Fundamental Systems Equipment

3-1

Chapter 3

Fundamental Systems Equipment

Learning Objectives

Upon completion of this chapter you will be able to: 1. State the function of a radio communications handset, a radio set control, and a transfer switchboard. 2. Describe the functions and interrelationships of a radio transmitter. 3. Describe the functions of receive and transmit multicouplers. 4. Describe the differences between the codes used for manual telegraphy and teletypewriter transmissions. 5. Describe the two basic modes of teletypewriter operation. 6. Describe the two types of teletypewriter dc circuits.

7. State the two types of radio teletypewriter shift systems and describe their basic differences. 8. Describe the functions and interrelationships of radio-frequency-carrier shift send and receive systems. 9. Describe the signal flow in an audio-frequency-tone shift system. 10. State the function of the tone terminal set in an audio-frequency-tone shift system. 11. Describe the basic multiplexing process. 12. Describe the three operations performed by a facsimile system. 13. Describe the functions and interrelationships of facsimile equipment.

14. Describe the countermeasures that can be used to eliminate compromising emanations.

Equipment Purposes

A communications system is a collection of equipment used together to do a specific job. You may see this equipment used to send or receive voice communications, or both, or to send, receive, or send and receive teletypewriter information. Figure 3-1 is a basic block diagram of a voice system. You can see how this equipment is interconnected to form a basic communications system. We are going to look at several of the equipment blocks in detail. 3-2 Figure 3-1.—Voice system. HANDSET The handset converts acoustical energy (your voice) to electrical energy for use in modulating a radio transmitter. It also converts electrical energy to acoustical energy for reproduction of a received signal. When the push-to-talk button is depressed on the handset, the dc keying circuit to the transmitter is closed, placing the transmitter on the air.

Handsets are normally connected to a radio set control unit.

Radio Set Control Unit

The radio set control unit shown in figure 3-2 provides a capability to remotely control some radiophone transmitter functions and the receiver output. Some of the controls are used for turning the transmitter on and off. Others are used for voice modulating the transmission (or keying when cw operation is desired). You can even control the audio output level of the receiver and silence the receiver when transmitting. 3-3 Figure 3-2.—Radio set control unit. Under standard operating conditions up to four of these units can be used in parallel with a single transmitter and receiver group to provide additional operating positions. This setup is often found aboard ship where a transmitter and/or receiver is controlled and operated from several locations such as the bridge or the combat information center.

Transfer Switchboards

A transmitter transfer switchboard provides the capability to transfer remote control station functions and signals to transmitters. Figure 3-3 is a representative transfer switchboard that provides the capability for selectively transferring any one, or all, of ten remote control station functions and signals to any one of six transmitters. The cabinet has ten rotary switches arranged in two vertical rows of five each. Each switch has eight positions. The circuitry is arranged so that you cannot parallel transmitter control circuits; that is, you cannot connect more than one transmitter to any remote control location.

Figure 3-3.—Transmitter transfer switchboard. Each switch operating knob corresponds to a remote control station. Each switch position (1 through 6) corresponds to a transmitter. One switch position, X, provides for transfer of all circuits to additional 3-4 transmitter transfer switchboards when more than six transmitters are installed in the system. When the rotary switch is placed in the OFF position the remote control station is removed from the system. Let's look at an example of one transfer switchboard application. When remote control station number two is to have control of transmitter number three, the switch knob designated number two is rotated until its pointer indicates position three on its dial plate.

The receiver transfer switchboard permits the operator to transfer the audio output from a receiver to a remote control station audio circuit. A representative receiver transfer switchboard is shown in figure 3- 4. This switchboard contains ten seven-position switches. Each switch is connected to a remote control station, and each switch position (one through five) is connected to a receiver. Figure 3-4.—Receiver transfer switchboard. The X position on each switch allows transfer of circuits to additional switchboards just like with the transmitter transfer switchboard.

Q1. What are the basic functions of a handset? Q2. What capability does a transmitter transfer switchboard provide? Q3. What function does a receiver transfer switchboard perform?

Transmitters

You learned earlier that transmitters may be simple with low power (milliwatts) capabilities. These may be used to send voice messages a short distance. You may also use highly sophisticated units that use thousands (even millions) of watts of power to send many channels of data (for example voice, teletypewriter, television, telemetry) simultaneously over long distances. Let's look at a complete transmitter set. 3-5 Radio Transmitting Set The applications, configurations, and components you will become familiar with here are typical of most general purpose transmitter systems used in the Navy. A specific transmitter is used only for ease of illustration and example.

We will be discussing a 1,000 watt, single-sideband radio transmitting set that is available to the Navy in any one of four setups. The normal configuration has a transmitter capable of voice, continuous wave, and radio teletypewriter transmissions in the 2- to 30-megahertz frequency range. Exact spacing and number of channels available within the frequency spectrum, modes of operation, and frequency range depend on the model of equipment and how it is configured for use. Stack or rack mounting is used in a ship or shore permanent installation with accessory equipment (for example an rf amplifier, coupler control unit, or power supply) to form a complete communications system. One of three different three- phase primary power sources can be used (depending on whether the transmitter is land, air, or shore based) to provide operating power to the set. Combinations available are 115 volts, 400 hertz; or 208/440 volts, 60 hertz.

General Description Figure 3-5 shows the major units of this set. They are the radio transmitter, the radio frequency amplifier, the power supply, and the electrical equipment shock mount base. An antenna coupler group (consisting of a coupler and coupler control unit) is normally used to match the impedance of the system to a 50-ohm transmission line. If you want to operate with any 50-ohm antenna system, terminating connections are available. Figure 3-5.—Radio transmitting set. 3-6 The transmitter unit provides an upper sideband (usb), lower sideband (lsb), independent sideband (isb), cw, fsk, or compatible AM signal. The output of the transmitter has enough power to drive the radio frequency amplifier.

Depending on the model, the transmitter tunes across the frequency range in 100- or 500-hertz increments. Digital circuitry is used to accomplish this process. Transmitter outputs are also applied to the rf amplifier to automatically tune it to the correct frequency. We will go through a detailed breakdown of the transmitter unit later in this chapter. RADIO FREQUENCY AMPLIFIER.—The rf amplifier unit is a two-stage linear power amplifier that produces an output of 1,000 watts with a nominal input of 100 milliwatts. Nineteen frequency bands are used to cover the operating frequency range. The operating band is automatically selected by digital coding generated by the transmitter. The code controls two motor-driven band switch assemblies.

Automatic control circuits protect the unit against overload and compensate for variations in system gain, mode of operation, and loading. All low voltages required for operation (except two of the relay control voltages) are internally produced. The high voltages required in the amplifier stages are produced by the associated power supply (when using 60 hertz primary power) or the optional internally mounted power supply (when using 400 hertz primary power). Let's take a look at figure 3-6 to see all the operating controls and indicators located on the front panel. Some controls are used only for initial setup and are protected by a hinged access cover. All connections are made at the rear of the case. The amplifiers and the associated interstage broadband transformer assemblies are cooled by forced ventilation. Cooling air is drawn through a filter on the front panel and exhausted through a port on the rear of the case. You should always take particular care to clean or replace any filter in electronic equipment as a regular part of your preventive maintenance program.

