NAVY ELECTRICITY AND ELECTRONICS TRAINING SERIES MODULE 17-RADIO-FREQUENCY COMMUNICATIONS PRINCIPLES

NEETS MODULE 17-Radio-Frequency Communications Principles UNCLASSIFIED

1-1 UNCLASSIFIED 1 RADIO-FREQUENCY COMMUNICATIONS LEARNING OBJECTIVES After you finish this chapter, you should be able to do the following: 1. Define electrical telecommunications. 2. Describe the use of radiotelegraph, radiotelephone, teletypewriter, and facsimile. 3. Define and describe the interrelationships of the system, set, group, unit, assembly, subassembly, part, and reference designations. 4. State the frequency ranges of the various frequency bands and describe the most common uses of those bands by the Navy. 5. Describe a strategic communications link. 6. Describe a tactical communications link. 7. Describe the five basic communications modes of operation. 8. Describe a switched communications network. 9. Describe the purpose of the two Navy-only networks.

1.1 INTRODUCTION TO NAVAL TELECOMMUNICATIONS When the wireless (radiotelegraph) was invented, the Navy saw a possible use for it. It could be used for communications from shore stations to ships along the coast. In 1899, the first official naval radio message was sent from ship to shore. It only traveled a distance of 20 miles but that was a start. The next advance was in 1916 when the Navy first used radiotelephone between ships. Three years later the first airborne radio was used to communicate with a ground station. In the early years, communications was not the best because of poor tuning techniques. Receivers often did not pick up the signal. This problem was almost eliminated in 1931 when the first superheterodyne receivers were installed in the fleet. In 1944, another important event took place. The first successful radio teletypewriter transmissions between ships were completed. The first successful use of radiophoto (facsimile) occurred in 1945 with the transmission of the surrender document signing that ended World War II. Naval communications has grown tremendously in size and complexity since then.

The fleets of our modern Navy travel faster and are spread over greater areas of ocean than any seagoing force of the past. Commanders and their subordinates throughout the Department of the Navy use the facilities of naval communications as a primary method of communicating.

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1-2 UNCLASSIFIED Naval communications relies on top performance from all of its assigned personnel. Reliable, secure, and timely transmission and receipt of information, based on wartime requirements, is the ultimate goal.

Previous modules have discussed electronic components or circuitry in individual units. In this chapter we will tie up some loose ends for you and discuss radio-frequency communications.

We will cover the considerations involved in receiving or transmitting a radio-frequency signal between two or more geographic locations. Let's start by defining telecommunications.

TELECOMMUNICATIONS refers to communications over a distance and includes any transmission, emission, or reception of signs, signals, writings, images, or sounds. Intelligence produced by visual means, oral means, wire, radio, or other electromagnetic systems are also included. Electrical, visual, and sound telecommunications are all used in the Navy. In this chapter we will talk only about electrical types of telecommunications.

1.2 ELECTRICAL The types of electrical communications are radio and wire. Radio uses electromagnetic waves to transmit and receive intelligence. The waves are not guided by a physical path between sender and receiver. Wire uses conductors to carry these waves. Radio is the most important method the Navy has of communicating between widely separated forces. The transmission methods we will be discussing are radiotelegraph, radiotelephone, teletypewriter, and facsimile.

1.2.1 Radiotelegraph Radiotelegraph transmissions are referred to as continuous wave (cw) telegraphy. Cw is a manual or automatic system of transmitting signals using a wave of radio-frequency (rf) energy. The radio operator separates a continuously transmitted wave into dots and dashes based on the Morse code. This is accomplished by opening and closing a telegraphic hand key.

Radiotelegraphy was the first means of radio communications that had military and commercial importance. Radiotelegraph still is used as a means of communication to, from, and among widely separated units of the Navy.

NEETS MODULE 17-Radio-Frequency Communications Principles UNCLASSIFIED

1-3 UNCLASSIFIED Relative slow speed of transmission and the requirement for experienced operators are the major disadvantages of radiotelegraph. The main advantage is reliability. A thinking person at both sending and receiving stations provides a capability of being understood not present in automated systems.

1.2.2 Radiotelephone Radiotelephone is one of the most useful military communications methods. Because of its directness, convenience, and ease of operation, radiotelephone is used by ships, aircraft, and shore stations. It has many applications and is used for ship-to-shore, shore- to-ship, ship-to-ship, air-to-ship, ship-to-air, air-to-ground, and ground-to-air communications. Modern means of operation make it possible to communicate around the world by radiotelephone. One of the most important uses of radiotelephone is short- range tactical communications. This method permits tactical commanders to communicate directly with other ships. Little delay results while a message is prepared for transmission, and acknowledgments can be returned instantly. Radiotelephone equipment for tactical use usually is operated on frequencies that are high enough to have line-of-sight characteristics; that is, the waves do not follow the curvature of the earth. As you know, these characteristics limit the usual range of radiotelephone from 20 to 25 miles. This is important because it reduces the chances of the enemy intercepting the message. Radiotelephone procedures can be learned easily by persons with no other training in communications.

Radiotelephone has some disadvantages. You may find transmissions unreadable because of static, enemy interference, or high local noise level caused by shouts, gunfire, and bomb or shell bursts. Wave propagation characteristics of radiotelephone frequencies sometimes are unpredictable, and tactical transmissions may be heard from great distances. Most radiotelephone messages are in plain language, and if information is to be kept from the enemy, users must keep their messages short, stick to the proper procedures, and be careful of what they say.

Q1. What are the two types of electrical communications? Q2. What is the main advantage of radiotelegraph communications?

NEETS MODULE 17-Radio-Frequency Communications Principles UNCLASSIFIED

1-4 UNCLASSIFIED Q3. Why is radiotelephone one of the most useful methods of military communications? Q4. What are the disadvantages of radiotelephone communications? 1.2.3 Teletypewriter Teletypewriter (tty) signals may be transmitted by either landline (wire), cable, or radio. The landline tty is used both by the military services and by commercial communication companies. The Navy uses radio teletypewriter (rtty) mainly for high-speed automatic communications across ocean areas. The tty unit is equipped with a keyboard similar to a typewriter. When the operator presses a key, a sequence of signals is transmitted. At receiving stations, the signals are fed into terminal equipment that translates the sequences of signals into letters, figures, and symbols and types the messages automatically.

The rtty mode of transmission and reception is rapidly becoming more efficient and reliable for communications between ships and from ship-to-shore. Ships copy what is known as "fleet broadcast" messages on rtty. The speed at which message traffic is transmitted on rtty circuits depends on the equipment in use. Normal speed of operation is 100 words per minute, but it may be faster or slower. You may find high-speed equipment, capable of printing a line or even a page at a time, in some communications centers. The use of rtty has brought about a considerable savings in manpower.

1.2.4 Facsimile Facsimile (fax) is the process used to transmit photographs, charts, and other graphic information electronically. The image to be transmitted is scanned by a photoelectric cell. Electrical changes in the cell output, corresponding to the light and dark areas being scanned, are transmitted to the receiver. At the receiver, the signal operates a recorder that reproduces the picture. The fax signals may be transmitted either by landline or radio.

NEETS MODULE 17-Radio-Frequency Communications Principles UNCLASSIFIED

1-5 UNCLASSIFIED Facsimile transmissions suffer distortion from all of the common sources of interference experienced with ordinary radiotelegraph and radio teletypewriter. Certain characteristics of TIF transmission make it less susceptible to complete loss of intelligence. For example, picture quality will be downgraded by any noise bursts, since facsimile recording is a continuous recording of signals coming from a receiver. However, because the machine scans material at the rate of about 100 lines per inch, each line is only 1/100th of an inch high. So you can see, if a noise burst interferes with the signal, it will distort a line only 1/100th of an inch high, leaving the image still readable. Under similar circumstances on a conventional rtty circuit, such distortion could cause a portion of the page copy to be unreadable.

Facsimile transmission is not intended to be a replacement for teletypewriter and other general methods of transmission. It is an important communications supplement and provides a means of handling certain types of graphic and pictorial intelligence by swift communications methods. It is widely used by the Navy weather information services and ship and station weather centers to obtain the latest weather maps. Chances are the photo you saw in the newspaper was transmitted by facsimile.

Q5. What is the main use of a radio teletypewriter? Q6. What is facsimile? 1.3 SYSTEM INTRODUCTION Until recently, RADIO COMMUNICATIONS brought to mind either telegraphy (cw), voice (AM), or possibly radio teletypewriter (rtty) communications. Today, radio communications has become a highly sophisticated field of electronics. Even small Navy ships have the capability to "come up" on the commonly used ship-to-ship, ship-to-air, and ship-to-shore communications circuits. These circuit operations are accomplished through the use of compatible and flexible communications systems.

A communications system (as you will see later in this chapter) consists of two or more equipment sets (sets will be explained a little later). Communications systems follow the system sub1division shown in figure 1-1.

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1-6 UNCLASSIFIED Systems are arranged and interconnected to perform a circuit operation that cannot be performed by any single piece of equipment. Navy communications systems vary from the simple to the very complex, depending upon the circuit operations involved. Because a Navy ship must use every inch of available space, the communications equipment may be spread over several portions of the ship, for instance, receivers in one location, transmitters in another, and terminal equipment in another. The equipment must be installed in such a manner that it is flexible and can be used interchangeably with other installed communications equipment. Consequently, large numbers of sets which make up the shipboard communications system are installed and are capable of operating separately and simultaneously. Flexibility is provided through a complex arrangement of interconnections. These allow the physically separated equipment to be selectively switched (patched) by you into different circuit configurations.

Figure 1-1 System subdivision

NEETS MODULE 17-Radio-Frequency Communications Principles UNCLASSIFIED

1-7 UNCLASSIFIED As naval electronics has grown in capability and complexity, an orderly plan of equipment designations has been adopted. The largest designator, system, describes equipment that work together for a specific function. For example, the radar system of a ship includes every item of electronics equipment used in or with a radar on board that ship. The smallest designator, part, describes one single piece, such as a bolt or a resistor.

1.3.1 System A SYSTEM is a combination of sets, units, assemblies, subassemblies, and parts joined together to perform a specific operational function or several functions. Examples are communications systems, radar systems, or navigation systems. You will find the majority of troubleshooting done in the Navy is system oriented. Figure 1-2 is a pictorial view of a typical communications system containing the components necessary for transmission and reception of voice, telegraphy, and teletypewriter signals. Figure 1-3 is a block diagram of the same communications system with the arrows showing the direction of signal flow.

Figure 1-2 Communications system pictorial view

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1.3.2 Set A SET consists of a unit or units and the assemblies, subassemblies, and parts connected or associated together to perform a specific function. A good example of this is a radio receiving set or a radio transmitting set.

Figure 1-4 is a block diagram of a radio transmitting set. It consists of a radio-frequency amplifier unit (1), a radio transmitter unit (2), a power supply unit (3), and an antenna coupler group.

Figure 1-3 Communications system block diagram

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1.3.3 Group A GROUP is a collection of units, assemblies, subassemblies, and parts. It is a subdivision of a set or system, but it is not capable of performing a complete operational function. The coupler requires power and signals from the radiofrequency amplifier unit for operation. An example is the antenna coupler group in figure 1-4.

1.3.4 Unit A UNIT is an assembly or any combination of parts, subassemblies, and assemblies mounted together. A unit is normally capable of independent operation in a variety of situations. An example of a unit might be a power supply.

Figure 1-4 Radio transmitting set

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1-10 UNCLASSIFIED 1.3.5 Assembly An ASSEMBLY is a number of parts or subassemblies, or any combination thereof, joined together to perform a specific function. Figure 1-5 shows a unit (2) with its six assemblies. The assembly (A6) contains six subassemblies.

Figure 1-5 Unit and assembly

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1-11 UNCLASSIFIED 1.3.6 Subassembly A SUBASSEMBLY consists of two or more parts that form a portion of an assembly or a unit. It is replaceable as a whole, but some of its parts are individually replaceable.

The distinction between an assembly and a subassembly is not always exact; an assembly in one application may be a subassembly in another when it forms a portion of an assembly. Figure 1-6 shows a printed circuit board subassembly and some of the parts which may be mounted on it.

Figure 1-6 Typical subassembly

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1-12 UNCLASSIFIED 1.3.7 Part A PART is one component or two or more components joined together. A part is not normally subject to disassembly without destruction. Resistors, capacitors, and transistors are examples of parts.

