (ILS) capability. The transmission principle of the VOR is based on creating a phase difference between two signals. RMI.– The RMI is a bearing indicator, usually with two pointers and a movable compass rose. The compass rose rotates as the aircraft turns, indicating the compass heading of the aircraft under the top of the index at all times. Therefore, all bearings taken from an RMI are magnetic. BDHI.– The BDHI is similar to the RMI in that a pointer provides magnetic bearing information. Additional information concerning the BDHI is contained in the TACAN section. HSI.– The HSI gives the pilot a visual indication of the navigational situation of the aircraft. Tactical Air Navigation System (TACAN) The tactical air navigation (TACAN) system provides the crew with information needed for precise positioning within 200 nautical miles. As with VOR, TACAN provides an infinite number of radials radiating outward from the station. In addition, distance measuring equipment (DME) provides continuous slant-range distance information. TACAN operates in the UHF band and has 126 channels available in the X-mode pulse code. Pulse coding gives ground equipment the capability of an additional 126 channels in the Y mode. The station identifier is usually trans- mitted at 37.5-second intervals in international Morse code. Airborne DME transmits on 1025 to 1150 MHz; associated ground-to-air frequencies are in the 962 to 1024 MHz and 1151 to 1213 MHz ranges. Channels are separated at 1-MHz intervals in these bands. TACAN DME is designed to provide range information to a maximum distance of 200 to 300 nmi, depending on aircraft equipment. The air-to-air (A/A) function is provided to give distance information between two aircraft, working in the same manner as a regular ground- based TACAN station. Some sets provide only DME information. Newer sets provide both distance and bearing information to other aircraft. To obtain useful information, the A/A function should be selected by both aircraft with a 63-channel frequency separation, In addition, each aircraft must have the same mode (X or Y) selected. If one aircraft sets A/A channel 4 and the other sets A/A channel 67 in the X band, useful information should be obtained. TACAN bearing is presented on an RMI (bearing), a BDHI, and a HSI (bearing and DME). The BDHI and HSI combine an RMI with a distance or range indicator, which saves space by displaying TACAN information on a single instrument. 5-20

Long Range Navigation (Loran) The name loran is derived from the words long range navigation, which describes the hyperbolic system of electronic navigation, It provides lines of position over the surface of the earth. Over water, usable loran signals can be received at ranges up to 2,800 miles. The loran system consists of a series of synchronized chain (set) of radio transmitting stations. These stations broadcast pulse signals similar to those used in radar with a constant time interval between them. The transmitting stations are the foci. The aircraft has a combination radio receiver and time difference measuring device. The measurements made by this equipment are used to make entries in tables or charts that identify the hyperbola on which the receiver is located. The loran receiver is similar to an ordinary radio receiver, except that it has no speaker. The output of the receiver is fed to a loran base indicator. The base indicator is an electronic device capable of measuring the time difference between the receptions of the master and secondary signals with high precision. This indicator measures the time difference by one of the following methods: 1. 2. 3. Using a CRT to provide a visual display of the incoming signals. By visually aligning these signals, a reading of the time difference measurement is obtained. Automatically, by the loran set. It provides readings of the time difference. Integrating with a computer to display latitude and longitude. Readings obtained by these methods are plotted on a loran plotting chart, or, in the case of direct latitude/longitude readouts, they are plotted on any chart. OMEGA Navigation System Loran has significantly improved navigation over water and is very accurate up to 800 nmi. At distances over 1,000 nmi, sky waves must be used. Sky wave use causes a loss in position accuracy. Omega is an accurate long-range system that overcomes these problems. The very low frequency (VLF) used by Omega transmitters increases range. To get an accurate fix, a navigator obtains simultaneous signals from three different Omega stations. There are only eight Omega stations worldwide; yet, they provide worldwide coverage (fig. 5-20). These eight Figure 5-20.-Omega transmitter locations. stations actually operate at 10 to 13 kHz and use a signal phase difference rather than a time-of- arrival signal. Omega transmitting stations operate in the internationally allocated very low frequency (VLF) navigational band between 10 and 14 kHz. The VLF lets Omega provide navigational signals at much longer ranges than other ground-based navigational systems. The eight transmitting stations provide worldwide coverage with an inherent potential fixing accuracy of 2 to 4 nautical miles 95 percent of the time. Navigational Computer Systems When automatic sensing devices are tied into a navigation computer system, the navigator is automatically provided current readings of present latitude and longitude, ground speed, and heading. The navigation computer system eases the navigator’s workload and frees him or her to make the decisions that are beyond the capability of computers. To handle the many flight conditions at the speed of sound or faster, the navigator uses automatic navigation computers. The navigational 5-21

computer system consists of the following com- handle, the navigator simultaneously changes the ponents: position of the cross hairs and the corresponding coordinate measurements (east-west and north- The data-gathering units (sensors) such as south) being fed to the navigation computers. The radar, Doppler, INS, LORAN, and function is completed almost instantaneously. TACAN When the navigator positions the cross hairs on a given return, the computers determine the Computer units where the computations distance between the aircraft and the return. If and comparisons are made the coordinates of the return have been set in the computer, the computer can maintain a running Navigation panels containing the dials and account of the aircraft latitude and longitude. controls that give the navigator a system- monitoring and control capability SENSORS.– Sensors are data-gathering units such as radar, Doppler, INS, LORAN, and TACAN. Radar.– When a radar set is incorporated into the computer system, movable electronic cross hairs are displayed on the radarscope so that range and direction of radar returns are measured and inserted into the computer (fig. 5-21). The cross hairs consist of a variable range mark and a variable azimuth mark. They are maneuvered with a cross hair control handle. On the radarscope, they resemble a single fixed-range mark and a heading mark. By moving the cross hair control Doppler.– Doppler radar’s contribution to the computer system is ground speed and drift angle. These two outputs are put to several uses in the computer system. Doppler ground speeds is used to drive the present position latitude and longitude counters. Doppler outputs are used in platform leveling and in checking inertial ground speed in an inertial system. Doppler radar is an essential part of many navigation computer systems. INS.– The INS is used to feed velocity information into the computers. Once the inertial sensor is leveled and in operation, it is used to continually update the present position counters. Loran.– Loran fits in well with an automatic computer system. Some computer systems have Figure 5-21.-Radar cross hairs. 5-22

the coordinates of loran stations stored in them. During flight, the navigator selects the stations, and the computer does the rest. Fixing is automatic and occurs in the same way that the navigator takes a celestial fix. An assumed position is determined by the computers; then, the loran position is applied to this assumed position. A series of credibility checks and approximations are applied automatically to the computer. The result is an accurate loran fix. When the computer functions in the loran mode, continuous present position and ground speed information is still available. TACAN.– TACAN can easily be added to a computer system. Since the TACAN output is given in the form of a range and bearing, the computers only need the coordinates of the TACAN station being used. This data is set into the computer before the mission begins. Some corrections must be applied to TACAN outputs to increase accuracy. The bearings received from TACAN are magnetic; therefore, the computer must have an accurate magnetic variation value at all times. This is usually built into the computer. TACAN range output is expressed in slant range. The computer applies absolute altitude above the station to the slant range to produce exact ground range. COMPUTER UNITS.– The two basic types of navigation computers are the analog and the digital computer. graphic replica of the problem to be solved is constructed to find the answer, The analog computer is generally larger than the digital computer because many components must be added to solve a wide variety of problems. The analog computer has one main advantage—it is not as sensitive to temperature and pressure changes as the digital system. Digital.– The digital computer is generally lighter and more compact than the analog system. In some cases, the digital computer weighs less than 100 pounds. It computes navigation problems in the same way as the analog computer. It is unnecessary to design a digital computer expressly for the navigation problems it is to solve. Properly programmed, the same computer could be used in fields other than navigation. This is possible because the digital computer deals strictly with numbers. This requires that all inputs be changed to a numerical value before they are sent to the computer. Likewise, all outputs must be converted back to terms that are meaningful to the navigator. NAVIGATION PANELS.– The navigation panels make up the greatest part of the computer system visible to the navigator. Panel appearance and operation vary with each computer system. The multitude of counters, dials, switches, buttons, control knobs, and selectors give the navigator maximum use and control of the system. Selectors that determine which sensors are used and which readouts are given let the Analog.– An analog computer is comparable navigator switch from one mode of operation to to the navigator’s handheld computer because a another, as shown in figure 5-22. Figure 5-22.-Typical control display unit. 5-23

The computer system helps the navigator. Most modern computers have limits built into them so they will not accept unreasonable information. For instance, if the coordinates of a fix point are set 1 degree of latitude in error, the computer rejects the fix because the information is totally incompatible with information already in the computer. A rapid change in ground speed from a sensor might be rejected and that sensor output no longer used because it would be considered unreliable. So far in this discussion, only basic navigation has been considered. A sophisticated computer system can solve ballistic problems and auto- matically release bombs and missiles. If the system is installed on a transport-type aircraft, cargo drops and notification of bailout time to paratroops can be controlled by the navigation computer. Q21. Q22. Q23. Q24. Q25. Q26. Q27. Q28. Q29. Describe differentiation. Define a self-contained navigation system. State the transmission principle of the VOR. What is the frequency range of the transmitted airborne TACAN DME? With the addition of X and Y modes to the TACAN system, what total number of channels are available? Loran determines the difference by measuring time intervals between the arrival of the first signal and the arrival of a second signal. What type of measurements can be used? State the basic reason for incorporating the navigational computer in aircraft . List the data-gathering units of a typical navigational computer system. What other uses can the sophisticated computer system provide? COMMUNICATIONS AND DATA LINK Learning Objectives: Identify communica- tions and data link systems and recognize their purpose. Recognize the interface structure between, and the operating features of participating units of a data link system. Radio communications is a highly sophisti- cated field of electronics. Even small Navy aircraft have the capability to come up on the commonly used communication circuits. Some common circuits include ship-to-ship, ship-to-air, air-to-air, air-to-ground, and ship-to-shore. Telecommunications refers to communica- tions over a distance. It includes any transmission, emission, or reception of signs, signals, writings, images, or sounds. It also includes intelligence produced by visual means, oral means, wire, radio, or other electromagnetic systems. Electrical, visual, and sound telecommunications are all used in the Navy. The basic equipment used to communicate are the transmitter and receiver, Transmitters and receivers each perform two basic functions. The transmitter generates a radio- frequency (RF) signal of sufficient power at the desired frequency and has a means of varying (or modulating) the basic frequency so it can carry an intelligible signal. The receiver selects the desired RF signal you want to receive and rejects all unwanted RF signals. In addition, the receiver detects the intelligence of the signal and amplifies the weak incoming signal to overcome the losses the signal suffers in its travel through space. Navy Frequency Band Use Table 5-1 shows the radio-frequency (RF) spectrum broken down into bands used by the Table 5-1.-Radio-Frequency Spectrum FREQUENCY DESCRIPTION 30 GHZ—300 GHZ extremely high frequency 3 GHZ—30 GHZ superhigh frequency 300 MHZ—3 GHZ ultrahigh frequency 30 MHZ—300 MHZ very high frequency 3 MHZ—30 MHZ high frequency 300 KHZ—3 MHZ medium frequency 30 KHZ—300 KHZ low frequency 3 KHZ–30 KHZ very low frequency 300 HZ—3 KHZ voice frequency up to 300 HZ extremely low frequency 5-24

military. Propagation of radio waves varies widely at different frequencies. Frequencies and equipment are chosen to meet the communications application desired. The frequency bands of particular interest to the Aviation Electronics Technician (AT) are discussed in the following paragraphs. For information on the other bands, refer to Navy Electricity and Electronics Training Series (NEETS), module 17, Radio-Frequency Communications Principles. MEDIUM-FREQUENCY (MF) BAND COM- MUNICATIONS.– The medium-frequency (MF) band of the radio-frequency spectrum includes the international distress frequencies (500 kHz and about 484 kHz). Only the upper and lower ends of the MF band have naval use. Frequencies in the lower portion of the MF band (300 to 500 Hz) are normally used for ground-wave transmission. They provide for transmission over moderately long distances over water and for moderate to short distances over land. Transmission in the upper MF band is generally limited to short-range communications (400 miles or less). HIGH-FREQUENCY (HF) COMMUNICA- TIONS.– Successful transmission of HF signals over long distances depends on the refraction of radio waves by layers of the ionosphere. Ultra- violet radiation from the sun determines the height and density of these layers. They vary significantly with the time of day, season of the year, and the 11-year cycle of sunspot activity. Naval communications within the HF band fall into groups of four general types of services. They include point-to-point, ship-to-shore, ground-to-air, and fleet broadcast. All of these services, except the fleet broadcast service, normally operate with two-way communications. Some of these services involve ships and aircraft that present special problems because of their physical characteristics and mobility. These special problems of HF performance are at least partially offset by powerful transmitters and sensitive receiving systems at the ship/shore terminals. VERY-HIGH-FREQUENCY (VHF) AND ABOVE COMMUNICATIONS.– Normally, frequencies above 30 megahertz are not subject to refraction (bending) by the atmosphere, and ground-wave range is minimal. This normally limits the use of this frequency spectrum to line of sight. However, you can increase range through tropospheric scatter techniques, Some com- munications using VHF and above frequencies use a technique called forward propagation by tropospheric scatter. Certain atmospheric and ionospheric con- ditions can also extend the normal line-of-sight range. Frequencies at the lower end of this band are capable of overcoming the shielding effects of hills and structures to some degree. However, as the frequency increases, the problem becomes more prominent. Reception is notably free from atmospheric and man-made static. The very-high- frequency (VHF) and ultra-high-frequency (UHF) bands are within the line-of-sight transmission bands. Amplitude-Modulated Systems Amplitude modulation (AM) is a method used to vary the amplitude of an electromagnetic carrier frequency according to the intelligence carried by the carrier. The carrier frequency is a radio- frequency (RF) wave suitable for modulation by the intelligence to be transmitted. One form of amplitude modulation is to interrupt the carrier using a prearranged code. The on-off keying of a continuous-wave (CW) carrier (fig. 5-23) frequency is one way to modulate a carrier. The intervals of time when a carrier is present or absent carries the desired intelligence. As applied to a continuously oscillating RF source, on-off keying is known as CW signaling, or as an interrupted continuous wave (ICW). The primary disadvantages of AM modulation are susceptibility to noise interference and the inefficiency of the transmitter. To overcome the susceptibility to noise interference, angle modulation was developed. Angle Modulation Angle modulation is modulation in which the angle of a sine-wave carrier is varied by a Figure 5-23.-Continuous-wave modulation. 5-25

modulating wave, Frequency modulation (FM) signals from 190 kHz to 550 kHz and from 2 MHz and phase modulation (PM) are two types of angle to 25 MHz, in five frequency bands, A mechanical modulation. In FM, the modulating signal causes type counter, located on the front panel of the the carrier frequency to vary. These variations are receiver (fig. 5-24), shows the frequency, in MHz, controlled by both the frequency and amplitude of received signals. It can receive signals that are of the modulating wave. In PM, the phase of the of the amplitude modulated (AM), unmodulated carrier is controlled by the modulating wave form. In frequency modulation (FM), an audio signal is used to shift the frequency of an oscillator at an audio rate. Frequency-shift key (FSK) is the simplest form of FM, and it is similar to CW keying in AM transmissions. For more information on AM, FM, and pulse modulation principles, refer to Navy Electricity and Electronics Training Series (NEETS), module 12, Modulation Principles, NAVEDTRA 14184. General-Purpose Receiver A typical general-purpose receiver, consisting of a receiver and its mounting, is a super- heterodyne receiver. It is capable of receiving RF Figure 5-24.-Megahertz frequency indicator. ANSWERS FOR REVIEW QUESTIONS Q21. THROUGH Q29. A21. Differentiation is the process of investigating or comparing ho w one physical property varies with respect to another. A22. A self-contained system is complete in itself; it does not depend on the transmission of data from ground installations. A23. The VOR transmission principle is based on creating a phase difference between two signals. A24. 1025 MHz to 1150 MHz. A25. 126 channels in X and 126 channels in Y, 252 total channels available. A26. CRT display, automatically by the loran set, integrating with the computer. A27. To handle the many flight conditions at the speed of sound or above. A28. Radar, Doppler, INS, loran, and TACAN. A29. Solving of ballistic problems, automatic release of bombs and missiles, cargo drops, and notification of bailout times are just a few. 5-26

continuous wave (CW), or frequency shift keyed (FSK) types. HF Transceiver The typical HF transceiver transmits and receives communications in the high-frequency (HF) band and can operate on a frequency range from 2.0 to 29.999 MHz. The set may include a radio receiver-transmitter (RT), radio set control, and mounting. The RT unit is usually of modular construction and easy to maintain. In addition to the set components, the complete aircraft installation may require a headset, microphone, key, antenna coupler, and antenna. VHF Transceivers The main purpose of VHF transceivers is to provide two-way communications between aircraft, ships, and shore stations. They normally operate within the frequency range of 116 MHz to 149.95 MHz. Some VHF transceivers are dual purpose. Their receivers also work with the VHF omnidirectional rapid range (VOR) navigation systems. When used for this purpose, the frequency range of the receiver extends to cover 108 MHz to 151.95 MHz. UHF Transceivers There are two main types of UHF trans- ceivers—frequency modulated (FM) and ampli- tude modulated (AM). Typical FM UHF transceivers operate between 225.0 MHz to 399.9 MHz, with channels spaced 100 kHz apart. Typical AM UHF transceivers operate between 225.0 MHz to 399.975 MHz, with a fixed guard frequency of 243. MHz. Intercommunications Systems All aircraft intercommunication systems perform essentially the same basic functions. They deliver audio to one or more selected stations on board the aircraft to permit crew members to speak to each other. They also provide control of the communication facilities so various members of the crew may receive incoming radio messages or transmit messages with the aircraft transmitters. It is also necessary for the intercommunication system to contain facilities for operating recording equipment. This lets you make permanent records of the various receptions and transmissions occurring during flight. Communication Antennas An antenna is a special type of electrical circuit intentionally designed to radiate and/or receive electromagnetic energy. In an ordinary circuit, the inductance (L), capacitance (C), and resistance (R) properties lump together and are constant. Therefore, the electromagnetic field is confined to the circuit where it performs useful work. In an antenna, the L, C, and R properties spread out, and the electromagnetic field tends to escape or radiate. It is this radiated field that provides the link between a transmitter and receiver. While the simplest type of antenna is the bidirectional dipole, limitations in directivity, frequency bandpass, and gain somewhat restrict its use. Other dipole configurations such as the ram’s horn and the corner reflector are for special applications. Although the crossed dipole, the whip, the top-loaded vertical, or the J antennas are in use, the ground plane antenna is probably the most popular. This is especially true when reception or transmission must be equally effective in all directions (omnidirectional), For much higher frequencies, the biconical or the disc horn is an excellent antenna. The log periodic, helical, and flat-spiral antennas have an extremely wide (as high as 20: 1) operating frequency range. When space is not a controlling factor, the rhombic and the V type provide high gain and directivity. They can be unidirectional by terminating the ends of the legs with a non- inductive resistor. The V can be unidirectional by use of another V spaced an odd number of quarter wavelengths behind the original. Typical legs for the rhombic are three to four wavelengths; for the V type, legs of eight wavelengths are not uncommon. The parabolic antenna can produce high gain and excellent directivity. Although screen mesh, or even a grid or rod, provides increased stability where wind resistance is a design factor, the reflector element generally consists of a solid surface. Physically, the reflector should be several wavelengths in diameter. The radiating element may be a dipole, a horn, or other suitable radiator. Mounting a hemispherical reflector in front of the dipole may increase gain providing its surface area does not appreciably shadow the rear parabolic reflector. 5-27

Data Link System Interfacing and Operation The data link system is a communications link that provides computer-to-computer exchange of information. A typical link may include tactical ASW data between an aircraft and other partici- pating units (PUS) and reporting units (RU) via RF transmission. Data link transmission includes communication and navigation information, voice communications, secure (coded) voice communi- cations, sonobuoy information, and computer data. A data link system is an integrated communi- cations system that uses the functions and components of various communications systems to provide the data link capability. A modern data link system has the following components installed on the aircraft. A general-purpose digital computer (GPDC) A switching logic unit (SLU) A data terminal set (DTS) An integrated radio control (IRC) A secure data keyer A communication system (HF and UHF radio equipment discussed earlier) ELECTRONIC COUNTERMEASURES (ECM), ELECTRONIC SUPPORT MEASURES (ESM), AND WEAPON CONTROL Learning Objectives: State the meaning and purpose of the two basic categories of ECM—electronic and nonelectronic. Identify various types of deception and jamming devices used in ECM and recognize their characteristics. Describe the weapon control fundamentals to include the primary problem, ballistics, and trajectory. The purpose of ECM equipment is to detect, analyze, locate, and degrade the use of an enemy’s electronic warfare equipment. To do this, the Navy uses two basic categories of airborne ECM systems—passive ECM (PECM) or ESM and electronic and nonelectronic ECM (designed to jam or block an electronics system). Because ECM equipment is classified, no in-depth theory or circuitry is discussed in this TRAMAN. ESM Indicator Units ESM operations are not directly detectable by the enemy because they do not transmit. The purpose of ESM equipment is to detect (receive), plot (locate), and analyze the signal characteristics of a suspected enemy’s communications, navi- gation, and radar equipments. To do this, an ESM system must have receivers that cover the entire frequency spectrum and a direction-finder (DF) type of antenna system. They also require indicators with circuitry to analyze and display the various signal characteristics. You may know ESM as passive electronic countermeasures (PECM). The terms PECM and ESM are synonymous. ESM indicators give the operator a visual picture or digital readout of the received signal, let the operator analyze and determine the required signal characteristics, and plot the location of the transmitting station. There are three basic classes of indicators—panoramic adapters, digital display indicators, and pulse analyzers. Electronic Countermeasures (ECM) The design of defensive ECM equipment is primarily to protect a single aircraft from an enemy radar. This equipment is also referred to as a deceptive ECM system because it deceives rather than jams a radar system. The two basic categories of ECM equipment are electronic and nonelectronic ECM. ELECTRONIC ECM EQUIPMENT.– Various types of electronic ECM equipments deceive various types of radars, such as search, fire-control, etc. The method of deception (such as time delay for search radar and frequency shifting for fire-control radar) may vary, but the operating concept is the same. For example, to deceive a threat radar signal, false information is injected, and the signal is retransmitted with increased power. The ECM equipment receives the threat radar signal, amplifies it, detects the pulse, delays the pulse a few seconds, and retransmits the pulse. Some ECM equipment not only injects time delays, but transmits multiple pulses that show up as multiple targets on a radar’s indicator. 5-28