3-7 Figure 3-6.—Rf amplifier unit. POWER SUPPLIES.—One power supply produces operating voltages for the amplifier when operating from a 60-hertz power source. All components of the power supply, except the power transformers, are mounted on a chassis and panel assembly that is hinge-mounted to a metal case. The power transformers are constructed as part of the case and there are no operating controls. The other power supply produces operating voltages for the rf amplifier when a 400-hertz, three- phase, 115-volt primary power source is used.

ANTENNA COUPLER GROUP.—The antenna coupler group is an automatic antenna tuning system. However, the equipment design includes provisions for manual or semiautomatic tuning. This makes the system adaptable for use with other radio transmitters. The manual tuning capability is useful when a failure occurs in the automatic tuning circuitry. Tuning can also be accomplished without the use of rf power (SILENT TUNING). This method is useful in installations where radio silence must be maintained except for brief transmission periods.

The antenna coupler matches the impedance of a 15-, 25-, 28-, or 35-foot whip antenna to a 50-ohm transmission line at any frequency in the 2- to 30-megahertz range. Control signals from the associated antenna coupler control unit automatically tune the matching network in less than five seconds. During manual and silent operation, tuning is accomplished by the operator with the controls mounted on the antenna coupler control unit. A low power (not to exceed 250 watts) cw signal is required for tuning. Once tuned, the coupler is capable of handling 1,000 watts peak envelope power (pep).

The coupler is enclosed in an aluminum, airtight, pressurized case. Six mounting feet enable the unit to be attached to the mast of a ship at the base of a whip antenna. The coupler is pressurized with dry nitrogen to aid internal heat transfer and to prevent corona and arcing. All components of the coupler are secured to a chassis that is mounted to the case so that an air duct exists between the chassis plate and the 3-8 case. An internal fan circulates the nitrogen over and through the heat-producing elements and then through the air duct. While passing through the air duct, the nitrogen loses its heat to the bottom of the case. This heat is then transferred by convection through fins on the bottom of the case and by conduction through the mounting feet.

Figure 3-7 shows the antenna coupler control unit. This unit provides the power and control signals required to tune the coupler. Control signals are either automatically produced by the coupler control when a tune cycle is initiated or manually produced with the front panel controls. Figure 3-7.—Antenna coupler control unit. All dc operating voltages are produced from a 115-volt, 48- to 63- or 350- to 450-hertz, single-phase primary power source. Meter and protection circuits are used to give you complete control of the coupler from the remotely positioned coupler control unit.

Q4. If the rf amplifier discussed has an 80 milliwatt input, what would be the maximum output? Q5. What are the tuning modes for the coupler group discussed? Q6. What is the purpose of an antenna coupler? Q7. Why is the coupler pressurized with nitrogen? RADIO TRANSMITTER.—Figure 3-8 shows the front panel of the radio transmitter unit. The radio transmitter accepts audio or coded intelligence and uses it to modulate one of 280,000 possible operating radio frequencies in the 2.0- to 29.999-megahertz frequency range. Tuning is accomplished digitally by means of five control knobs and a switch located on the front panel. The transmitter has a normal rf output level of at least 100 milliwatts and is designed to be used with an associated rf power amplifier.

3-9 Figure 3-8.—Radio transmitter unit. When the AM and ssb transmit modes of operation are used, the output from a handset is applied to the transmitter. The voice signals are amplified and used to modulate a 500-kilohertz local carrier that produces a 500-kilohertz IF. The resulting double sideband signal is filtered in the AM mode, amplified, and converted by a triple-conversion process to the desired rf operating frequency. The rf signal is amplified to a nominal 100 milliwatt level. In cw operation, the 500-kilohertz local carrier is inserted directly into the IF amplifiers. The signal is further processed in the same manner as the voice signals in the AM or ssb modes of operation. In fsk operation, the loop current is converted to audio frequencies representing marks and spaces. These audio signals are applied to the audio circuits of the transmitter.

Thereafter, these signals are processed in the same manner as the voice signals in AM or ssb modes of operation. A typical radio transmitting set block diagram is shown in figure 3-9. 3-10 Figure 3-9.—Typical radio transmitting set block diagram.

Receivers

The receiver we will discuss is a triple-conversion superheterodyne, tunable from 2 to 30 megahertz. Triple conversion uses three IF frequencies to give better adjacent-channel selectivity and greater image- frequency suppression. Figure 3-10 shows the front panel of this receiver where tuning is done digitally by five controls and a switch. A display window directly above each control provides a digital readout of the frequency setting. The displayed frequency can be changed in 1-kilohertz increments. The front panel switch allows the operating frequency to be changed in 100- or 500-hertz increments depending on the model. This will provide you with 280,000 discrete frequencies locked to a very accurate frequency standard. You can continuously tune each 1,000-hertz increment by selecting the VERNIER position of the hertz switch. When using the vernier, the full accuracy of the frequency standard is sacrificed. The receiver demodulates and provides audio outputs for the lsb, usb, isb, AM, cw, and fsk types of received signals.

3-11 Figure 3-10.—Typical radio receivers Q8. What are the transmitter operating modes? Q9. What type of tuning does the receiver use?

Antenna Distribution Systems

Receiving antenna distribution systems operate at low power levels and are built to fit a standard 19- inch rack. Each piece of distribution equipment is fitted with termination or patch fittings designed for ease of connecting and disconnecting. A basic patch panel is shown in figure 3-11. Even a fundamental distribution system has several antenna transmission lines and several receivers. Normally a patch panel consists of two basic patch panels. One panel is used to terminate the antenna transmission lines and the other the lines leading to the receivers. Any antenna can be patched to any receiver through the use of patch cords.

Figure 3-11.—Basic rf receive patch panel. Many distribution systems are more complex. A complex distribution system to cover most situations is illustrated in figure 3-12. In this system you can patch four antennas to four receivers, or you can patch one antenna to more than one receiver via the multicouplers (multicouplers are covered later in 3-12 this chapter). You can also patch rf and audio from one compartment to another. A frequency standard is connected (through a distribution amplifier not shown) to the receivers.

Figure 3-12.—Complex distribution system. Transmitting antenna distribution systems perform the same functions as receiving systems. However, because of the higher power levels, design and fabrication problems are more difficult. The ideal design would be to have all the transmission lines designed for the highest power level. But because high-power patch cords are expensive, large, and difficult to handle, this approach is seldom followed. In practice, the basic patch panel we just looked at in figure 3-11 is practical for low power levels.

Another type of transmitter patch panel is shown in figure 3-13. 3-13 Figure 3-13.—Transmitting antenna patch panel. This type of transmitting antenna patch panel is interlocked with the transmitter so that no open jack connection can be energized and no energized patch cord can be removed. This provides you with a greater degree of personnel and equipment safety. Receive Multicoupler Figure 3-14 is a filter assembly multicoupler that provides seven radio frequency channels in the 14- kilohertz to 32-megahertz range. Any or all of these channels may be used independently of any of the other channels, or they may operate simultaneously. You can make connections to the receiver by means of coaxial patch cords, which are short lengths of cable with plugs attached to each end.