1.3.8 Reference Designations Reference designations consist of letters, numbers, or both and are used for identification purposes. Reference designations can be used in several different ways. One important way you will use them is as a cross-reference for locating supply stock numbers. When you know the reference designator, ordering the correct replacement for a failed component is easy. You will also use them frequently in corrective and preventive maintenance. Reference designators will help you to locate test points and adjustments; they will also help you to move back and forth between various technical manuals, schematics, tables, or other references.

Each set within a system is assigned an “AN” nomenclature. Each unit, assembly, subassembly, and part of a set has an assigned reference designation. Systems, sets, and groups have no reference designation. The unit is the highest level assigned a reference designator.

Each unit is assigned an identifying number. This number begins with the number 1 and runs consecutively for all units of a system or a set. Let's look back at the radio transmitting set AN/URT-xx with the unit numbers 1, 2, 3 on figure 1-4. You should note that these units may also have an “AN” nomenclature, such as T-xxx/URT. The T indicates the equipment is a transmitter. The xxx would be replaced by 3 digits that indicate the model number.

By examining the reference designator of a unit, you will be able to determine in which group, if any, the unit is contained. Let's look at a complete reference designator for a unit. A good example for us to break down is the reference designator 2A2A3C1 on figure 1-7.

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The first indicator, 2, is numeric and refers to unit 2. The next indicator, A2, is alphanumeric and refers to assembly A2. The next indicator, A3, is also alphanumeric and refers to subassembly A3. The last indicator, C1, like the two previous, is alphanumeric and refers to the part C1. We have just located capacitor C1 on subassembly A3, which is on assembly A2, which is in unit 2 of the equipment.

Reference designations may be expanded or reduced to as many levels as required to identify a particular part. Let's look at a couple of examples on our figure. The designator 2J1 identifies jack J1, which is mounted directly on unit 2. The designator 2A4C3 identifies capacitor C3, which is on assembly A4 in unit 2.

Partial reference designations are used to save space on diagrams. For example, refer back to figure 1-6. Partial reference designations are placed near the parts on subassembly A15, and a note indicates the reference designation prefix is added. Capacitor C3 on subassembly A15 has the complete reference designation 2A7A15C3.

Figure 1-7 Reference designations

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1-14 UNCLASSIFIED Q7. A system is subdivided into what levels? Q8. In the example 1A6CR3, what is the assembly designator? 1.4 NAVY FREQUENCY BAND USE Rapid growth in the quantity and complexity of communications equipment and increased worldwide international requirements for radio frequencies have placed large demands upon the radiofrequency spectrum. These demands include military and civilian applications such as communications, location and ranging, identification, standard time and frequency transmission, and industrial, medical, and other scientific uses.

The allocation, assignment, and protection of all frequencies used by any component of the Navy are the responsibility of Commander Naval Telecommunications Command (COMNAVTELCOM). Table 1-1 shows the radio-frequency spectrum broken down into nine bands used by the military. Propagation of radio waves varies widely at different frequencies. Frequencies and equipment are chosen to meet the communications application desired. We will discuss the radio-frequency spectrum in the following paragraphs.

FREQUENCY DESCRIPTION 30 GHZ - 300 GHz extremely high frequency (EHF) 3 GHZ - 30 GHz super high frequency (SHF) 300 MHZ - 3 GHz ultra high frequency (UHF) 30 MHZ - 300 MHz very high frequency (VHF) 3 MHZ - 30 MHz high frequency (HF) 300 KHZ - 3 MHz medium frequency (MF) 30 KHZ - 300 KHz low frequency (LF) 3 KHZ - 30 KHz very low frequency (VLF) 300 HZ - 3 KHz voice frequency Up to 300 Hz extremely low frequency (ELF) Table 1-1 Radio-Frequency Spectrum

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1-15 UNCLASSIFIED 1.4.1 Extremely Low-Frequency Communications The purpose of the EXTREMELY LOW-FREQUENCY (elf) communications system is to send short "phonetic letter spelled out" (PLSO) messages from operating authorities in the continental United States (CONUS) to submarines operating at normal mission speeds and depths. Elf has the ability to penetrate ocean depths to several hundred feet with little signal loss. This ability allows submarines to be operated well below the immediate surface and enhances submarine survivability by making detection more difficult.

This is a one-way communications system from the operating authority to submarines at sea. The large size of elf transmitters and antennas makes elf transmission from submarines impractical.

1.4.2 Very-Low-Frequency Communications The communications commitments of the Navy now cover the face of the earth. New sea frontiers to the north have opened a four-million-square-mile, ice-covered ocean of strategic importance. Our Navy must maintain control of the operating forces in an ever expanding coverage area. This additional area requires changes in communications capacity, range, and reliability. Additional needs have been particularly great in the North Atlantic and the newly opened Arctic Ocean. High-frequency circuits are too unreliable in these areas because of local atmospheric disturbances.

VERY-LOW-FREQUENCY (vlf) transmissions provide a highly reliable path for communications in these northern latitudes as well as over and under all oceans and seas of the world. At present, practically all Navy vlf transmitters are used for fleet communications or navigation. The vlf transmission is normally considered a broadcast, that is, one-way transmission, no reply required. The vlf transmitter normally transmits single-channel rtty.

Vlf is currently used for communications to large numbers of satellites and as a backup to shortwave communications blacked out by nuclear activity. Our Navy depends on vlf for crucial communications during hostilities.

Secondary applications of the vlf range include worldwide transmission of standard frequency and time signals. Standard frequency and time signals with high accuracy over long distances have become increasingly important in many fields of science. It is essential for tracking space vehicles, worldwide clock synchronization and oscillator calibration, international comparisons of atomic frequency standards, radio navigational aids, astronomy, national standardizing laboratories, and communications systems.

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1-16 UNCLASSIFIED A vlf broadcast of standard time and frequency signals has more than adequate precision for the operation of synchronous cryptographic devices, decoding devices, and single- sideband transmissions.

1.4.3 Low-Frequency Communications The LOW-FREQUENCY (lf) band occupies only a very small part of the radio- frequency spectrum. This small band of frequencies has been used for communications since the advent of radio.

Low-frequency transmitting installations are characterized by their large physical size and by their high construction and maintenance costs. Another disadvantage is low- frequency signal reception being seriously hampered by atmospheric noise, particularly at low geographical latitudes. Over the years, propagation factors peculiar to the low- frequency band have resulted in their continued use for radio communications. Low- frequency waves are not so seriously affected during periods of ionospheric disturbance when communications at the high frequencies are disrupted. Because of this, the Navy has a particular interest in the application of low frequencies at northern latitudes.

The Navy's requirement to provide the best possible communications to the fleet requires operation on all frequency bands. Constant research is being done to improve existing capabilities and to use new systems and developments as they become operationally reliable.

In the past, the fleet broadcast system provided ships at sea with low-frequency communications via cw telegraph transmissions. As technology advanced, the system was converted to single-channel radio teletypewriter transmission. Today If communications is used to provide eight channels of frequency division multiplex rtty traffic on each transmission of the fleet multichannel broadcast system.

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1-17 UNCLASSIFIED 1.4.4 Medium-Frequency Communications The MEDIUM-FREQUENCY (mf) band of the radio-frequency spectrum includes the international distress frequencies (500 kilohertz and approximately 484 kilohertz). Some ships have mf equipment. If desired the distress frequencies may be monitored. When this is done the transmitter usually is kept in the standby position. Ashore, the mf receiver and transmitter equipment configuration is usually affiliated with search and rescue organizations, which are generally located near the coast.

Only the upper and lower ends of the mf band have naval use because of the commercial broadcast band (AM) extending from 535 to 1,605 kilohertz. Frequencies in the lower portion of the mf band (300 to 500 kilohertz) are used primarily for ground-wave transmission for moderately long distances over water and for moderate to short distances over land. Transmission in the upper mf band is generally limited to short-haul communications (400 miles or less).

1.4.5 High-Frequency Communications The Navy began using HIGH FREQUENCIES for radio communications around World War I when only a few communications systems were operated on frequencies near 3 megahertz. When we look at the extensive present-day use of high frequencies for long- distance communications, the fact that those Navy systems were intended for very short- range communications of a few miles seems curious. The general belief at the time was that frequencies above 1.5 megahertz were useless for communications purposes.

One of the prominent features of high-frequency, long-distance communications is the variable nature of the propagation medium. (You studied this in NEETS, Module 10, Wave Propagation, Transmission Lines, and Antennas). Successful transmission of hf signals over a long distance is dependent upon refraction of radio waves by layers of the ionosphere. The height and density of these layers is formed mainly by ultraviolet radiation from the sun. They vary significantly with the time of day, season of the year, and the eleven-year cycle of sunspot activity. Because of these variations, you must generally use more than a single frequency, sometimes up to four or five, to maintain communications on a circuit.

In spite of the difficulties we encounter with hf propagation, the economic and technical advantages of using high frequencies have led to rapid expansion of the use of the hf band. Because the number of users has increased, the hf spectrum is approaching saturation.

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1-18 UNCLASSIFIED The hf band is shared by many domestic and foreign users, and only portions scattered throughout the band are allocated to the military services. Like other agencies, Navy requirements have grown; the capacity of the Navy's assigned portion of the hf spectrum has become severely taxed. The use of single-sideband equipment and the application of independent sideband techniques have increased the capacity, but not enough to catch up with the demand. Some predict that satellite communications will eventually relieve congestion in the hf band and that, for some types of service, it will replace hf for long- distance communications. We will present more information to you concerning satellite communications in chapter 3. Even with new technology the hf spectrum most likely will continue to be in high demand for some time.

Naval communications within the hf band can be grouped into four general types of services: point-to-point, ship-to-shore, ground-to-air, and fleet broadcast. All but the fleet broadcast are normally operated with two-way communications. Some of these services involve ships and aircraft that present special problems because of their physical characteristics and mobility. Generally, the less than optimum hf performance of this shipboard equipment is at least partially offset by powerful transmitters and sensitive receiving systems at the shore terminals.

POINT-TO-POINT.—Point -to-point systems are established to communicate over long- distance trunks or links between fixed terminals. A trunk is normally a message circuit between two points that are both switching centers or individual message distribution points. A link is a transmitter-receiver system connecting two locations.

Generally, enough real estate is acquired at the terminals to permit the use of large, high- gain antennas aimed at opposite terminals of each link. This increases the effective radiated power and the sensitivity of the receiving system; it also reduces susceptibility of a circuit to interference.

With the path length and direction fixed, other propagation factors are simplified and highly reliable communications can be achieved.

SHIP-TO-SHORE.—This application of the hf band is more difficult than point -to-point since the ship is moving and constantly changing its position. In ship-to-shore the path length and direction are variable. Aboard ship, limited space and other restrictions prohibit installation of large, efficient hf antennas. Because of the mobility of ships, shipboard antennas are designed to be as nearly omnidirectional as possible.

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1-19 UNCLASSIFIED Our problems are not as severe at the shore terminal where we have sufficient space for efficient omnidirectional antennas or arrays designed for coverage of large areas of the earth. At shore stations, rotatable, high-gain antennas or fixed, point-to-point antennas are used. For example, a rhombic antenna ashore may work well for long-haul, ship-to-shore communications when the ship is within range of the antenna.

Several frequencies are usually assigned for each circuit. Therefore, a frequency can be selected that best matches the propagation path conditions between the shore terminal and the ship.

GROUND-TO-AIR.—The use of hf radio for ground- to-air communications is similar to ship-to-shore. The only exception is an aircraft moves more rapidly than a ship. All major circuit improvements must be made at the ground station. For example, higher powered transmitters, lower noise receivers, and more efficient antennas must be used on the ground.

FLEET BROADCASTS.—As the name implies, this service involves broadcast area coverage from shore-based transmitters to ships at sea. Messages to be sent to ships are delivered by various means to the proper broadcast station. They are then broadcast for shipboard reception. To overcome propagation problems, naval communicators send the messages on several frequencies at once. This is known as frequency-diversity transmission. This type of transmission allows the ship to choose the best frequency for reception. Space-diversity with physically separated receive antennas also helps to overcome this problem.