Regardless of the type of deception used, the threat radar cannot plot the correct location of the aircraft. Fire-control radar will not be able to “lock-on” the aircraft, ACTIVE ECM/JAMMING.– Active ECM is a term given to ECM electronic equipment designed to jam communications, navigation, and radar receivers. These jammers are high-power, noise-modulated transmitters that transmit random noise over a given band of frequencies. This high-powered noise overdrives (jams) the receiver of the target equipment and makes it useless. In threat radar, the jammer signal will cause the indicator to blossom. It blossoms because the jammer’s powerful noise signal overdrives the radar receiver’s circuits. When the radar receiver’s circuits are overdriven, the receiver puts out a constant video signal for an area where the noise signal is stronger than the receiver’s maximum sensitivity. In this way, one ECM jammer can protect (hide) a group of aircraft over a large area. NONELECTRONIC ECM.– Another means of deceiving a threat radar is by using chaff. Chaff is the general name given to packaged strips of metal foil that resembles confetti. When chaff ejects from an aircraft, it disperses into the air and causes multiple echo signals (targets) on the radar’s indicator. The metal foil is cut to the correct wavelength of the radar transmitting frequency, so it will reflect maximum echo signals back to the radar receiver. Weapon Control Fundamentals The primary problem of aircraft weapons systems is to accurately determine the correct position and attitude in which to place the aircraft. Correct positioning of the aircraft gives reasonable assurance of a hit on the target. No matter how difficult or how simple the problem, two terms are always present in the solution of the problem—ballistics and trajectory. Ballistics refers to the science of the motion of projectiles. It is a study of all the various forces, both controllable and uncontrollable, that govern the movements of projectiles. The study of ballistics includes two branches— interior and exterior. The study of interior ballistics involves the movement of projectiles inside a gun barrel or bore. The study of exterior ballistics involves the motion of the projectile in free air after it leaves the bore of the gun or the launcher. Exterior ballistics is the branch of ballistics with which you are concerned. To understand exterior ballistics, you must fully understand the term trajectory. Trajectory is the curve a projectile describes in space as it travels to the target. For guns, trajectory is from the muzzle to the first point of impact . For rockets and missiles, the actual ballistic trajectory is that portion of the distance to the target under free flight (after burn time). For bombs, the trajectory is from the time of release to the time of impact. Weapons Systems Concept As aircraft altitudes increased and speeds reached the supersonic regions, the ability of the attacking aircraft to perform its mission became more difficult. To engage a target at supersonic speeds was impossible when depending only on the operator for accuracy. The result of solving these problems was the current aircraft—a completely integrated machine. Each of the separate systems are subsystems interconnected and dependent, to some extent, on each of the others. For example, the navigation system depends on the radar system; and the automatic flight control system depends on a computer. The computer depends on both the radar and naviga- tion systems for proper operation. A weapons system includes the following: Units that detect, locate, and identify the target. Units that direct or control the delivery unit or the weapon, or both. Units that deliver or initiate delivery of the weapon to the target. Units that destroy the target when in contact with it or near it; these units are usually termed weapons. 5-29

Q30. Q31. Q32. Q33. Q34. Q35. Q36. Q37. ASW To what NEETS module should you refer for information on radio frequency com- munications principles? What two transmission bands are contained within the line-of-sight transmission band? What is meant by the statement “some VHF transceivers are dual purpose?” To what NEETS module should you refer for more information on AM, FM, and pulse modulation principles? What type of communications antenna is excellent for higher frequencies? What term is given to electronic ECM equipment designed to jam communica- tions, navigation, and radar receivers? Describe ballistics . Define trajectory. ACOUSTIC AND RECORDER SYSTEMS Learning Objectives: Recognize the operat- ing principles of magnetic anomaly detec- tion (MAD). Recognize the classification, specifications, and operating principles of sonobuoys currently in use. Recognize the functions of and the relationship between components comprising magnetic tape recorder systems used on Navy ASW aircraft. The most feasible method of detecting a submerged submarine was to detect its disturbance of the local magnetic field of the earth. The development of the sonobuoy has made it possible to detect submarines using sound-ranging equip- ment (sonar) by aircraft. Principles of Magnetic Detection Light, radar, and sound energy cannot pass from air into water and return to the air in any degree that is usable for airborne detection. However, the lines of force in the earth’s magnetic field pass through the surface of the ocean essentially undeviated and undiminished in strength. The change of medium from water to air or air navigation has little or no effect on magnetic lines of force. Consequently, detection of an object under the water can occur from a position in the air above it if the object has magnetic properties that distort the earth’s magnetic field. A submarine has sufficient ferrous mass and electrical equipment to cause a detect- able distortion (anomaly) in the earth’s field. Detection of this anomaly is the function of magnetic anomaly detection (MAD) equipment. The maximum range of submarine detection is a function of both the intensity of its magnetic anomaly and the sensitivity of the detector. NOTE: A magnetometer is the detector in MAD equipment. A submarine’s magnetic moment (magnetic intensity) determines the intensity of the anomaly. The magnetic moment depends mainly on the submarine’s alignment in the earth’s field, its size, its detected latitude, and the degree of its permanent magnetization. ANOMALY STRENGTH.– A submarine’s anomaly is usually so small that MAD equipment must be capable of detecting a distortion of about one part in 60,000. This is because the direction of alignment of the earth’s magnetic lines of force rarely change by more than one-half of 1 degree in a submarine anomaly. COMPENSATION.– Regardless of its source, strength, or direction, any magnetic field may be defined in three axial coordinates. That is, it must act through any or all of three possible directions—longitudinal, lateral (transverse), or vertical—in relation to the magnetometer detector. Compensation for magnetic noises is necessary to provide a magnetically clean environment. This ensures the detecting system will not be limited to the magnetic signal associated with the aircraft itself. Under ideal conditions, all magnetic fields acting on the magnetometer head are completely counterbalanced. In this state, the effect on the magnetometer is the same as if there are no magnetic fields at all. This state exists only when the following ideal conditions exist: 1. 2. 3. The aircraft is flying a steady course (no maneuvers) through a magnetically quiet geographic area. Electric or electronic circuits remain either on or off during compensation. Direct current of the proper intensity and direction flow through the compensation coils, so all stray fields are balanced. 5-30

To approximate these conditions, the com- pensation of MAD equipment usually occurs in flight, well at sea. In this way, the equipment compensation occurs under operating conditions, which closely resemble those of actual ASW search flights. Sonobuoys and Associated Receivers and Recorders Sonobuoys are aircraft-deployed, expendable sonar sets that contain a VHF radio transmitter to relay acoustic information to the deploying aircraft. The detection, localization, and identification of potentially hostile submarines is the primary mission of the U.S. Navy airborne antisubmarine warfare (ASW) forces. The ASW capability of the fleet and the Navy operational readiness to deal with the submarine threat critically depends on sonobuoys. Sonobuoys detect underwater sounds, such as submarine noise and fish sounds. These audio frequency (AF) signals modulate an oscillator in the RF transmitter portion of the sonobuoy. The output of the transmitter is an FM-modulated, VHF signal that is transmitted from the sonobuoy antenna. The signal is received by the aircraft that dropped the sonobuoy. This signal is detected and processed by a sonobuoy receiver. By analyzing the detected sounds, the ASW operator can determine various characteristics (such as propeller shaft speed) of the detected submarine. The use of several sonobuoys operating on different VHF frequencies in a tactical pattern lets the ASW operator localize, track, and classify a submerged submarine. Sonobuoys may be grouped into three cate- gories—passive, active, and special-purpose. Passive sonobuoys are used in LOFAR and DIFAR systems. Active sonobuoys are used in CASS and DICASS systems, and special-purpose sonobuoys ( BTS and DLC) are used for missions other than ASW. PASSIVE SONOBUOY.– The passive sonobuoy is a listen-only sonobuoy. The basic acoustic sensing system that uses the passive sonobuoy for detection and classification is the low-frequency analysis and recording (LOFAR) system. LOFAR System. In the LOFAR system, sounds emitted by the submarine are detected by a hydrophone from a passive omnidirectional sonobuoy. Data on the frequency and amplitude of these sounds are then transmitted by the sonobuoy antenna to a receiving station. At this station, normally located on board the deployment aircraft, the sound data is analyzed, processed, displayed, and recorded, The basic LOFAR display plots the frequency of the sound waves against the intensity of their acoustic energy and against the duration of the sound emission. This data can be displayed on a video screen and printed out. The data is also recorded on magnetic tape for storage and retrieval when desired. DIFAR System. The directional low-frequency analysis and recording (DIFAR) system is an improved passive acoustic sensing system. Using the passive directional sonobuoy, DIFAR operates by detecting directional information, and then it frequency multiplexes the information (data) to the acoustic data transmitted by the sonobuoy to the deployment aircraft. This information undergoes processing by the aircraft’s acoustic analysis equipment to compute a bearing and display it. Subsequent bearing information from the sonobuoy can pinpoint, by triangulation, the location of the sound or signal source. ACTIVE SONOBUOY.– The active sono- buoy is either self-timed or commendable. The self-timed sonobuoy generates a sonar pulse at a fixed pulse length and interval. The commandable sonobuoy generates a sonar pulse, as determined by a UHF command signal from the controlling aircraft. An active sonobuoy uses a transducer to radiate a sonar (sound) pulse that is reflected from the hull of the submarine. The time between the ping (sound pulse) and the echo return to the sonobuoy is measured. Taking into account the Doppler effect on the pulse frequency, this time-measurement data helps to calculate both range and speed of the submarine relative to the sonobuoy. CASS sonobuoys. The command active sonobuoy system (CASS) allows the sonobuoy to remain silent until it receives a command signal from the aircraft to radiate a sound pulse. This technique allows the aircraft to surprise the submarine. 5-31

DICASS sonobuoy. A CASS sonobuoy, equipped with a directional hydrophone, is a directional commandable sonobuoy (DICASS). A DICASS sonobuoy lets the aircraft acoustic analysis equipment determine both range and bearing to the target with a single sonobuoy. DICASS sonobuoys are replacing the older RO and CASS sonobuoys. SPECIAL-PURPOSE SONOBUOYS.– Currently there are two categories of special- purpose sonobuoys in use by the fleet — the bathythermobuoy (BTS), and the Down-Link Communication (DLC) special-purpose sonobuoys. These sonobuoys are NOT for use in sub- marine detection or localization. Bathythermobuoy. The bathythermobuoy (BTS) measures the water temperature versus depth. The time of descent of a temperature probe determines the water depth. Once the BTS enters the water, this probe (fig. 5-25) descends automatically at a constant 5 feet per second. The probe uses a thermistor, a temperature- dependent electronic component, to measure the temperature. The electrical output of the probe goes to a voltage-controlled oscillator, whose output signal frequency modulates the sonobuoy transmitter. The frequency of the transmit signal, which is recovered at the sonobuoy receiver in the aircraft, is linearly proportional to water temperature. The water temperature and depth are recorded on graph paper that is visible to the ASW operator. DLC. The down-link communition (DLC) buoys are for communication between air- craft and submarines. The DLC buoy is not com- manded and provides down-link communications only by a preselected code. Sonobuoy Receivers The sonobuoy receiver has many functions. It receives RF signals from deployed sonobuoys, ANSWERS FOR REVIEW QUESTIONS Q30. THROUGH Q37. A30. Module 17. A31. UHF and VHF. A32. Module 12, Modulation Principles. A33. Their receivers also work with VHF and VOR. A34. The biconical or disc horn. A35. Active ECM. A36. The science of motion of projectiles. A37. The curve of a projectile describes in space as it travels to the target. 5-32

Figure 5-25.-Bathythermograph sonobuoy deployment. detects intelligence on the signals, provides intelligence to various onboard equipment for acoustic analysis and recording and for navigating or navigation purposes. SONOBUOY RECEIVER SET.– One com- monly used sonobuoy receiver set includes 31 radio receivers that receive FM-modulated signals in the VHF range. Thus, simultaneous reception, demodulation (detection), and audio output of up to 31 RF channels are possible. These channels may each be any one of 31 preselected channels. Each audio output provides two levels—high audio and standard audio. The equipment is primarily for (but not limited to) installation in either fixed- or rotary-wing aircraft. Although capable of being an inde- pendent operating unit, normally, the equipment is used with some combination of several types of sonobuoys and a signal processor. Newer sonobuoy receiver groups provide the capability of simultaneously receiving 20 sonobuoy signals. To accomplish this they use 20 subassemblies. Each subassembly may be independently and automatically tuned to any 1 of 99 sonobuoy RF channels now in use, and those that are in development for future deployment. 5-33

SONAR COMPUTER-RECORDER GROUP.– The sonar computer-recorder group (DIFAR system) analyzes, records, and generates a permanent printed display of the passive and active sonobuoy signals processed by the receiver system. This display can provide information for identifying and locating the source of the sound. The system uses low-frequency analysis, directional-frequency analysis, broadband- frequency analysis, directional listening, and active ranging or Doppler techniques to detect, classify, and localize the underwater target, The four basic modes of operation of the DIFAR system are as follows: 1. 2. 3. 4. OMNISEARCH—omnidirectional signal from a passive buoy with NO directional capabilities ALI-LOFAR—integrated omnisearch display using a directional or nondirectional passive buoy DIFAR—directional frequency analysis and recording—will give a bearing to the target using directional buoys Range—gives the range in yards to the target using an active range only buoy Magnetic Recorders Magnetic recorders are used throughout the Navy in various forms and types. They may be a simple audio recorder or the most complex data recorder; however, all of them provide a handy, compact means of storing and retrieving large amounts of information. OPERATION OF A MAGNETIC RE- CORDER.– Operation of a magnetic recorder involves three basic processes—recording, reproducing, and erasing. In analog systems, reproducing is playback or play. In digital systems, record is write, and reproducing is read back or read. Keep in mind that analog recording and digital recording refer to recording techniques and not to the information recorded. DIGITAL RECORDING.– The basic difference between analog and digital recording is in the method and degree of magnetizing of the recording media. For analog recording, linearity and low distortion are the primary requirements. However, for digital recording (as in most digital systems) there are only two states—0 or 1, ON or OFF, TRUE or FALSE, or whatever names are convenient. ERASING.– The term erasing refers to an electromagnetic process, or demagnetizing procedure, that removes signals previously recorded without affecting the magnetic tape in any other way. The action is a realignment (or polarizing) of the oxide particles on the tape so all modulation (recorded data) is removed, making it possible to reuse the same tape. Q38. Q39. Q40. Q41. Q42. What two factors determine the maximum detection range of a submarine? What is the purpose of compensation? What recorder system plots the frequency of the sound waves against the intensity of their acoustic energy and duration of the sound emission from an omnidirectional passive sonobuoy? List the types of sonobuoys. List the four basic modes of the DIFAR system. SUPPORT EQUIPMENT Learning Objective: Identify various support equipment, including aircraft power generation, conversion, control, regulation, and protection equipment. Support equipment has become as important to the assigned mission of naval aviation activities as the aircraft itself. Many different types of support equipment are required for handling, servicing, loading, testing, and maintaining aircraft. Although your rating is not responsible for the upkeep and maintenance of support equipment, you, as a user, must have a basic knowledge of the equipment’s capacity and operation. You must understand the capabilities and limitations of the auxiliary power sources provided for use in ground servicing and maintenance of aircraft. You must observe and enforce all safety precautions and regulations concerning the use of the units, You must also know the requirements for cooling the various electronic equipment while on the ground. You must be familiar with the sources of auxiliary air and cooling, and you must know the capabilities and limitations of the various cooling units. 5-34

This chapter discusses these topics. In most sections, the discussion is general. In a few instances, details are presented as they pertain to specific items of equipment. Coverage of the equipment is limited to those expected to be in common usage during the life of this training manual. AIRCRAFT POWER The electrical power system of an aircraft consists of the power source and its associated controls, the generation system and its associated controls and regulation, the conversion units, the feeder and distribution system and its component parts, and the various protective devices used throughout the installation. As part of the overall effort to standardize aircraft and electronic installations, the supply and distribution of power offered a logical starting point. The first step was to standardize the supply voltages and power frequencies and to use generators that would provide the required power. Later in the standardization program, the generation of dc power was discontinued, and the primary power became exclusively ac. The dc requirement was supplied through transformer- rectifiers. This reduced the number of voltages generated, reduced the number of rotary devices, and allowed the use of smaller conductors in the distribution system. The result was a drastic decrease in the total weight of a given installation, which, in turn, permitted a more complex installation for a given weight allowance. To be of any real value, a partial listing of the considerations involved in any discussion of aircraft electrical systems must include the following items: 1. A main generating source refers to all generator units driven by a specified engine; thus, a single-engine aircraft can have only one main source. 2. Multiengine aircraft may have a main generating subsystem for each engine. This is the usual practice, but it is not universally followed. 3. Adequate frequency regulation and stability in ac generation systems require some method of speed control of the generator’s rotor drive mechanisms. 4. Provisions must be made to ensure that adequate power is available in each mode of operation. In the event of failure of the aircraft engine or its associated generation system, the maximum amount of power that can be produced is decreased. In the case of single-engine aircraft, this automatically constitutes an emergency situation. 5. The failure of a single generator or engine in a multiple installation does not constitute the same degree of emergency as the same failure in a single-engine installation. Although some restrictions are placed on operational capabilities, some degree of safety may usually be maintained with the remaining engines and generators. 6. Provisions should be made to enable use of external power sources for starting the engines while on the ground and for ground operation without using the aircraft engines. The aircraft electrical system must include provisions to prevent applying both internally generated power and externally furnished power to the system at the same time. Aircraft Electrical Systems The electrical system of each model aircraft has some features peculiar to it alone, while other features are common to most models. In this sect ion, you are presented with a general discussion of the electrical system of a typical aircraft. SOURCE OF POWER.— The basic source of power for the electrical system is the aircraft engine. An ac generator requires a constant rotational speed to produce a constant frequency output. In most modern aircraft, a constant-speed drive (CSD) unit is inserted between the aircraft engine and the ac generator for this purpose. GENERATION SYSTEM.— The heart of the electrical generation system is the constant-speed, wye-connected ac generator. This unit normally produces a three-phase output voltage of about 120/208 volts at 400 Hz, which is subsequently regulated to 115/200 volts. The basic theory of ac generators is discussed in Navy Electricity and Electronics Training Series (NEETS), module 5. DC Generator.— In most older aircraft, all electrical power was generated as dc voltage. In most of the newer aircraft, no dc voltage is generated. The dc requirements are met by transforming and rectifying the ac. In some operational aircraft presently in service, however, the main power generation system provides both ac and dc voltages from a common unit. In other aircraft models, a separate generator is used to provide the dc power required for operation of 5-35

the dc components. This method is not common in airborne applications because of the limited number of engines available. The basic theory of dc power generators is presented in NEETS, module 5. Emergency Generators.— In the event of failure or shutdown of the aircraft engines or main generators, the electrical system becomes inoperative. The aircraft must have electrical power to maintain adequate flight control. All naval aircraft incorporate an auxiliary or emergency generator that operates independently of the aircraft engine. System Voltage Regulation Voltage regulators are incorporated in all electrical generation systems. Although similar in basic purpose, the configuration and details of operation vary with each type. A typical solid- state voltage regulator may consist of a sensing circuit with input rectifiers, a temperature- compensated Zener diode reference and error- detecting bridge, and a three-stage transistor amplifier. The output of the bridge circuit is a voltage inversely proportional to the difference between the generator voltage and the regulator set voltage, and it is referred to as the error signal. External Power All aircraft have provisions for application of electrical power from an external source for starting the aircraft engines and/or for ground servicing and maintenance without operating the engines. This power, while not generated within the aircraft, is part of the overall electrical system of the aircraft. All aspects must be compatible with the power generated within the aircraft. Under no circumstances may the internal and external power be used at the same time. This is one of the functions of the distribution system, which is discussed briefly in the following text, The equipment used to supply power in the external mode of the electrical system is discussed briefly in a later portion of this chapter. Distribution Systems Once the electrical power has been generated and some of it transformed, it must be distributed to the various components and equipment where it is to be used. In a simple system, with comparatively few equipment and requiring only a single form of electrical power, a simple distribution system could be used. In modern naval aircraft, however, with the complex electrical and electronic installations requiring many forms of power, an extremely complex distribution system is required. Each model aircraft has different electrical requirements; therefore, each distribution system must differ from all others under individual requirements. The major area of difference between distribution systems of different model aircraft lies in the switching arrangement used to change electrical loads from one source to another in the event of a malfunction. Power Conversion Devices In most naval aircraft, the main electrical power generation system produces three-phase ac power at 400 Hz. All aircraft require various levels and quantities of dc power. In many instances, ac power of a different frequency is also required. In these cases, various devices are needed to ANSWERS FOR REVIEW QUESTIONS Q38. THROUGH Q42. A38. Its magnetic anomaly and the sensitivity of the detector. A39. To provide a magnetically clean environment and ensure the detecting system will not be limited by the aircraft itself. A40. LOFAR and DIFAR. A41. Passive, active, CASS, DICASS, and special purpose. A42. Omnisearch, ALI-LOFAR, DIFAR, and range. 5-36

convert the power from the forms generated into the forms required for the specific application. A few important conversion devices are discussed briefly in the following paragraphs. TRANSFORMER-RECTIFIERS.— The most common conversion device for changing ac to dc is the transformer-rectifier. The three-phase, 115-volt ac is reduced in a step-down transformer, and then rectified to produce the 28-volt dc required for operation of various relays, lights, instruments, and mechanical devices. Specific transformer-rectifier units are discussed in the electrical section of the maintenance instructions manual (MIM) for each model aircraft. The fundamental theory of transformers is discussed in NEETS, module 2. INVERTERS.— An inverter is a rotating electromechanical device used to convert low- voltage dc into ac. It consists essentially of a speed-governed dc motor, an armature and brush assembly, and a permanent magnet inductor-type ac generator all within a single unit. The armature and the permanent magnet rotor are usually mounted on a common shaft. The inverter’s output frequency and voltage should be checked periodically to assure that they are within prescribed limits. Should adjustment be required, the electrical shop is notified, since adjustment of inverters is a responsibility of the AE rating. FREQUENCY CHANGERS.— When ac volt- ages of a frequency different from that produced by the main generator are required, suitable motor-generator combinations are used. Main electrical power frequency is usually 400 Hz, Many aircraft provide a 60-Hz source for test equipment and an 800-Hz source for certain instruments or components. Q43. To what NEETS module should your refer for information on ac generators? Q44. What is the purpose of external power? Q45. List some power conversion devices. CIRCUIT PROTECTION AND CONTROL The electrical system of an aircraft is protected from damage and failure by fuses, current limiters, and circuit breakers. Control and distribution of power are accomplished by the use of switches and relays. Each of these components is available in many styles and sizes, some of which are ideally suited for use in aircraft, while others are limited to use in shop installations. In the following section, you will be presented with a brief discussion of these components. Fuses Fuses provide a controlled, intentionally weakened link in an electrical circuit. They serve as safety devices in the event of undesired overloads. Fuse sizes are available with ratings as low as a few milliamperes to several hundred amperes. Fuses of most ratings are available for normal, slow-acting, or fast-acting operation. A fuse is a heat-sensitive, heat-operated device. When operated at the rated current, it consumes electrical power, and then dissipates this power in the form of heat. Under normal operating conditions, the dissipated heat is not sufficient to cause the fuse to open (blow). However, when the fuse is operated above the normal current rating, the overload current generates additional heat, which melts the fusible element. 1. Voltage rating. A fuse can be operated at any circuit voltage if it is mounted in a sufficiently well-insulated holder (as long as the fusible element is able to open without suffering arc damage). When a fuse blows due to excessive current, the full-circuit voltage appears across the open fuse. If inductance is present in the circuit, a surge is generated that may cause a destructive arc to be formed within the fuse. Under these conditions, intense heat and pressure develop, and the fuse may literally explode. 2. Blow-time characteristics. The blow-time characteristics of a fuse depend on the percent of rated current and thermal inertia of the fuse. Overload currents (currents larger than the maximum value for which the fuse is rated), when flowing through a fuse, heat the element beyond normal capacity. After a period of time, the fusible element opens. Fuse elements with a large thermal inertia increase the length of time before blowing. Fuses containing such elements are known as slow- acting, slow-blow, or time-delay fuses. Slow- acting fuses are constructed with a compound element—a thermal cutout and a fusible link that melts on short circuits on very high overloads. 5-37