Figure 3-14.—Electrical filter assembly. 3-14 A set of nine plug-in type filter assemblies is furnished with the equipment and covers the entire vlf, lf, mf, and hf bands. Only seven of the assemblies may be installed at one time, and you have the option of selecting those you need to cover the most used frequency bands. Figure 3-12 illustrates how the filter assembly is used in combination with other units to pass an rf signal from an antenna to one or more receivers. Transmit Multicouplers Most multicouplers for the hf range are designed for use with either transmitters or receivers, although some are used with both. There are a large number of channels in a multicoupler so that many transmitters can be used at the same time on one antenna. This is especially true in the 2- to 12-megahertz range.

Figure 3-15 shows you an antenna coupler group designed primarily for shipboard use. Each coupler group permits several transmitters to operate simultaneously into a single, associated, broadband antenna. You can see this reduces the total number of antennas required in the limited space aboard ship. Figure 3-15.—Antenna coupler group. 3-15 These antenna coupler groups provide a coupling path of prescribed efficiency between each transmitter and its associated antenna. They also provide isolation between transmitters, tunable bandpass filters, and matching networks.

Teletypewriter And Facsimile Equipment

In previous areas we have discussed different methods of voice communications. At times, however, the message is too long for practical transmission by voice. To get information or an idea across to another person far away, you may also need a chart, map, or photograph. Teletypewriter (tty) and facsimile equipment allow us to do just that, with ease. Let's see how this is done.

Basic Principles

To give you an idea of how intelligence is sent via teletypewriter, let's take a look at the manual telegraph circuit. This circuit, shown in figure 3-16, includes a telegraph key, a source of power (battery), a sounder, and a movable sounder armature. If the key is closed, current flows through the circuit and the armature is attracted to the sounder by magnetism. When the key is opened, the armature is retracted by a spring. With these two electrical conditions of the circuit, intelligence can be transmitted by means of a teletypewriter code. These two conditions of the circuit are referred to as MARKING and SPACING. The marking condition occurs when the circuit is closed and a current flows; the spacing condition occurs when it is open and no current flows.

Figure 3-16.—Manual telegraph circuit. If the key at station A is replaced by a transmitting teletypewriter and the sounder arrangement at station B is replaced by a receiving teletypewriter, the basic teletypewriter circuit (loop) shown in figure 3-17 is formed. 3-16 Figure 3-17.—Simple teletypewriter circuit. If a teletypewriter signal could be drawn on paper, it would resemble figure 3-18. This is the code combination for the letter R. Shaded areas show intervals during which the circuit is closed, and the blank areas show the intervals during which the circuit is open. The signal has a total of seven units. Five of these are numbered and are called INTELLIGENCE units. The first and last units of the signal are labeled START and STOP. They are named after their functions: the first starts the signal, and the last stops it.

These are a part of every teletypewriter code signal: the START unit is always spacing, and the STOP unit is always marking. Figure 3-18.—Mark and space signals. The teletypewriter signal is theoretically a perfect signal. The time between each unit remains the same during transmission of the signal. The shift from mark to space (and vice versa) is called a TRANSITION. A transition occurs at the beginning and end of each unit when it shifts from mark to space or space to mark; a character may have two, four, or six transitions.

When figuring the time duration of a signal character, no allowance for transition time is made since the transition is instantaneous and is considered to have zero time duration. The time duration for each unit is measured in milliseconds. Q10. What is the function of an antenna patch panel? Q11. What are the functions of a multicoupler? Q12. What are the terms used to describe an open or closed telegraph circuit? Q13. How many units are in a tty signal and what are they? 3-17 Codes Two of the codes the Navy uses are found in manual telegraphy and in teletypewriter operation. One is very easy to understand while the other is more complex. Let's look at these two types and how they work.

MANUAL TELEGRAPHY.—In manual telegraphy, the most widely used code is the Morse code. In this code, two distinctive signal elements are employed-the dot and the dash. The difference between a dot and a dash is its duration, a dash being three times as long as a dot. Each character is made up of a number of dots and/or dashes. The dot and dash elements making up any character are separated from each other by a time interval equal to the duration of one dot. The time interval between the characters for each word is equal to the duration of three dots. The interval between words is equal to seven dots. (A signal-man uses the Morse code to send visual flashing-light messages. The radioman uses the Morse code to send messages electrically.) TELETYPEWRITER MESSAGE TRANSMISSION.—In teletypewriter operation, the code group for each character is of uniform length. Since the Morse code is an uneven length code, it cannot be used in teletypewriter operation without additional code converters.

The FIVE-UNIT (five-level) CODE has been the most commonly used in modern printing telegraphy and is universally used in teletypewriter operation. This is also known as the Baudot code. The mechanical sending device in the teletypewriter divides the sending time for each character into five short code elements (impulses) of equal duration. The five-unit code is an example of what is called an even length or constant length code (one in which the number of signal elements for a character is the same for every character and the duration of each element is constant). In the five-unit code, each character consists of a combination of five signal elements; each element may be either a mark or a space. A total of thirty-two combinations of signal elements are possible with this arrangement.

The thirty-two possible combinations available from the five-unit code are insufficient to handle the alphabet and numbers since twenty-six combinations are required for the letters of the English alphabet alone. This leaves only six combinations for numerals, symbols, or nonprinting functions. This number of combinations is obviously inadequate; therefore, two of the thirty-two combinations are used as shift signals. The shift signals are often referred to as case-shift signals (one case is a letter shift, and the other a figure shift.) These two shift signals permit the remaining code combination to be used as letter-shift signals for letters and as figure-shift signals for numerals, function signs, and so forth. When a letter shift is transmitted, it sets the receiving instrument in a condition to recognize any letter signal combination. It will recognize letter combinations until a figure shift is received. Then the receiving instrument sets itself in a condition to recognize any figure signal combination received. The interpretation of a signal combination is determined by the previous shift signal. This plan enables 30 of the 32 available combinations to have two meanings.

Q14. There are not enough combinations of the five-unit code to handle the alphabet, symbols and so forth. What is used to increase the number of available code combinations? Modes of Operation The two basic modes of teletypewriter operation are ASYNCHRONOUS (start-stop) and SYNCHRONOUS. The most common mode used in teletypewriter operation is the start-stop mode. Synchronous operation is used more in high-speed data systems. Let's examine their differences. ASYNCHRONOUS.—In the start-stop mode of operation, the receiving device is allowed to run for only one character. It is then stopped to await the reception of a start signal indicating the next character is 3-18 about to start. In this manner any difference in speed between the transmitting and receiving devices can accumulate only during the duration of one character. However, you should note that a penalty must be paid for this advantage. The length of each character must be increased to include a unit (element) to start the receiving device and another to stop it.

The start unit precedes the first intelligence unit and is always a space signal. Its purpose is to start the receiving machine. The stop unit follows the last code unit and is always a mark signal. Its purpose is to stop the receiving machine in preparation for receiving the next character. The start unit must be equal to at least one unit of the code. The standard mode uses a stop unit that is 1.42 times the length of one intelligence unit. It is common practice to refer to a code unit as an element and to use the terms interchangeably. You will also hear duration of a unit referred to as the unit interval.