1.4.6 Very-High-Frequency and Above Communications Frequencies above 30 megahertz are not normally refracted by the atmosphere and ground-wave range is minimal. This normally limits our use of this frequency spectrum to line of sight. The exception to this is increased range through the use of tropospheric scatter techniques. Some communications using vhf and above frequencies use a technique called forward propagation by tropospheric scatter (fpts). This method will be discussed in more detail in chapter 5.

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1-20 UNCLASSIFIED Certain atmospheric and ionospheric conditions can also cause the normal line-of-sight range to be extended. Frequencies at the lower end of this band are capable of overcoming the shielding effects of hills and structures to some degree; but as the frequency is increased, the problem becomes more pronounced. Reception is notably free from atmospheric and man-made static. (The VERY-HIGHFREQUENCY (vhf) and ULTRAHIGH-FREQUENCY (uhf) bands are known as line-of-sight transmission bands.) Because this is line-of-sight communications, the transmitting antenna is in a direct line with the receiving antenna and not over the horizon. The line-of-sight characteristic makes the vhf band ideal for amphibious operations (beach landing from sea craft) and the uhf well suited for tactical voice transmissions (maneuvering of ships traveling together). The SUPERHIGH-FREQUENCY (shf) band is used for radar and satellite communications, whereas the EXTREMELY HIGH-FREQUENCY (ehf) band is used only in the experimental stage.

Q9. The majority of vlf transmitters are used for what purpose? Q10. Today the Navy uses lf communications as a segment of what operational system? Q11. Why does the Navy only use the upper and lower ends of the mf band? Q12. What are the four general types of communications services in the hf band? Q13. A message transmitted on several frequencies at the same time is an example of what type of transmission? Q14. Physically separating receive antennas is an example of what technique? Q15. When using frequencies above 30 megahertz, you are normally limited to using what range?

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1-21 UNCLASSIFIED 1.5 COMMUNICATIONS FUNDAMENTALS Now that we have learned the Navy's fundamental use of the various frequency bands, we should look at the types of communications links and their modes of operation. The Navy uses many modes of operation; the type used is based upon the function of the circuit or network. These modes (or functions) are combined to form a communications link. We will also discuss some of the actual networks the Navy uses on a daily basis.

1.6 COMMUNICATIONS LINKS A complex of links forms a major communications system. The naval communications system is broken down into strategic and tactical groups.

1.6.1 Strategic Strategic communications are generally world-wide in nature. They are operated on a common-user (Navy, Army, Department of Defense, and so on) or special-purpose basis. A strategic system may be confined within a specified area or limited to a specific type of traffic, but the configuration is designed so that combined operations with other strategic systems are possible. As an example, we will look at the automatic voice network, automatic digital network, and the defense special security communications system later in this chapter.

1.6.2 Tactical Tactical communications are usually limited to a specific area of operations and are used to direct or report the movement of specific forces. Some tactical networks are used only for operational traffic; others may be used for operational and administrative traffic. For instance, the task force, task-group, and air-control networks are ordinarily used for operational traffic. Ship-to-shore networks and broadcast networks serve both types of traffic.

1.6.3 Modes of Operation Communications links have numerous modes of operation. In our discussion, a mode of operation is identified as a link or path between two or more points that is capable of providing one or more channels for the transmission of intelligence. Let's take a look at the five most common modes of operation.

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1-22 UNCLASSIFIED SIMPLEX.—The simplex (splx) mode uses a single channel or frequency to exchange information between two or more terminals. Communications is in one direction only.

HALF DUPLEX.—The half -duplex (hdx) mode has one-way flow of information between terminals. Technical arrangements often permit transmission in either direction, but not simultaneously. This term must be qualified to show s/o (send only), r/o (receive only), or s/r (send or receive).

SEMIDUPLEX.—The semiduplex (sdx) uses an arrangement of equipment where one terminal is simplex configured and the other uses two channels or frequencies in full duplex. A clarifying example is a ship in a simplex mode terminated full duplex with a shore station. The ship may send or receive but not do both at the same time.

FULL DUPLEX.—The full -duplex (fdx) mode is a method of operation in which telecommunications between stations takes place simultaneously in both directions using two separate frequencies. In other words, a ship may send and receive different messages at the same time. The term "full duplex" is synonymous with "duplex."

BROADCAST.— Broadcast (bc) is the type of operation in which one station transmits information on one or more channels directed to more than one station and/or unit. The broadcast system has no provision for receipt or reply; however, special arrangements may require the receiving station to reply or receipt for the message at a later time by other means. Broadcasts are the primary means of delivering messages to the fleet. Since Navy units copying broadcasts are not required to receipt for messages received, they can maintain radio silence while still receiving essential messages.

Message traffic is normally sent to the fleet by three methods: broadcast, intercept, and receipt. The first two are "do not answer" methods; the third, as its name implies, requires a receipt from the addressee (addee) for each message. Broadcast and intercept methods allow the fleet to preserve radio silence, which is a great advantage from the standpoint of security. By the intercept method, a shore radio station transmits messages to another shore station that repeats them back. Ships intercept and copy all of this message traffic.

Broadcast is preferable to intercept chiefly because it is faster. It is the method by which nearly all fleet traffic is handled. It uses radiotelegraph, radiotelephone, radio teletypewriter, and facsimile.

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1-23 UNCLASSIFIED There is some similarity between civilian and naval broadcasts. Just as commercial stations in the broadcast band transmit programs to radio receivers in the homes in their communities, Navy communications stations broadcast messages to fleet units in their particular geographic areas. The resemblance between Navy and commercial stations ceases there. Information broadcast by naval communications stations is contained in chronologically numbered messages addressed to the ships. The messages are copied by the fleet units, which check the serial numbers to ensure they have a complete file. This checks and balances system ensures the ship has not missed any of the broadcast message traffic.

Fleet broadcasts follow regular schedules. Messages are placed on the schedules in order of precedence. If a message of higher precedence is given to a transmitter station while a lower precedence message is being transmitted, the latter message may be interrupted to transmit the message of higher precedence. All ships copy all messages appearing on the broadcast schedule they are guarding.

Messages are normally transmitted on several frequencies to make sure they are received. This gives the receiving station the choice of frequency selection when considering time of day and atmospheric conditions for best reception.

Q16. The naval communications system is made up of what two groups of communications? Q17. What are the five most prominent communications modes of operation? 1.7 SWITCHED NETWORKS The defense communications system (DCS) is composed of all worldwide, long-haul, government owned and leased point-to-point circuits, trunks, terminals, switching centers, control facilities, and tributaries of military departments and other defense activities. In essence the DCS combines into a single system all the elements that make up the naval communications system and the Army and Air Force equivalent.

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1-24 UNCLASSIFIED The switched networks discussed in this section, automatic voice network, automatic secure voice communications, automatic digital network, and the defense special security communications system, are part of the DCS and are managed by the Defense Communications Agency (DCA). You should not confuse these DCS networks with the HICOM (high-command communications network) and NORATS (Navy operational radio and telephone switchboard) networks. We will discuss both of these Navy-only networks later in this chapter.

1.7.1 Automatic Voice Network (AUTOVON) The DCS AUTOVON offers rapid, direct interconnection of DOD and certain other government installations through worldwide telephone exchanges. AUTOVON is a worldwide, general-purpose direct dialing telephone system. The goal of the AUTOVON system is to complete connections between two points anywhere in the world in about two seconds and to complete regular connections with push-button speed.

The AUTOVON system is made up of several installations comparable in function to commercial telephone exchanges. An installation is referred to as an AUTOVON switch, or simply switch. Within individual areas we have local command, control, and administrative voice communications systems. These systems connect into the worldwide AUTOVON through manually operated telephone switchboards or automatic dial exchanges by using direct in and out dialing.

Normal AUTOVON service allows your station to call other stations on a worldwide basis for day to-day communications by using the telephone.

1.7.2 Automatic Secure Voice Communications (AUTOSEVOCOM) Another close relative to the AUTOVON system is the AUTOSEVOCOM a worldwide, switched telephone network. It provides authorized users with a means for exchanging classified information over communications security (COMSEC) circuitry or over other approved circuitry. The system consists of both manual and automated networks within a single system.

For subscribers to the AUTOSEVOCOM network, telephone directories containing subscriber listings, general instructions for placing calls, and trouble-reporting procedures are provided.

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1-25 UNCLASSIFIED 1.7.3 Automatic Digital Network (AUTODIN) The DCS AUTODIN is a fully automatic, digital system. The system converts word messages to digital form for transmission.

AUTODIN is used to furnish instantaneous, error-free, and secure communications around the world to several thousand directly connected subscriber terminals. Daily capacity of the system is about five million average-length messages.

AUTODIN switching centers are interconnected through a network of high-frequency radio channels, submarine cables, microwave and tropospheric channels, and a variety of wire lines.

The whole concept of AUTODIN is to reduce manual handling of messages to a minimum by the use of automated equipment. This system has reduced message delivery times and delay anywhere in the world to a matter of seconds rather than minutes or hours.

1.7.4 Defense Special Security Communications System (DSSCS) The defense special security communications system (DSSCS) was established for the purpose of integrating the critical intelligence communications (CRITICOMM) and the special intelligence communications (SPINTCOMM) networks into a single automated communications network. In effect, the integration of DSSCS subscribers into AUTODIN provides two separate systems within AUTODIN one system for special intelligence (SI) message traffic and the other for the AUTODIN regular message traffic.

1.8 NAVY-ONLY NETWORKS Some networks are used by the Navy only. As mentioned previously, these are the high command communications network (HICOM) and the Navy operational radio and telephone switchboard (NORATS) networks. Let's look at some of their functions and purposes.

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1-26 UNCLASSIFIED 1.8.1 High Command Communications Network (HICOM) The HICOM network provides a voice link between the Chief of Naval Operations (CNO) and all subordinate commands ashore, afloat, and airborne. CNO is the master control station and each fleet commander in chief has an area network control station. All naval communications stations are members.

In cases where a fleet unit is suffering communications difficulties with normal channels, HICOM is used on a not-to-interfere basis to restore communications. All naval communications stations are required to guard HICOM for their respective area networks and use this system.

1.8.2 Navy Operational Radio and Telephone Switchboard (NORATS) The NORATS meets our need for a connection between Navy tactical voice systems of the operating forces and the various fixed telephone services ashore. This system extends tactical voice to shore-based operational commands. NORATS provides a connecting point in the fleet center of each communication station. This point allows us to connect or patch all ship-to-shore voice circuits and designated local shore telephone systems and extensions. A combined HICOM/NORATS console exists at many naval communications stations.

Q18. What four switched networks are part of the defense communications system? Q19. What two elements support only designated Navy requirements? 1.9 SUMMARY Now that you have completed this chapter, a short review of what you have learned is in order. The following summary will refresh your memory of radio-frequency communications terms.

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1-27 UNCLASSIFIED TELECOMMUNICATIONS refers to transmission, emission, or reception of signs, signals, writings, images, or sounds. This is done by visual, oral, wire, radio, or other electromagnetic means.

RADIO COMMUNICATIONS is the term describing teletypewriter, voice, telegraphic, and facsimile communications.

SYSTEM is a combination of sets, units, assemblies, subassemblies, and parts joined together to form a specific operational function or several functions.

SET is a unit or units and the assemblies, subassemblies, and parts connected or associated together to perform a specific function.

GROUP is a collection of units, assemblies, subassemblies, and parts. It is a subdivision of a set or system but is not capable of performing a complete operational function.

UNIT is an assembly or any combination of parts, subassemblies, and assemblies mounted together. Normally capable of independent operation.

ASSEMBLY is a number of parts or subassemblies, or any combination thereof, joined together to perform a specific function.

SUBASSEMBLY consists of two or more parts that form a portion of an assembly or a unit.

PART is one component or two or more components joined together. It is not normally subject to disassembly without destruction.

EXTREMELY LOW FREQUENCY is the band of frequencies up to 300 hertz.

VERY LOW FREQUENCY is the band of frequencies from 3 kilohertz to 30 kilohertz.

LOW FREQUENCY is the band of frequencies from 30 kilohertz to 300 kilohertz.

MEDIUM FREQUENCY is the band of frequencies from 300 kilohertz to 3 megahertz.

HIGH FREQUENCY is the band of frequencies from 3 megahertz to 30 megahertz.

VERY HIGH FREQUENCY is the band of frequencies from 30 megahertz to 300 megahertz.