Small, light fuse elements reduce the thermal inertia; therefore, they are faster acting. This type of fuse is known as a fast-acting fuse and is used principally for the protection of sensitive instruments. In the selection of blow-time characteristics, both the steady state and the transient or surge currents are considered. If currents of 200 to 400 percent above normal can be tolerated for periods of 1 to 10 seconds, a slow-acting fuse is specified. If the circuit requires immediate protection for any current above normal, a fast-acting fuse is specified. If the current must be limited to 200 percent of the rating for periods less than 1 second, then a normal or medium blow-time characteristic is specified. (See fig. 5-2 6 .) When possible, a fuse should be operated at about 75 percent of its rated value. This provides a good balance between protection and reliability. 3. Vibration resistance. Fuse protection for equipment subject to vibration can be provided by special vibration-resistant construction. This type of fuse has a spring formation, with winglike extensions that bear on the inside wall of the glass body to decrease vibration of the fuse element. For slow-acting fuses, a different construction is used. This construction consists of a compound spring and link structure. On moderate overloads, as the compound element reaches the melting point, the spring pulls away from the link, while on short circuits, the link fails. Figure 5-2 6 .-Blow-time characteristics of fuses. 4. Identification coding. Fuses and their corresponding fuse holders are numbered according to a standardized system for easy identification. The numbering system is shown and explained in figure 5-2 7 . 5. Fuse holders. The most common class of fuse holders used in Navy equipment is the post- type holder, shown in figure 5-2 8 . It may be a screw in or a bayonet type. Both of these types are securely mounted to the chassis or front panel of the equipment. The purpose of the holder is the same, regardless of type—to hold the fuse securely with good electrical connection and physical stability for protection from mechanical vibration and electrical short circuit. You should use care to ensure that the fuse is of a physical size compatible with the holder. Fuses that are undersized allow physical movement and arcing. This results in a blown fuse, erratic opera- tion, or damaged holder. Fuses that are too large may cause cracking or breaking of the holder. Force should never be applied to either the fuse or the holder, since most are fragile devices. Post-type fuse holders are normally series connected in the line, with the end connection to the power source and the center connection to the load. When connected in this fashion, the equipment is protected in the event of a broken holder. A short circuit from a fuse holder to chassis ground will result in a blown fuse and excessive current will not flow. Reversed connections will not furnish this protection. Connection is normally made by solder, although some fuse holders are connected by the use of a screw or lug method. Current Limiters Devices somewhat similar to fuses, called current limiters, are used in aircraft circuits that carry high currents. (See fig. 5-2 8 .) The current limiter consists of a copper link of carefully predetermined sections. The sections melt when ANSWERS FOR REVIEW QUESTIONS Q43. THROUGH Q45. A43. Module 5. A44. Starting aircraft engines and/or ground servicing and maintenance. A45. Transformer-rectifier, inverter, and frequency changer. 5-38

Figure 5-27.-Identification coding: (A) fuses; (B) fuse holders. Figure 5-28.-Example of aircraft fuses and holders. abnormally high currents start to flow. The melting sections have a high-arc resistance to keep the circuit current within the capacity of the limiter. If the excessive current is only a temporary surge, the melting ceases, and the circuit continues to operate as if no abnormal current had been present. Repeated applications of excessive current or uninterrupted application for a period of several seconds melt through the sections and cause the limiter to function in the same manner as a fuse. Circuit Breakers In modern naval aircraft, circuit breakers have replaced fuses as the circuit protection devices for most of the wires and cables making up the electrical system. The circuit breaker is designed to open the circuit under short-circuit or overload conditions without injury to itself. Thus, it performs the same function as the fuse, but it has the advantage of being reset and used again. Circuit breakers are rated in amperes and volts. There are three basic types of circuit breakers—thermal, magnetic, and thermomag- netic. The following discussion is slanted toward the thermal type, because this type is more widely used. Circuit breakers are divided into three categories—the push-button reset type, the toggle type, and the automatic reset type (sometimes called a circuit protector). The push-button reset type (fig. 5-29) consists of a bimetallic, thermally actuated, spring-loaded Figure 5- 29.-Thermal circuit breaker. 5-39

device that connects two electrical contacts when set. An excessive current through the device causes an uneven expansion of the bimetallic mechanism (thermal release). This action releases a trigger escapement and permits the spring-loading to separate the contact members. A visual indication of the automatic opening is provided by causing the push button to move to an easily noticed “tripped” position. In this position, the button is fully extended and the white ring on the button is showing. This type of push- button breaker has a pullout feature that permits manual opening of the circuit. Another type of circuit breaker uses a toggle lever instead of the push button. It operates in the same manner as the push-button reset-type breaker, except that the tripped condition is indicated by the toggle lever being in the OFF position. This type of circuit breaker has the apparent advantage of also being used as a switch. Manual resetting of the circuit breaker may be accomplished by means of the actuator (either push button or toggle lever) whenever the bimetallic thermal element cools sufficiently for the trigger to engage its latching mechanism. In connection with resetting, there are two classifications for circuit breakers—trip-free and nontrip-free. In the trip-free class, the contacts cannot be kept closed by holding the actuator in the closed (or reset) position as long as an overload condition persists, which would otherwise cause normal tripping. The nontrip-free circuit breakers can be prevented (by the operator’s action) from tripping, even though a tripping condition exists. This should be done only in an emergency. Since this action is apt to change the calibration, the breaker should be replaced as soon as conditions permit. This type of breaker is no longer being installed in new aircraft, but it is still found on some older models. A disk type of thermal circuit breaker is shown in figure 5-3 0. This breaker consists of a conductive, snap-acting bimetallic disk that Figure 5-3 0 .-Disk type of thermal circuit breaker. bridges two electrical contacts. When the disk is heated by the excess current through it, it snaps to the reverse position, opening the contacts and breaking the circuit. In circuit breakers having low ratings, a resistance wire is inserted. Current through this wire provides the heat necessary to snap the disk. These breakers are reset by pressing a button that restores the disk to its original position. When circuit breakers of this type are closed, they cannot be reopened manually. They are also nonindicating; that is, the position of the break (open or closed) cannot be determined by visual inspection. The automatic reset type of circuit breaker is similar to the bimetallic-disk type just described, except that it has no reset push button. It resets itself automatically. After a short time, when the disk has cooled sufficiently, it will bend back and close the circuit, resetting itself. If a constant overload exists, the breaker will intermittently break the circuit. Another type of circuit breaker is the switch toggle variety, which is based on magnetic instead of thermal operation. This type can be made to open almost instantly when more than the rated current flows in the circuit. An electromagnet is placed in series with the spring-loaded contacts. The contacts are mounted so that an armature acts as a latch to hold them closed. When an excess current flows, the armature is pulled toward the electromagnet, releasing the contacts and opening the circuit. To reset the circuit breaker, the contacts are closed manually, and the spring- loaded armature returns to its normal position. MOBILE ELECTRIC POWER PLANTS The electrical power requirements for starting and servicing modern aircraft are extremely high. Even in aircraft equipped with batteries, and with the batteries fully charged, the capacity is not sufficient to withstand the heavy load of starting an aircraft engine or the power drain of prolonged operational ground checks. CAUTION Batteries are not to be used to start aircraft reciprocating engines except in an extreme emergency. The purpose of an aircraft battery is to operate specific instruments and radios in case of a loss of aircraft generator power. Aircraft are being manufactured that have no internal source of electrical power unless the 5-40

engines are operating. This presents problems when electrical power is required to perform maintenance. Running the aircraft engines to provide electrical power for maintenance purposes is also poor practice. There is the danger of turning propellers, jet intake and exhaust blast, or the expense of operating high-powered engines for long periods when only electrical power is required. To make maintenance easier and to provide instrumentation for monitoring engine performance during starts, an external source of electrical power is necessary. Although the AT is not responsible for the upkeep and maintenance of mobile electric power plants (MEPPs), you must have a basic knowledge of their capacity and operation. On all of the mobile electric power plants described in this chapter, the ac frequency is automatically controlled by a governor that controls the speed of the power plant. The voltage is controlled by a voltage regulator. If the power plant does not regulate to the proper speed (frequency), it must be serviced by the support-equipment work center. The term mobile electric power plant (MEPP) is limited to portable units not installed aboard the aircraft. The units may be self-propelled, towable, or merely transportable. They may be powered by diesel fuel, jet fuel, gasoline, or electricity. Identification of MEPPs There are four categories of MEPPs—(1) self- propelled vehicular, (2) gasoline- or diesel-engine driven trailer-mounted, (3) electrically driven trailer-mounted, and (4) gasoline-/diesel-engine or electrically driven dolly/skid-mounted. These power plants are further identified by prefix letters NA, NB, and NC, These letters indicate the type of power available from the unit as follows: NA—dc output power only NB—ac output power only NC—ac/dc output power The NC-2A is discussed here. For information on other MEPPS, you should refer to specific MIMs. The NC-2A (fig. 5-3 1) is a self-propelled diesel-engine-powered unit. It is front-axle driven, steered by the two rear wheels, and easily maneuverable in congested areas. The front axle is driven by a 28-volt dc, reversible, variable-speed motor, capable of propelling the unit up to 14 mph on level terrain, and has a turning radius of approximately 11 feet. Figure 5-3 1 .-MEPP NC-2A. This unit supplies 30 kVA, 120/208-volt, 400-Hz, three-phase power for servicing, starting, and maintaining jet aircraft. A dc generator produces 28 volts up to 500 amperes. Mobile Motor-Generator Sets Mobile motor-generator sets (MMGs) perform the same function as the mobile electric power plants. However, they are not self-contained and require an external source of electrical power for operation. The MMGs are primarily used in hangars on shore stations or on the hangar decks of aircraft carriers where running an internal combustion engine is not practical, and where external power is readily available. Only the MMG-1A is described in this section. For information about other MMGs, refer to the applicable publications. The MMG-1A (fig. 5-3 2 ) is a small, compact, trailer-mounted, electric-motor-driven generator Figure 5-3 2 .-MMG-1A. 5-41

set, used to provide 115/200-volt, three-phase, 400-Hz ac power for ground maintenance, calibration, and support for various types of aircraft systems and equipment. Operation oft he unit requires a three-phase, 60-Hz, 220- or 440-volt ac external power source. The unit must be towed or manually moved. Additional Support Equipment Other power systems and support equipments available to the AT include the deck-edge power system, the flight-line distribution system, and ground-cooling equipment. DECK-EDGE POWER.— The primary func- tion of the deck-edge electrical power system installed on aircraft carriers is to provide a readily accessible source of servicing and starting power to aircraft at almost all locations on the carrier’s flight and hangar decks. FLIGHT-LINE ELECTRICAL DISTRIBU- TION SYSTEM.— The flight -line electrical distri- bution system (FLEDS) is an electrical distribution system for servicing aircraft on the flight line. Figure 5-3 3 shows the major parts of the FLEDS. It consists of three-way junction boxes, inter- connecting ramps, aircraft service point castings, and aircraft connector plug assemblies. The total system capability is 24 aircraft. (See fig. 5-3 3 .) Each service point can service one aircraft with 115/200-volt, three-phase, 400-Hz power, The FLEDS accepts power from a mobile elec- trical power plant (MEPP) capable of supplying 115/200-volt, three-phase, 400-Hz power. Power is applied at the junction boxes and branches into the service point castings to the aircraft connector plug assemblies. The cables connecting the junc- tion boxes, service point castings, and aircraft connector plugs are installed underneath the interconnecting ramps for protection. Figure 5-33 .-FLEDS. 5-42

GROUND-COOLING EQUIPMENT.— The purpose and need for ground cooling varies. A primary reason for using ground-cooling equipment is that electronic equipment produces large quantities of heat. This heat must be dissipated or the equipment could get so hot that the equipment would be damaged and a fire hazard would be created. When a large quantity of air is required for cooling, a common source for this air is the aircraft’s ventilation system. Line maintenance, ground operational checks, and functional checks are usually performed without the aircraft’s operating ventilation system since this system is driven by the aircraft engines. Therefore, a substitute air supply must be provided for the air distribution system. The mobile air-conditioner (fig. 5-3 4 ) was designed for this purpose. Mobile air-conditioners include the NR-2B, NR-5C, and NR-10A. For information about these air- conditioners, refer to the applicable MIM. Additional SE information can be found in specific MIMs and Airman, NAVEDTRA 1 4014. OPNAVINST 4790.2 (series) has established the support equipment operator/organizational maintenance program. This program emphasizes and formalizes the responsibilities and procedures required in connection with the operation of support equipment (SE). (Support equipment is also referred to as ground support equipment Figure 5-34.-NR-2B mobile air-conditioning unit. [GSE], and you may see this terminology and abbreviation used in many publications.) During recent years, the improper use of SE has resulted in far too many ground-handling accidents, excessive repair and replacement costs amounting to millions of dollars annually, and reduced operational readiness. Investigation has shown the major reasons for improper use of this equipment to be lack of effective training for the individuals who operate and maintain the equipment. Also, the lack of effective supervision and leadership by the officers, chief petty officers, and petty officers/noncommissioned officers directly responsible for such operation and maintenance at the various activities contribute to the problem. CAUTION An SE operator’s license, OPNAV Form 4790/102, is required of all personnel who operate SE regardless of rate or rating. It is emphasized that the SE training program is intended to teach support-equipment operation and organizational-level maintenance only. This training does not qualify the individual to operate equipment on the aircraft. Q46. What components protect an aircraft electrical system? Q47. At what potential should a fuse be operated? For what reason? Q48. What advantage does a circuit breaker have have over a fuse? Q49. List the three basic types of circuit breakers. Q50. MEPP refers to what types of units? How are these units powered? Q51. What MEPPs identification would indicate dc output power, as output power, and ac/dc output power, respectively? Q52. What is the difference between an MEPP and an MMG? 5-43

ANSWERS FOR REVIEW QUESTIONS Q46. THROUGH Q52. A46. A47. A48. A49. A50. A51. A52. Fuses, current limiters, and circuit breakers. At about 75 percent of its rated value, it provides a good balance between protection and reliability. They can be reset and used again. Thermal, magnetic, and thermomagnetic. Portable units not installed aboard aircraft; they are powered by either diesel fuel, jet fuel, gasoline, or electricity. NA, NB, and NC. MMGs are not self-contained and require an external electrical power source for operation. 5-44

CHAPTER 8 TEST EQUIPMENT The operational theory of equipment is only one part of the knowledges you need to maintain avionics equipment. You also need a knowledge of avionics drawings, schematics, and test equipment. You use many publications to properly maintain a weapons system in modern-day aircraft because they are so complex. Just the list of the electronics equipment installed in modern-day aircraft is lengthy. It is impossible for each individual to know all the various types of electronics equipment presently in use. However, with a good general background on electronic principles and circuit theory and a little study, you, the Aviation Electronics Technician, can rapidly become familiar with any specific system or test equipment. In this chapter, you will learn about some common test equipment used by Aviation Electronic Technicians (ATs). This information is in addition to modules 3 and 16 of the Navy Electronic and Electricity Training Series (NEETS) on test equipments. Review and refer to the NEETS modules as necessary for additional information about the test equipment described in this chapter. No in-depth theory beyond that necessary to describe the operation of the test set under discussion is included here. When you use a piece of test equipment with which you are not familiar, always use the appropriate instruction manual. These publications contain detailed and specific information about the particular equipment. CARE AND USE OF AVIONICS SUPPORT EQUIPMENT Learning Objective: Identify the proper care and use of avionics support equipment to include calibration, repair, and handling requirements. All electronic maintenance shops have and require many pieces of test equipment to maintain different types of electronic units. However, there are very few spare test sets. When a test set becomes inoperative, shop maintenance suffers. Therefore, each person should use the test equipment properly and only for its designed purpose. Protect the equipment from physical harm that may result from dropping, falling, or any other careless misuse, and always observe proper operating techniques. One of the chief causes of test set failure is carelessness. The user can be careless in an operating procedure or in handling the set. Improper range selection for the measured quantity is the most common mistake in an operating procedure. Such an error might be to try to measure 250 volts on the 50-volt scale of a meter. If you aren’t sure about proper use of a test set, refer to the manual issued with the set. Improper handling causes damage to test equipment. Often, technicians place test sets near the edge of the bench where they can be easily knocked or pulled off. Read the instructions for proper handling and operating procedures, and think when you use a piece of equipment. Refer to NEETS, modules 3 and 16, for further information on test equipment operation and theory. CALIBRATION Test sets require checks to determine if they are within operating tolerances. Some test sets are used as frequency standards and require periodic calibration. You should always follow the recom- mendations of the manual or pamphlet issued with the set, unless current instructions change those recommendations. Normally, personnel in an intermediate-level maintenance shop perform calibration using special-purpose calibration equipment. Personnel at the organizational level of maintenance seldom calibrate test equipment. 8-1

REPAIR The using activity normally makes any minor repair of test sets not requiring calibration. Repairs are usually limited to the replacement of test leads and fuses. Before you make any repair, consult current instructions on repair of test equipment. Personnel assigned to an intermediate-level maintenance activity repair test equipment on a wider scale. Repair can vary from the replacement of circuit components to modules, depending on the authorized level of repair. However, most test equipment work at this level consists of calibrating equipment. HANDLING PRECAUTIONS Some equipments require special handling; however, several precautions apply to test equipments in general. Rough handling, moisture, and dust all affect the useful life of test equipment. For example, bumping or dropping a test instrument can destroy the calibration of a meter or short circuit the elements of an electronic tube within the instrument. Creasing or denting coaxial test cables alter their attenuating effect, affecting the accuracy of any RF measurements made with these cables. To reduce the danger of corrosion to untreated parts, always store test equipment in a dry place when not in use. Excessive dust and grime inside a test equipment affect its accuracy. Be sure all assembly screws that hold the case of the test equipment in place are tight and secure. As an added precaution, place all dust covers on test equipments when they are not in use. Meters are the most delicate part of test equipments. To make sure the meter maintains its accuracy, you should follow these additional precautions: 1. 2. 3. Make certain the amplitude of the input signal under test is within the range of the meter. Keep meters as far away as possible from strong magnets. When servicing an item of electronic equipment that contains a meter, dis- connect the meter from the circuit before making resistance or continuity tests. This precaution should prevent the possibility of burning out the meter. The instruction manuals that come with a piece of equipment contain the procedures for properly stowing test equipment cables and other accessories. Read these manuals carefully and follow the equipment instructions. Improper stowage of accessories could change cable charac- teristics and cause intermittent shorts in cables and leads. Improper stowage causes unreliable test equipment indications. Q1. Q2. Q3. Q4. Name one of the chief causes of test set failure. Although test equipment is repaired at the intermediate-level maintenance activity, most work performed at this level on test equipment consists of What is the most delicate part of a piece of test equipment? List the basic measuring parameters of electronic equipment. MEASURING INSTRUMENTS Learning Objective: Recognize types and uses of measuring equipment to include electronic meters, frequency measurement, and power measurement. In this chapter, the term measuring instruments includes only the class of test equipments that measure the basic parameters of an electronic equipment. The basic parameters are voltage, current, resistance, power, and frequency. METER OPERATION There must be some source of power available to operate a meter. Some meters use batteries installed in the meter case as a power source; others may use an electrical power cord plugged into a power receptacle. A vacuum tube voltmeter (VTVM) is an example of the second type. The power to operate some meters (such as meggers) is self-produced by manual operation of a handcrank. Most meters provide the means to measure more than one electrical quantity; these are multimeter. Before discussing any one particular type of meter, a brief review of each of the basic meters is necessary. For more details refer to NEETS, modules 3 and 16. 8-2

Ammeter The amplitude of current flow through the basic meter mechanism limits it to measuring a fixed range of only a fraction of an ampere. A current shunt overcomes this limitation and protects the mechanism. The current shunt is actually a resistance of low value, permitting the instrument to serve as a dc ammeter that can measure relatively large direct currents. The current distribution between meter movement and shunt is inversely proportional to their individual resistances. Thus, the shunt, which has less resistance, carries most of the current. Since the meter coil carries only a small portion of the circuit current, it can indicate relatively large values of circuit current. The instrument provides a variety of current ranges by the use of shunts of different values. Figure 8-1 shows a simplified schematic diagram of an ammeter section taken from a typical volt-ohm-milliammeter (VOM). Ohmmeter The midscale deflection of an ohmmeter occurs when the current drawn by the meter is one-half the value of the current at full-scale (zero ohms) deflection. This condition exists when the measured resistance is equal to the total meter circuit resistance. Analysis of the circuit in figure 8-2 shows that full-scale deflection occurs when shorting the meter probes together. Less than full-scale deflection occurs when the resistance to be measured, Rx, is connected into the circuit. If the meter now reads one-half of its former current, the total circuit resistance Figure 8-1.-Simplified schematic diagram of an ammeter. Figure 8-2.-Series-type ohmmeter basic circuit. has doubled. This indicates that RX is equal to the total meter circuit resistance. Since the ohms-calibrated scale is nonlinear, the midscale portion represents the most accurate portion of the scale. The usable range extends with reasonable accuracy on the high end to 10 times the midscale reading. However, on the low end it decreases to one-tenth of the midscale reading. To extend the range of an ohmmeter, the proper values of shunt and series resistors and battery voltages are connected into the circuit. The proper values let you read the meter full scale with the test leads shorted. Figure 8-3 shows a Figure 8-3.-Simplified schematic diagram of an ohmmeter. 8-3

simplified schematic diagram of an ohmmeter section taken from a typical VOM. Voltmeter Adding a voltage-multiplying resistor makes the basic meter mechanism suitable for use when measuring dc voltages. The voltage-multiplying resistor is placed in series with the coil (fig. 8-4) and limits the flow of current to a safe value. Since the value of the resistor is constant for any given application, the flow of current through the coil is proportional to the voltage under measurement. By properly calibrating the dial, the instrument indicates voltage. However, it is actually the current that activates the meter. The use of different values of multiplying resistors establishes the voltage ranges of the instrument. MULTIMETER Much of the work that you do using a VOM can be done with a multimeter. The name multimeter comes from multiple meter, which is exactly what a multimeter is. It is an ohmmeter, a dc and an ac milliammeter, and a voltmeter. A typical multimeter is shown in figure 8-5. Figure 8-5.-Typical multimeter. In many shops, you might use a portable, battery-operated multimeter such as a TS-352, USM-311, Simpson 260, or Simpson 160 for field use (troubleshooting in the aircraft, for instance). As an AT, however, you will often need a more sensitive meter—one that gives more accurate readings and has wider ranges. Often, equipment schematics and wiring diagrams specify that voltages indicated at test points were obtained with a meter of a certain sensitivity, such as a 20,000-ohms-per-volt meter. Figure 8-4.-Simplified schematic diagram of a dc voltmeter. You should use a meter with the same sensitivity 8-4