The length of time required to transmit the entire character is called the CHARACTER INTERVAL. Character interval becomes very important in some transmissions because certain items of equipment are character length conscious or code conscious. Stop unit intervals of various lengths are used or produced by various equipment (1.0, 1.27, 1.5, 1.96, 2.0, and so forth). Basically, the only difference between them is the length of time required to transmit one character. SYNCHRONOUS.—Synchronous teletypewriter operation does not in all cases have to rely upon elements of the transmitted character to maintain proper position in relation to the receiving device.

External timing signals may be used that allow the start and stop elements to be discarded. You will then see only the elements necessary to convey a character. Synchronous systems have certain advantages over asynchronous systems. The amount of time taken to transmit stop and start elements is made available for information transmission rather than for synchronizing purposes. Only the intelligence elements are transmitted. In start-stop signaling, the ability of the receiving device to select the proper line signal condition is dependent upon signal quality. For example, suppose the stop-to-start transition arrives before it should; then, because of atmospherics, all subsequent selection positions in that character will appear earlier in time in each code element. A synchronous system has a higher capability for accepting distorted signals because it does not depend on a start-stop system for synchronization.

Modulation Rate Several terms are used to refer to teletypewriter modulation rates or signaling speeds. These include BAUD RATE, BITS PER SECOND, and WORDS PER MINUTE. Baud is the only term that is technically accurate. The other terms are either approximations or require explanation. The word baud by definition is a unit of modulation rate. You will sometimes see it used to refer to a signal element, but this reference is technically incorrect. Baud rate is the reciprocal of the time in seconds of the shortest signal element. To find the modulation rate of a signal in bauds, you must divide the number 1 by the time duration of the shortest unit interval present in the signal. For example, 22 milliseconds (.022 seconds) is the time interval of the shortest unit in the five-unit code at 60 words per minute. To find the number of bauds corresponding to 60 words per minute, divide 1 by .022. Rounding off the result of the division gives us the number 45.5, which is the baud equivalent of 60 words per minute. Each increase in words per minute will correspondingly decrease the signal unit time interval.

(The defense communications system standard speed for teletypewriter operation is 100 words per minute or 75 baud.) Words per minute is used only when speaking in general terms for an approximation of speed. The term 100 words per minute means 100 five letter words with a space between them can be transmitted in a 60-second period. However, you can obtain this nominal words-per-minute rate in several systems by 3-19 varying either modulation rate or the individual character interval (length). For this reason, the modulation rate (baud) method of reference rather than words per minute is used.

Formula for baud rate and words per minute are as follows BIT is an acronym for the words binary digit. In binary signals, a bit is equivalent to a signal element. Because of the influence of computer and data processing upon our language, modulation rate is sometimes expressed in bits per second. When you understand all signal elements being transmitted are of equal length, then the modulation rate expressed in bits per second is the same as the modulation rate expressed in baud. Dc Circuits You were told the two conditions mark and space may be represented by any convenient means. The two most common are NEUTRAL and POLAR operation. In neutral, current flow represents the mark, and no current flow represents the space; in polar operation, current impulses of one polarity represent mark, and impulses of the opposite polarity of equal magnitude represent the space.

NEUTRAL.—Neutral circuits make use of the presence or absence of current flow to convey information. A neutral teletypewriter circuit is composed of a transmitting device, a battery source to supply current, a variable resistor to control the amount of current, a receiving device, and a line for the transmission medium. POLAR.—Polar operation differs from neutral operation in two ways. Current is always present in the polar system, and it is either positive or negative. A polar teletypewriter circuit contains the same items as a neutral circuit plus an additional "battery" source. The battery referred to here is not an actual battery but is a solid-state dc power supply. It provides variable current to the teletypewriters. The reason for having an extra battery source is because polar circuits use positive battery for marks and negative battery for spaces.

You will find in polar operation that the distortion of a signal is almost impossible through low line currents, high reactance, or random patching of signal circuits or equipment. In polar signaling when you experience a complete loss of current (a reading of zero on a milliammeter), you know you have line or equipment trouble; whereas the same condition with neutral signaling may indicate a steady space is being transmitted. This gives us a condition called RUNNING OPEN. Under this condition, the teletypewriter appears to be running because the machines is decoding the constant space as the Baudot character blank and the type hammer continually strikes the type box but there is no printing or type box movement across the page.

Q15. What are the two teletypewriter modes of operation? Q16. Define baud. Q17. Define bit. Q18. What are the two types of dc operations used to represent mark and space conditions? 3-20

Basic Systems

When two ttys are connected by communications wire or cable (over short or long distances), the exchange of information between them is direct. When the teletypewriters are not physically joined, exchange of information is more involved. Direct-current mark and space intervals cannot be sent through the air. The gap between the machines must be bridged by radio using a radio transmitter and receiver. The transmitter produces a radio frequency carrier wave to carry the mark and space intelligence. A KEYER is needed to change the dc pulses from the tty into corresponding mark and space modulation for the carrier wave in the transmitter. The radio receiver and a CONVERTER are required to change the radio frequency signal back to dc pulses.

Radio Teletypewriter Systems The Navy uses two basic radio teletypewriter (ratt) systems. These are the TONE-MODULATED SYSTEM, referred to as audio-frequency tone shift (afts), and the CARRIER-FREQUENCY SHIFT SYSTEM, referred to as radio-frequency-carrier shift (rfcs). The rfcs system is also called frequency-shift keying (fsk). Figure 3-19 shows a modulated carrier wave with audio tone impulses impressed on the radio- frequency carrier wave. These correspond to dc mark and space signals. Figure 3-19.—Modulated carrier wave with audio tone for mark and space.

We can best explain the rfcs signal by comparing it to the on-off cw signal. Cw signals are essentially a constant frequency with no variations along the frequency axis. Figure 3-20, view A, is an example. The complete intelligence is carried as variations in the signal amplitude. Figure 3-20, view B, shows the same signal as a shift in frequency between the mark and space. 3-21 Figure 3-20A.—Cw compared to an rfcs teletypewriter signal. Figure 3-20B.—Cw compared to an rfcs teletypewriter signal. AUDIO FREQUENCY TONE SHIFT.—Tone-modulated (afts) systems use amplitude modulation to change dc mark and space impulses into audio electrical impulses.

A basic tone-modulated system is shown in figure 3-21. Conversion to audio tones is accomplished by an audio oscillator in the tone converter. Rapid varying of the tone, according to the characters transmitted from the teletypewriter equipment, amplitude modulates the carrier wave in the transmitter. The receiver receives the modulated signal and separates the audio signal from the carrier. This process of separating the modulated signal is known as detection or demodulation. 3-22 Figure 3-21.—Basic tone modulated (afts) system.