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1-28 UNCLASSIFIED ULTRAHIGH FREQUENCY is the band of frequencies from 300 megahertz to 3 gigahertz.

SUPERHIGH FREQUENCY is the band of frequencies from 3 gigahertz to 30 gigahertz.

EXTREMELY HIGH FREQUENCY is the band of frequencies from 30 gigahertz to 300 gigahertz.

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1-29 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q19. A1. Radio and wire.

A2. Reliability.

A3. It is direct, convenient and easy to use.

A4. Static, enemy interference or a high local noise level.

A5. High speed automatic communications across ocean areas.

A6. The process used to transmit photographs, charts and other graphic information electronically.

A7. Set, group, unit, assembly, subassembly, and part.

A8. A6.

A9. Fleet communications or navigation.

A10. Fleet Multichannel Broadcast System.

A11. Due to the commercial broadcast (AM) band.

A12. Point-to-point, ship-to-shore, ground-to-air, and fleet broadcast.

A13. Frequency-diversity.

A14. Space-diversity.

A15. Line of sight.

A16. Strategic and tactical.

A17. Simplex, half-duplex, semiduplex, duplex, and broadcast.

A18. AUTOVON, AUTOSEVOCOM, AUTODIN, and DSSCS.

A19. HICOM and NORATS.

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2-1 UNCLASSIFIED 2 INTRODUCTION TO COMMUNICATIONS THEORY LEARNING OBJECTIVES After you finish this chapter, you should be able to do the following: 1. Describe the four basic types of transmitters. 2. Describe the two basic types of single-sideband circuits. 3. Describe the three basic types of teletypewriter circuits. 4. List the four primary functions of a basic receiver. 5. Describe the four primary functions of a basic receiver. 6. State the four characteristics of a basic receiver. 7. Evaluate the four characteristics of a basic receiver. 8. Describe the fundamental heterodyning process. 9. Describe the basic difference between an AM and an fm receiver. 10. Describe single-sideband suppressed carrier communications. 11. State the purpose of carrier reinsertion and how it is used in single-sideband communications. 12. Describe the basic theory and functions of receiver control circuits. 13. Describe the basic frequency synthesis process. 14. Describe the basic audio reproduction process.

2.1 INTRODUCTION In the previous chapter you learned the fundamentals of U.S. naval telecommunications and communications. Now, let’s look at the equipment and systems that are used to communicate in the Navy. The fundamental equipment used to communicate are the transmitter and receiver.

Transmitters and receivers must each perform two basic functions. The transmitter must generate a radio frequency signal of sufficient power at the desired frequency. It must have some means of varying (or modulating) the basic frequency so that it can carry an intelligible signal. The receiver must select the desired frequency you want to receive and reject all unwanted frequencies. In addition, receivers must be able to amplify the weak incoming signal to overcome the losses the signal suffers in its journey through space.

Representative transmitters and their fundamental features are described for you in this module.

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2-2 UNCLASSIFIED 2.2 TRANSMITTER FUNDAMENTALS Basic communication transmitters include continuous wave (cw), amplitude modulated (AM), frequency modulated (fm), and single sideband (ssb) types. A basic description of each of these transmitters is given in this chapter.

2.2.1 Continuous Wave Transmitter The continuous wave is used principally for radiotelegraphy; that is, for the transmission of short or long pulses of rf energy to form the dots and dashes of the Morse code characters. This type of transmission is sometimes referred to as interrupted continuous wave. Cw transmission was the first type of radio communication used, and it is still used extensively for long-range communications. Two of the advantages of cw transmission are a narrow bandwidth, which requires less output power, and a degree of intelligibility that is high even under severe noise conditions. (For example, when the receiver is in the vicinity of rotating machinery or thunderstorms.)

A cw transmitter requires four essential components. These are a generator, amplifier, keyer, and antenna. We have to generate rf oscillations and have a means of amplifying these oscillations. We also need a method of turning the rf output on and off (keying) in accordance with the intelligence to be transmitted and an antenna to radiate the keyed output of the transmitter.

Let’s take a look at the block diagram of a cw transmitter and its power supply in figure 2-1. The oscillator generates the rf carrier at a preset frequency and maintains it within close tolerances. The oscillator may be a self-excited type, such as an electron-coupled oscillator, or a quartz crystal type, which uses a crystal cut to vibrate at a certain frequency when electrically excited. In both types, voltage and current delivered by the oscillator are weak. The oscillator outputs must be amplified many times to be radiated any distance.

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2-3 UNCLASSIFIED

The buffer stage or first intermediate power amplifier stage (referred to as the ipa) is a voltage amplifier that increases the amplitude of the oscillator signal to a level that drives the power amplifier (pa). You will find the signal delivered by the buffer varies with the type of transmitter and may be hundreds or thousands of volts.

The buffer serves two other purposes. One is to isolate the oscillator from the amplifier stages. Without a buffer, changes in the amplifier caused by keying or variations in source voltage would vary the load of the oscillator and cause it to change frequency. It may also be used as a frequency multiplier, which is explained later in this text.

As you can see in the figure, a key is used to turn the buffer on and off. When the key is closed, the rf carrier passes through the buffer stage; when the key is open (buffer is turned off), the rf carrier is prevented from getting through.

The final stage of a transmitter is the power amplifier (referred to as the pa). In chapter 3 of NEETS, Module 1, Matter, Energy, and Direct Current, you learned that power is the product of current and voltage (P = IE). In the power amplifier a large amount of rf current and voltage is made available for radiation by the antenna.

The power amplifier of a high-power transmitter may require far more driving power than can be supplied by an oscillator and its buffer stage. One or more low-power intermediate amplifiers are used between the buffer and the final amplifier that feeds the antenna. The main difference between many low and high-power transmitters is in the number of intermediate power-amplifier stages used.

Figure 2-1 Cw transmitter block diagram

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2-4 UNCLASSIFIED Figure 2-2 is a block diagram of the input and output powers for each stage of a typical medium power transmitter. You should be able to see that the power output of a transmitter can be increased by adding amplifier stages capable of delivering the power required. In our example, the .5 watt output of the buffer is amplified in the first intermediate amplifier by a factor of 10, (this is a times 10 [× 10] amplifier) giving us an input of 5 watts to the second intermediate amplifier. You can see in this example the second intermediate amplifier multiplies the 5 watt input to it by a factor of 5 (× 5) and gives us a 25 watt input to our power (final) amplifier. The final amplifier multiplies its input by a factor of 20 (× 20) and gives us 500 watts of power out to the antenna.

Q1. What are the four basic transmitter types? Q2. What is the function of the oscillator in a cw transmitter? Q3. What is the final stage of a transmitter?

Figure 2-2 Intermediate amplifiers increase transmitter power

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2-5 UNCLASSIFIED 2.2.2 Amplitude Modulated Transmitter In AM transmitters, the instantaneous amplitude of the rf output signal is varied in proportion to the modulating signal. The modulating signal may consist of many frequencies of various amplitudes and phases, such as the signals making up your own speech pattern.

Figure 2-3 gives you an idea of what the block diagram of a simple AM transmitter looks like. The oscillator, buffer amplifier, and power amplifier serve the same purpose as those in the cw transmitter. The microphone converts the audio frequency (af) input (a person’s voice) into corresponding electrical energy. The driver amplifies the audio, and the modulator further amplifies the audio signal to the amplitude necessary to fully modulate the carrier. The output of the modulator is applied to the power amplifier. The pa combines the rf carrier and the modulating signal in the power amplifier to produce the amplitude-modulated signal output for transmission. In the absence of a modulating signal, a continuous rf carrier is radiated by the antenna.

Figure 2-3 AM radiotelephone transmitter block diagram

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2-6 UNCLASSIFIED 2.2.3 Frequency Modulated Transmitter In frequency modulation (fm) the modulating signal combines with the carrier to cause the frequency of the resultant wave to vary with the instantaneous amplitude of the modulating signal.

Figure 2-4 shows you the block diagram of a frequency-modulated transmitter. The modulating signal applied to a varicap causes the reactance to vary. The varicap is connected across the tank circuit of the oscillator. With no modulation, the oscillator generates a steady center frequency. With modulation applied, the varicap causes the frequency of the oscillator to vary around the center frequency in accordance with the modulating signal. The oscillator output is then fed to a frequency multiplier to increase the frequency and then to a power amplifier to increase the amplitude to the desired level for transmission.

2.2.3.1 Harmonics True harmonics are always exact multiples of the basic or fundamental frequency generated by an oscillator and are created in amplifiers and their associated circuits. Even harmonics are 2, 4, 6, and so on, times the fundamental; odd harmonics are 3, 5, 7, and so on, times the fundamental. If an oscillator has a fundamental frequency of 2,500 kilohertz, the harmonically related frequencies are:

Figure 2-4 Fm transmitter block diagram

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2-7 UNCLASSIFIED 5,000………………second harmonic

7,500………………third harmonic

10,000………………fourth harmonic

12,500………………fifth harmonic

You should note that the basic frequency and the first harmonic are one and the same.

The series ascends indefinitely until the intensity is too weak to be detected. In general, the energy in frequencies above the third harmonic is too weak to be significant.

In some electronics books, and later in this chapter, you will find the term SUBHARMONIC used. It refers to a sine wave quantity (for example, an oscillator output) that has a frequency that is a submultiple of the frequency of some other sine wave quantity it helped make. For example, a wave that is half the fundamental frequency of another wave is called the second subharmonic of that wave; one with a third of the fundamental frequency is called a third subharmonic; and so forth.

Q4. What purpose does a microphone perform in an AM transmitter? Q5. In an fm transmitter, when does an oscillator generate only a steady frequency? Q6. What is a harmonic? Q7. If the fundamental frequency is 200 megahertz, what is the third harmonic?

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2-8 UNCLASSIFIED 2.2.3.2 Frequency Multiplication Designing and building a stable crystal oscillator is difficult. As operating frequencies increase, the crystal must be ground so thin that it often cracks while vibrating. You will find that you can get around this problem by operating the oscillators in most transmitters at comparatively low frequencies, sometimes as low as 1/100 (.01) of the output frequency. You raise the oscillator frequency to the required output frequency by passing it through one or more frequency multipliers. Frequency multipliers are special power amplifiers that multiply the input frequency. Stages that multiply the frequency by 2 are called doublers; those that multiply by 3 are triplers; and those multiplying by 4 are quadruplers.

You will find the main difference between low-frequency and high-frequency transmitters is the number of frequency-multiplying stages used. Figure 2-5 shows the block diagram of the frequency multiplying stages of a typical Navy uhf/vhf transmitter. The oscillator in this transmitter is tunable from 18 megahertz to 32 megahertz. You have multiplier stages that increase the oscillator frequency by a factor of 12 through successive multiplications of 2, 2, and 3.

Figure 2-6 is a block diagram of an fm transmitter showing waveforms found at various test points. In high-power applications you often find one or more intermediate amplifiers added between the second doubler and the final power amplifier.

Figure 2-5 Frequency multiplying stages of a typical vhf/uhf transmitter

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2.2.4 Single-Sideband Transmitter You should remember the properties of modulation envelopes from your study of NEETS, Module 12, Modulation Principles. A carrier that has been modulated by voice or music is accompanied by two identical sidebands, each carrying the same intelligence. In amplitude-modulated (AM) transmitters, the carrier and both sidebands are transmitted. In a single-sideband transmitter (ssb), only one of the sidebands, the upper or the lower, is transmitted while the remaining sideband and the carrier are suppressed. SUPPRESSION is the elimination of the undesired portions of the signal.

Figure 2-7 is the block diagram of a single-sideband transmitter. You can see the audio amplifier increases the amplitude of the incoming signal to a level adequate to operate the ssb generator. Usually the audio amplifier is just a voltage amplifier.

Figure 2-6 Block diagram of an fm transmitter and waveforms

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The ssb generator (modulator) combines its audio input and its carrier input to produce the two sidebands. The two sidebands are then fed to a filter that selects the desired sideband and suppresses the other one. By eliminating the carrier and one of the sidebands, intelligence is transmitted at a savings in power and frequency bandwidth.

In most cases ssb generators operate at very low frequencies when compared with the normally transmitted frequencies. For that reason, we must convert (or translate) the filter output to the desired frequency. This is the purpose of the mixer stage. A second output is obtained from the frequency generator and fed to a frequency multiplier to obtain a higher carrier frequency for the mixer stage. The output from the mixer is fed to a linear power amplifier to build up the level of the signal for transmission.