in repairing that equipment to obtain accurate readings because of circuit loading. NOTE: For a review of the basic theory and operation of the multimeter, refer to NEETS, module 3. MILLIOHMMETER One of the most common and troublesome problems is finding the exact location of a short circuit in a power distribution circuit involving many parallel paths. This and several trouble- shooting problems are easier to solve with a milliohmmeter. A milliohmmeter is a low-range ohmmeter that can measure resistances in the milliohm range or less. The AN/USM-21A is a typical milliohmmeter used in the fleet. It can measure resistances in the range of 10 milliohms or less. Most ohmmeters read zero at such a low value. When using a milliohmmeter, you may encounter several problems. These problems include stray circuit resistances, such as contact resistance, test lead resistance, and switching resistance. In the conventional low-range ohmmeters, the primary problem is in the contact resistance at the test probes. The design of the AN/USM-21A overcomes the contact resistance problem. MEGOHMMETER (MEGGER) The megohmmeter, commonly called the megger, is an instrument that applies a high voltage to the component under test and measures the current leakage of the insulation. This lets you check a capacitor or an insulated cable for leakage under much higher voltages than an ohmmeter can supply. The megger consists of a hand-driven dc generator and an indicating meter. It measures resistances of many megohms. There are various resistance ratings of meggers with full-scale values as low as 5 megohms and as high as 10,000 megohms. Figure 8-6 shows the scale of a 100-megohm, 500-volt megger. Notice that the upper limit is infinity and that the upper end of the scale is also crowded. The first scale marking below infinity represents the highest accurate value the instrument can provide. Thus, if the pointer goes to infinity while you are making a test, it means that the resistance is higher than the range of the set. There are also various voltage ratings of meggers, such as 100, 500, 750, 1,000, and 2,500. The most common type is the one with a 500-volt rating. This voltage rating refers to the maximum output voltage of the megger. The output voltage depends on the turning speed of the crank and armature. When the megger’s armature rotation reaches a predetermined speed, a slip clutch maintains the armature at a constant speed. The voltage rating is important. If too high a voltage is applied, it will cause even a good component to break down. Therefore, do not use a 500-volt megger to test a capacitor rated at 100 volts. You can use meggers to test the insulation resistance of conductors that may be shorting or breaking down under high voltage. In some situations, you can use meggers in the prevention of unnecessary breakdowns. You could maintain a record of insulation resistance of power and high-voltage cables, motor and generator windings, and transmission lines. These records reflect fluctuations in resistance and help Figure 8-6.-Scale of a 100-megohm, 500-volt megger. 8-5

determine when to replace the components to prevent a breakdown. Meggers are used for testing capacitors whose peak voltages are not below the output of the megger. They are also used for testing for high-resistance grounds or leakage on devices such as antennas and insulators. The following are precautions you should take when using meggers: 1. When you are making a megger test, do not energize the equipment. Disconnect it entirely from the system before testing. 2. Observe all safety rules in preparing equipment for test and in testing, especially when testing installed high-voltage apparatus. 3. Use well-insulated test leads, especially when using high-range meggers. Check the leads after connecting them to the megger and before connecting them to the component under test. Operate the megger and make sure there is no leak between the leads. The reading should be infinity. Check the leads by touching the test ends of the leads together while turning the crank slowly. The reading should be about zero. If the indication reads differently, you may have a faulty lead or a loose connection. 4. When using high-range meggers, take proper precautions against electric shock. There is enough capacitance in most electrical equip- ment to store up energy from the megger generator to give a very disagreeable and even dangerous electric shock. Because there is a high protective resistance in the megger, its open circuit voltage is not as dangerous as it would otherwise be; still, be careful. 5. Discharge equipment having considerable capacitance before and after megger tests. This should help you avoid receiving a dangerous shock. You can do this by grounding or short circuiting the terminals of the equipment under test. The AN/PSM-25, shown in figure 8-7, is a common megger used through the fleet. For more information on meggers, refer to NEETS, module 16. ELECTRONIC METERS Electronic meters and nonelectronic meters are used for the same purposes; however, they do have some differences. In the electronic multi- meter and corresponding nonelectric measuring devices, the current- and resistance-measuring Figure 8-7.-AN/PMS-25 megger. circuits function in the same way. However, when an electronic multimeter is used to measure voltage, an amplifier is involved. Therefore, the electronic meter requires calibration before it is used. The proper calibration and use of the instruments vary slightly, according to model. You should refer to the operation instruction manual for the specific details of each model. The ordinary voltmeter cannot be accurately used to make voltage measurements in high- impedance circuits. For example, you need to measure the plate voltage of a pentode amplifier. (See fig. 8-8.) When you connect the meter between the plate and cathode of the electron tube, the meter resistance is in parallel with the effective plate resistance Thus, the plate resistance is lowered. The effective plate resistance is in series with the plate load resistor and this series circuit appears across the supply voltage as a voltage divider. Since the overall 8-6

Figure 8-8.—Loading effect created by meter resistance. resistance is now lower, the current through R L will increase. This causes the voltage drop across R L to also increase, and the voltage drop across R eff will decrease. The result is an incorrect indication of plate voltage and is called the loading effect. The lower the sensitivity of the meter, the greater the loading effect and the higher the incorrect indication (error) will be. A meter having a sensitivity of 20,000 ohms per volt and a 250-volt maximum scale reading would introduce an error of about 1 percent. However, in circuits with very high impedances, even a meter with a 20,000-ohm-per-volt sensitivity would impose too much of a load on the circuit. VACUUM TUBE VOLTMETER Another limitation of the ac, rectifier-type voltmeter is the shunting effect at high frequencies of the relatively large capacitance of the meter’s rectifier. This shunting effect may be greatly reduced by replacing the usual metallic oxide rectifier with a diode electron tube. The output of the diode goes to the grid of an amplifier, in which the plate circuit contains the dc meter. Such a device is an electron tube voltmeter or a vacuum tube voltmeter (VTVM). Voltage measurements are extremely accurate with this type of meter, even at frequencies up to 500 megahertz and sometimes higher. The VTVM model that is used determines its frequency limitation. The input impedance of a VTVM is large; therefore, the current drawn from the circuit voltage being measured is small and in most cases negligible. The main purpose of a VTVM is to reduce the loading effect by taking advantage of the VTVM’s extremely high input impedance. The TS-505 multimeter contains a VTVM, and it is used extensively in electronics maintenance. You should refer to figure 8-9 as you read this section. The VTVM measures dc voltages from 0.05 volt to 1,000 volts (in nine ranges) and ac voltages from 0.05 volt to 250 volts rms (in seven ranges) at frequencies from 30 Hz to 1 MHz. Using the RF adapter with the dc voltage measurement circuit lets you measure RF voltages from 0.05 volt to 40 volts rms at frequencies from 500 kHz to 500 MHz. You may measure resistances from 1 ohm to 1,000 megohms. Figure 8-9.—TS-505 multimeter front panel. 8-7

The accuracy of this meter is ±5 percent for dc voltages and ±6 percent for ac and RF voltages. The meter movement requires 1 mA for full-scale deflection. The input impedance to the meter is 6 megohms at audio frequencies, 40 megohms on the 1,000-volt dc range, and 20 megohms on all other ranges. The power require- ment is 98 to 132 volts, single phase, 50 to 1,000 Hz, at about 21 volt-amperes. The removable cover of the TS-505 contains accessories such as alligator clips, an RF adapter, and miniature probe tips. The miniature tips slip over the regular tips for work in confined areas. Operating Controls The following are the controls you use when operating the meter (fig. 8-9): FUNCTION switch—Selects the type of multimeter operation desired and turns the multimeter on or off. RANGE switch—Selects the various voltage or resistance measurement ranges. ZERO ADJ. control—Controls the pointer of the indicating meter. Use it to set the meter pointer at zero on the +DC, –DC, AC, or OHM scale, or at midscale on the ±DC scale. OHMS ADJ. control—Controls the pointer of the indicating meter. Use it to set the meter pointer at on the OHMS scale when the FUNCTION switch is set on OHMS position. Meter—Indicates the value of voltage or resistance measured. AC LINE cord—Connects the multimeter to the ac power source. COMMON probe—Connects the ground or common circuit of the multimeter to the equipment under test. DC probe—Connects the equipment under test to the dc measuring circuit of the multimeter OHMS probe—Connects the equipment under test to the ohmmeter circuit of the multimeter AC probe—Connects the equipment under test to the ac measuring circuits of the multimeter. Pilot light indicator—Lights when power is applied to the multi meter. Techniques for Use The TS-505 multimeter is not difficult to operate. However, do not try to use this instrument unless you have studied the technical manual that contains the operating procedures, or unless you have received instruction in its proper use from your shop supervisor. There are two peculiarities of this meter that you need to know about. 1. It must warm up before it gives accurate readings. This usually takes about 10 minutes. During this period, the meter pointer may drift rapidly. This is normal. 2. You cannot read voltage measurements directly off the meter scale when the function switch is in the ±DC position. The purpose of the ±DC position (zero center scale) is to determine the polarity of an unknown dc voltage. It also indicates a zero dc voltage input to the multimeter CAUTION The maximum input dc voltage to the multimeter when in the ±DC position is one-half of the range switch voltage setting. The major difference between any VTVM and a conventional multimeter is that the VTVM uses a vacuum tube in its input. For a detailed explanation of the circuitry of the TS-505 VTVM, consult the manufacturer’s manual or the operation and service instruction manual. PHASE ANGLE VOLTMETER The overall accuracy of many electronic equipments is determined by measuring phase angles. In the past, the phase shift or phase angles between signals were measured by observing patterns on an oscilloscope. It was hard to determine small angles and difficult to translate various points into angles and sines of angles using this method. Also, using oscilloscope patterns is 8-8

a limiting factor if one of the signals contains harmonic distortion or noise. In any complex waveform containing a fundamental frequency and harmonics, measuring phase shifts presents problems. In most applica- tions, the primary interest is the phase relationship of the fundamental frequency, regardless of the phase relationship of any harmonics that are present. Therefore, one requirement of a phase- measuring device is its ability to measure the phase difference between two discrete frequencies, regardless of the phase and amplitude of other components of the waveform. Figure 8-10 shows the basic block diagram of a phase angle voltmeter. There are two inputs— the signal and the reference. Each channel contains a filter that passes only the funda- mental frequency and highly attenuates all other frequencies. Each channel has a variable amplitude control and amplifiers to increase the variety of signals that you can check. A calibrated phase shifter is inserted into one channel. That channel signal can then be phase shifted to correspond to the other channel. The phase detector detects this and indicates it on the meter. The calibrated phase shifter is a switch (whose position corresponds to the 0-degree, 90-degree, 180-degree, and 270-degree phase shift) and a potentiometer (whose dial is calibrated from 0 to 90 degrees). The total phase shift is the sum of the two readings. The phase detector is a balanced diode, bridge- type demodular. Its output is proportional to the signal frequency amplitude times the cosine of the angle of phase difference between the signal input and the reference input. If the shifted reference input is in phase or 180 degrees out of phase with the signal input, the output from the phase detector is proportional to the signal input amplitude. The cosine of the angle is unity. If the shifted reference input is 90 degrees or 270 degrees from the signal input, the phase detector output will be zero (the cosine of the angle is zero). The point at which the two signals are in phase or 180 degrees out of phase is the point of Figure 8-10.-Phase angle voltmeter block diagram. 8-9

maximum deflection on the meter. The difference Q8. between the in-phase and the 180-degree out-of- phase points is in the direction in which the needle swings—not the distance it swings. Upon approaching the point of maximum deflection, the rate of change of the meter reading decreases because the cosine has a small rate of change near 0 degrees. This makes it difficult to read the exact point of maximum deflection. The cosine’s maximum rate of change occurs as it approaches 90 degrees (and thus gives a better indication on the meter). Therefore, most commercial voltmeters are set to determine the point at which the signals are 90 degrees out of phase, known as quadrature. However, this requires converting the phase shifter reading so it shows the correct amount of phase shift rather than 90 degrees more or less than the actual amount. Different manufacturers use different methods to determine the signal quadrant, which leads to some confusion. Also, manufacturers differ on whether the final reading is a leading or a lagging phase shift. This means that you, the technician, must know the phase angle voltmeter you are using. The Navy has several phase angle voltmeters and each operates differently. You cannot assume that the method you use to determine the phase angle on one type of meter is the method you should use to determine it on another. Also, you cannot assume that because one meter gives a leading angle between signal and reference waveforms, another meter will also give a leading phase shift. Q9. Q10. Q5. Q6. Q7. What is the most accurate portion of the ohmmeter scale, and why? When repairing equipment, you should use a meter with the same sensitivity as specified in schematics and wiring diagrams. What is the reason for doing this? Name the piece of test equipment that consists of a hand-driven dc genera- tor, applies a high voltage to the com- ponent under test, and measures current leakage. Loading effect is the result of a meter’s sensitivity, and it causes incorrect voltage indications. What relationship exists between a meter’s sensitivity and its loading effect? What is the major difference between a VTVM and a conventional multimeter? A phase angle voltmeter is used to determine the overall accuracy of electronic equipment by measuring phase angles. What is actually measured by the phase angle voltmeter? DIFFERENTIAL VOLTMETER The differential voltmeter is a reliable precision piece of test equipment. Its general function is to compare an unknown voltage with an internal reference voltage and to indicate the difference in their values. A common differential voltmeter is the 883A (fig. 8-11), manufactured by the John Fluke Co. The Fluke 883A has many capabilities and uses. You may use it as 1. 2. 3. a conventional transistor voltmeter for measuring voltages from 0 volt to 1,100 volts dc, a differential voltmeter for precision (0.01 percent of input voltage) measurement of dc voltages in this range, or as an accurate ac voltmeter and a megohmmeter for measuring resistance from 10 megohms to 11,000 megohms. The Fluke Model 883A is accurate enough for precision work in calibration laboratories yet rugged enough for general shop use. For more information on the Fluke Model 883A, you should refer to NEETS, module 16. FREQUENCY MEASUREMENT Often, frequency measurements are an essential part of preventive and corrective maintenance for electronic equipment. You may have to determine rotation frequencies of some mechanical devices. For example, you have to 8-10

Figure 8-11.-Fluke Model 883A differential voltmeter. check the output frequency of electric power generators when starting the engine and during preventive maintenance routines. Equipment that operates in the audio-frequency range requires adjusting to operate at the correct frequencies. Accurate tuning of radio transmitters to their assigned frequencies provides reliable com- munications. Tuning also avoids interfering with radio circuits operating on other frequencies. Radar sets also require proper tuning to get satisfactory performance. A stroboscope can measure the rotation frequency of rotating machinery such as radar antennas, servomotors, and other types of electric motors. Stroboscopic methods compare the rate of one mechanical rotation or vibration with another or with the frequency of a varying source of illumination. Tachometers can also measure the rotation frequency of armatures in electric motors, dynamotors, and engine-driven generators. Vibrating-reed, tuned-circuit, or moving-disk meters directly measure the electrical output frequency of ac power generators. The vibrating- reed device is the simplest frequency meter, and it is rugged enough to mount directly on generator control panels. You may also use it to check the line voltage in the shop to be sure the proper 8-11

frequency is available to the equipment and/or test sets. Frequency Meters The term frequency meter refers to an item of test equipment used to indicate the frequency of an external signal. Although some frequency meters generate signals having a basic frequency, you should not confuse them with test equipment known as signal generators. The frequency meter measures the frequency of a signal developed in an external circuit. Some frequency meters generate a signal frequency; others do not. Those that don’t generate an internal frequency are known as wavemeters. There are two basic types of wavemeters—reaction and absorption. Frequency meters that do generate an internal frequency may use either electronic or mechanical oscillation as the frequency generator. Measurement Methods You in the parison may make frequency audio-frequency range method or by using a measurements by the com- direct-reading frequency meter. You may make frequency comparisons by use of a calibrated audio- frequency signal generator with either an oscilloscope or a modulator and a zero-beat indicator device. Instruments using series frequency-selective electrical networks, bridge test sets having null indicators, or counting-type frequency meters can make direct-reading frequency measurements. Since the wavemeter is relatively insensitive, it is very useful in determining the fundamental frequency in a circuit generating multiple harmonics. You may check the calibration of test equipment that measures signals in this frequency range by comparing them with standard frequency signals broadcast by the National Bureau of Standards. The signal frequencies of radar equipment that operate in the UHF and SHF ranges can be measured by resonant cavity-type wavemeters, resonant coaxial line-type wavemeters, or Lecher-wire devices. When properly calibrated, resonant cavity and resonant coaxial line wavemeters are more accurate. They also have better stability than wavemeters used for measurements in the LF to VHF range. These frequency-measuring instruments often come as part of communication and electronic equipment, but they are also available as general-purpose test sets. 8-12

Heterodyne Meters Heterodyne frequency meters are available in several varieties. Although they all function in the same general manner, some differences exist in how they accomplish their purpose. Test instruments of this class generate a signal within the test set. This signal mixes with a signal from the equipment under test to obtain a beat frequency. The frequency of one signal is then changed to obtain a zero beat. The beat frequency is the difference frequency that results from heterodyning two signals. A zero beat results when heterodyning two signals of the same frequency. You may determine the frequency of the unit under test by reading the frequency indicator of the test set. A heterodyne frequency meter (fig. 8-12) usually consists of the following parts: A heterodyne oscillator An RF harmonic amplifier A crystal-controlled oscillator A A mixer or detector modulator An AF output amplifier A means for indicating frequency Most models come with a set of calibration charts giving the dial readings for the frequencies listed and a table of the crystal harmonics. The table and charts give complete and accurate frequency coverage over the set’s range. Some models indicate the frequency directly on dials. The crystal-controlled oscillator operates at a fixed frequency. However, it is also capable of emitting various harmonic frequencies of the crystal for use as check frequencies. These checkpoints provide a measure for adjusting the heterodyne oscillator, thus ensuring more accurate operation. Provisions are usually made within the crystal-controlled oscillator for precise adjustment to its assigned fundamental frequency. Figure 8-12.-Crystal-calibrated heterodyne frequency meter block diagram. 8-13

Wavemeters Wavemeters are calibrated, resonant circuits used to measure frequency. Although not as accurate as heterodyne frequency meters, wave- meters are comparatively simple and easy to carry. You may see any type of resonant circuit in wavemeter applications. The exact kind of circuit depends on the frequency range for which the meter is intended. Resonant circuits consisting of coils and capacitors are used with low-frequency wavemeters. VHF and microwave instruments have butterfly circuits, adjustable transmission line sections, and resonant cavities. There are three basic kinds of wavemeters— the absorption, the reaction, and the transmission types. The absorption wavemeter consists of the basic resonant circuit, a rectifier, and a meter for indicating the amount of current induced into the wavemeter. In use, this type of wavemeter loosely couples to the measured circuit. Then, you adjust the resonant circuit of the wavemeter until the current meter shows a maximum deflection. You determine the frequency of the circuit under test from the calibrated dial of the wavemeter. The reaction wavemeter gets its name from having to be adjusted until a marked reaction occurs in the circuit being measured. For example, the wavemeter is loosely coupled to the grid circuit of an oscillator, and the tuning circuit of the wavemeter is adjusted until it is in resonance with the oscillator frequency. The setting of the wavemeter dial is made by observing the grid- current meter in the oscillator. At resonance, the wavemeter circuit takes energy from the oscillator, causing the grid current to dip sharply. The frequency of the oscillator is then determined from the calibrated dial of the wavemeter. This type is commonly referred to as a grid-dip meter. The transmission wavemeter is an adjustable coupling link. When inserted between a source of radio-frequency energy and an indicator, energy is transmitted. However, energy to the indicator only occurs when the wavemeter is tuned to the frequency of the source. Transmission wavemeters are commonly used to measure microwave frequencies. Units of this type are also found in echo boxes. The additional provisions for echo boxes permit additional testing functions. Many types of wavemeters are used for various functions. The cavity-type wavemeter (fig. 8-13) is the type most commonly used for measuring microwave frequencies; therefore, it is the one covered in this chapter. The device employs a resonant cavity that effectively acts as Figure 8-13.-Typical cavity wavemeter. 8-14

a high-Q, LC tank circuit. The resonant frequency of the cavity varies by means of a plunger, which mechanically connects to a micrometer mechanism. Movement of the plunger into the cavity reduces the cavity size and increases the resonant frequency. Conversely, an increase in the size of the cavity (made by withdrawing the plunger) lowers the resonant frequency. The microwave energy from the equipment under test goes into the wavemeter through one of two inputs—A or D. The crystal rectifier then detects (rectifies) the signal, and the current meter (M) indicates the rectified current. You can use the cavity wavemeter as either a transmission-type or an absorption-type wavemeter. When used as a transmission wavemeter, the unknown signal couples into the circuit through the A input. When the cavity is tuned to the resonant frequency of the signal, energy is coupled through coupling loop B into the cavity and out through loop C to the crystal rectifier. It is rectified, and current flow resulting from this rectification is indicated on the meter. At frequencies off resonance, little or no current flows in the detector, and the meter reading is small. Vary the micrometer and attached plunger until you get a maximum meter reading. Compare the resulting micrometer setting with a calibration chart supplied with the wavemeter to determine the unknown frequency. When the unknown signal is relatively weak, such as the signal from a klystron oscillator, the wavemeter functions as an absorption wavemeter. Connect the instrument at the D input. The RF loop C then acts as an injection loop to the cavity. When the cavity is tuned to the resonant frequency of the klystron, the cavity absorbs maximum energy and the meter will dip. This indicates a reduction of current. When the cavity is not at the resonant frequency of the klystron, the current meter will indicate high current. Therefore, tune the cavity for a minimum reading, or dip, in the meter, and determine the resonant frequency from the micrometer setting and the calibration chart. Potentiometer R1 adjusts the sensitivity of the meter from the front panel of the instrument. J1 is a video jack for observing video waveforms with a test oscilloscope. A directional antenna is used with the instrument for making relative field strength measurements of radiated signals for use in measuring the frequency of radar transmitters. This setup is also used for constructing radiation patterns of transmitting antennas. In radiation pattern measurements connect the directional antenna to the wavemeter input and tune the instrument to the frequency of the system under test. The cavity will then lock on this frequency by an automatic frequency control (AFC) system. For reliable results, the output signal must be continuous and constant. This is necessary for any variation in the meter reading caused directly by a change in the actual field strength. That is the signal field strength when the position of the wavemeter changes with respect to the transmitting antenna. After establishing a reference level on the meter, change the position of the wavemeter by moving it around the radiating antenna, maintaining a fixed distance from it. To determine the field pattern, record the wavemeter readings at various positions around the transmitting equipment on polar graph paper. COUNTER-TYPE FREQUENCY METER The counter type of frequency meter is a high-speed electronic counter, with an accurate, crystal-controlled time base. This type of combination provides a frequency meter that automatically counts and displays the number of events (hertz) occurring in a precise interval, The frequency meter itself does not generate any signal, it merely counts the recurring pulses fed to it. The Hewlett-Packard Model 5245L electronic counter (figs. 8-14 and 8-15) is a high-fre q uency general-purpose electronic counter. The Model 5245L measures frequencies from 0 to 50 MHz, periods from 1 µsec to 10 seconds, and period averages from 10 to 100,000 periods. Also, it can measure the ratio of two frequencies and the multiplied ratio of two frequencies. The Model 5245L provides the following additional features: Decade scaling to for any frequency to 50 MHz Standard output frequencies from 0.1 Hz to 10 MHz, in decade steps Four-line, binary-coded-decimal (BCD) output to drive digital recorder (Hewlett- Packard Model 562A), digital-to-analog converter (Hewlett-Packard Model 580A/581A), remote readout, or data processing equipment Remote control by external contact closure 8-15