RADIO-FREQUENCY-CARRIER SHIFT.—For frequency-shift (fsk) systems, the transmitter provides a source of radio-frequency excitation. Figure 3-22 illustrates a basic frequency-shift keyed system. In modern systems, the keyer is built into the transmitter. The keyer shifts the signal box below or above the assigned frequency to correspond with the mark or space required to transmit tty characters. Normally the keyer is adjusted for an 850-hertz spread, 425 hertz above and 425 hertz below the assigned frequency. A spacing impulse will be 425 hertz above the operating frequency, and a marking impulse will appear 425 hertz below.

3-23 Figure 3-22.—Basic radio-frequency-carrier shift system (rfcs). In both the tone-modulated system and the carrier-frequency shift system, all tty signals pass through the tty panel that controls the looping current in all the circuits. Looping current is the current supplied by the tty battery. The tty panel integrates the tone-modulated and the carrier-frequency shift systems. It provides every possible interconnection of available tty equipment. With this configuration maximum operational flexibility is achieved with the least amount of circuitry and equipment.

Q19. What is the function of a keyer? Q20. What is the function of a converter? Q21. Basically describe an afts system. Q22. Basically describe an rfcs system. Rfcs Send System Figure 3-23 shows an rfcs teletypewriter transmit communications system. You should refer to this figure frequently while reading the functional descriptions of the equipment shown. 3-24 Figure 3-23.—Rfcs transmit (send) system. TELETYPEWRITER SETS.—Most of the teletypewriter sets used by the Navy belong to one family of tty equipment. This equipment features various weights and sizes, quiet operation, and high operating speeds. They present relatively few maintenance problems. Because of this they are well suited for severe shipboard conditions of roll, vibration, and shock.

These teletypewriters operate at various speeds. Conversion from one speed to another is usually only a matter of changing the gears that are located within the equipment. Teletypewriters may be send/receive units or receive only units. They may be designed as floor models, table models, or rack and wall-mounted sets. The teletypewriter shown is a send/receive floor model. The teletypewriter receives messages and prints them on page-size copy paper. In addition, it can receive and record messages on perforated tape. You can use the keyboard or perforated tape to send messages. Page print monitoring is available with both methods. The set shown can prepare perforated and printed tape for separate transmission. It does this with or without simultaneous transmission and page-print monitoring. The combinations of services available are extensive.

The tty set may include a CABINET, KEYBOARD, PAGE PRINTER, TYPING PERFORATOR, TRANSMITTER DISTRIBUTOR, TYPING REPERFORATOR, power distribution panels, and a power supply. In operation, the components are linked by electrical or mechanical connections. You are given a wide range of possibilities for sending, receiving, or storing tty messages. All equipment components are housed within the cabinet. Transmission signals are initiated through the keyboard (kybd) or through the 3-25 transmitter distributor (td). Signals received or local transmissions can be monitored on the page printer.

The typing perforator and typing reperforator are devices for preparing tapes on which locally initiated or incoming tty messages can be stored for future transmission through the td. COMMUNICATION PATCHING PANELS.—Ttys are provided flexibility by jacks that are used to terminate all ttys and associated equipment. The jacks are wired in communications patching panels, usually referred to as tty patch panels. You are able to connect any combination of equipment electrically by means of patch cords. The plugs on the patch cords are inserted into the jacks at the front of the panel. These plugs have three different parts. They are the tip, ring, and sleeve. The tip carries the intelligence signal while the ring carries the synchronizing (step) or timing signals. The sleeve carries an alarm signal that indicates (both visually and audibly) a problem to the operator. The problem may be equipment failure, loss of loop current, or improper patching. Commonly used combinations of equipment are often wired together within the panel (called normal-through). Individual pieces of equipment are wired on jacks to allow you to use them alone or in combination.

Tty patch panels also furnish a central point for connecting the dc voltage supply into the tty circuits. One source of supply can be used for all circuits passing through a particular panel. RED and BLACK are used on patch panels to identify whether that panel is used for passing secure or nonsecure information. Red indicates that secure (encrypted) information is being passed through the panel. Black indicates that nonsecure (unencrypted) information is being passed. Patch panels through which secure information is passed are indicated by a red sign on the front that has inch high white block letters that say "RED PATCH PANEL." Panels through which nonsecure information is passed are indicated by two black signs on the front with inch high white block letters. One sign says "BLACK PATCH PANEL" and the other "UNCLAS ONLY." Each panel contains six channels. Each channel has its own series circuit of looping jacks, set jacks, and a rheostat for adjusting line current. The number of looping and set jacks in each channel varies with the panel model. Each panel includes a meter and rotary selector switch for measuring the line current in any channel. There are six miscellaneous jacks. Any tty equipment not regularly assigned to a channel, may be connected to one of these jacks.

If the desired tty equipment is wired in the same looping channel as the radio adapter used, no patching is required. But, if the desired tty is not wired in the same looping channel as the keyer or converter, it must be patched. For example, let's put a tty on channel 1 and a converter on channel 3. If you want to receive, you must insert one end of the patch cord in the set jack for channel 1 and the other end in either one of the two looping jacks of channel 3. In any switching operation between the plugs and jacks of a tty panel, the cord plug must be pulled from the looping jack before you remove the other plug from the set (machine) jack. Pulling the plug from the set jack first opens the circuits to the channel, causing all tty messages in the channel to be interrupted.

WARNING Removing the set (machine) jack before the looping jack exposes a dangerous dc voltage on the exposed plug. Q23. Most Navy tty sets operate at what speeds? Q24. A receive tty set provides outputs in what formats? 3-26 Q25. What does the color red indicate on a tty patch panel? CRYPTOGRAPHIC EQUIPMENT.—Cryptographic equipment is used to ENCRYPT and DECRYPT tty messages that require security handling. (Encrypting is the method used to code a transmitted message; decrypting is used to decode a received message.) To code or decode any message, the send and receive cryptographic equipment must be compatible.

REMOTE TRANSMITTER CONTROL UNIT.—The remote transmitter-control unit is mounted close to the kybd and permits remote control of the transmitter (xmtr). It has a transmitter power on-off switch, a power-on indicator lamp, a carrier-on indicator lamp, and a three-position rotary selector switch. For rfcs operation you set the switch to CFS SEND to transmit and to CFS REC to receive. Use the TONE S/R position for both transmitting and receiving afts signals. An audio frequency tone-shift system will be discussed later in this chapter.

TRANSMITTER TRANSFER SWITCHBOARD.—The transmitter transfer switchboard is used in this system to connect the remote transmitter control unit to the radio transmitter. RADIO TRANSMITTER.—The radio transmitter transmits the tty signal. You should be careful when tuning the transmitter for rfcs operation. The carrier frequency setting is critical and must be properly set to ensure a correct output from the transmitter. Q26. What are the functions of cryptographic equipment? Rfcs Receive System Figure 3-24 shows the rfcs receive system used to receive the transmitted signal and translate it back to a usable output. You should look at this figure while studying the units in this section.

3-27 Figure 3-24.—Rfcs receive system. ANTENNA FILTER ASSEMBLY.—The antenna filter assembly is connected to the antenna and receives the rf signal from the antenna. It filters out any unwanted rf signals and allows the desired band of frequencies to pass. RADIO RECEIVER.—The radio receiver takes the rf signal passed on by the antenna filter and translates it to an audio signal. RECEIVER TRANSFER SWITCHBOARD.—The receiver transfer switchboard is used to tie the receiver to any converter unit connected to it. This allows you a wide selection of equipment for connection to the same receiver.