2.2.4.1 Suppressed Carrier In ssb the carrier is suppressed (or eliminated) at the transmitter, and the sideband frequencies produced by the carrier are reduced to a minimum. You will probably find this reduction (or elimination) is the most difficult aspect in the understanding of ssb. In a single-sideband suppressed carrier, no carrier is present in the transmitted signal. It is eliminated after modulation is accomplished and is reinserted at the receiver during the demodulation process. All rf energy appearing at the transmitter output is concentrated in the sideband energy as "talk power."

Figure 2-7 Ssb transmitter block diagram

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2-11 UNCLASSIFIED After the carrier is eliminated, the upper and lower sidebands remain. If one of the two sidebands is filtered out before it reaches the power amplifier stage of the transmitter, the same intelligence can be transmitted on the remaining sideband. All power is then transmitted in one sideband, instead of being divided between the carrier and both sidebands, as it is in conventional AM. This provision gives you an increase in power for the wanted sideband. You should note in figure 2-8 that the bandwidth required for the ssb suppressed carrier, view B, is approximately half that needed for conventional AM, view A. This enables us to place more signals in a smaller portion of the frequency spectrum and permits a narrower receiver bandpass.

Figure 2-8 Comparison of bandwidths of conventional AM and ssb voice channels

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2-12 UNCLASSIFIED 2.2.4.2 Applications Single-sideband transmission is the most common communications mode used today. Some of the ssb applications used in naval communications are described for you in the following paragraphs.

SSB VOICE CIRCUITS.—The high command (HICOM) network uses ssb as a means of communications between fleet commanders; and fleet commanders use it for communications between their subordinates and adjacent commands.

Ssb is generally used whenever special voice communications circuits are necessary between shore activities or between ships and shore activities because it is less susceptible to atmospheric interference than amplitude modulation.

SSB TELETYPEWRITER CIRCUITS.—With few exceptions, you will find ssb used on all longhaul (great distance) teletypewriter circuits, which includes ship-to-shore. Most of these systems are covered circuits; that is, an electronic cryptographic device on both ends of the circuit automatically encrypts and decrypts message traffic. These devices are used on point-to-point, ship-to-shore, ship-to-ship, and broadcast circuits.

Point-to-Point.—Most point -to-point, long-haul circuits between naval communications stations quickly use up the available frequency spectrum that ssb provides. Independent sideband (isb) transmission is normally used to compensate for the deficiency. Isb is used extensively in naval communications to expand our traffic capabilities. You will find there is a similarity between ssb and isb. Isb uses outputs from two sideband generators; it suppresses both carriers and then filters out an upper sideband from one and a lower sideband from the other. We then combine the two remaining sidebands and transmit an envelope with upper and lower sidebands that contain different intelligence. Isb can be used with MULTIPLEXING (a method for simultaneous transmission of two or more signals over a common carrier wave) to transmit a lot of intelligence on one circuit. Independent sideband and multiplexing will be discussed in more detail in chapter 3.

Ship-to-Shore.—Many ships handle enough message traffic to justify ship- to-shore teletypewriter circuits. Depending on traffic load, these circuits may contain from one to four (minimum) teletypewriter circuits on one sideband circuit. If the traffic load warrants more than one teletypewriter circuit, we usually use time division multiplex or frequency division multiplex (mux) equipment. This equipment is capable of handling many incoming and outgoing circuits. One circuit normally is used as an ORDERWIRE CIRCUIT for operator-to-operator service messages and for making frequency changes when necessary. The remaining circuits are available for handling official message traffic.

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2-13 UNCLASSIFIED Q8. Why are frequency multipliers used? Q9. What are two advantages of ssb transmission? Q10. What is the purpose of an order-wire circuit? Ship-to-Ship.—Ship- to-ship ssb teletypewriter circuits are in wide use today. Their main application is with task force or task group networks or with several ships in company. By using this type of network, ships can send their outgoing messages to another ship that relays traffic ashore. You can see this procedure saves manpower and circuit time, prevents individual ships from overcrowding ship-to-shore circuits, and conserves the frequency spectrum. Depending on the number and types of ships in company, the guard can be shifted to other ships from time to time. A major advantage of these circuits is that electronic cryptographic devices can be used to send classified messages without need for manual encryption. These circuits are used for incoming as well as outgoing traffic, and they can use either hf or uhf communications equipment.

2.3 RECEIVER FUNDAMENTALS An AM receiver processes amplitude-modulated signals received by its antenna. It delivers an output that is a reproduction of the signal that originally modulated the rf carrier at the transmitter. The signal can then be applied to some reproducing device, such as a loudspeaker, or to a terminal device, such as a teletypewriter. Actual AM receivers vary widely in complexity. Some are very simple; others contain a large number of complex circuits.

2.3.1 Functions Whatever its degree of sophistication, a receiver must perform certain basic functions to be useful. These functions, in order of their performance, are reception, selection, detection, and reproduction.

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2-14 UNCLASSIFIED 2.3.1.1 Reception Reception occurs when a transmitted electromagnetic wave passes through the receiver antenna and induces a voltage in the antenna.

2.3.1.2 Selection Selection is the ability of the receiver to select a particular frequency of a station from all other station frequencies appearing at the antenna of the receiver.

2.3.1.3 Detection Detection is the action of separating the low (audio) frequency intelligence from the high (radio) frequency carrier. A detector circuit is used to accomplish this action.

2.3.1.4 Reproduction Reproduction is the action of converting the electrical signals to sound waves, which can then be interpreted by your ear as speech, music, and the like. An example of this might be the stereo speakers in your car.

2.3.2 Receiver Characteristics Sensitivity, noise, selectivity, and fidelity are important receiver characteristics. These characteristics will be useful to you when performing receiver tests. They can help you to determine whether a receiver is working or not or in comparing one receiver to another.

2.3.2.1 Sensitivity The ability of a receiver to reproduce weak signals is a function of the sensitivity of a receiver. The weaker a signal that can be applied to a receiver and still produce a certain value of signal output, the better the sensitivity rating. Sensitivity of a receiver is measured under standardized conditions. It is expressed in terms of the signal voltage, usually in the microvolts that must be applied to the antenna input terminals to give an established level of the output. The output may be an ac or dc voltage measured at the detector output or a power measurement (measured in decibels or watts) at the loudspeaker or headphone terminals.

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2-15 UNCLASSIFIED 2.3.2.2 Noise All receivers generate a certain amount of noise, which you must take into account when measuring sensitivity. Receiver noise may originate from the atmosphere (lightning) or from internal components (transistors, tubes). Noise is the limiting factor of sensitivity. You will find sensitivity is the value of input carrier voltage (in microvolts) that must be applied from the signal generator to the receiver input to develop a specified output power.

2.3.2.3 Selectivity Selectivity is the degree of distinction made by the receiver between the desired signal and unwanted signals. You will find the better the ability of the receiver to reject unwanted signals, the better its selectivity. The degree of selection is determined by the sharpness of resonance to which the frequency determining circuits have been engineered and tuned. You usually measure selectivity by taking a series of sensitivity readings. As you take the readings, you step the input signal along a band of frequencies above and below the circuit resonance of the receiver; for example, 100 kilohertz below to 100 kilohertz above the tuned frequency. As you approach the tuned frequency, the input level required to maintain a given output level will fall. As you pass the tuned frequency, the required input level will rise. Input voltage levels are then compared with frequency. They can be plotted on paper or you might view them on an oscilloscope. They would appear in the form of a response curve. The steepness of the response curve at the tuned frequency indicates the selectivity of the receiver.

2.3.2.4 Fidelity The fidelity of a receiver is its ability to accurately reproduce, in its output, the signal that appears at its input. You will usually find the broader the band passed by frequency selection circuits, the greater your fidelity. You may measure fidelity by modulating an input frequency with a series of audio frequencies; you then plot the output measurements at each step against the audio input frequencies. The resulting curve will show the limits of reproduction.

You should remember that good selectivity requires that a receiver pass a narrow frequency band. Good fidelity requires that the receiver pass a broader band to amplify the outermost frequencies of the sidebands. Receivers you find in general use are a compromise between good selectivity and high fidelity.

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2-16 UNCLASSIFIED 2.3.3 Superheterodyne Receiver The superheterodyne is the type receiver most familiar to you. You probably see one daily in your home in the form of an AM and/or fm radio. We will discuss the basic workings of both AM and fm types and their differences.

2.3.3.1 Amplitude Modulation Receiver Figure 2-9 shows a block diagram with waveforms of a typical AM superheterodyne receiver developed to overcome the disadvantages of earlier type receivers. Let’s assume you are tuning the receiver. When doing this you are actually changing the frequency to which the rf amplifier is tuned. The rf carrier comes in from the antenna and is applied to the rf amplifier. The output of the amplifier is an amplified carrier and is sent to the mixer. The mixer also receives an input from the local oscillator. These two signals are beat together to obtain the IF through the process of heterodyning. (Heterodyning will be further discussed later in this chapter and was covered in NEETS, Module 12, Modulation Principles.) At this time you should note the dotted lines connecting the local oscillator, rf amplifier, and the mixer. This is used on block diagrams and schematics to indicate GANGED TUNING. Ganged tuning is the process used to tune two or more circuits with a single control. In our example, when you change the frequency of the receiver all three stages change by the same amount. There is a fixed difference in frequency between the local oscillator and the rf amplifier at all times. This difference in frequency is the IF. This fixed difference and ganged tuning ensures a constant IF over the frequency range of the receiver.

Figure 2-9 AM superheterodyne receiver and waveforms

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2-17 UNCLASSIFIED The IF carrier is applied to the IF amplifier. The amplified IF carrier is then sent to the detector. The output of the detector is the audio component of the input signal. This audio component is then passed through an audio frequency amplifier. The amplified audio component is sent to a speaker for reproduction. This allows you to hear the signal.

You should note that a superheterodyne receiver may have more than one frequency- converting stage and as many amplifiers as needed to obtain the desired power output. (Additional amplifiers are not shown.)

HETERODYNING.— As you know the intermediate frequency is developed by a process called heterodyning. This action takes place in the mixer stage (sometimes called a converter or first detector). Heterodyning is the combining of the incoming signal with the local oscillator signal. When heterodyning the incoming signal and the local oscillator signal in the mixer stage, four frequencies are produced. They are the two basic input frequencies and the sum and the difference of those two frequencies. The amplifier that follows (IF amplifier) will be tuned to the difference frequency. This difference frequency is known as the intermediate frequency (IF). A typical value of IF for an AM communications receiver is 455 kilohertz. The difference frequency is a lower frequency than either the rf input or oscillator frequencies. This lower frequency gives slightly better gain but does increase the chances of image frequency interference. Image frequencies will be discussed later in this chapter.

DETECTION.—Once the IF stages have amplified the intermediate frequency to a sufficient level, it is fed to the detector. When the mixer is referred to as the first detector, this stage would be called the second detector. The detector extracts the modulating audio signal. The detector stage consists of a rectifying device and filter, which respond only to the amplitude variations of the IF signal. This develops an output voltage varying at an audio-frequency rate. The output from the detector is further amplified in the audio amplifier and is used to drive a speaker or earphones.

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2-18 UNCLASSIFIED 2.3.3.2 Frequency Modulated Receiver The function of a frequency-modulated receiver is the same as that of an AM superheterodyne receiver. You will find some important differences in component construction and circuit design caused by differences in the modulating technique. Figure 2-10 is a block diagram showing waveforms of a typical fm superheterodyne receiver. Comparison of block diagrams in figures 2-9 and 2-10 shows that in both AM and fm receivers, the amplitude of the incoming signal is increased in the rf stages. The mixer combines the incoming rf with the local oscillator signal to produce the intermediate frequency, which is then amplified by one or more IF amplifier stages. You should note that the fm receiver has a wide-band IF amplifier. The bandwidth for any type of modulation must be wide enough to receive and pass all the side-frequency components of the modulated signal without distortion. The IF amplifier in an fm receiver must have a broader bandpass than an AM receiver.

Sidebands created by fm differ from the AM system. You should recall that the AM system consists of a single set of side frequencies for each radio-frequency signal modulated. An fm signal inherently occupies a wider bandwidth than AM because the number of extra sidebands that occur in an fm transmission is directly related to the amplitude and frequency of the audio signal.