Figure 8-14.-Model 5245L electronic counter front panel. 8-16

Figure 8-15.—Model 5245L electronic counter rear panel. 8-17

Display storage that permits reading display while making a new count Eight-digit display using rectangular (narrow) digital display tubes, with decimal point position and measurement units displayed automatically Operation with plug-in units that extend the basic range and performance of the counter The Model 5245L features solid-state design, low-power consumption, small size (5 1/4-inch panel height), light weight (32 pounds), easy conversion for rack mounting, and modular plug- in circuit boards for simplified maintenance. To increase the range of measurement, five plug-in units (not shown) are available. The Model 5245L measures frequency, period average, ratio of two frequencies, and total events. A FUNCTION selector switch selects measurement function, and a TIME BASE selector switch selects time base or multiplier. A SAMPLE RATE control selects the sampling rate, and a SENSITIVITY control adjusts instrument sensitivity. Direct readout is available in both PERIOD and FREQUENCY functions with measurement units displayed and with decimal point auto- matically positioned. In the MANUAL function the display is a direct read. The decimal point will not light. Note that the only difference between ratio and period measurements is the use of an external frequency instead of the internal 1-MHz oscillator. Two factors determine the basic counter accuracy, One factor is the aging rate of the 1-MHz crystal standard in the time base, which is less than 2 parts in per week. A second factor is the inherent error of ±1 count present in all counters of this type. This error is due to phasing between the timing pulse that operates the electronic gate and the pulses that pass through the gate to the counters. The chart in figure 8-16 shows the errors possible for frequency or period measurements, The three factors contributing to the accuracy of period measurements are as follows: 1. 2. 3. The aging rate of the l-MHz standard, which is less than 2 parts in per week The ambiguity of the ±1 count The ± trigger error (for one period, and a signal-to-noise ratio of 40 dB, this trigger error is 0.3 percent at rated sensitivity) Figure 8-16.-Model 5245L electronic counter measurement accuracy. Frequencies of 0.1 Hz to 1 MHz are available in decade steps at the TIME BASE EXT connector as selected by the TIME BASE switch. This output is subject to the following restrictions, Frequencies of 0.1 Hz through 10 MHz are available in decade steps at the rear-panel OUTPUT connector as selected by the rear-panel OUTPUT switch. This output is subject to the following restrictions. All frequencies are available one at a time in the MANUAL function without interruption. 1 kHz is continuously available for all functions except 100K PERIOD AVERAGE. The 10 kHz to 10 MHz is continuously available in all functions. NOTE: The accuracy and stability of these outputs are the same as those of the time base oscillator. The Hewlett-Packard Model 525 1A frequency converter extends the frequency range of the Model 5245L to 100 MHz. The Model 5251A mixes a selected 10-MHz harmonic (between 20 and 90 MHz) with the input signal. The resulting difference-frequency signal receives amplification and goes to the basic counter for counting and display. Because the selected 10-MHz harmonic 8-18

is from a harmonic generator driven by a 10-MHz output from the basic counter, the stability and accuracy of the basic counter remains. The Hewlett-Packard Model 5253B frequency converter extends the frequency range of the Model 5245L to 512 MHz. To retain the stability and basic accuracy, multiply a 10-MHz signal, from the counter’s internal time base, to a known harmonic frequency. When this harmonic frequency mixes with the input signal frequency, the difference frequency that results is within the range of the basic counter, and the counter displays the difference frequency. The Hewlett-Packard Model 5254A frequency converter provides the Model 5254L with a frequency range from 300 to 3,000 MHz. To retain the stability and accuracy of the basic counter, use a 50-MHz multiple of the crystal- oscillator signal from the counter to beat with the measured signal. The difference frequency produced is within the display range of the basic counter. The converter has an indicator that aids in frequency selection and indicates the output level to the counter. The required input signal level is 50 mV rms to 1 V rms. The input connector is a type N female. The Hewlett-Packard Model 5261A video amplifier unit extends the sensitivity of the Model 5245L to 1.0 millivolt over the frequency range of 10 Hz to 50 MHz. Input impedance increases to 1 megohm and can increase to 10 megohms by using an accessory 10:1 divider probe (Hewlett- Packard 10003A) for signals greater than 10 mV. A 50-ohm output is used for oscilloscope moni- toring of the amplified signal. The Hewlett-Packard Model 5262A time interval unit provides start and stop pulses. These pulses start by electrical inputs to the main count gate in the Model 5245L, enabling it to make time measurements. Time intervals from 1 microsecond to 10 8 seconds are measured with a resolution of 0.1 microsecond. Basic counter accuracy remains when the signal counted is from the internal oscillator. Q11. Q12. Q13. Describe the general function of a differential voltmeter. What item of test equipment is used to indicate the frequency of an external signal? List the parts of most heterodyne frequency meters. Q14. Q15. Q16. Q17. Wavemeters are calibrated resonant circuits used to measure frequency. List the three basic kinds of wavemeters. Of the three basic wavemeters, which one is commonly used to measure microwave frequencies? The counter frequency meter is a high-speed electronic counter, with an accurate, crystal- controlled time base. What does this combination provide? What does the Model 5245L counter frequency meter measure? POWER MEASUREMENTS You must check the power consumption and the input and output signal power levels of electronic equipment. It is easy to determine dc power; the unit of power (the watt, P) is the product of the potential in volts (E) and the current (I) in amperes, or, P = IE. You can take a few basic circuit measurements and compute the power using Ohm’s law. It is not as easy to determine ac power. To make ac power measurements, you must consider the phase angle of the voltage and current. Measurement is further complicated by the frequency limitations of various power meters. If there is no phase difference, compute ac power in the same manner as dc power—by determining the average value of the product of the voltage and current. Electric power at a line frequency of approxi- mately 60 Hz is directly measured by a dyna- mometer type of wattmeter. This type of meter indicates the actual power. Therefore, the phase angle of the voltage and current does not have to be determined. Normally, the exact power consumption of equipment is not necessary for maintenance, and a current measurement is enough to decide whether the power consumption is within reasonable limits. Many ac voltmeters have scales calibrated in decibels (dB) or volume units. Such meters are used to make measurements where direct indication in decibels is desired. Remember, these are voltmeters and that power measurements are not meaningful unless the circuit impedance is known. The topic of decibels is discussed in chapter 1 of Aviation Electronics Technician 3, NAVEDTRA 1 4028, NEETS, modules 11 and 16, and in the Electronics Installation & Maintenance 8-19

Book Test Methods and Practices , NAVSHIPS 0967-LP-000-0130. For more information on decibels, refer to these publications. At radio frequencies below the UHF range, power is usually determined by voltage, current, and impedance measurements. One common method used to determine the output power of RF oscillators and radio transmitters consists of connecting a known resistance to the equipment output terminals. After measuring the current flow through the resistan ce, you then calculate the power as the product of I 2R. Since the power is proportional to the current squared, the meter scale can indicate power units directly. A thermocouple ammeter is used to measure RF current. The resistor used to replace the normal load is of special design. It has to have low reactance and the ability to dissipate the required amount of power. Some common names for such resistors are dummy loads or dummy antennas. In the UHF and SHF portions of the RF spectrum, it is more difficult to accurately measure voltage, current, and impedance. These basic measurements may change greatly at slightly different points in a circuit. Also, small changes in the placement of parts near the tuned circuits may affect their measurements. Test instruments that convert RF power to another form of energy, such as light or heat, can measure the power output of microwave radio or radar transmitters indirectly. One method measures the heating effect of a resistor load on a stream of passing air. To achieve accurate measurement of large magnitude power, you can measure the temperature change of a water load. The most common type of power meter for use in this frequency range uses a bolometer. The bolometer is a loading device that undergoes changes of resistance as changes in the power dissipation occur. Measure the resistance before and after applying RF power; the change in resistance determines the power. The Model 432A power meter operates with Hewlett Packard (HP) temperature-compensated thermistor mounts, such as the 8478B and 478A coaxial and 486A waveguide series. The frequency range of the 432A with these mounts in 50-ohm coaxial systems is 10 MHz to 18 GHz. Its frequency range in waveguide systems is 2.6 GHz to 40 GHz. Full-scale power ranges are 10 microwatts to 10 milliwatts (-20 dBm to +10 dBm). The total measurement capacity of the instrument is divided into seven ranges, selected by a front-panel RANGE switch (fig. 8-17). The COARSE ZERO and FINE ZERO controls zero the meter. Zero carry-over from the most sensitive range to the other six ranges is within ±0.5 percent. When setting the RANGE

A11. Its general function is to compare an unknown voltage with an internal reference voltage and to indicate the difference in their values. A12. Frequency meter . A13. A heterodyne oscillator, RF harmonic amplifier, crystal-controlled oscillator, a mixer or detector, a modulator, an AF output amplifier, and a means for indicating frequency. A14. Absorption, reaction, and transmission . A15. Transmission . A16. A frequency meter that autom atically counts and displays the number of events (hertz) occurring in a precise interval. A17. Frequency, period average, ra tio of two frequencies, and total events.

8-20

8-21 Figure 8-17.—Model 432A power meter front panel. switch to COARSE ZERO, the meter indicates thermistor bridge unbalance. Adjust the front panel COARSE ZERO adjust for initial bridge balance. For best results, FINE ZERO the 432A o n the particular meter range in use. The CALIBRATION FACTOR switch provides discrete amounts of compensation for measurement uncertainties related to standing wave ratio (SWR) and thermistor mount efficiency. The calibration factor value permits direct meter reading of the RF power delivered to an impedance equal to the characteristic impedance (ZO) of the transmission line between the thermistor mount and the RF source. The label of each 8478B, 478A or 486A thermistor mount contains calibration factor values. The MOUNT RESISTANCE switch o n the front panel compensates for three types of thermistor mounts. You can use Model 486A waveguide mounts by setting the MOUNT RESISTANCE switch to 100 o r 200Ω, depending o n the thermistor mount. The 200Ω position is for use with Models 478A and 8478B thermistor mounts. The rear panel baby N connector (BNC) labeled RECORDER (fig. 8-18) provides an output voltage that is Figure 8-18.-Model 432A power meter rear panel. linearly proportional to the meter current. One volt fed into an open circuit equals full-scale meter deflection. This voltage develops across a 1-kilohm resistor. Therefore, when a recorder with a 1-kilohm input impedance is connected to the RECORDER output, about 0.5 volt will equal full-scale deflection. This loading of the RECORDER output has n o effect on the accuracy of the 432A panel meter. You may connect a digital voltmeter to the rear panel RECORDER output for more resolution of power meter readings. When connecting a voltmeter with an input impedance greater than 1 megohm to the RECORDER output, 1 volt equals full-scale deflection. The 432A has two calibration jacks ( VRF and VCOMP) on the rear panel. You can use them for precision power measurements. Instrument error can be reduced from ±1 percent to ±0.2 percent of reading + 5 µW. This depends on the care taken when measuring and on the accuracy of auxiliary equipment. Some factors affect the overall accuracy of power measurement. The major sources of error are mismatch error, RF losses, and instrumentation error. In a practical measurement situation, both the source and thermistor mount have SWR, and the

source seldom matches the thermistor mount unless using a tuner. The amount of mismatch loss in any measurement depends on the total SWR present. The actual thermistor mount impedance, the electrical length of the line, and the characteristic impedance of the line will determine the impedance that the source sees. In general, neither the source nor the thermistor mount has impedance, and the actual impedances are only reflection coefficients, mismatch losses, or SWR. The power delivered to the thermistor mount, hence the mismatch loss, can only be described as being somewhere between two limits. The uncertainty of power measurement due to mismatch loss increases with SWR. Limits of mismatch loss are generally determined by means of a chart. To determine the total mismatch loss uncertainty in power measurement, algebraically add the thermistor mount losses to the uncertainty caused by source and thermistor mount match. RF losses account for the power entering the thermistor mount but not being dissipated in the detection thermistor element. Such losses may be in the walls of a waveguide mount or in the center conductor of a coaxial mount. Losses may also be from the capacitor dielectric, poor connections within the mount, or be due to radiation. The degree of inability of the instrument to measure the substitution power supplied to the thermistor mount is called power meter accuracy or instrumentation error. Instrumentation error of the Model 432A is ±1 percent of full scale, 0°C to +55°C. Calibration factor and effective efficiency are correction factors for improving power measurement accuracy. Both factors are marked on every HP thermistor mount. The calibration factor compensates for thermistor mount VSWR and RF losses whenever connecting the thermistor mount to an RF source without a tuner. Effective efficiency compensates for thermistor mount RF losses when using a tuner in the measurement system. Set the 432A CALIBRATION FACTOR selector to the appropriate factor indication on the thermistor mount. This resulting power indicates the power that would go from the source to a load impedance equal to The calibration factor does not compensate for source VSWR or for multiple reflections between the source and the thermistor mount. You can minimize mismatch between the source and the thermistor mount without a tuner. Insert a low SWR precision attenuator in the transmission line between the thermistor mount and the source. Since the mount impedance (and corresponding SWR) deviates significantly only at the high and low ends of a microwave band, it is unnecessary to use a tuner. A tuner or other effective means of reducing mismatch error is recommended when the source SWR is high or when more accuracy is necessary. The HP Model 478A coaxial thermistor mount (fig. 8-19) is designed for use with HP Models 431 and 432 power meters. It can measure microwave power from 1 µW to 10 µW. The mount design minimizes adverse effects from environmental temperature changes during measurement. For increased measurement accuracy, effective efficiency and calibration factor are measured for each mount and at selected frequencies across the operating range. The results are marked on the label of the instrument. The Model 478A operates over the 10-MHz to 10-GHz frequency range. Throughout the range, the mount terminates the coaxial input in a 50-ohm impedance and has a SWR of not more than 1.75 without external tuning. Each mount contains two matched series pairs of thermistors, which cancel the effects of drift with ambient temperature change. Thermal stability is accomplished by mounting the leads of all four thermistors on a common thermal conductor to ensure a common thermal environ- ment. This conductor is thermally insulated from the main body of the mount. The thermal insulation makes sure thermal noise or shocks applied externally to the mount, such as those Figure 8-19.-Model 478A thermistor mount. 8-22

from handling the mount manually, cannot significantly disturb the thermistor. The thermal immunity lets the thermistors be used to measure microwave power down to the microwatt region. Q18. By what method is dc power determined? Q19. You use a resistor that is specially designed to dissipate the required amount of power and replace normal loads. List the two types of resistors used for this purpose. Q20. List the major sources of error that affect the overall accuracy of power measure- ments. SEMICONDUCTOR TESTERS Since semiconductors have replaced vacuum tubes, the testing of semiconductors is vital. In this section, three basic types of equipment are discussed—the Huntron Tracker 1000, Huntron Tracker 2000, and the Automatic Transistor Analyzer Model 900 in-circuit transistor tester. Huntron Tracker 1000 You will test components with Huntron Tracker 1000 using a two-terminal system, where two test leads attach to the leads of the component under test. The 1000 tests components in-circuit, even when there are several components in parallel. The following types of devices are tested using the Huntron Tracker 1000: Semiconductor diodes Bipolar and field effect transistors Bipolar and MOS integrated circuits (both analog and digital) Resistors, capacitors, and inductors The 1000 is used on boards and systems with ALL voltage sources in a power-off condition. A 0.25 ampere signal fuse (F1) connects in series with the channel A and B test terminals. Accidentally contacting test leads to active voltage sources (for example, line voltage, powered-up boards or systems, charged high-voltage capacitors, etc.) may cause this fuse to open, making replacement necessary. When the signal fuse blows, the display shows open circuit signatures, even with the test leads shorted together. CAUTION The device to be tested must have all power turned off and have all high-voltage capacitors discharged before connecting the 1000 to the device. The line fuse (F2) should only open when there is an internal failure inside the instrument. Therefore, you should always locate and correct the problem before replacing F2. The front panel of the 1000 makes function selection easy. The 1000 uses interlocking push- button switches for range selection. A toggle switch is used for channel selection, and integral LED indicators show the active functions. The CRT displays the signatures of the parts under test. The display has a graticule consisting of a horizontal axis that represents voltage, and a vertical axis that represents current. The horizontal axis is divided into eight divisions, which lets you estimate the voltage at which signature changes occur. This is mainly useful in determining semiconductor junction voltages under either forward or reverse bias. Push in the power on/off switch. The 1000 should come on line with the power LED illuminated. Before you can analyze signatures on the CRT, you must focus the 1000. To do this, turn the intensity control to a comfortable level. Now, adjust the focus control (back panel) for the narrowest possible trace. Aligning the trace is important in determining the voltages at which changes in the signature occur. With a short circuit on channel A, adjust the horizontal control until the vertical trace is even with the vertical axis. Open channel A and adjust the vertical control until the horizontal trace is even with the horizontal axis. Once set, you should not have to adjust these controls during normal operation. Turn the power off by pushing the power switch in. When you turn the power on again, the same intensity setting will be present. The 1000 has three impedance ranges—low, medium, and high. To select these ranges, press the appropriate button on the front panel. Always start with the medium range; then you can adjust for other ranges. If the signature on the CRT is close to an open (horizontal trace), try the next higher range for a more descriptive signature. If the signature is close to a short (vertical trace), try the next lower range. There are two channels (channel A and channel B) that you can select by moving the 8-23

toggle switch to the desired position. When using a single channel, plug the red probe into the corresponding channel test terminal. Then plug the black probe into the common test terminal. When testing, connect the red probe to the positive terminal of the device (that is, anode, +V, etc.). Connect the black probe to the negative terminal of the device (that is, cathode, ground, and so forth.). By following this procedure, the signature will appear in the correct position on the CRT display. The alternate mode of the 1000 provide-s automatic switching back and forth between channel A and channel B. This allows easy comparison between two devices or the same point on two circuit boards. You select the alternate mode by moving the toggle switch to the ALT position. The alternate mode is useful when comparing a known good device with the same device whose quality is unknown. The signal section applies the test signal across two terminals of the device under test. The test signal causes current to flow through the device and a voltage drop across its terminals. The current flow causes a vertical deflection of the signature on the CRT display. The voltage across the device causes a horizontal deflection of the signature on the CRT display. The combined effect produces the current-voltage signature of the device on the CRT display. An open circuit has zero current flowing through the terminals and a maximum voltage across the terminals. In the LOW range, a diagonal signature from the upper right to the lower left of the CRT (fig. 8-20, view A) represents an open circuit. In the HIGH and MEDIUM ranges, an open circuit shows as a horizontal trace from the left to the right (fig. 8-20, view B). When you short the terminals together, the maximum current flows through the terminals, and the voltage at the terminals is zero. A vertical trace from the top to the bottom of the CRT graticule in all ranges shows this short (fig. 8-20, view C). The CRT deflection drivers boost the low-level outputs from the signal section to the higher voltage levels needed by the deflection plates in the CRT. The HORIZONTAL and VERTICAL controls on the front panel adjust the position of the trace on the CRT display. 288X Figure 8-20.-Circuit signatures: View A—Low-range open circuit; view B—medium- and high-range open circuit; and view C—all ranges short circuit. 8-24

You use three other CRT controls to adjust the brightness and clarity of the trace— INTENSITY, FOCUS, and ASTIGMATISM. The front panel intensity control is the primary means of adjusting the visual characteristics of the trace. The focus control is on the back panel and is operator adjustable. The astigmatism trim pot is inside the 1000 on the main printed circuit board. The pot is factory adjusted to the correct setting. Huntron Tracker 2000 The Huntron Tracker 2000 (fig. 8-21) is a versatile troubleshooting tool having the following features: Multiple test signal frequencies (50/60 Hz, 400 Hz, 2000 Hz) Four impedance ranges (low, medium 1, medium 2, high) Automatic range scanning Range control: high lockout Adjustable rate of channel and/or range scanning alteration Dual polarity pulse generator for dynamic testing of three terminal devices LED indicators for all functions Dual channel capability for easy comparison Large CRT display with easy to operate controls GENERAL OPERATION.— You will test components using the 2000 t wo-terminal system. It also has a three-terminal system when using the built-in pulse generator. When using this system, you place two test leads on the leads of the component under test. The 2000 tests components in-circuit, even when there are several parts in parallel. Use the 2000 only on boards and systems with all voltage sources in a power-off condition. A 0.25 ampere signal fuse connects in series with the channels A and B test terminals. Accidental contact of the test leads to active voltage sources, such as line voltage, powered-up boards or systems, and charged high-voltage capacitors may cause this fuse to open, making replacement necessary. When the signal fuse blows, the 2000 displays short circuit signatures even with the test leads open. CAUTION The device under test must have all power turned off and all high-voltage capacitors discharged before connecting the 2000 to the device. 288X Figure 8-21.-Huntron Tracker 2000. 8-25

Table 8-1.-Front Panel Controls and Connectors 288X 8-26

Figure 8-22.-Front panel. 288X The line fuse should only open when there is an internal failure inside the instrument. Always locate the problem and correct it before replacing this fuse. Front Panel.— The front panel of the 2000 makes function selection easy. All push buttons are the momentary action type. Integral LED indicators show which functions are active. Look at figure 8-22 and table 8-1 for details about each item on the front panel. Back Panel.— Secondary controls and connectors are located on the back panel (fig. 8-23 and table 8-2). Figure 8-23.-Back panel. Table 8-2.-Back Panel Controls and Connectors 288X 288X 8-27

CRT Display.— The signature of the part under test is displayed on the CRT. The display has a graticule consisting of a horizontal axis that represents voltage, and a vertical axis that represents current. The axes divide the display into four quadrants. Each quadrant displays different portions of the signatures. Quadrant 1 displays positive voltage (+V) and positive current (+I). Quadrant 2 displays negative voltage (-V) and positive current (+I). Quadrant 3 displays negative voltage (-V) and negative current (–I). Quadrant 4 displays positive voltage (+V) and negative current (–I). The horizontal axis divides into eight divisions, which allows the operator to estimate the voltage at which changes in the signature occur. This is useful in determining semiconductor junction voltages under either forward or reverse bias. OPERATION OF PANEL FEATURES.— The following section explains how to use the front and back panel features. Turn the power/intensity knob clockwise. The 2000 comes on with the LEDs for power, channel A, 50/60 Hz, low range, and pulse/DC illuminated. Focusing the 2000 display is an important part of analyzing the test signatures. First you adjust the intensity control to a comfortable level. Then, adjust the focus control (back panel) for the narrowest possible trace. Aligning the trace is important in determining which quadrants the portions of a signature are in. With a short circuit on channel A adjust the trace rotation control until the trace is parallel to the vertical axis. Adjust the horizontal control until the vertical trace is even with the vertical axis. Open channel A and adjust the vertical control until the horizontal trace is even with the horizontal axis. Once set, you should not have to readjust these settings during normal operation. Range Selection.— The 2000 has four impedance ranges—low, medium 1, medium 2, and high. You select these ranges by pressing the appropriate button on the front panel. Start with one of the medium ranges; that is, medium 1 or medium 2. If the signature on the CRT display is close to an open (horizontal trace), select the next higher range for a more descriptive signature, If the signature is close to a short (vertical trace), select the next lower range. The high lockout feature, when activated, prevents the instrument from entering the high range. This feature works in either the manual or auto mode. The auto feature scans through the four ranges—three with the HIGH LOCKOUT activated at a speed set by the RATE control. This feature allows you to see the signature of a part in different ranges while freeing your hands to hold the test leads. Channel Selection.— There are two channels on the 2000-channel A and channel B. You select a channel by pressing the appropriate front panel button. When using a single channel, plug the red probe into the corresponding channel test terminal. Plug the black probe into the common test terminal. When testing, connect the red probe to the positive terminal of the device; that is, anode, +V, etc. Connect the black probe to the negative terminal of the device; that is, cathode, ground, and so forth. Following this procedure should assure that the signature appears in the correct quadrants of the CRT display. The ALT mode is a useful feature of the 2000. It lets you compare a known good device with a device of unknown quality. In this test mode, you use common test leads to connect two equivalent points on the boards to the common test terminal. The ALT mode of the 2000 allows you to automatically switch back and forth between channel A and channel B so you can easily compare two devices. You may also compare the same points on two circuit boards. Select the ALT mode by pressing the ALT button on the front panel. You may vary the alternation frequency by using the RATE control. NOTE: The black probe plugs into the channel B test terminal. When using the alternate and auto features simultaneously, each channel is displayed before the range changes. Figure 8-24 shows the sequence of these changes. Frequency Selection.— The 2000 has three test signal frequencies—50/60 Hz, 400 Hz and 2000 Hz. You can select these by pressing the appropriate button on the front panel. In most cases, you should start with the 50/60 Hz test 8-28

signal. 288X Figure 8-24.-Auto/alternate sequence. Use the other two frequencies to view small amounts of capacitance or large amounts of inductance. Pulse Generator.— The built-in pulse generator of the 2000 allows dynamic, in-circuit testing of certain devices in their active mode. In addition to using the red and black probes, you use the pulse generator. The output of the pulse generator connects to the control input of the device under test with one of the blue micro clips provided. The pulse generator has two outputs, G1 and G2, so you can test three terminal devices in the alternate mode. A variety of output waveforms is available using the pulse generator selector buttons. First select the pulse mode or the dc mode using the PULSE/DC button. In the pulse mode, the LED flashes at a slow rate. In dc mode, the LED is continuously on. Then select the polarity of output desired using the positive (+) and negative (–) buttons. All three buttons function in a push-on/push-off mode, and only interact with each other to avoid the NOT ALLOWED state. After selecting the specific output type, set the exact output using the LEVEL and WIDTH controls. The LEVEL control varies the magni- tude of output amplitude from zero to 5 volts (peak or dc). During pulse mode, the WIDTH control adjusts the duty cycle of the pulse output from a low duty cycle to 50 percent maximum (square wave). The start of a pulse is triggered by the appropriate zero crossing of the test signal. This results in the pulse frequency being equal to the selected test signal frequency. The WIDTH control setting that selects the duty cycle determines the end of a pulse. The WIDTH control has no effect when in the dc mode. Troubleshooting Tips You will use the Huntron Tracker 1000 and the Huntron Tracker 2000 to test various types of devices and circuits. Some troubleshooting tips are given in this section. Perform most tests using the medium or low range. Use the high range only for testing at a high impedance point, or if higher test voltages are required (that is, to test the Zener region of a 40-volt device). Sometimes, component defects are more obvious in one range than another. If a suspect device appears normal for one range, try the other ranges. Use the low range when testing a single bipolar junction, such as a diode, a base- emitter junction, or a base-collector junction. It offers the best signature. Use a higher range to check for reverse bias leakage. When performing in-circuit testing, do a direct comparison to a known good circuit. The 1000 test leads are not insulated at the tips, Be sure to make good contact to the device(s) under test. (NOTE: This tip pertains to the 1000 only.) When you troubleshoot, try relating the failure mode of the circuit under test to the type of defect the 1000 shows. For example, expect a catastrophic printed circuit board failure to have a dramatic signature difference from that of a normal device of the same type. A marginally operating or intermittent board may have a failed part that shows only a small pattern difference from normal. If you cannot relate a system failure to a specific area of the printed circuit board, begin by examining the signatures at the connector pins. This method of troubleshooting shows all the inputs and outputs. It will often lead directly to the failing area of the board. 8-29