CONVERTER-COMPARATOR GROUP.—The converter-comparator group is used with receivers in either space or frequency diversity operation. When diversity operation is not required, each converter can be used separately with a single receiver. Each converter has its own COMPARATOR circuitry. This built-in design feature results in a considerable reduction in size from older units. The comparator was located in a separate chassis in the older units. Size has been further reduced through the use of microelectronics. Figure 3-25 shows the basic method we use to convert a frequency-shift rf signal into a signal that controls the dc loop of a tty. The frequency shifts of the af output from the receiver are converted into dc pulses by the af discriminator. The dc pulses are then fed into the keyer. The keyer opens and closes the dc loop of the tty according to the mark and space characters received.

3-28 Figure 3-25.—Frequency shift receiving system simplified block diagram. In diversity operation the comparator section of the converter-comparator group (shown in figure 3- 24) compares the strength of the signals from two receivers. Signals from each converter are fed into a comparator circuit that compares the signals. This comparison is displayed on a crt on the front of the equipment. The comparison is in the form of LISSAJOUS PATTERNS. A lissajous pattern is a combined, simultaneous display of the amplitude and phase relationships of two input signals. One signal is applied to the vertical and the other to the horizontal deflection circuits. Lissajous patterns have many applications in electronics. They have operational uses as well as uses in corrective and preventive maintenance. Further coverage on lissajous patterns can be found in NEETS, Module 19, The Technician's Handbook. Figure 3-26 shows several typical lissajous monitoring patterns for the converter- comparator group. Once we have a correctly tuned signal, the comparator feeds it to the communication patching panel for patching to the tty. Now let's refer back to figure 3-24 while we discuss the rest of the units in the system.

Figure 3-26.—Typical lissajous monitoring patterns. COMMUNICATION PATCH PANEL.—The communication patch panel serves the same functions on the receive side of the rfcs system as it did on the transmit side. It routes the dc signal to the 3-29 proper cryptographic equipment. It also routes the decoded teletypewriter signal from the cryptographic equipment to the selected tty. CRYPTOGRAPHIC EQUIPMENT.—The cryptographic equipment converts the transmitted coded signal to a decoded signal that can be printed out in its original state.

TELETYPEWRITER.—The tty equipment is used to convert the dc signal received from the communication patch panel to a printed copy of the original transmitted message. The tty shown is used only for receive and does not have the ability to transmit. Q27. What are the functions of a converter-comparator group? Afts System Figure 3-27 is a simplified block diagram of a HALF-DUPLEX (send or receive) uhf, audio- frequency-tone shift system. A half-duplex communications circuit permits two-way communications between stations. Communications can be in either direction but not simultaneously. The term half-duplex is qualified by adding send only, receive only, or send or receive. Let's use the block diagram to trace a signal through the system.

Figure 3-27.—Half-duplex afts teletypewriter system. SIGNAL FLOW.—On the transmit side, dc signals from the tty set are fed to the communication patching panel. From the panel they are patched to the tone terminal set. The tone terminal set converts 3-30 the dc signals into audio tone-shift signals. These signals are then patched to the transmitter section of the transceiver through the transmitter transfer switchboard. The audio tone-shift signals modulate the rf carrier generated by the transmitter (xmtr). The rf tone-modulated signals are then radiated by the antenna.

On the receive side, the rf tone-modulated signals are received at the antenna. You then patch the signal via the multicoupler to the receiver section of the transceiver. Demodulation takes place at this point. The resulting audio tone-shift signals are then patched through the receiver transfer switchboard. The signals now go from the switchboard to the tone terminal set, where they are converted back to dc signals. The dc signals are then patched through the communication patching panel to the tty for printing. TONE TERMINAL SET.—In tone modulation transmission, the tty pulses are converted into corresponding audio tones. These tones amplitude modulate the rf carrier in the transmitter. Conversion to audio tones is accomplished by an audio oscillator in the tone converter.

An internal relay in the tone converter closes the control line to the transmitter. This keys the transmitter on the air when the operator begins typing a message. The transmitter remains keyed until after the message has been transmitted. On the receive side, the tone converter accepts the mark and space tones coming in from a receiver and converts them into signals suitable to operate a relay in the converter. The make and break contacts of the relay are connected in the local tty dc loop circuit. This causes the teletypewriter to print in unison with the mark and space signals from the distant tty.

Multiplexing Equipment The number of communications networks in operation throughout any given area is increasing. As a result, all areas of the rf spectrum have become highly congested. The maximum number of intelligible transmissions taking place in the radio spectrum is being increased through the use of MULTIPLEXING. Multiplexing is the simultaneous transmission of a number of intelligible signals (messages) in either or both directions using only a single rf carrier. You may use two methods of multiplexing. These are TIME-DIVISION and FREQUENCY-DIVISION.

TIME-DIVISION.—With AM voice and tone communications, we want to transmit and receive for 360 degrees of each sine wave. However, an audio signal may be transmitted and received satisfactorily by periodically sampling the signal. The sampling process yields a received signal like the one shown in figure 3-28. There is no limit to the maximum number of samples that may be made, but you must sample at least twice per cycle of audio to get satisfactory results. In practical systems, 2.4 samples per cycle are usually taken. This concept of sampling forms the basis for time-division multiplex (tdm) operation.

3-31 Figure 3-28.—Components of a sine wave. Figure 3-29, view A, illustrates, the fundamental principle of tdm. Let's look at an example. Assume that a 3,000-hertz tone is applied to each of the six channels in the transmitter. Assume also that the rotating switch turns fast enough to sample, in turn, each of the six channels 2.4 times during each cycle of the 3,000-hertz tone. The speed of rotation of the switch must then be 2.4 × 3,000 or 7,200 rotations per second. This is the optimum sampling for a practical system.

3-32 Figure 3-29.—Fundamental principle of time-division multiplexing. When the transmitter and receiver switches are synchronized, the signals will be fed in the proper sequence to the receiver channels. The samples from transmitter channel one will be fed to receiver channel one. In this way, many channels of audio are combined to form a single output (multiplexed) chain. Time spacing occurs between the components of the separate channels. The chain is transmitted (via wire or radio path) to distant demultiplexing receivers. Each receiving channel functions to select and reconstruct only the information included in the originally transmitted channel.

In most present day applications, electronic switching is used as the sampling component. The main advantage to electronic sampling is the longer life of an electronic switch when compared to an electromechanical switch. We use a mechanical system in our example to make this concept easier for you to see. Now let's look at figure 3-29, view B, where channel one is shown sampled four times. (This is the output of channel one in our transmitter.) Figure 3-29, view C, shows all six channels being sampled four times during each cycle. (This is the output of the rotating switch in our transmitter.) What you see here is a continuous, time-sharing waveform.