Figure 2-10 Block diagram of an fm receiver and waveforms

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2-19 UNCLASSIFIED You should observe that only two fundamental sections of the fm receiver are electrically different from the AM receiver. These are the discriminator (detector) and the limiter.

Beyond the IF stage, the two receivers have a marked difference. AM demodulation involves the detection of variations in the amplitude of the signal; fm demodulation is the process of detecting variations in the frequency of the signal. In fm receivers a DISCRIMINATOR is a circuit designed to respond to frequency shift variations. A discriminator is preceded by a LIMITER circuit, which limits all signals to the same amplitude level to minimize noise interference. The audio frequency component is then extracted by the discriminator, amplified in the af amplifier, and used to drive the speaker.

ADVANTAGES.— In normal reception, fm signals are almost totally absent of static while AM signals are subject to cracking noises and whistles. Fm followed AM in development and has the advantage of operating at a higher frequency where a greater amount of frequencies are available. Fm signals provide much more realistic sound reproduction because of an increase in the number of sidebands. This increase in the number of sidebands allows more of the original audio signal to be transmitted and, therefore, a greater range of frequencies for you to hear.

As you can see, fm requires a wide bandpass to transmit signals. Each transmitting station must be assigned a wide band in the fm frequency spectrum. During fm transmissions, the number of significant sidebands that must be transmitted to obtain the desired fidelity is related to the deviation (change in carrier’ frequency) divided by the highest audio frequency to be used. At this point you may want to review chapter 2 of NEETS, Module 12, Modulation Principles. For example, if the deviation is 40 kilohertz and the highest audio frequency is 10 kilohertz, the modulation index is figured as shown below:

In this example, a modulation index of 4 equates to 14 significant sidebands. Because the audio frequency is 10 kilohertz and there are 14 side-bands, the bandwidth must accommodate a 140-kilohertz signal. You can see this is considerably wider than the 10- to-15-kilohertz bandpass used in AM transmitting.

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2-20 UNCLASSIFIED FREQUENCY CONVERSION.—Frequency conversion is accomplished by using the heterodyne principle of beating two frequencies together to get an intermediate frequency. So far, you have only become familiar with single conversion; however, some receivers use double or triple conversion. These methods are sometimes referred to as double or triple heterodyning. Receivers using double or triple conversion are very selective and suppress IMAGE SIGNALS to yield sharp signal discrimination. (Image signals are undesired, modulated carrier signals that differ by twice the intermediate frequency from the frequency to which the superheterodyne receiver is tuned.) Double and triple conversion receivers also have better adjacent channel selectivity than can be realized in single conversion sets.

In military communications receivers you may sacrifice fidelity to improve selectivity. This is permitted because intelligence (voice, teletypewriter) can be carried on a fairly narrow band of frequencies. Entertainment receivers, on the other hand, must reproduce a wider band of frequencies to achieve their high-fidelity objective.

Q13. What frequency conversion principle is used to develop the IF? Q14. What is the function of the detector? Q15. What is the major disadvantage of an fm signal as compared to an AM signal? 2.3.4 Single-Sideband You know from studying the single-sideband transmitter material in this chapter you may transmit only one sideband of an AM signal and retain the information transmitted. Now you will see how a single-sideband signal is received.

2.3.4.1 Advantages Figure 2-11 illustrates the transmitted signal for both AM and ssb. Ssb communications has several advantages. When you eliminate the carrier and one sideband, all of the transmitted power is concentrated in the other sideband. Also, an ssb signal occupies a smaller portion of the frequency spectrum in comparison to the AM signal. This gives us two advantages, narrower receiver bandpass and the ability to place more signals in a small portion of the frequency spectrum.

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Ssb communications systems have some drawbacks. The process of producing an ssb signal is somewhat more complicated than simple amplitude modulation, and frequency stability is much more critical in ssb communication. While we don’t have the annoyance of heterodyning from adjacent signals, a weak ssb signal is sometimes completely masked or hidden from the receiving station by a stronger signal. Also, a carrier of proper frequency and amplitude must be reinserted at the receiver because of the direct relationship between the carrier and sidebands.

Figure 2-11 Comparison of AM and ssb transmitted signals

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2-22 UNCLASSIFIED Figure 2-12 is a block diagram of a basic ssb receiver. It is not significantly different from a conventional superheterodyne AM receiver. However, a special type of detector and a carrier reinsertion oscillator must be used. The carrier reinsertion oscillator must furnish a carrier to the detector circuit. The carrier must be at a frequency which corresponds almost exactly to the position of the carrier used in producing the original signal.

Rf amplifier sections of ssb receivers serve several purposes. Ssb signals may exist in a small portion of the frequency spectrum; therefore, filters are used to supply the selectivity necessary to adequately receive only one of them. These filters help you to reject noise and other interference.

Ssb receiver oscillators must be extremely stable. In some types of ssb data transmission, a frequency stability of ±2 hertz is required. For simple voice communications, a deviation of ±50 hertz may be tolerable.

These receivers often employ additional circuits that enhance frequency stability, improve image rejection, and provide automatic gain control (agc). However, the circuits contained in this block diagram are in all single-sideband receivers.

Figure 2-12 Basic ssb receiver

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2-23 UNCLASSIFIED 2.3.4.2 Carrier Reinsertion The need for frequency stability in ssb operations is extremely critical. Even a small deviation from the correct value in local oscillator frequency will cause the IF produced by the mixer to be displaced from its correct value. In AM reception this is not too damaging, since the carrier and sidebands are all present and will all be displaced an equal amount. Therefore, the relative positions of carrier and sidebands will be retained. However, in ssb reception there is no carrier, and only one sideband is present in the incoming signal.

The carrier reinsertion oscillator frequency is set to the IF frequency that would have resulted had the carrier been present. For example, assume that a transmitter with a suppressed carrier frequency of 3 megahertz is radiating an upper sideband signal. Also assume that the intelligence consists of a 1-kilohertz tone. The transmitted sideband frequency will be 3,001 kilohertz. If the receiver has a 500-kilohertz IF, the correct local oscillator frequency is 3,500 kilohertz. The output of the mixer to the IF stages will be the difference frequency, 499 kilohertz. Therefore, the carrier reinsertion oscillator frequency will be 500 kilohertz, which will maintain the frequency relationship of the carrier to the sideband at 1 kilohertz.

Recall that 1 kilohertz is the modulating signal. If the local oscillator frequency should drift to 3,500.5 kilohertz, the IF output of the mixer will become 499.5 kilohertz. The carrier reinsertion oscillator, however, will still be operating at 500 kilohertz. This will result in an incorrect audio output of 500 hertz rather than the correct original 1-kilohertz tone. Suppose the intelligence transmitted was a complex signal, such as speech. You would then find the signal unintelligible because of the displacement of the side frequencies caused by the local oscillator deviation. The local oscillator and carrier reinsertion oscillator must be extremely stable.

Q16. What two components give a ssb receiver its advantages over an AM superheterodyne receiver?

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2-24 UNCLASSIFIED 2.3.5 Receiver Control Circuits This section deals with circuits that control receiver functions. We will explain how some of the basic manual and automatic receiver control functions work.

2.3.5.1 Manual Gain Control (mgc) You learned previously that high sensitivity is one of the desirable characteristics of a good receiver. In some cases high sensitivity may be undesirable. For example, let’s suppose the signals received from a nearby station are strong enough to overload the rf sections of your receiver. This may cause the audio output to become distorted to the point of complete loss of intelligibility. To overcome this problem, you can use manual gain control of the rf section. By using the manual gain control, you can adjust the receiver for maximum sensitivity and amplify weak input signals. When you receive a strong input signal, the rf gain may be reduced to prevent overloading. A typical manual gain control circuit for a receiver is illustrated in figure 2-13. Let’s go through the basic circuitry.

Figure 2-13 Typical rf gain control

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2-25 UNCLASSIFIED C1 is an emitter bypass capacitor. Resistors R1 and R2 develop emitter bias for the amplifier. C2 provides dc isolation between the tank and the base of transistor Q1. You should recall from your studies of NEETS, Module 7, Solid-State Devices and Power Supplies, and Module 8, Amplifiers, that amplifier gain may be varied by changing bias. Potentiometer R2, the rf gain control, is nothing more than a manual bias adjustment. When the wiper arm of R2 is set at point B, minimum forward bias is applied to the transistor. This causes the amplifier to operate closer to cutoff and reduces gain. When you move the control toward point A, the opposite occurs. R1 limits the maximum conduction of Q1 when R2 is short circuited. You may run into an alternate biasing method when the transistor is operated near saturation. In that case, a large change in gain would again be a function of bias.

2.3.5.2 Manual Volume Control (mvc) Figure 2-14 shows the circuitry for a common method of controlling volume in a superheterodyne receiver. C1 and R1 form an input signal coupling circuit and are also the means of controlling the level applied to the audio amplifier. R1, R2, and R3 develop forward bias and set the operating point for the transistor amplifier. R4 is the collector load resistor for Q1, and C3 is the output coupling capacitor. Potentiometer R1 in the circuit shown causes the input impedance of the stage to remain fairly constant. The signal from the preceding stage is felt across R1. By adjusting R1, you can change the input level to Q1 and vary the output amplitude.

Figure 2-14 Typical manual gain/volume control

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2-26 UNCLASSIFIED 2.3.5.3 Automatic Gain/Volume Control (agc/avc) Output volume variations of a receiver often result from variations in the input signal strength. Changes in input signal strength occur when we change stations or when we experience fading because of changing atmospheric conditions. The function of an AUTOMATIC GAIN CONTROL, also referred to as an AUTOMATIC VOLUME CONTROL, is to limit unwanted variations in the output of the receiver caused by variations in strength of the received signal input. A receiver without agc would require continuous manual readjustment to compensate for received signal changes so that it could maintain a constant output level.

Signals from stations operating at the same power level may not reach the receiver antenna with the same power. This is because of differences in transmission distances, carrier frequencies, atmospheric conditions, and obstructions between the transmitter and receiver antennas.

You might draw the conclusion that an agc network is not necessary when the receiver is operating on a single station. However, this is not true; atmospheric conditions may cause the signal strength to vary (fade in and out), or the antenna may receive components of the signal which have traveled along different paths. For example, one component may travel directly from the antenna, and another may have been reflected from a distant object. The two signals will sometimes be in phase and at other times be out of phase, thus tending to reinforce or cancel each other. The result is a variation in signal strength at the receiver antenna. This variation in signal strength is often referred to as FADING. The effect of fading in the output signal voltage of an rf stage is best demonstrated by the following example: An rf amplifier connected to a receiving antenna has a voltage gain of 100. If the antenna receives an input signal of 10 microvolts, the output voltage is computed as follows:

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2-27 UNCLASSIFIED With the output voltage equal to 1 millivolt, and if fading is to be avoided, the output voltage must remain at 1 millivolt. However, if a reflected signal is received that is approximately one-half the strength (5 microvolts) of the original and is in phase with the original signal, the total input signal to the receiving antenna will increase to 15 microvolts. To maintain the desired 1 millivolt of output signal, you must somehow reduce the gain of the rf amplifier. With an input of 15 microvolts and a desired output of 1 millivolt (1,000 microvolts), the gain of the amplifier must be reduced to:

When the 10-microvolt original signal and the 5-microvolt reflected signal are out of phase with each other, the signal strength at the receiving antenna will decrease to 5 microvolts. If we want to maintain our original 1,000-microvolt output signal, the voltage gain of the amplifier must be increased as follows:

A variation of amplifier gain, similar to the example, is necessary if we are going to compensate for input signal strength variations. The required amplifier gain variations can be accomplished automatically by the addition of an agc circuit within the receiver. Let’s take a look at the methods and circuits used to produce agc and the manner in which agc (avc) controls receiver gain.

CIRCUITRY.— Figure 2-15 is a block diagram representing agc feedback to preceding stages. The detector circuit has a dc component in the output that is directly proportional to the average amplitude of the modulated carrier. The agc circuitry uses this dc component by filtering the detector output to remove the audio and IF components and by applying a portion of the dc component to the preceding stages. This agc voltage controls the amplification of any or all of the stages preceding the detector stage. Solid-state receivers may use either positive or negative voltage for agc. The type of transistors used and the elements to which the control voltage is applied determine which type we will have.