Devices made by different manufacturers, especially digital integrated circuits, are likely to produce slightly different signatures. This is normal and may not show a failed device. Remember, leakage current doubles with every 10-degree Celsius rise in temperature. Leakage current shows up as a rounded transition (where the signatures show the change from zero current flow to current flow) or by causing curvature at other points in the signatures. Leakage current causes curvatures due to its nonlinearity. Never begin the testing of an integrated circuit using the low range. If you initially use the low range, confusion can result from the inability of this range to display the various junctions. Always begin testing using the medium range. If the signature is a vertical line, switch to the low range. Here you can check for a short or low impedance (less than 500 ohms). Switch to the low range if the device is suspect and appears normal in the medium range. This will reveal a defective input protection diode not evident when using the medium range. NOTE: The 2000 test leads are conductive only at the tips. Be sure to make good contact with the device(s) under test. When testing analog devices or circuits, use the low range. Analog circuits contain many more single junctions. Defects in these junctions show more easily when using the low range. Also, the 54-ohm internal impedance in the low range makes it less likely that parts in parallel with the device under test will sufficiently load the tester to alter the signature. When testing an op amp in-circuit, compare it directly to a known good circuit. This is because the many different feedback paths associated with op amps can cause an almost infinite number of signatures. Often when checking a Zener diode in-circuit, it will not be possible to examine the Zener region due to circuit leakage. If you must see the Zener region under this condition, unsolder one side of the diode to eliminate the loading effects of the circuit. HUNTRON TRACKER 1000.— Bipolar integrated circuits containing internal shorts produce a resistive signature (a straight line). This line begins in the 10 o’clock to 11 o’clock position. It ends in the 4 o’clock to 5 o’clock position on the display when using the low range. This type of signature is always characteristic of a shorted integrated circuit. It results from a resistive value of 4 to 10 ohms, typical of a shorted integrated circuit. A shorted diode, capacitor, or transistor junction always produces a vertical (12 o’clock) straight line using the low range. HUNTRON TRACKER 2000.— Bipolar integrated circuits containing internal shorts produce a resistive signature (a straight line) beginning in the 1 o’clock to 2 o’clock position. This signature ends in the 7 o’clock to 8 o’clock position when using the low range. This type of signature is characteristic of a shorted integrated circuit. This results from a resistive value of 4 to 10 ohms. A shorted diode, capacitor, transistor junction, etc., always produces a vertical (12 o’clock) straight line when using the low range. Automatic Transistor Analyzer Model 900 You can use this instrument to test bipolar transistors and diodes in any one of three different modes. Two modes, the VIS and SND, can be used either in-circuit or out-of-circuit. In the VIS mode, red and green lights flashing in or out of phase with the amber light show the condition of the device under test. In the SND mode, the Sonalert™ also indicates good devices by beeping out of phase with the amber light. The intent of the SND mode is to permit the operator to perform in-circuit tests on transistors or diodes without having to look at the light display. The third mode is the METER mode. You can only use this for out-of-circuit testing. In the METER mode, you may measure Beta,

and material identity. Also, you can measure emitter base voltage, base current (Ib), and collector current (Ic). There are four ranges for the Beta mode—one for small signal transistors, two ranges for medium-power transistors, and one for large-power transistors. In the VIS mode and the SND mode, the maximum voltage, current and signal levels applied to the device under test are within safe limits. Therefore, the device under test will receive no damage nor will any adjacent circuitry. This instrument will test transistors and diodes in-circuit in the VIS or SND mode if the total dynamic shunt impedances across the junctions are not less than 270 ohms. Also, the total 8-30

dynamic shunt for the emitter to collector must not be less than 25 ohms. If such should occur, the test set will give the indication for a SHORT. The 8-inch meter, which reads from left to right, has two scales marked 0-10 and 0-50. The 0-10 range is used in the leakage collector current and Vbe (IDENT) modes. The 0-50 range is used in the BETA modes. Notice a mark on the meter just short of half-scale with the nomenclature GERMANIUM and SILICON. This mark is the reference in the IDENT mode. As the meter markings show, those readings below the mark show the device material is germanium. The readings above the mark show the device is silicon. On the slanting horizontal panel immediately in front of the meter face are the appropriate test sockets and two push-button switches. One switch is ZERO and the other BETA. On the vertical front panel immediately below the push-button switches are knobs marked ADJ and CAL. At the top center is the POLARITY switch marked PNP and NPN. In the center of the vertical front panel is the RANGE switch, the FUNCTION switch, and the Sonalert™. Near the bottom of the vertical front panel are the probe jacks. The slide switch for turning the instrument on and off is also in this location. VIS MODE: TRANSISTOR.— To test transistors with the visual indication only, turn the FUNCTION switch to the XSTR-VIS mode. The amber light should flash at about a 1-second rate. Insert the transistor under test in the proper socket. In this mode, you perform two tests on the transistor. The amber light shows the performance of each test. When the amber light is out, this is the EB-BC test mode. When the amber light is on, this is the emitter-collector test mode. The test shows good transistors by one pair of similarly colored lights (green for NPN and red for PNP) when the amber light is off. When the amber light is on, no lights show good transistors. The left-hand lights show the condition of the base-collector. The absence of one or all lights in the EB-BC test mode shows an open or opens. The occurrence of both a red and a green light on either side in the EB-BC test mode shows a short. For more information about the Model 900 tracker, refer to the Maintenance Manual, All Levels for Automatic Transistor Analyzer Model 900, ST810-AD-0PI-010, for patterns other than those just discussed. There are 96 possible patterns listed, VIS MODE: DIODE.— You cannot properly test diodes in the XSTER mode. To test a diode, insert the diode in the proper socket and turn the FUNCTION switch to the DIODE/VIS mode. If the diode is good, a pair of green lights will flash out of phase with the amber. If a pair of red lights flash out of phase with the amber light, the diode is either installed improperly or marked improperly. If the diode has a short, additional lights will flash out of phase with the amber. No lights will flash in phase with the amber. You cannot properly test transistors in the DIODE mode. When testing transistors, only one transistor should be in the test socket at one time. Do not leave any diodes in the diode socket while testing transistors. When testing diodes, do not leave transistors in the transistors sockets. If you do not observe these precautions and the devices left in the socket are defective, incorrect light indications will occur. These indications may mislead the operator into believing the device under test is defective. WARNING Unit being tested must be disconnected from ac outlet, and all capacitors capable of storing electricity should be discharged. IN-CIRCUIT TESTING.— When testing diodes in-circuit, attach the emitter lead to the anode of the diode. Attach the collector lead to the cathode. When testing transistors, attach the leads to the right terminals as shown by the schematic. If the operator happens to fasten the leads to the transistor in the wrong order, an erroneous display will result. However, if the transistor is good, the instrument will give a good indication. The indication will be for the transistor of the opposite type. A good NPN improperly connected will give good PNP indications and vice versa. If the device is bad, the instrument will give a bad indication. You cannot make a qualitative analysis of the kind of failure unless you attach the proper leads to the correct terminals. To ensure the instrument will show the correct type of transistor (PNP or NPN), you must identify the base lead. Use the following procedure to identify the base lead: 1. Disconnect the lead to the emitter terminal on the instrument. Only the light repre- senting the emitter junction should go out. 2. Reconnect the emitter lead. 8-31

3. 4. 5. Disconnect the lead to the collector terminal of the instrument. Only the light representing the collector junction should go out. Should both lights go out during the tests, the connections are incorrect. Rearrange the leads on the transistor and perform-the tests again. You should now see the proper results. There are six possible combinations for the connection of these leads. Four of these combinations are incorrect. These will cause the instrument to give an incorrect indication as to transistor type (PNP or NPN). The other two combinations will give proper indications, but you still may not know which leads are the emitter and collector terminals. You will know whether the transistor is good and whether it is an NPN or a PNP transistor. If you must know which leads are the emitter and collector terminals, it is possible to find out after identifying the base lead using the meter mode for Beta. SND Mode.— In either the XSTR/SND mode or DIODE/SND mode, light patterns showing good devices will have an accompanying beeping sound from the Sonalert™. The beeping will be out of phase with the amber light. METER Mode.— Before testing a transistor in any of the METER modes, you should test the transistor in one of the visual modes. This will tell you whether the transistor is an NPN or a PNP. After determining this, put the POLARITY switch in the proper position to agree with the indication in the visual mode. Beta.— To test the Beta of the transistor, set the FUNCTION switch to the BETA position. Next, set the RANGE switch to the appropriate position according to the power capability of the transistor under test. After the RANGE switch is in the proper position, operate the push-button switch marked ZERO. Now adjust the ADJ knob for a zero reading on the meter. Next, actuate the push-button switch marked BETA and adjust the CAL knob for full-scale deflection, Release the BETA push-button switch; now the Beta of the device will show on the meter. Take care in selecting the Beta range to test the transistor. It is possible to damage small signal transistors should you try to test them in the 2 mA Ib (LG. PWR. XSTR) mode. Leakage: or To test a transistor for or set the FUNCTION switch on the proper position. Next, set the RANGE switch to the 100 mA position. Then push the switch marked ZERO and adjust the ADJ knob for a zero reading on the meter. Now release the ZERO button. Set the RANGE switch on the lowest leakage range, which will still permit less than full-scale deflection on the meter. You may now read the leakage directly off the meter. Read the first and then Use this order because the meter will read down scale when switching from to Also, you can increase the meter sensitivity. However, if you read first and then switch to the meter will read up scale. It is now possible to peg the meter. Although the meter has protection, avoid undue abuse. Material Identity: Transistor.— To use this instrument in the IDENTITY mode, set the FUNCTION switch to IDENT. Check the ZERO ADJUST on the meter as mentioned before. After setting the ZERO, release the ZERO push button. Now note whether the needle reads above or below the mark on the meter face just short of half scale, If the meter reads below the mark, the device is a germanium transistor. If it reads above the mark, it is a silicon transistor. This information can be extremely useful when trying to substitute transistors. Leakage: Diode. — To test the reverse leakage of diodes, install the diode in the diode socket. You now determine whether the diode is good by testing the device in the visual mode. Once you determine that the diode is good, place the POLARITY switch to NPN. Turn the FUNC- TION switch to the mode, and set the RANGE switch to 100 mA. Now check to see that the meter is at zero, as mentioned before. After zeroing the meter, set the RANGE switch on the lowest range possible that still permits less than full-scale deflection on the meter. Read the leakage on this range. Material Identity: Diode.— To test the material identity of a diode with the diode properly installed in the socket, place the POLARITY switch in the PNP position (zero the meter) and the FUNCTION switch in the IDENT position. Using the leads, short the base and collector terminals together. The meter will show either germanium or silicon as described before in the IDENT mode for transistors. 8-32

CAUTION Do not identity test transistor material with the base and collector leads shorted together. This may create an erroneous reading. Model 109 Probe The Model 109 probe, used with the Model 900 tester, is easy to use, having one-hand operation. It automatically adjusts to any spacing between one-thirty second inch to five-eighths inch. You can rotate each probe point in a full 360-degree circle. The points are individually spring loaded for proper contact. You can connect the probe to three printed circuit board terminations. The probe has the extremely low contact resistance of less than .005 ohm. The use of the probe eliminates unsoldering while making in-circuit tests of transistors, diodes, ICs, and other components. Finally, the retractable cord stretches to a full 12 feet. DESCRIPTION.— The Model 109 three-point probe speeds servicing of printed circuit assemblies that have transistors, diodes, and most other board-mounted components. You can make instant connections to three points on a printed circuit board. You will make rapid evaluation of transistors using the Model 109 probe with the Model 900 automatic transistor analyzer in-circuit. You can accomplish a complete test of all stages in a piece of electronic equipment in a matter of minutes. You can also use the Model 109 to make temporary component substitutions on the printed circuit board. OPERATION.— Connect the leads of the Model 109 probe to an appropriate piece of test equipment. Determine the connection points on the printed circuit board to connect to the test equipment. Apply the Model 109 probe points to the circuit board. Press the probe toward the board to ensure a good connection. The Model 109 probe green point is slightly shorter than the yellow and blue probe points. This allows connection of the collector and emitter before the base to provide maximum ease of use. The Model 109 probe is a valuable aid when making resistance and voltage measurements using a conventional VOM or VTVM. Use the yellow and blue probe points as the negative and positive meter feeds. You can make rapid evaluations of entire circuits faster than with any other method because each point pierces through conventional resist coatings and solder residues. Q21. Q22. Q23. Q24. Q25. The Huntron Trackers 1000 and 2000 are for use on circuit boards and systems with all voltage sources in what condition? What mode on the automatic transistor analyzer Model 900 has the Sonalert™? What type signal display does the Huntron Trackers 1000 and 2000 show when the signal fuse is open and the test leads shorted together? When using the Huntron Tracker 2000, why must you make good contact with the test leads? What is the minimum total shunt im- pedance across the junction of the diode or transistor under test using the automatic transistor analyzer Model 900 to ensure a good test reading? SIGNAL GENERATORS Learning Objective: Recognize charac- teristics and identify the uses of signal generators to include frequency-modulated and pulse-modulated signal generators. Standard sources of RF energy are used to maintain airborne electronic equipment. These energy sources are called signal generators. The principal function of the signal generator is to produce an alternating voltage of the desired frequency and amplitude. The generated signal may be modulated or unmodulated, depending on the test or measurement in question. When using the signal generator, the output signal couples into the circuit under test. You trace its progress through the equipment by using a high-impedance device such as a VTVM or an oscilloscope. RF SIGNAL GENERATORS Radio-frequency signal generators comprise a rather large and very useful class of test equipment. Because of the extremely wide frequency range in the RF region of the spectrum, many signal generators, with different RF ranges 8-33

as well as other instrument refinements, are available. FREQUENCY-MODULATED RF SIGNAL GENERATORS Many types of frequency-modulated (FM) signal generators are available for your use; however, some are used for special applications. The following discussion of FM generators provides basic information that applies to most FM generators. An FM signal is one in which the output frequency varies above and below a center frequency. The overall frequency variation is known as the frequency swing (or deviation). The rate at which this swing recurs is controllable at any audio- or video-frequency rate for which the generator is capable. The frequency change of the output is accomplished by the mechanical variation of either the capacitance or inductance of the oscillator circuit or by the use of a reactance tube connected to the oscillator circuit. In the latter case, changes of the voltage impressed on the grid of the reactance tube change the amount of reactance introduced into the oscillator-tuned circuit. As a result, it causes the output frequency to change. The frequency of the signal on the grid of the reactance tube thereby controls the rate of frequency deviation. The amplitude of the signal voltage controls the amount of the deviation. A sweep generator is a form of an FM signal generator. Its carrier deviation is adjustable by a sweep-width control. The sweep generator differs from the ordinary FM signal generator because it maintains the rate of carrier deviation at a fixed frequency. The voltage used to effect the deviation is either a sine wave or a sawtooth waveform. You use an oscilloscope to observe the patterns formed when the passband of interest is swept by this type of generator. The oscilloscope time base must use (or be synchronized with) the same waveform used to produce the deviation. The horizontal (or time) axis of the pattern represents the instantaneous frequency of the generator output. The vertical axis shows the response characteristic of the circuit under test for each frequency. Sweep generators are widely used for observing the response characteristics and the visual alignment of tuned circuits. The sweep generator is used to check the bandwidth of IF amplifiers used in radar receivers. Deviation of the carrier may occur either electromechanically or electronically. The electro- mechanical method consists of mechanically varying the capacitance or the inductance of the oscillator tank circuit, causing the frequency to vary accordingly. The electronic method makes use of a reactance-tube modulator. A sweep generator produces patterns con- taining a considerable number of instantaneous frequencies. Marker signals, which are super- imposed on the trace, are introduced. These signals orient passband characteristics (or center frequency) of the circuit under test with respect to frequency. The circuit that produces the marker signals may be an integral part of the instrument, or the marker signals may come from an external source. Most modern frequency-swept signal genera- tors use a reactance-tube method of modulation. Modulation of this type results in greater flexibility. Also, the equipment is lighter and more compact than rotating capacitor equipment. The reactance tube and its associated com- ponents are connected across the tank circuit of the oscillator in the signal generator. Often, the ac power line, which provides an excellent oscilloscope-synchronizing medium, couples to the grid of the reactance tube to control the rate 8-34

of the sweep. The reactance-tube modulator has an advantage over electromechanical modulators because it can be excited by an external variable AF signal generator. The electromechanical modulator is usually limited to single-frequency operations. PULSE-MODULATED RF SIGNAL GENERATORS A pulse-modulated (PM) RF signal generator is similar to the conventional RF signal generator. It differs in its output, which consists of RF energy in the form of pulses that occur at an audio rate. The generator controls can vary the pulsewidth (duration of each pulse) and the repetition rate (number of pulses per second). The PM generator is commonly used to check receiver performance of many radar systems that have a pulse-type emission. A conventional oscillator circuit generates a constant RF carrier to produce pulse-modulated RF signals. This energy goes to the grid of a mixer stage, which has at the same time impressed on its suppressor grid a square wave generated in a separate circuit. The positive half-cycles of the square wave allow the mixer tube to conduct, and the negative half-cycles cut the tube off. During the conducting intervals, the RF signal on the control grid varies the plate current. Therefore, pulses of RF current, corresponding to the positive half-cycles of the square wave, appear in the mixer plate circuit. The pulses normally go to one or more amplifier stages. Controls in the square wave circuit vary pulse time and repetition rate. The Model 628A SHF signal generator (fig. 8-25) is a general-purpose broadband signal generator that produces RF output voltages from 15 GHz to 21 GHz. A single control determines the output frequency, which is directly read on a dial calibrated to an accuracy of ±1 percent or better. The 628A signal generator has some versatile modulation characteristics. It is possible to frequency modulate, square-wave modulate, or pulse modulate the output by internally or externally generated signals. The 628A also provides synchronizing pulses for use with external equipment. Figure 8-25.-Model 628A SHF signal generator front panel. 8-35

In addition to producing an accurate and controllable RF signal, you can use the 628A signal Q26. Q27. Q28. Q29. generator to test pulse systems, measure sensitivity and selectivity of amplifiers, receivers, and other tuned systems, measure signal-to-noise ratio of RF signals, make slotted line measurements, investigate microwave impedances and other transmission line characteristics, measure frequency response of microwave systems, and determine resonant frequency and Q of waveguide cavities . What is the principal function of the signal generator? While various types of FM signal generators are available, many are restricted to special applications. What type is used for general applications? Most frequency-swept signal generators use a reactance-tube method of modulation. What is the reason for this? What is a common application for pulse- modulated generators? SIGNAL ANALYZERS Learning Objective: Identify signal analyzers to include signal analysis and waveform measurements including O scope, synchroscope, spectrum analyzers, and distortion analyzers. Signal analyzers, while used in many different situations, are normally used for one purpose— to check the response of an equipment under simulated conditions of specific operations. WAVEFORM MEASUREMENT Waveform measurements are made by observ- ing displays of voltage and current variations with respect to time or by harmonic analysis of complex signals. Waveform displays are particularly valuable for adjusting and testing pulse-generator, pulse-former, and pulse-amplifier circuits. The waveform visual display is also useful for determining signal distortion, phase shift, modulation factor, frequency, and peak-to-peak voltage. You can use harmonic analysis test sets to determine the energy distribution in electrical signals. Frequency-selective circuits separate the signals into narrow frequency bands. The energy in each band is indicated by a meter or displayed on a CRT. By connecting a group of frequency- selective circuits in parallel, you can manually or automatically tune a single frequency-selective circuit. You can also use a heterodyne method (using a sweep generator and fixed-tuned circuit) to select electrical power present in a narrow frequency band. OSCILLOSCOPE An oscilloscope or O scope is an electronic test set that displays information on the face of its CRT. There are many ways you can use an oscilloscope; however, its primary use is in troubleshooting and aligning electronic equip- ment. You do this by observing and analyzing waveform shape, amplitude, and duration. The maintenance instruction manual (MIM) for the particular equipment specifies the waveforms that you should see at the various test points throughout the equipment. Waveforms at any one selected test point may differ, depending on whether the operation of the equipment is normal or abnormal. Figure 8-26 is a typical display you may see on a cathode-ray oscilloscope. This illustration shows the instantaneous voltage of the wave plotted against time. The elapsed time equates to the horizontal distance (view A), from left to right, across the etched grid (graph) placed over the face of the tube. The amplitude of the wave is the vertical measure (view B) on the graph. The oscilloscope also provides picture changes in quantities other than voltages in electric circuits. If an electric current waveform is of interest, you can usually send the current through a small series resistor and look at the voltage wave across the resistor with the oscilloscope. There are also suitable transducers that change other quantities such as temperature, pressure, speed, and acceleration into voltage for display on the oscilloscope. 8-36