More than six channels (perhaps 24 or more) may be used. As we increase the number of channels, the width of each sample segment must be reduced. The problem with reducing the width of the pulse is that the bandwidth (bw) necessary for transmission is greatly increased. Decreasing the pulse width decreases the minimum required rise time of the sampling pulse and increases the required bandwidth. When you increase the number of channels, you increase the bw. The bw is also affected by the shape of the sampling pulse and the method used to vary the pulse.

3-33 Common methods of time-division multiplexing include PULSE AMPLITUDE MODULATION (pam), PULSE WIDTH or PULSE DURATION MODULATION (pwm or pdm), PULSE POSITION MODULATION (ppm), and PULSE CODE MODULATION (pcm). We have been studying an example of pulse amplitude modulation. (These methods of tdm were discussed in NEETS, Module 12, Modulation Principles.) FREQUENCY DIVISION.—Frequency division multiplexing (fdm), unlike tdm, transmits and receives for the full 360 degrees of a sine wave. Fdm used presently by the Navy may be divided into two categories. One category is used for voice communications and the other for tty communications.

The normal voice speaking range is from 100 to 3,500 hertz. During single channel AM voice communications, the audio frequency amplitude modulates a single rf (carrier frequency). However, in voice fdm, each voice frequency modulates a separate frequency lower than the carrier frequency (subcarrier frequency). If these subcarrier frequencies are separated by 3,500 hertz or more, they may be combined in a composite signal. This signal modulates the carrier frequency without causing excessive interference. In figure 3-30, the output of channel one is the voice frequency range of 100 to 3,500 hertz. The output of channel two is the combination of a different voice frequency with a subcarrier frequency of 4,000 hertz. The output of channel three is another voice frequency. This voice frequency combined with a subcarrier frequency of 8,000 hertz gives you an output frequency range of 8,100 to 11,500 hertz. The overall bw for the composite modulation package shown is 100 to 15,500 hertz. Each separate channel occupies its own band of frequencies. The composite signal is used to modulate the carrier frequency of the transmitter.

Figure 3-30.—Block diagram of a frequency-division multiplexing system. Multichannel broadcast and ship/shore terminations use tty fdm. With this system, each channel of the composite tone package of the broadcast is assigned an audio frequency. By multiplexing tty circuits, up to 16 circuits may be carried in any one of the 3,000 hertz multiplexed channels described above. Don't confuse the two types of multiplexing. In the first case, 3,000 hertz audio channels have been combined. In the second case, a number of dc tty circuits are converted to tone keying and combined in a single 3,000-hertz audio channel. Figure 3-31 illustrates a 16-channel, tty-multiplexing system. The output of the dc pulsed circuits is converted to audio keying. Each channel has a separate audio center frequency. Channel frequencies range from 425 hertz for the lowest channel to 2,975 hertz for the highest 3-34 channel. A mark in an individual tty loop keys an audio tone 42.5 hertz below the center frequency. A space in the input signal keys an audio tone 42.5 hertz above the center frequency. Let's look at an example. The mark and space frequencies for channel one are calculated as 382.5 hertz and 467.5 hertz, respectively (425 ± 42.5). Combining these keyed tones into a composite signal results in a tone package within a standard 3,000-hertz bandwidth. By occupying no more than 3,000 hertz of the audio spectrum, the output signal is suitable for transmission via radio or landline.

Figure 3-31.—Block diagram of modulator units. Q28. What is the function of a tone terminal set? Q29. What are the two types of multiplexing? Q30. What is the purpose of multiplexing? 3-35 Facsimile FACSIMILE (fax) is a method of transmitting still images over an electrical communications system. The images, called "pictures" or "copy" in fax terminology, may be weather maps, photographs, sketches, typewritten or printed text, or handwriting. Figure 3-32 shows a facsimile transceiver. You must realize that the still image serving as the fax copy or picture cannot be transmitted instantly in its entirety.

Three distinct operations are performed. These are (1) scanning, (2) transmitting, and (3) recording or receiving. Figure 3-32.—Facsimile transceiver. Scanning consists of subdividing the picture in an orderly manner into a large number of segments. This process is accomplished in the fax transmitter by a scanning drum and phototube arrangement. The picture you want to transmit is mounted on a cylindrical scanning drum. This drum rotates at a constant speed and at the same time moves longitudinally along a shaft. Light from an exciter lamp illuminates a small segment of the moving picture and is reflected by the picture through an aperture to a phototube. During picture transmission, the light crosses every segment of the picture as the drum slowly spirals past the fixed lighted area.

The amount of light reflected back to the phototube is a measure of the lightness or darkness of the segment of the picture being scanned. The phototube changes the varying amounts of light into electrical signals. These are used to amplitude modulate the constant frequency output of a local oscillator. The modulated signal is then amplified and sent to the radio circuits. Signals received by the fax receiver are amplified and actuate a recording mechanism. This recorder makes a permanent recording (segment by segment) on paper. The paper is attached to a receiver drum similar to the one in the fax transmitter. The receiver drum rotates synchronously with the transmitter drum. Synchronization of the receiver and transmitter is done to reduce distortion. Synchronization is obtained by driving both receiver and transmitter drums with synchronous motors operating at the same speed. Drum rotation continues until the original picture is reproduced. The recording mechanism may reproduce the picture photographically by using a modulated light source shining on photographic paper or film. It may also reproduce directly by burning a white protective coating from specially prepared black recording paper.

The receiver drum is FRAMED with respect to the transmitter drum by a series of phasing pulses that are transmitted just before transmission. The pulses operate a clutch mechanism that starts the 3-36 scanning drum in the receiver. This ensures proper phasing with respect to the starting position of the scanning drum in the transmitter. Figure 3-33 is a block diagram of the equipment necessary for radio facsimile operation. View A shows the receiving system. This system consists of a standard radio receiver, a frequency-shift converter, and a facsimile recorder. View B shows two systems for transmitting TIF signals. The upper row of blocks is for carrier-frequency shift transmission. This system consists of a facsimile transceiver, a keyer adapter, a frequency shift keyer and a transmitter capable of fsk emission. The lower row of blocks is for audio-frequency shift transmission and uses a fax transceiver, a radio modulator, and an AM transmitter.

Figure 3-33.—Radio facsimile systems.

Security, Quality Monitoring, And Safety

Security, quality monitoring, and safety are important areas that you must be aware of. If the fundamentals are followed, you will see higher quality communications. You will also help meet the communications goals of the Navy. Let's find out what these fundamentals are and what they will do for you. TEMPEST Compromising emanations (ce) are, generally referred to as TEMPEST. These signals may be unintentional, data-related, or intelligence-bearing. If intercepted or analyzed, these signals could disclose classified information. TEMPEST problems are associated with material transmitted, received, handled, or otherwise processed by electrical information processing equipment or systems. Any electrical information processing device may cause problems. Even your electric typewriter or a large, complex data processor may emit interceptable compromising emanations. Some countermeasures taken to ensure against TEMPEST problems are listed below: 3-37 • Design of equipment in which ce is suppressed • Approved installation criteria that limits interaction between classified and unclassified signal lines, power lines, grounds, equipment, and systems • Low level keying and signaling • Shielded enclosures for equipment installations • Proper shipboard grounding of equipment, including ground straps

Transmission Security

Transmission security includes all measures designed to protect transmission from interception, traffic analysis, and imitative deception. Every means of transmission is subject to interception. In radio transmission, it should be assumed that all transmissions are intercepted. Speed Versus Security Three fundamental requirements of a military communications system are reliability, security, and speed. Reliability is always first. Security and speed are next in importance and, depending on the stage of an operation, are interchangeable. During the planning phase, security is more important than speed.