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The circuit shown in figure 2-16 produces a positive agc voltage. Transformer T1, diode CR1, capacitor C1, and resistor R1 comprise a series diode detector. The agc network is made up of R2 and C2. With normal detector operation and the positive (+) potential shown at the input, CR1 conducts. Conduction of the diode will cause a charging current (shown by the dashed line) to flow through agc capacitor C2 and agc resistor R2. This charging current develops a voltage across C2. When the potential across T1 reverses, the diode will be reverse biased and will not conduct. When this happens, the charging current ceases and C2 begins to discharge. The discharge path for C2 is shown by the solid arrows. The discharge path time constant of C2, R1, and R2 is chosen to be longer than the period (1/f) of the lowest audio frequency present in the output of the detector. Because of the longer time constant, C2 will not discharge much between peaks of the modulating signal, and the voltage across C2 will be essentially a dc voltage. This voltage is proportional to the average signal amplitude. Now, if the signal strength varies, C2 will either increase or decrease its charge, depending on whether the signal increases or decreases. Since the charge on the agc capacitor responds only to changes in the average signal level, instantaneous variations in the signal will not affect the agc voltage.

Figure 2-15 Block diagram showing agc application

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You should remember that, depending on transistor types, the receiver may require either a positive or a negative agc voltage. A negative agc voltage could be easily obtained by reversing CR1. Once the values for R2 and C2 have been selected, the voltage divider action of the components is fixed, and the circuit operates automatically without further adjustment. If the average amplitude of the signal increases, the charge on C2 will also increase. If the signal amplitude decreases so does the charge on C2.

The agc voltages in a receiver provide controlled degenerative feedback. By adjusting the operating point of an amplifier, you can control the gain. Under no-signal conditions, bias of the rf and IF amplifiers is developed by standard means, such as self bias. With an applied signal, an agc voltage is developed, which in conjunction with normal biasing methods develops the operating bias for the amplifiers.

Figure 2-16 Series diode detector and simple agc circuit

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2-30 UNCLASSIFIED TRANSISTOR AMPLIFIER GAIN.—You have seen how a dc voltage that is obtained at the output of the agc network is proportional to, and will reflect, the average variations of the average signal level. Now all we have to do is use this agc voltage to control the amplification of one or more of the preceding amplifiers. Figure 2-17 illustrates a common-emitter amplifier with agc applied to the base element. A change in the agc voltage will change the operating point of the transistor and the dc emitter current. In this circuit, R1 and R4 form a voltage divider and establish no-signal (forward) bias on the base. Since a pnp transistor is used, the base has a negative potential. The agc voltage from the detector is positive with respect to ground and is fed to the base through dropping resistor R2. You will find when the dc output of the detector increases (because of an increase in the average signal level) the agc voltage will become more positive. This increased positive potential is applied to the base of Q1, which decreases the forward bias of Q1 and decreases the gain of the amplifier. Agc, in this application, works with controlled degenerative feedback. Use of an npn transistor, in the same configuration, would require the agc voltage to possess a negative potential.

FORWARD AND REVERSE AGC.—When we use an agc voltage to cause degeneration by driving the amplifiers toward cutoff, it is referred to as REVERSE agc. Figure 2-18 shows the type of agc circuitry normally used with this method. A second method that uses agc is an application called FORWARD agc. In the case of forward agc, you'll find the amplifier is driven toward the saturation region of its characteristic curve. (Sometimes referred to as an energy diagram.)

Figure 2-17 Common emitter amplifier with agc

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Let’s look at our example. Assume the agc voltage is negative. Under this condition, Q1 is self biased; under no-signal conditions, it is operating well up on its characteristic curve. When a signal is applied, negative agc voltage is developed in the detector circuit and fed back to Q1, which increases forward bias. Any increase in signal level causes an increase in agc voltage. An increase in agc voltage increases conduction, which in turn drives the transistor to or near saturation. As the transistor approaches saturation, its gain is correspondingly reduced.

On the other hand, if the input signal level decreases, the negative agc voltage decreases. The forward bias is then reduced, and the transistor operates on a lower portion of its characteristic curve where gain is higher.

Forward agc provides you with better signal-handling capabilities; however, reverse agc is simpler to use, causes less loading of the tuned circuits, and produces smaller variations in input and output capacitance.

Figure 2-18 Reverse agc

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2-32 UNCLASSIFIED Q17. What does manual gain control do to strong and weak signals, respectively? Q18. What is the purpose of agc/avc in a receiver? 2.3.5.4 Delayed Automatic Gain Control The disadvantage of automatic gain control, attenuating even the weak signal, is overcome by the use of delayed automatic gain control (dagc). Let’s take a look at the typical dagc circuitry in figure 2-19. This type of system develops no agc feedback until an established received signal strength is attained. For signals weaker than this value, no agc is developed. For sufficiently strong signals, the delayed agc circuit operates essentially the same as ordinary agc.

Figure 2-19 Delayed agc action

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2-33 UNCLASSIFIED Our circuit uses two separate diodes; one is the detector diode and the other the agc diode. The agc diode is connected to the primary of the last IF transformer and the detector diode to its secondary. A positive bias is applied to the cathode of the agc diode. This keeps it from conducting until a prearranged signal level has been reached. The adjust delay control allows manual control of the agc diode bias. Manual control allows you to select the signal level at which agc is applied. If mostly weak stations are to be received, the setting should be high (no agc until the signal level is high). However, you should set it as low as possible to prevent overloading of the last IF amplifier by stronger signals.

Finally, you must have two diodes to obtain delayed agc. If only one diode were used, the agc would be developed from the detector diode, and there would be no delayed action. Or, if a signal diode were biased to provide the delaying action desired, no signal would pass to the audio amplifier until the bias was exceeded by the input signal.

2.3.5.5 Beat-Frequency Oscillator The beat-frequency oscillator (bfo) is necessary when you want to receive cw signals. Cw signals are not modulated with an audio component, you remember, so we must provide one. The action of the rf amplifier, mixer, local oscillator, and IF amplifier is the same for both cw and AM; but the cw signal reaches the detector as a single frequency signal with no sideband components. To produce an af output, you must heterodyne (beat) any cw signal with an rf signal of the proper frequency. This separate signal is obtained from an oscillator known as a beat-frequency oscillator.

Figure 2-20 is a block diagram of a superheterodyne receiver capable of receiving and demodulating a cw signal. The bfo heterodynes at the detector and produces an af output. The detector (second detector) is used primarily because the mixer (first detector) is normally used as the source of agc.

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If the intermediate frequency is 455 kilohertz and the bfo is tuned to 456 kilohertz or 454 kilohertz, the difference frequency of 1 kilohertz is heard in the output. Generally, you will tune the bfo from the front panel of a receiver. When you vary the bfo control, you are varying the output frequency of the bfo and will hear changes in the tone of the output audio signal.

2.3.5.6 Squelch The sensitivity of a receiver is maximum when no signal is being received. This condition occurs, for example, when a receiver is being tuned between stations. At this time background noise is picked up by the antenna, and you will hear noise greatly amplified. This noise is highly annoying and occurs because receiver gain is maximum without a signal. You can often overcome this problem by using a circuit called a SQUELCH, NOISE SILENCER, NOISE SUPPRESSOR, or NOISE LIMITER. All of these noise type circuits just clip the peaks of the noise spikes. Squelch will actually eliminate noise. Figure 2-21 is a typical circuit of this type. The circuit cuts off receiver output when no input signal is being received. It accomplishes this by blocking either the detector or audio amplifier when no signal is present. Let’s take a look at the theory involved in this process.

Figure 2-20 Placement of the beat frequency oscillator

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The squelch diode CR1 connects the output of the first af stage to the input of the second. Amplifier Q1 serves as the control transistor for the circuit. The anode and cathode voltages of CR1 are normally biased positive with respect to ground.

With no input signal, R9 is adjusted until Q1 draws enough collector current to reduce its collector voltage and the anode voltage of CR1 to a value below the voltage on the cathode of CR1. At this point the anode voltage of the squelch diode is negative with respect to its cathode, and conduction ceases. Audio output is now reduced to zero and the receiver is silent.

The base of Q1 is connected to the automatic volume control (avc) line. Anytime a signal enters the receiver, a negative avc voltage is applied to the base of Q1. This reduces the collector current and increases the collector voltage, which in turn increases the anode voltage of CR1 until the anode becomes positive with respect to the cathode. Once again diode CR1 will conduct, and the signal will be passed to the second af amplifier. Diode CR1 is effectively a switch controlled by the avc voltage.

Q19. What is a disadvantage of agc? Q20. What is the main difference between agc and dagc?

Figure 2-21 Squelch circuit

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2-36 UNCLASSIFIED Q21. What is the function of the bfo? Q22. What is the purpose of a squelch circuit? 2.3.5.7 Audio Tone The tone of the sound reproduced in the audio section of a receiver depends on several factors. The frequency response of the audio amplifiers determines the degree of amplification provided to different frequencies in the sound spectrum. The size and quality of any loudspeaker used will determine its response to various frequencies. Response of the human ear is the final judge of tonal quality, and that varies with the individual.

Because of these variables, some form of tone control is sometimes used in Navy receivers. Treble tones are defined as the audio frequencies above approximately 3,000 hertz and bass tones are those below approximately 300 hertz. Although several methods of tone control can be used, we are only going to mention the attenuation method. With this method, a decrease in the intensity of one tone can produce an apparent increase in the intensity of another tone. As an example, let’s look at tones of 400 and 4,000 hertz produced by a speaker with the same intensity. If we reduce the intensity of the 4,000- hertz tone, the 400-hertz tone will appear to be louder, even though its intensity has not actually changed. You should see from this example that bass emphasis can be accomplished by attenuating treble tones.

The simplest type of tone control is illustrated in figure 2-22. Fixed capacitor C1 parallels the primary winding of the output transformer, effectively shunting the higher frequencies to ground. The size of C1 determines the lowest frequency to be affected. When you select the BASS position of the tone control, C1 is connected and improves bass response by de-emphasizing the treble tones. You can often use this circuit to improve the output of a small speaker with poor treble response.

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A continuously variable tone control is illustrated in figure 2-23. Tone control R1 and bypass capacitor C1 act as a variable RC filter. With the wiper arm of R1 in the upper (BASS) position, C1 bypasses the higher frequencies to ground and provides better bass response. When the wiper arm of R1 is in the lower (TREBLE) position, the resistance of R1 is placed in series with C1, which reduces the shunting effect of C1 to high frequencies and improves the treble response. This method gives you the advantage of smooth, continuous tone control at all points between maximum bass and maximum treble response.

Figure 2-22 Fixed capacitor tone control Figure 2-23 Variable tone control using an RC filter

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2-38 UNCLASSIFIED A switch-type, variable tone control is illustrated in figure 2-24. With this method we use a three position switch to provide fixed degrees of tone control. When tone control S1 is in the BASS position, capacitor C1 bypasses the high frequencies and provides bass emphasis. With S1 in the normal (NORM) position, C2 acts as the bypass, and a moderate amount of high-frequency attenuation is accomplished. This position provides balanced bass and treble response. When S1 is in the TREBLE position, C3 acts as the bypass and provides minimum high-frequency attenuation and maximum treble emphasis. As a rule of thumb, you can figure the capacitance of C2 is approximately five times the value of C3, and C1 is approximately ten times the value of C3. For example with C3 at .001 microfarads, C2 would be .005 microfarads, and C1 would be .01 microfarads.

2.3.5.8 Crystal Filters A quartz crystal, used as a selective filter in the IF section of a communications receiver, is one of the most effective methods of achieving maximum selectivity. It is especially useful when the channel is crowded and considerable noise (both external and internal) is present.

One possible circuit arrangement is shown in figure 2-25. Let’s look at the theory involved in understanding this circuit. You can see a crystal in one leg of the bridge circuit. The secondary of the input transformer (T1) is balanced to ground through the center tap connection. The crystal acts as a high Q series resonant circuit. It allows signals within the immediate vicinity of resonance to pass through the crystal to the output coil (L3). The desired signal appears between the center tap of L3 and ground.