Figure 8-26.-Typical waveform display: (A) measure- ment of elapsed time; (B) measurement of voltage difference. Interpreting the Display As you read this paragraph, look at figure 8-26. Find the elapsed time between two points on the graph (view A, points A and B). Multiply the horizontal distance between these points in major graduated divisions by the setting of the TIME/DIV (time per division) control. This control sets the horizontal sweep rate of the oscilloscope. The distance between points A and B is 4.5 major divisions. If the TIME/DIV control is set at 100 microseconds per division, then the elapsed time between points A and B is 4.5 x 100 = 450 microseconds. In general, elapsed time = horizontal distance (in divi- sions) x TIME/DIV setting. If you are using the MULTIPLIER control with the TIME/DIV control, multiply the above result by the setting of the MULTIPLIER. If a MAGNIFIER is in operation, divide the result by the amount of magnification. Again, look at figure 8-26. To find the voltage difference (view B, points A and B) between any two points on the graph, multiply the vertical distance between these points (in major graduated divisions) by the setting of the VOLTS/DIV control. This control sets the vertical deflection factor, or sensitivity, of the oscilloscope. The vertical distance between points A and B is 4.0 divisions. If using the VOLTS/DIV control at 0.5 volt per division, then the voltage difference between points A and B must be 4.0 x 0.5 = 2.0 volts. You can express the quantity called pulse repetition rate (or pulse repetition frequency) for periodic pulses as the number of pulses per unit of time. For example, 10 pulses per second and 50 pulses per microsecond. In using the oscilloscope to measure the frequency or repetition rate of periodic waveforms, you read the horizontal distance in major divisions between corresponding points on two succeeding waves first. This is the horizontal distance occupied by one cycle of the wave. Multiply this by the setting of the TIME/DIV control in seconds, milli- seconds, or microseconds. Determine the reciprocal of this product; that is, divide 1 by the product. The result is the desired frequency or repetition rate. Square waves, rather than other forms of waves, are usually used to test equipment. By using square waves, you can see more than just a defect’s presence; you can see the nature of the defect. The nature of the defect is suggested by the kind of distortion that occurs on a square wave. By observing the square wave response, you, the technician, can easily tell whether the transmission of low or high frequencies is affected. However, this observation is not so clear with regard to frequency with waves other than square waves. Linear devices that give identical responses to square wave inputs generally give responses similar to each other when other waveforms are input to them. Information Contained in a Square Wave A periodic wave contains the following components: 1. A fundamental wave, which is a sine wave having a frequency equal to the repetition frequency of the square wave. 2. An infinite series of odd harmonics—sine waves having frequencies that are equal to whole numbers multiplied by the funda- mental frequency. The harmonics must be in phase and in amplitude to the fundamental. 8-37

Waveform D of figure 8-27 depicts a periodic rectangular wave (square wave). With the square wave, the only harmonics present are the odd harmonics (those whose frequencies are equal to the fundamental frequency multiplied by odd whole numbers). The strengths of the harmonics vary in inverse proportion to the frequencies of the harmonics, the fifth harmonic being one-fifth as strong as the fundamental, for example. Figure 8-27 suggests a way in which these waves combine to make up a square wave. By looking at the four curves shown in figure 8-27, you can see that 1. curve A is the fundamental sine wave, 2. curve B is the sum of the fundamental and third harmonic, 3. curve C is the sum of the fundamental plus third and fifth harmonics, and 4. waveform D is the ultimate square wave. You can see by looking at figure 8-27 that the first few harmonics combine with the fundamental to provide an approach to an actual square wave. Figure 8-27-Addition of harmonics to a fundamental waveform. Additional harmonics, of higher frequencies, would cause the leading edge of the wave to rise more rapidly. This will produce a sharper corner between the leading edge and the top of the wave. It would require an infinite range of harmonics to produce a truly vertical leading edge and an actual sharp corner. Although this situation is physically impossible to produce, waves can be generated that are very close to this ideal. (The same considerations apply to the falling edge of the waveform and to the following corner.) You can find information about the amplitude and phase relationships of the higher harmonics within the leading-edge steepness and in the sharpness of the corner. If low-frequency components (fundamental and the first few harmonics) are not present in the proper amounts and in the correct phase relationships, the flat top of the square wave is affected. Refer to figure 8-28. View A shows the Figure 8-28.-Information found in a square wave. 8-38

location of the low- and high-frequency low-frequency components have lagging phase information in a square wave. Low-frequency angles and are accentuated. defects appear in the form of slope or general curvature in the top (views B and C). In view B, Oscilloscope Block Diagram the low-frequency components have leading phase Figure 8-29 is a block diagram of a typical angles and are attenuated. In view C, the oscilloscope, omitting power supplies. The Figure 8-29.-Typical oscilloscope block diagram. 8-39

waveform (A) is input into the vertical amplifier input. The calibrated VOLTS/DIV control sets the gain of this amplifier. The push-pull outputs (B and C) of the vertical amplifier go through a delay line to the vertical deflection plates of the cat bode-ray tube. The time base generator or sweep generator develops a sawtooth wave (E) that is a horizontal deflection voltage. The rising or positive-going part of this sawtooth, called the runup portion of the wave, is linear. It rises through a given number of volts during each unit of time. This rate of rise is set by the calibrated TIME/DIV control. The sawtooth voltage goes to the time base amplifier. This amplifier supplies two output sawtooth waveforms (G and J) simultaneously— one of them positive-going, like the input, and the other negative-going. The positive-going sawtooth goes to the right horizontal deflection plate of the CRT, and the negative-going sawtooth goes to the left deflection plate. As a result, the cathode-ray beam sweeps horizontally to the right through a given number of graduated divisions during each unit of time. The TIME/DIV CONTROL establishes the sweep rate. To maintain a stable display on the CRT screen, each horizontal sweep must start at the same point on the waveform. To accomplish this, a sample of the displayed waveform goes to a trigger circuit, which gives a negative output voltage spike (D) at some selected point on the displayed waveform. This triggering spike starts the rising portion of the time base sawtooth. As far as the display is concerned, then, triggering is synonymous with the starting of the horizontal sweep of the trace at the left side of the grid. The rectangular unblanking wave (F) is derived from the time base generator goes to the grid of the CRT. The duration of the positive part of this rectangular wave corresponds with the duration of the positive-going or rising part of the time base output. The beam is switched on during its left-to-right travel and switched off during its right-to-left retrace. Often, the leading edge of the displayed waveform actuates the trigger circuit. However, it may be desirable to observe this leading edge on the screen—and the triggering and unblanking operations require a measurable time (P), often about 0.15 microsecond. To see the leading edge, a delay (Q) of about 0.25 microsecond is introduced by the delay line in the vertical deflection channel. The delay occurs after the point where the sample of the vertical signal is tapped off and fed to the trigger circuit. The purpose of the delay line is to retard the application of the observed waveform to the vertical deflection plates. This occurs until the trigger and time base circuits have had an opportunity to begin the unblanking and hori- zontal sweep operations. This permits viewing the entire desired waveform—even though the leading edge of that waveform was used to trigger the horizontal sweep. If the delay line were not used, only that portion of the waveform following the instant (T) in waveform (B) could be seen. Oscilloscope Probe The input circuit to the vertical amplifier (fig. 8-30) of an oscilloscope can be simulated by a high resistance (R) shunted by a small shunt capacitance (C). In some applications, even this high resistance and small capacitance can produce undesirable loading on the circuit whose waveforms are being examined by means oft he oscilloscope. Loading can cause the oscilloscope presentations to be different from the waveforms that would be present with the oscilloscope disconnected. Use of a passive probe reduces this resistive-capacitive loading on the circuit under investigation. The probe (fig. 8-31) includes a resistor shunted by a capacitor This combination is connected in series with the inner conductor of the cable to the oscilloscope input. The result is that when connecting the probe to the circuit under investigation, a new effective loading capacitance smaller than the original capacitance (C) and a new effective loading resistance larger than the original resistance (R) occurs. Thus, the probe reduces the loading effect of the oscilloscope input circuit on the circuit under investigation. A second effect of the probe is to reduce the amount of signal voltage applied directly to the Figure 8-30.-Oscilloscope vertical amplifier input circuit. 8-40

Figure 8-31.-Oscilloscope vertical amplifier using a passive probe input. oscilloscope input connection for a given amount of original signal voltage. This occurs because of the voltage-divider action of and R. This effect is taken into account in the attenuation ratio marked on the probe. Thus, if the probe is a 10 x ATTEN, all oscilloscope voltage indications must be multiplied by 10. If an oscilloscope equipped with a probe is used to look at a square wave, and the probe capacitor is too small, some of the high- frequency components of the square wave are bypassed around the oscilloscope input terminals by the input capacitance (C). Thus, the steepness of the leading edge of the displayed square wave (fig. 8-32, view A) is reduced. If the probe capacitor is adjusted to the correct value, a compensating amount of high-frequency information is bypassed around the probe resistor Figure 8-32 .-Effects of probe adjustment. (fig. 8-31). To makeup for the loss through C (fig. 8-31), the leading edge of the displayed square wave is restored to its original steepness (fig. 8-32, view B). If (fig. 8-31) is made too large, the high-frequency response of the circuit is overcompensated and applies too much high- frequency information to the oscilloscope input connection. This results in an overshoot in the displayed waveform (fig. 8-32, view C) that was not present in the original waveform. (fig. 8-31) is adjusted to its correct value by using the probe to display the square wave generated by the voltage calibrator, which is a part of the oscilloscope. Adjustment is made to display a square wave with as flat a top as possible. You must check the probe adjustment whenever you use a probe with an oscilloscope or a plug-in preamplifier. This is especially important if the previous use was with an input capacitance different from that of the instrument to which you are now connecting the probe. NOTE: As indicated in figure 8-31, the attenuation achieved is a result of R as well as Though you may swap probes with other types of oscilloscopes, the calibration may be in error even though the waveform distortion may adjust out. 8-41

SYNCHROSCOPE The synchroscope is an adaptation of the oscilloscope. Its normal use is for radar applications. A trace occurs only with an input trigger, as contrasted with the continuous sawtooth sweep provided by the oscilloscope. Synchroscope circuits are similar to oscilloscope circuits, with the exception of the signal and the sweep channels. Figure 8-33 shows these circuits in block diagram form. The signal channel of a typical synchroscope includes an input circuit that is usually in the form of a 72-ohm adjustable-step attenuator. Various degrees of attenuation are available, and the calibrated dial indicates how much attenuation is present. The attenuator makes sure all signals, regardless of amplitude, produce about the same input level to the amplifier section. Following the attenuator is an artificial delay line. This low-pass filter has a cutoff frequency higher than the highest passed frequency and an impedance of 72 ohms. The delay line terminates into a 72-ohm gain control. One purpose of the delay line is to delay presentation of the observed signal. The delay lasts until an undelayed portion of the input signal initiates the sweep trace. Without the delay line, the initial portion of the waveform would not appear on the trace. This would occur because a certain amount of time is necessary for the input signal voltage to rise to the level needed to trigger the sweep circuit. With the delay line in use, the signal does not reach the amplifier until one-half microsecond after the trace starts. As a result, you can see the entire pulse. A secondary purpose of the delay line is to provide, by reflection, a series of accurately spaced pulses suitable for calibration of short time intervals. A switch causes a mismatch in the termination of the delay line, causing the secondary purpose. When a sharp pulse is input into the line, a series of reflections occurs similar to those shown in figure 8-34. Since the time required for a pulse to travel down the line and back is 1 microsecond, a series of pulses occurring 1 microsecond apart occur. Each successive pulse is smaller because of the losses in the delay line, but enough pulses are visible for most high-speed calibration purposes. The gain control feeds a wideband or video amplifier, which connects to the vertical deflection plates. In addition, an external connection is provided to the vertical plates. The horizontal circuit consists of a sync switch for either internal or external sync, a sync amplifier with a gain control, and a start-stop sweep generator. The sweep generator will not develop a sweep voltage until it receives a pulse of enough amplitude. The duration of the sweep, or sweep speed, is adjustable from a very few microseconds to about 250 microseconds. The sweep generator connects to a conventional horizontal amplifier. Since the trace is triggered by the input signal, the synchroscope may be used to observe nonperiodic pulses; for example, the Figure 8-33.-Typical synchroscope block diagram. 8-42

Figure 8-34 .-Pulse reflection on a mismatched line. nonperiodic pulses occurring in a radar system with an unstable PRF generator. In later designs, provisions are commonly made for calibration of input voltages and sweep time. Voltage calibration is made by comparing the unknown voltage with a variable-voltage pulse of known value, generated internally. The calibrating pulse is adjusted so it is equal in amplitude to the unknown voltage. You can then read the value from the dial that controls the calibrating pulse. Sweep time calibration occurs with the help of marker pulses produced by accurately adjusted tuned circuits. The marker pulses appear on the trace as a series of bright dots spaced at intervals chosen by the operator. In a typical synchroscope, you may select marker intervals of 0.2, 1, 10, 100, and 500 microseconds, depending on the time duration of the pulse under test. Q30. Q31. Q32. Q33. Signal analyzers can be used in many applications. It is used for what function? What determination can you make by observing the square wave response? Look at figure 8-28. At what point on a square wave does low- and high-frequency information appear? An oscilloscope probe reduces the loading effect of the O-scope input circuit on the circuit under test. What is the second purpose of the probe? Q34. The synchroscope is an adaption of the— oscilloscope. What is the difference of the trace on the synchroscope and oscilloscope? SPECTRUM ANALYZER When a radio-frequency carrier wave is modulated by keying, speech or music, or pulses, the resulting wave contains many frequencies. The original carrier is present, together with two groups of new frequencies (sideband com- ponents). One group of sidebands is displaced in frequency below the carrier. The other group is displaced above the carrier. The distribution of these frequencies, when shown on a graph of voltage or power against frequency, is called the spectrum of the wave. A spectrum analyzer is a device used to exhibit the spectrum of modulated waves in the radio- frequency range and the microwave region. In principle, the spectrum analyzer operates by tuning through the frequency region in question, using a narrow band receiver. A cathode-ray oscilloscope usually measures the output of the receiver, and the plot on the screen is a graph of voltage versus frequency. The device is essentially a superheterodyne receiver with a very narrow- band intermediate frequency amplifier section. The local oscillator frequency varies between two values at a linear rate. The frequency-control generator governs the frequency of the local oscillator. It also produces the horizontal sweep voltage for the CRT deflection plates. (See 8-43

Figure 8-35.-Typical spectrum analyzer block diagram. fig. 8-35.) As a result, each position of the beam corresponds to a definite frequency value, and the display is a graph in which the X-axis is interpreted in terms of frequency. The output of the receiver detector is amplified and goes to the vertical deflection plates. The beam deflects vertically by an amount pro- portional to the voltage developed in the detector (and amplifier). The signal for analysis goes into the mixer stage of the receiver. The local oscillator changes in frequency at a linear rate, beating with each of the signal frequency components in succession to form the intermediate frequency of the narrowband amplifier. The output of the IF amplifier is detected, amplified, and applied to the vertical deflection plates. Spectrum analyzers designed for analysis of microwave signals have klystron tubes in the local oscillator stage. Analyzers adapted for lower frequency RF signals use triode oscillators that vary through reactance-tube modulators. Spectrum analyzers are the main tool for studying the output of pulse-radar transmitter tubes, such as magnetrons. In this kind of analysis, unwanted effects, such as frequency 8-44

Figure 8-36.-Frequency spectra. modulation of the carrier, are easy to detect. In pure amplitude modulation of a carrier wave by a square pulse, the spectrum is symmetrical about the carrier frequency. Lack of symmetry indicates the presence of frequency modulation. Look at view A of figure 8-36. It shows a spectrum representing the ideal condition. Views B and C show examples of undesirable magnetron spectra. These forms indicate trouble in the modulator, the tuning system, or in the magnetron tube itself. The best definition of carrier frequency is the center frequency in a symmetrical spectrum (fig. 8-36, view A). Some analyzers use this principle as a means of carrier frequency measurement. A sharply resonant circuit in the receiver acts as a trap to prevent an extremely narrow range of frequencies from appearing in the output of the IF amplifier. The result of its use is a gap that appears in the display, and the gap corresponds to the resonant frequency of the trap. The adjustment of the trap is calibrated in frequency, and the circuit can be adjusted to make the gap occur in the center of the spectrum. You can then read the frequency of the carrier from the calibration of the trap. For more information about spectrum analyzers, refer to NEETS, module 16. In addition, the EIMB Test Methods and Practices, NAVSHIPS 0967-LP-000-0130, contains detailed discussions of spectrum analysis techniques. Echo BOX The echo box is for use in field testing, troubleshooting, and adjusting pulsed-type radar systems. Although simple in construction and operation, it has many applications. If properly used within its design limitations, the echo box can frequently eliminate the need for a complex test setup and an elaborate step-by-step testing procedure. The echo box uses passive circuitry, which does not require any external power other than the radar set whose signal is under analysis. External power requirement is a critical factor with most other test sets. The echo box is similar in operation to a tuned cavity frequency meter; however, it has different capabilities. The tuned cavity frequency meter can measure the frequency of CW or pulsed RF signals in the microwave range. The echo box, however, has no practical application in the testing or analysis of CW equipment signals. Figure 8-37 indicates the basic functional elements of a typical echo box. Energy from the radar transmitter goes through the directional couplers to the resonant Figure 8-37.-Typical echo box functional circuit. 8-45

cavity. When the cavity length is properly adjusted, resonant oscillations are set up by each successive pulse of microwave energy. Maximum amplitude of oscillation occurs when the cavity is tuned precisely to the signal frequency. The crystal diode detects these cavity oscillations and indicates them on the meter as an average dc current. The amplitude of oscillation and the average current reading are proportional to the transmitter power output. Oscillations in the tuned cavity also couple back to the radar set under test, where they are processed as an echo signal. This signal, when viewed on the indicator CRT, permits analysis of the radar pulse and presents an indication of the general operating condition of the radar set. Since energy builds up in the cavity, saturation of the cavity is possible. If saturation does occur, distortion of the waveform and erroneous values of the measurements result. If the directional couplers do not prevent cavity saturation, there must be some additional attenuation. Analysis of the displayed waveform can provide a fairly complete functional analysis of the operational condition of a radar set. Among the most important factors it can determine are frequency and bandwidth, power and frequency spectra, sensitivity, pulsewidth and condition, and recovery time. Analysis of the waveform can also prove helpful in locating the cause of malfunctions within the radar set. You need to remember, however, that the echo box presents only relative (rather than absolute) values of power and sensitivity and only rough values of frequency. These quantities are not as accurate as the corresponding values obtained by using a spectrum analyzer. The primary value of the echo box lies in its regular usage. For maxi- mum benefit, you must compare the values from a given test to corresponding values from a test on a radar set you know is operating properly. In general, however, the echo box is an extremely valuable instrument. When used in a continuing maintenance program, it lets the operator maintain the equipment in peak operating condition. Also, it gives indications of deterioration before actual malfunctions occur. Distortion Analyzer The Hewlett-Packard Model 332A distortion analyzer (fig. 8-38) is a solid-state instrument for measuring distortion and ac voltages. The Model 332A includes a high-impedance AM detector that operates from 500 kHz to greater than 65 MHz. Distortion levels of 0.1 percent to 100 percent full scale are measured in seven ranges for any fundamental frequency of 5 Hz to 600 kHz. Harmonics are indicated up to 3 MHz. The high sensitivity of these instruments requires only 0.3 V rms for the 100 percent set level reference. The OUTPUT connectors provide a low distortion output for monitoring with an oscilloscope, a true rms voltmeter, or a wave analyzer. The instruments are capable of an isolation voltage of 400 volts above chassis ground. You can also use the transistorized voltmeter contained in the Model 332A separately for general-purpose voltage and gain measurements. The voltmeter has a frequency range of 5 Hz to 3 MHz (20 Hz to 500 kHz for the 300 µV range), and a voltage range of 300 µV to 300 V rms full scale. The AM detector is a broadband dc restoring peak detector consisting of a semiconductor diode and filter circuit. AM distortion levels as low as 0.3 percent can be measured on a 3 V to 8 V rms carrier modulated 30 percent in the standard broadcast band. Also, lower than 1 percent distortion can be measured at the same level of the carrier up to 65 MHz. The Model 332A distortion analyzer has two modes of operation— the distortion mode and the voltmeter mode. Total harmonic distortion measurements from 5 Hz to 600 kHz are possible. The distortion mode can indicate harmonics up to 3 MHz. Distortion measurement accuracy is determined by the overall effect of harmonic frequency measurement accuracy, elimination characteristics, distortion introduced by the instrument, and meter accuracy. In the voltmeter mode, the transistorized voltmeter provides a full- scale sensitivity of 300 µV rms (residual noise <25 µV). The voltmeter frequency range is 5 Hz to 3 MHz (20 Hz to 500 kHz on the 300 µV range). The distortion measurement accuracy of the 332A is a result of the sharp elimination characteristic of the rejection amplifier circuit and the low level of distortion introduced by the instrument. The fundamental reject ion is at least 80 dB, which is small compared to the distortion introduced by the instrument. Thus, low-level harmonic content in the input signal can be measured accurately. You can use the 332A with a wave analyzer for extremely sensitive (>80 dB down in the audio-frequency range) measurements of odd harmonics. 8-46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ON switch turns instrument ac power on. Pilot lamp glows when instrument is turned ON. NORM-RF DET switch selects front panel INPUT connectors or rear panel RF INPUT connector. INPUT terminals provide connections for input signals. FUNCTION selector selects mode of operation of the instrument. MECHANICAL ZERO ADJUST mechanically zero-sets meter before turning instrument on. DISTORTION/VOLTMETER indicates distortion level and voltage levels of input signals. SENSITIVITY selector provides 0 to 50 dB attenuation of input signal in 10 dB steps in SET LEVEL and DISTORTION positions of FUNCTION selector. SENSITIVITY VERNIER control provides fine adjustment of attenuation level selected by SENSITIVITY selector. METER RANGE selector selects full-scale range of meter in percentage, dB, and rms volts. FREQUENCY RANGE selector selects frequency range to correspond to fundamental frequency of input signal. COARSE BALANCE control provides coarse adjustment for balancing the Wien bridge circuit. FINE BALANCE control provides a vernier adjustment for balancing the Wien bridge circuit. Frequency vernier control provides fine adjustment of FREQUENCY dial. FREQUENCY dial selects fundamental frequency of input signal. OUTPUT connectors provide means of monitoring the output of the meter circuit with an oscilloscope, a true rms voltmeter, or a wave analyzer. RF INPUT connector provides input connection for AM RF carrier input signal. FUSE provides protection for instrument circuits. LINE VOLTAGE (115 V/230 V) switch sets instrument to operate from 115 V or 230 V ac. AC power connector provides input connections for ac power. BATTERY VOLTAGE (+28 to +50 VDC and –28 to –50 VDC) terminals provide connections for external batteries. Figure 8-38.-Model 332A distortion analyzer front and rear panels. 8-47