Figure 2-24 Switch tone control

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The capacitance that exists between the crystal holder plates (C3) may bypass unwanted signals around the crystal; therefore, some method must be provided to balance out this capacitance. You accomplish circuit balancing by applying out-of-phase voltages to neutralize any undesired signal voltages. The tap on L3 permits proper impedance matching and is the balanced input to the next stage.

Q23. What does a tone control circuit in a receiver do to the audio signal? Q24. What is the function of a crystal filter in a receiver? 2.3.5.9 Automatic Frequency Control Afc circuits are used in situations where you must accurately control the frequency of an oscillator by some external signal. Basically, this type circuit does two things: It senses the difference between the actual oscillator frequency and the frequency that is desired and produces a control voltage proportional to the difference; it also uses the control voltage to change the oscillator to the desired frequency. Afc circuits are used to control the frequency of sinusoidal oscillators and nonsinusoidal oscillators. Only sinusoidal afc circuits will be covered here. Afc circuits are used in radio receivers, fm transmitters, and frequency synthesizers to maintain frequency stability. Figure 2-26 is a block diagram illustrating afc operation in a receiver. Let’s run through the applicable parts of this block diagram.

Figure 2-25 Crystal filter used in the IF section of a superheterodyne receiver

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The frequency discriminator controls the varicap in this receiver. A varicap is used to keep the IF stable. You may want to review varicap theory in chapter 3 of NEETS, Module 7, Solid-State Devices and Power Supplies at this point. The varicap application here produces an apparent reactance, which is included in the oscillator frequency control circuitry. For example, let’s assume the IF is 455 kilohertz and the local oscillator (lo) is tracking below the incoming station. When the lo output decreases slightly in frequency, the IF will rise. This causes the output of the discriminator to increase the capacitive reactance of the varicap, which increases the oscillator frequency to the desired value. Now let’s assume the lo output increases. The IF will then decrease. This causes the discriminator output to decrease the capacitive reactance of the varicap. This will cause the oscillator frequency to decrease.

Figure 2-27 shows another widely used type of afc and its circuitry. This type is commonly referred to as a BALANCED-PHASE DETECTOR or PHASE- DISCRIMINATOR. This circuit uses fixed capacitors and the varying conductance of the diodes to achieve a variable reactance. As you have seen in the block diagram, an afc circuit requires two sections, a frequency detector and a variable reactance. Our detector output is a dc control voltage proportional to the amount of frequency change. This dc voltage is applied directly to the oscillator. The phase inverter input signals are discriminated IF outputs fed to the two diodes 180 degrees out of phase.

Figure 2-26 Block diagram of receiver showing automatic frequency control

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A reference voltage is also applied to both diodes. The diodes are biased to conduct only during the peak portions of the input signals. Any change in oscillator frequency will alter the phase relationship between the sawtooth reference voltage and the incoming signals. If this happens, one diode will conduct more than the other and produce a control signal. This system remains unbalanced at all times because any change in frequency is instantaneously corrected. The network between the diodes and oscillator is essentially a low-pass filter. This filter prevents discriminator pulses from reaching the oscillator.

2.4 FREQUENCY SYNTHESIS In present day communications systems, long term accuracy of one part in a million is required from many of the frequency generators (local oscillators) used in communications equipment. Variable frequency oscillators cannot practically achieve this high degree of stability. Therefore, a system known as FREQUENCY SYNTHESIS has been developed to meet the stringent demands for stability. This system uses circuitry that produces a signal frequency through a heterodyning and frequency selection process. This signal is not harmonically related to any of the signals used in the heterodyning process. It is also not related to the selected crystal frequency. This makes the signal unique.

Figure 2-27 Automatic frequency control (phase discriminator)

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2-42 UNCLASSIFIED Figure 2-28 is a multiple crystal, frequency synthesizer that produces desired output frequencies by mixing frequencies from several crystal oscillators. Each oscillator uses ten or more crystals to control its operating frequency. This provides for a large number of output frequency combinations. Figure 2-29 is a practical frequency synthesizer in which the harmonics and subharmonics of a single standard oscillator are combined to provide a wide multichoice of output signals. Each of these signals is harmonically related to a subharmonic of the standard oscillator. You will find the primary difficulty encountered in the frequency synthesizers is the presence of spurious signals generated in the "combining mixers." Extensive filtering and extremely careful selection of operating frequencies are required for even the simplest circuits. Spurious frequency problems increase and channel spacing decreases as the range of operating frequencies increases.

Figure 2-28 Multiple crystal frequency synthesizer Figure 2-29 Simple crystal frequency synthesizer

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2-43 UNCLASSIFIED Q25. What is the primary function of an afc circuit? Q26. What is frequency synthesis? 2.5 AUDIO REPRODUCTION DEVICES The purpose of audio reproduction devices, such as loudspeakers and headphones, is to convert electrical audio signals to sound power. Figure 2-30 shows you a diagram of a loudspeaker called the PERMANENT MAGNET SPEAKER. This speaker consists of a permanent magnet mounted on soft iron pole pieces, a voice coil that acts as an electromagnet, and a loudspeaker cone connected to the voice coil. The audio signal has been previously amplified (in terms of both voltage and power) and is applied to the voice coil. The voice coil is mounted on the center portion of the soft iron pole pieces in an air gap so that it is mechanically free to move. It is also connected to the loudspeaker cone; as it moves, the cone will also move. When audio currents flow through the voice coil, the coil moves back and forth proportionally to the applied ac current. As the cone (diaphragm) is attached to the voice coil, it also moves in accordance with the signal currents; in so doing, it periodically compresses and rarefies the air, which produces sound waves.

Figure 2-30 Permanent magnet speaker

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2-44 UNCLASSIFIED Most speakers of the above type receive their input by means of transformer coupling. This is necessary because of the normal, low impedance of the voice coil. You will find the standard impedance values for this type speaker are 4, 8, 16, and 32 ohms. Other impedance values may be obtained, but those listed are the most common.

While permanent magnet speakers perform reasonably well in the audio range, they have limitations. Most Navy speakers reproduce low audio frequencies quite well, mid-band frequencies fairly well, and high frequencies quite poorly. Let’s see why. When the speaker is constructed, only a limited number of turns may be built into the voice coil. This gives us a fixed inductance. At low frequencies, the inductive reactance of the voice coil is relatively low, and large audio currents flow. This provides a strong magnetic field around the voice coil and a strong interaction with the field of the permanent magnet. Low frequency response is excellent. At midband frequencies, inductive reactance increases and less current flows in the voice coil. This produces less magnetic field and less interaction. Midband response is still acceptable in a properly designed speaker. At high audio frequencies inductive reactance is quite high, and little current flows in the voice coil. This results in a greatly reduced voice coil field and little interaction with the permanent magnetic field. Also at high frequencies the interwinding capacitance of the voice coil tends to shunt some of the high audio frequencies, which further reduces the high frequency response.

Frequency response of most permanent magnet speakers falls off at the higher audio frequencies. This problem is normally overcome either by the use of an expensive, specially designed speaker, or through the use of two speakers, one of which is designed to operate well at the higher audio frequencies (tweeter) and one at the lower frequencies (woofer).

As shown in figure 2-31, an electromagnet may be used in place of a permanent magnet to form an electromagnetic dynamic speaker. When we do this, sufficient dc power must be available to energize the field electromagnet. The operation otherwise is much the same as that of the permanent magnet type. This type of speaker is seldom used in Navy equipment.

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Figure 2-32 shows a diagram of typical headphones used with Navy equipment. The device consists of a permanent magnet and two small electromagnets through which the signal currents pass. A soft iron diaphragm is used to convert the electrical effects of the device into sound power. When no signal currents are present, the permanent magnet exerts a steady pull on the soft iron diaphragm. Signal current flowing through the coils mounted on the soft iron pole pieces develops a magnetomotive force that either adds to or subtracts from the field of the permanent magnet. The diaphragm thus moves in or out according to the resultant field. Sound waves are then reproduced that have an amplitude and frequency (within the mechanical capability of the reproducer) similar to the amplitude and frequency of the signal currents.

Figure 2-31 Electromagnetic speaker

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As compared to permanent magnet speakers, standard headphones are considered to be high impedance devices. Headphone electromagnets are normally wound with many turns of small wire, which provide the larger impedance. Because of the physically small size and inflexibility of the metal diaphragm, the headphones often give poor response to the lower audio frequencies. In the voice range of audio, most standard issue headphones are adequate.

2.6 SUMMARY In this chapter you learned transmitter and receiver fundamentals. We also discussed modes of operation and special controls circuits. Let’s review some of these areas.

A HARMONIC is an exact multiple of the fundamental frequency. Even harmonics are 2, 4, and so on, times the fundamental. Odd are 3, 5, and so on, times the fundamental frequency.

A SUBHARMONIC is an exact submultiple of the fundamental frequency. Even subharmonics are one-half, one-quarter, and so on. Odd subharmonics are one-third, one- fifth, and so on, of the fundamental frequency.

Figure 2-32 Headphone

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2-47 UNCLASSIFIED SUPPRESSION is the process of eliminating an undesired portion of a signal.

MULTIPLEXING is a method for simultaneous transmission of two or more signals over a common carrier wave.

An ORDER-WIRE CIRCUIT is a circuit between operators used for operations control and coordination.

RECEPTION is when an electromagnetic wave passes through a receiver antenna and induces a voltage in that antenna.

DETECTION is the separation of low-frequency (audio) intelligence from the high (radio) frequency carrier.

REPRODUCTION is the process of converting electrical signals to sound waves. This sound is speech, music, and so on.

SENSITIVITY of a receiver is the ability to reproduce weak signals. The greater the receiver sensitivity, the weaker the signal that will be reproduced.

Receiver SELECTIVITY is the ability to select the desired signal and reject unwanted signals.

NOISE SILENCER, NOISE SUPPRESSOR, or NOISE LIMITER, are circuits that clip the peaks of the noise spikes in a receiver.

SQUELCH is a circuit that cuts off the output of a receiver when there is no input.

A BALANCED-PHASE DETECTOR or PHASE-DISCRIMINATOR is a circuit that controls the oscillator frequency (afc).

FREQUENCY SYNTHESIS is a signal-producing process through heterodyning and frequency selection.

A PERMANENT MAGNET SPEAKER is one with a permanent magnet mounted on soft iron pole pieces.

The FIDELITY of a receiver is the ability to accurately reproduce at its output the signal at its input.

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2-48 UNCLASSIFIED GANGED TUNING is the process used to tune two or more circuits with a single control.

HETERODYNING is the mixing of the incoming signal with the local oscillator frequency. This produces the two fundamentals and the sum and difference frequencies.

An IMAGE FREQUENCY is an undesired frequency capable of producing the desired frequency through heterodyning.

AUTOMATIC VOLUME/GAIN CONTROL is a circuit used to limit variations in the output signal strength of a receiver.

FADING is the variations in signal strength at the antenna of a receiver.

REVERSE AGC is when an amplifier is driven toward cutoff.

FORWARD AGC is when an amplifier is driven toward saturation.

A BEAT-FREQUENCY OSCILLATOR is an additional oscillator used in a receiver when the receiver is receiving a cw signal and provides an audible tone.

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2-49 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q26. A1. Am, fm, cw, ssb.

A2. It generates an rf carrier at a given frequency within required limits.

A3. Power amplifier.

A4. It converts audio (sound) into electrical energy.

A5. When no modulation is present.

A6. It is an exact multiple of the basic or fundamental frequency.

A7. 600 megahertz.

A8. To obtain higher carrier frequencies.

A9. It saves power and frequency bandwidth.

A10. For operator-to-operator service messages and frequency changes.

A11. Reception, selection, detection, and reproduction.

A12. Sensitivity, noise, selectivity, and fidelity.

A13. Heterodyning.

A14. To extract the modulating audio signal.

A15. Wide bandpass.

A16. A special type of detector and a carrier reinsertion oscillator.

A17. Attenuates the strong and amplifies the weak.

A18. To limit unwanted variations in the output.

A19. Weak signals produce bias, which could result in no usable receiver output.

A20. Dagc does not attenuate weak signals.

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2-50 UNCLASSIFIED A21. It is heterodyned with the rf to produce an audio frequency.

A22. It eliminates noise when no signal is being received.

A23. It controls the amount of bass and treble response.

A24. It is used to achieve maximum selectivity.

A25. It is used to accurately control the frequency of the oscillator.

A26. The process of selecting and/or heterodyning frequencies to produce a signal frequency.