Q35. Describe what factors a spectrum analyzer exhibits. Q36. Describe the purpose of the echo box. Q37. What limitation should you consider when you use the echo box? REFLECTOMETRY TEST SETS Learning Objectives: Recognize the basic theories of time- and frequency-domain reflectometry. Recognize the characteristics of resistive and reactive loads. Recognize TDR displays and identify range and resolution and the uses of analyzing terminations. Identify the advantages and disadvantages of FDR as compared to TDR testers. Recognize the purpose and use of FDR testers. Reflectometry test sets have many uses. They are primarily used to help the organizational maintenance technician verify and troubleshoot aircraft wiring, transmission lines, waveguides, and antenna systems. However, the intermediate maintenance technician can use reflectometry test sets to verify cable connectors, determine test cable impedances, and troubleshoot test equip- ment. There are two types of reflectometry test sets currently used by the Navy—time-domain reflectometer (TDR) and frequency-domain reflectometer (FDR) testers. TIME-DOMAIN REFLECTOMETRY (TDR) TEST SETS You will use time-domain reflectometer (TDR) test sets to check and troubleshoot aircraft wiring, transmission lines, and antenna systems for shorts, opens, crimps, bad couplings, etc. To do this, you will monitor TDR reflected waveforms. TDRs operate on the same principle as radar; that is, they send pulses of energy into a system to see what, if anything, is reflected. Like standing waves on an antenna line, if nothing is reflected, the impedance of the transmission line is uniform and properly terminated. However, if crimps, opens, bad couplings, and so forth, are present, a discontinuity exists, and in-phase or out-of- phase pulses return to the TDR test set. These reflections occur on its CRT as positive, negative, or simply fast-rising voltages, which show the known causes usually at fault. Impedances greater than 50 ohms appear to the TDR as in phase, while those less than 50 ohms appear out of phase. These are respectively classified (traditionally) as inductive and capacitive faults, which are explained by the basic equation: = where L = inductance, C = capacitance, and Z = impedance. TDR Basics The TDR analysis begins with the insertion of a step or pulse of energy (referred to as the incident signal into a system or cable. Then, at the point of insertion, you see the energy reflected by the system or cable under test. Figure 8-39 shows the typical TDR analysis. The output of the pulse generator is, a step signal with a rise time of about 110 picosecond. This signal (incident signal) goes through a sampling tee to the CRT of the sampling oscilloscope and to the system under test via a termination connector. The equivalent bandwidth of the CRT deflection circuits provides a system rise time of about 140 picosecond. This allows the TDR to give resolution (detect faults) as close as one-half inch apart. The reflected signal from the system under test reenters the TDR test set and returns via the sampling tee to the sampling oscilloscope CRT along with the incident signal. By comparing the magnitude, duration, and shape of the reflected signal, you can determine the nature of the impedance variation in the system under test. RESISTIVE LOADS.— With a pure resistive load on the output of the TDR, and a step signal applied, a signal whose amplitude is a function of the resistance (fig. 8-40) appears on the CRT. If the line terminates in its characteristic impedance (fig. 8-40), there is no reflected signal. The signal on the CRT will remain flat. However, if the impedance at the termination is greater or less than then reflections (standing-wave ratio [SWR]) exist. The amplitude of the reflected signal is proportional to the value of If is greater than (50 the reflected signal is in phase with the incident signal, and, when applied to the CRT, the reflected signal adds to the incident signal. If is less than the reflected signal is out of phase with the incident signal. When applied to the CRT, the reflected signal subtracts from the incident signal. The dotted lines in figure 8-40 represent various composite signals (incident ± reflected) that you would observe for various values of The time from the start of the incident (step) signal to the 8-48

Figure 8-39.-Typical TDR analysis. Figure 8-40.-Step signal-height variations resulting from different resistive loads. step created by the reflected signal represents twice cable. This moves the reflections away from the the distance to the discontinuity; that is, the time leading edge of the step (start of the incident it took the incident step to reach the discontinuity signal) and prevents overshoot and ringing from and return. Most TDRs are calibrated to read this appearing on the CRT signal. time in feet or inches to the discontinuity. You should separate the system under test REACTIVE LOADS.— The waveform of from the TDR test set by 8 inches of 50-ohm reactive loads (fig. 8-41) depends on the time Figure 8-41.-TDR reactive load characteristics (time constant = 1). 8-49

constant formed by the load and the 50-ohm source. The series RL network (fig. 8-41, view A) appears as an open the instant the step voltage reaches it. This is because the inductor L offers maximum impedance to the change in current caused by the step voltage. Therefore, the reflected signal is in phase with the step voltage and is additive. This explains the sharp rise in voltage. However, as soon as the inductor saturates, the only opposition to current is resistor R. Since L saturates at a nonlinear rate, the voltage drops at a nonlinear rate from the peak of the spike to the same level as the flat portion of the step voltage. At this time, the only load seen by the line is the 50-ohm resistor, which equals the characteristic impedance of the line. The reflections cease until the next step appears at the termination. Then, the cycle repeats itself. To understand the wave shape shown in figure 8-41, view B, you need to remember that L appears as an open to the fast-rising step voltage the instant it is felt at the termination. However, as the inductor saturates, it offers less and less opposition to current until it completely saturates (0 ohm). Since the inductor is parallel to R, the termination is a short, and the reflected wave is 180 degrees out of phase with the incident wave. Since L saturates at a nonlinear rate, the voltage declines at a nonlinear rate. Views C and D of figure 8-41 show a similar analysis of the transmission lines with the RC terminations. The analysis of these different types of discontinuities explains the usefulness of the TDR. Through proper analysis of the discontinuities, you can determine whether they are resistive, inductive, or capacitive and whether it is in series or parallel with the load. TDR in Practice TDR discontinuities have clear separations in time on the CRT. You can easily see the mismatch caused by a connector even if another bad discontinuity is present elsewhere in the system. By using the analysis explained before, you can establish which connector is troublesome and in what way. Once you determine that a discontinuity appears in a waveform, it is simple to locate it in the system. You can save time by calibrating the system so 1 centimeter on the horizontal axis equals a certain number of feet for the transmission system under test. The limiting factor is the system rise time, and any closely spaced discontinuities will appear as a single discontinuity. The finite rise time also limits the size of the distinguishable reactive impedance response. For example, a small shunt capacity in a 50-ohm system causes the waveform to depart from the ideal response (fig. 8-42). The maximum observable line length is a function of the repetition rate chosen. This rate determines the duration of the pulse after its rise. For example, a 200-kHz repetition rate permits the use of TDR devices with up to 1,000 feet of air dielectric cable or 670 feet of polyethylene dielectric coaxial cable. A system’s velocity constant determines the speed at which a wave travels through a transmission system. A wave travels faster through air than through polyethylene. This explains the difference in maximum checkable lengths of coaxial cable using a particular repetition rate on the TDR. The longer the cable, the lower the repetition rate must be. 8-50

Figure 8-42.-Small shunt capacity in system degrades ideal response. Range and Resolution Assuming that the total impedance equals 50 ohms, you may measure a resistance between 0.025 ohm and 100 kilohms. Because the height of the reflection is directly proportional to the resistance, you may determine the resistance by using a precalculated transparent overlay. One common use of the TDR is in analyzing a coaxial cable. The amount of impedance variation that is detectable in a long section of cable is a function of the flatness of the top of the incident step. If this step is flat within ±0.5 percent, it can detect an impedance variation of 0.5 ohm along the cable, corresponding to a 1 percent check on cable impedance. Thus, irregularities in cable makeup resulting from variations in the braiding process or tightness of the insulating jacket show up clearly. Analyzing Terminations A departure from 50 ohms in a termination or cable connector can cause some problems. For example, large reflections in a pulse system or a large voltage standing-wave ratio (VSWR) can occur in a system that carries primarily sinusoidal signals. Because of human errors in the assembly process, even the best connectors will cause reflections or a varying VSWR. Therefore, expensive connectors do not ensure freedom from unwanted reflections. However, the TDR helps you locate unacceptable connectors by rapidly showing where the mismatches are and how bad they are. The TDR also indicates if these connectors are resistive, capacitive, or inductive and whether series or shunt. Figure 8-43 shows a step being propagated from a section of RG9A/U into a load. The connector on the load and the cable are the general radio type 874. It shows four different cases with varying loads. These cases show how you can analyze the connection and the load by using the TDR. With different connectors and loads, the small mismatches (discontinuities) take on different Figure 8-43.-Waveforms resulting from the use of different loads. Horizontal scale 0.4 µsec/cm; vertical scale 0.5 percent/cm. 8-51

impedance characteristics and the reflected signals change. This change also appears in the wave shape viewed on the oscilloscope. You can compare these signals with those of a normal system by using an overlay showing the pattern of a normal system. The most convenient method to make precise measurements of cable impedance is to connect a section of air dielectric line (with precisely determined impedance) between the cable and the TDR. The step height through the air dielectric line section sets the 50-ohm level. You note any variations from this level in the test cable and calculate the impedance of the cable (fig. 8-44). In this test, the impedance level of the test line is where (Greek letter rho) is the reflection coefficient of the reflected mismatch, If the change in amplitude shows to be +0.03, then The impedance of a long section of coaxial cable would be exactly if there were no line losses. However, most cables have a small series loss and a negligible shunt loss. This series resistance adds to causing the impedance level (as observed at one end of a cable) to increase when adding longer sections of cable. The slope on the step height that results from the increasing impedance is evident in figure 8-45. There are other applications in which the TDR method of analysis is effective, including component characteristic analysis, antenna analysis, and aircraft wiring checks. You can place the components in an appropriate jig and use the TDR method to determine their shunt capacity and series inductance (fig. 8-46). Investigation of antennas reveals that the TDR pattern is not simple, but instead presents a Figure 8-44.-Oscillograph of step from air dielectric line into test cable. Figure 8-45.-Trace of cable shows construction irregularities and increasing series resistance. complex reactive profile (fig. 8-47). Once you determine the proper profile for a particu- lar antenna, you can detect any improper construction details and determine the proper corrective action. FREQUENCY-DOMAIN REFLECTOMETRY (FDR) TEST SETS Frequency-domain reflectometry (FDR) is a fast, simple, and reliable technique developed to Figure 8-46.-Resistor checked for shunt capacity special jig. Figure 8-47.-Scope trace of antenna with reactive profile. 8-52

locate defects in microwave cables and waveguide systems connecting receivers, transmitters, and antennas. Like the TDR, the FDR tester permits direct readout of cable distance, in feet, to the discontinuity (impedance fault). This system has an impressive record of reliability, reduced service time, and improved service standards. Because the FDR checks cables at their actual operating frequencies, discontinuities outside those fre- quencies do not affect the test. When measure- ments indicate a fault, you can precisely determine its location (in terms of distance in feet from the point of test). Therefore, you can make repairs quickly and efficiently. FDR vice TDR Until FDR testers, TDR was used as the primary test of cables; a system that has several limitations. For example, TDR measurements cover a spectrum determined by its pulse charac- teristics; therefore, it detects all discontinuities, including those outside the operating frequency range, which do not affect a system’s operation. With the FDR, however, the analysis is within the actual operating frequency band of the microwave system, which assures proper system performance at the operating frequencies. While the FDR works in waveguides and band-limited systems (including transmission networks that contain filters), the TDR cannot work in such systems. The TDR requires a transmission line that passes the whole spectrum from the fundamental frequency (2 MHz to 5 MHz) to the highest harmonic (15 GHz). Waveguides that act as high-pass filters cannot transmit TDR pulses. Similarly, the TDR cannot see through low-pass or bandpass filters because they eliminate the low-frequency harmonics and appear to display a discontinuity on the TDR’s CRT. FDR Testing The FDR identifies defective systems by injecting an RF signal into a system and using insertion-loss (attenuation in the line) and return- loss (VSWR) measurements. These measurements help to classify the system under test as good or in need of repair. There are various test setup configurations to measure these losses, based on the particular FDR equipment. Figure 8-48 Figure 8-48.-Typical setup for VSWR and insertion performance. 8-53

represents a typical test setup for VSWR and insertion-loss monitoring. Such a test configura- tion provides simultaneous measurement of the losses. If the input and output connectors of the device under test are accessible, an insertion-loss check verifies input to output performance across the band. For insertion-loss measurement, the network analyzer (fig. 8-48) (using its B and REF channels) indicates the ratio of output signal to input signal directly in dB. For tests of long cables whose ends are accessible, the FDR allows measurements from a connector end as far as 2,000 feet from the tester. In some tested systems, however, either the input or output connector may be inaccessible. For such systems, a return-loss measurement made on the accessible connector provides a total system check. For return-loss measurements, the network analyzer (using the A and REF channels) indicates (measures) the ratio of reflected power to incident power directly in dB. Incident power is the output of the RF sweep oscillator unit. Figure 8-48 shows how the signals in each case are sampled via directional couplers. Comparison of each measured signal with the incident power of the RF oscillator supplies automatic compensation for any swept-source power variations across the band. This gives a true graph of performance in dB versus frequency on the network analyzer CRT. Figure 8-49 shows an example of insertion-loss measurement on the network analyzer CRT. In this example, a loss of less than 10 dB is acceptable (as determined from previous tests of a good system). The cable, however, needs repair because a fault (discontinuity) is present, which produces an insertion loss greater than 35 dB at a frequency of 3.56 GHz. Figure 8-50 shows a return-loss measurement for the same cable. Here, a loss of 11 dB (as determined from a good system), which corresponds to a VSWR of 1.8, is acceptable. At 3.56 GHz, however, the return loss on the CRT indicates 5 dB, which corresponds to a VSWR of 3.6, and it is unacceptable. The dual-channel network analyzer in figure 8-48 permits the display of both measurements simultaneously, and both verify the discontinuity in the system cable under test. Single-channel FDR testers require individual test setups for measuring insertion and return losses and comparison of the individual graphs. DETERMINING CABLE LENGTHS OR DISTANCE TO FAULTS.— To determine cable length or fault (discontinuity) location measure- ments (fig. 8-51), a waveguide or a coaxial tee is added in the test setup. You then calibrate the FDR test setup with a calibration cable (provided with FDR set) to establish a known 0-foot reference on the CRT display, Then connect the system cable to the tee. The resulting CRT display of the network analyzer consists of a stationary pattern containing a series of half-dome ripples. A count of the total number of these ripples indicates the number of feet from the cable end to the fault, as shown in figure 8-52. The FDR display is from the cable that needs repairs (figs. 8-50 and 8-51). Multiply the 5 2/3 ripples by the Figure 8-49.-Insertion-loss display. 8-54 Figure 8-50.-Return-loss display.

Figure 8-51.-Test setup for fault location measurement. calibration factor of 2 feet per ripple (CRT to the same tee junction, discontinuities and/or calibrated that way). You can see that the location of the fault is 11 1/3 feet from the cable end connector (5 2/3 x 2 = 11 1/3 ft). Figure 8-53 shows a dual-channel display of the cable after completing the repairs. The insertion loss is less than 10 dB and the return loss is greater than 11 dB, indicating proper performance of the system cable. DETAILED FDR ANALYSIS.— With the sweep oscillator output, the transmission system under test, and the crystal detector all connected Figure 8-52.-Measuring a cable fault. termination mismatches in the system reflect some of the incident power. The reflected power combines with the incident signal at the crystal detector, resulting in a changing phase relation- ship that depends on both distance to the discontinuity and signal frequency. As the frequency is swept, it changes the number of wavelengths that occupy the fixed path from the tee to the point of reflection and back. The display Figure 8-53.-Dual-channel display of a repaired cable. 8-55

shows amplitude ripples that result from the summing of the incident and reflected signals. This relationship changes with frequency. Figure 8-54 shows how the magnitude of the vector sum of these signals, which is the signal level detected for display, varies with frequency. The resulting display of the varying-magnitude detected signal is actually a logarithmic SWR presentation. The ripple peaks are adjacent VSWR maxima that occur during the sweep. They occur at each frequency in which the round-trip length of the reflected wave path from the source to the defect has changed by one wavelength. The number of ripples appearing across the full width of the display is a measure of the distance from the discontinuity to the crystal detector. Therefore, a direct readout of fault distance is available when the swept source operates over a sweep width (AF). The sweep width is chosen to provide a display calibration (in terms of ripples per foot) compatible with the length of the transmission system under test. In a coaxial system, the distance to a discontinuity, which may be a fault or the cable end, is represented by the equation Where D is the distance to the fault or cable end in feet, 492 is the half wavelength in feet of a 1-MHz wave in free space transmission, K is the propagation constant that relates the propagation velocity in the coaxial system to the velocity in free space, N is the number of ripples observed in the display, and AF is the swept-frequency excursion (sweep width) of the signal source in MHz. You should note that for any type of cable, AF can be selected to equal 492K. The distance in feet is equal to the number of ripples (including the fractional ripples) shown in the display. Figure 8-54.-Magnitude of the vector sum. 8-56

In waveguide systems, the distance down the waveguide to the fault is represented by the same equation, with K as the relation

is the wavelength in free space and g is the wavelength in the waveguide) at the frequency of measurement. Q38. Q39. Q40. Q41. Q42. Q43. Q44. Q45. Describe some of the main uses for the TDR. Describe the basics of TDR. While you can determine different types of discontinuities with the TDR, what else can you determine through proper analysis? What factor determines the speed at which a wave travels through a transmission system? By what method does using a TDR help you locate an unacceptable connector? While TDR and FDR provide similar measurements, the FDR eliminates what limitation of the TDR? Describe the means by which the FDR identifies defective systems. When determining cable lengths or distance to faults, what means do you use to determine the number of feet from the cable end to the fault? VAST STATION Learning Objective: Identify features, components, and operating procedures of a typical ATE VAST station. U.S. Navy aircraft carriers and shore installations are equipped with automatic test equipments (ATEs), such as the Versatile Avionics Shop Test (VAST) station, AN/USM-247(V), and the Hybrid Automatic Test System (HATS), AN/USM-403. The VAST and HATS deal with the continually changing field of avionics testing. The use of these computerized ATEs has significantly reduced the space requirements of special- and manual-support test equipments, The discussion contained in this chapter deals with the VAST station. TYPICAL VAST STATION In its basic form, a VAST station is assembled from an inventory of functional building blocks. These building blocks furnish all the necessary stimuli and have the measurement capability to check current naval avionics equipment. As new equipment is developed and introduced, the test station configuration may be modified. As it becomes necessary, new building blocks furnish new parameters or greater precision to existing capabilities. A typical VAST station (fig. 8-55) consists of a computer subsystem, a data transfer unit Figure 8-55.-Typical carrier-based VAST station. 8-57

(DTU), and a stimulus and measurement section containing functional building blocks configured to meet the intended test application. A computer subsystem controls the test station, which executes test programs to assure accurate and satisfactory testing. The computer subsystem includes a general-purpose digital computer that executes test routines and has diagnostic and computational capabilities. Also, this subsystem processes data and furnishes a permanent record of test results. Two magnetic tape transports provide rapid access to avionics test programs and immediate availability of VAST self-check programs. The data transfer unit (DTU) (fig. 8-56) serves as the operator-machine interface. It synchronizes instructions and data flow between the computer and the functional building blocks. Also, it contains the display and control panels. The operator communicates with the com- puter and the stimulus and measurement section of the VAST system by using the DTU control panel, which has the keyboard and mode select key. The test station may be operated in three modes—manual, semiautomatic, or fully automatic. The DTU contains a maintenance panel that monitors station auto-check results and indicates building block faults. Transmission of instructions from the control computer is on a request/ acknowledge basis. Essentially, the stimulus and measurement section controls the response rate. This allows instructions to be transmitted at an asynchronous rate, corresponding to the 8-58

Figure 8-56.-Data transfer unit (DTU). maximum frequency at which a given building block or avionics unit can respond. Therefore, there is no requirement for immediate program storage in the DTU. FEATURES OF A VAST STATION A VAST station may have as many as 14 racks of stimulus and measurement building blocks (fig. 8-57). Large station configurations may contain as many as 17 core building blocks. Core building blocks are designated as a result of high-use factors or because they are needed for self-test requirements. Building blocks not in the core category are usually selected to meet the specific test requirements of shop operations or avionics equipment on board ship. In general, the location of such peripheral building blocks is flexible. To maintain standardization between VAST stations, the effects of building block interconnection cable losses and switches have to remain within predictable limits; this is the purpose of the core concept. Ease of maintenance is the main objective of the VAST station designed. In addition to the modularized design of VAST building blocks, there are three levels of fault detection, which ensure rapid confidence tests and easy fault location. The three levels of detection are auto-check. self-check, and self-test. Fault detection may be initially made through auto-check. The auto-check is inherent in the logic and control design of the test station and includes Figure 8-57.-VAST station with building blocks. 8-59

verification of instructions and fault monitoring. Auto-check is carried out on a continuous basis during station operation and, when a fault occurs, testing is interrupted. The second level of VAST fault detection is self-check. Self-check is a programmed sequence that is initiated by the VAST operator through the DTU keyboard. Self-check may be either internal or at the system level. Internal self-check measures the ability of a building block to perform against its own internal standards. System self-check requires the use of two or more building blocks in a test configuration selected to isolate faults within the test setup. The self-check philosophy used to verify the operation of VAST is based upon confirmation of key system elements first. Then, these elements are used to check the remaining building blocks. Fundamental core building blocks are checked by means of internal standards. Once satisfactory performance is assured, their capabilities are used to check the remaining building blocks, The checkout of noncore building blocks is accomplished by using any combination(s) of core measurement and stimulus building blocks. The final level of VAST fault detection is self-test. This is a series of test programs used to locate faults within a building block. If a building block has been found to contain a malfunction as a result of a self-check routine, then self-test programs are conducted. This is done by removing the faulty building block from the VAST rack and by connecting it to the test station in the same manner as if it were a unit under test. Avionics equipment must be designed to be adaptable to automatic testing to assure optimum support by VAST. Moreover, test programs must be prepared that are compatible with VAST performance characteristics. VAST-TO-UUT INTERCONNECTING DEVICE Included in the program design is the all- important interconnecting device design. In its simplest form, the interconnecting device consists of an adapter cable, which connects the unit under test (UUT) to the VAST interface. In some cases, however, it is necessary to introduce, as part of the electrical interface in the interconnecting device, passive and active circuits to change impedance levels or to amplify low signals, Ordinarily, this is not required if avionics equipment has been designed within the require- ments of VAST. Often, passive circuit functions are obtained through the use of standard plug-in modules. The last element of the test program is the instruction booklet or microfilm strip. This element details all the steps to follow when you test any given unit, from initial procedures, such as hookup and clearing operations, down to the final stages of disconnect and UUT closeout. OPERATION OF A VAST STATION In the typical VAST test procedure, ease of operation in the actual testing becomes apparent, The initial setup of the weapon replaceable assembly, including removal of dust covers, cooling provisions, and connections to interface device, may be made off station to minimize disruptions of station operators. Final connections between the VAST station’s interface panel and the UUT are made in a few moments at the station. The operator begins testing by selecting the code that initiates the test program. Before power or stimulus is applied to the UUT, continuity tests are run to make sure the proper test program has been selected and no condition exists that will damage the VAST station or the UUT once active tests are started. If everything checks out, the testing proceeds automatically, The operator only has to respond to instructions that appear on the CRT display. The program will not stop until a fault is encountered or a program halt is reached. The purpose of programmed halts is to allow manual intervention during the course of testing to make adjustments and observations. When the identification of faults and the operator’s instructions are required (such as interpreting a complex waveform), the operator may be referred to the test program instructions. Upon completion of the test program, the CRT display indicates closeout procedures. A VAST station is completely autonomous and normally operated under computer control in a fully automatic mode, stopping only as previously mentioned. Of course, the operator can select any one of the semiautomatic modes or a manual mode. The semiautomatic modes include a one- group, one-test, and one-step mode. These auxiliary modes permit detailed observation of various test sequences, and they are useful 8-60

in performing work-around procedures in reconciling differences in equipment and program mode status and in the verification of repairs. In the manual mode, the test station is completely off-line with respect to the computer. Instructions are introduced by the operator through the keyboard on a one-word-at-a-time basis. (See fig. 8-58.) Although the manual mode is never used for avionics testing, it is useful for debugging new programs, integrating new building blocks into the station, and performing self-check operations on some of the building blocks. Q46. List the elements of a typical VAST station. Q47. List the three levels of detection that ensure rapid confidence tests and easy fault detection. Q48. What is the purpose of programmed halts? Figure 8-58.-Typical VAST control panel. 8-61

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