During the execution phase, speed sometimes passes security in importance. Radio Transmission Security When a message is transmitted by radio, the originator may know some of those who are receiving it, but will never know all of those who are receiving the message. You must assume that an enemy receives every transmission. Property prepared messages using modern cryptographic systems may prevent an enemy from understanding a message. However, they can still learn a lot. For example, as time for a planned operation approaches, the number of messages transmitted increases. An enemy then knows that something will occur soon, and their forces are alerted. Strict radio silence is the main defense against radio intelligence.

The amount of radio traffic is not the only indicator used by an enemy. Statistical studies of message headings, receipts, acknowledgments, relays, routing instructions, and services are also used by an enemy. Communications experts can often learn much about an opponent from these studies. Direction finders are another aid the enemy can use to determine where messages originate. Radiotelephone Security Radiotelephone networks are operated so frequently that many operators tend to be careless. There are too many instances of interception of vhf and uhf transmissions at distances of many thousands of miles. You may have occasion to work on or around this type of equipment. If you are ever required to bring any transmitter on the air for any purpose, you must be familiar with and use all the correct procedures.

Q31. The transmission of still images over an electrical communications system is known as what? Q32. The term TEMPEST refers to what? Q33. What are the three fundamental requirements of a military communications system? Q34. Which of the above requirements is most important? 3-38

Shipboard Communications Systems Quality Monitoring (Qmcs)

In recent years the volume of shipboard communications has increased greatly. This expansion has led to the shipboard installation of sophisticated equipment. Factors such as frequency accuracy and dc signal distortion are critical to the operation of communications systems. These systems demand precise initial lineup and monitoring to ensure satisfactory operations are maintained. System degradation is often caused by many small contributing factors. When these factors are added together, the system becomes unusable.

Scheduled Maintenance When you perform scheduled, logical checks that ensure continuous, optimum performance of shipboard communications systems, you are doing SCHEDULED MAINTENANCE. In many cases this maintenance prevents outages before they occur. Some of the scheduled checks will include the following: • Transmitter/receiver frequency • Transmitter power out • Receiver sensitivity/bandwidth • Primary power (voltage, current, cycles) Electromagnetic Interference (emi) Many complex electronic systems are installed aboard Navy ships. In modern ships, complex systems with higher power and greater sensitivity are being crowded into a restricted and corrosive area.

Figure 3-34 is a Spruance class destroyer with its crowded (compact) communications environment. The ability of these systems to perform their individual functions without interference is known as ELECTROMAGNETIC COMPATIBILITY (emc). Emc is concerned with the structure of the ship and its electrical and electronic system. Compact environment is a major limitation to the effectiveness of a total ship system concept. Figure 3-34.—Total ship. 3-39 Operation of a total ship system in this unique shipboard environment presents a challenge to all concerned. You must always consider the effects that motion, temperature variations, and exposure to adverse elements will have on the performance of the total ship system. This is particularly true on those system components that are mounted topside.

On board ship, you will find much attention is given to keeping the topside cosmetically and mechanically shipshape. It is equally important to keep it electronically shipshape. Minor mechanical problems, such as loose connections, broken bond straps, or rusty junctions can cause serious communications problems. These sources of electromagnetic radiations reduce receiver performance and are known as ELECTROMAGNETIC INTERFERENCE (emi). Sources of emi can be divided into the following broad categories: • Functional. Emi can originate from any source designed to generate electromagnetic energy and which may create interference as a normal part of its operation. The interference may be unintentional or caused by other on board or adjacent platform systems. This interference also may be intentional or caused by electronic countermeasures (ECM).

• Incidental. Emi can originate from man-made sources. These are sources not designed specifically to generate electromagnetic energy but which do in fact cause interference. Examples of incidental emi sources include power lines, motors, and switches. • Natural. Emi can be caused by natural phenomena, such as electrical storms, rain particles, and solar and interstellar radiation. It is recognized by the following audible noises: – Intermittent impulses of high intensity that are caused by nearby electrical storms – Steady rattling or cracking caused by distant electrical storms – Continuous noise of precipitation static caused by electrically charged rain drops – A steady hiss at high frequencies caused by interstellar noise • Hull-generated. Emi can be caused by the interaction of radiated signals with elements of the hull and rigging of a ship. (The functional signals themselves do not cause interference.) The following are two general methods by which emi is transmitted: Conduction. Undesired energy from one equipment is coupled to interconnecting cables or components of another equipment. This energy is conducted via the wiring in the shielded enclosure that protects sensitive circuits. You will find proper design, adequate isolation, and shielding of cables and equipment can control this problem.

Radiation. Energy is beamed directly from the transmitting antenna, or source, to the victim receiving antenna. When this interference is picked up by a receiver, you have two solutions. Interfering energy can be eliminated at the source or you can filter, or blank it out at the victim equipment. Filtering is far less desirable. Interference may be on the same frequency as the desired signal and will not be eliminated without affecting the reception of all desired signals. Most unprotected shipboard receivers are susceptible to emi over a frequency range much wider than their normal bandpass. Off-frequency rejection rarely excludes strong, adjacent-channel signals. These signals enter the receiver and degrade receiver performance by being processed along with the desired, tuned signal. Usually, the presence of emi will be apparent to you. It has a bad effect. Upon the desired signal quality, such as that in CROSS-MODULATION where a spurious response occurs when the carrier 3-40 of a desired signal intermodulates with the carrier of an undesired signal. Extremely strong, off-frequency signals may even burn out the sensitive front-end stages of a receiver. Emi also can degrade overall receiver performance in a less noticeable way. It does this by desensitizing the receiver front end. The noise level is raised and effectively lowers the signal to noise ratio and thus the sensitivity. This causes a decrease in desired signal amplification. For these reasons, shipboard receive systems are designed to include protective circuitry between the antenna and receiver to filter out off-frequency signals. This prevents or limits interference, desensitization, or burnout. Depending upon the system, these protective devices may include filters, multicouplers, preselectors, and so forth. These devices can minimize interference caused by inadequate frequency separation or poor physical isolation between transmit and receive antennas.

Q35. What is the purpose of QMCS? Q36. What is emi? Q37. What are the two emi transmission methods?

Electromagnetic Radiation

Radio-frequency (rf) transmitting systems with high-power transmitting tubes and high-gain antennas have increased the possibility of injury to personnel working in the vicinity. An electromagnetic radiation hazard exists when electronic equipment generates a strong enough electromagnetic field to fall in a category listed below: • Causes harmful or injurious effects to humans and wildlife • Induces or otherwise couples currents and/or voltages of magnitudes large enough to initiate electroexplosive devices or other sensitive explosive components of weapons systems, ordnance, or other explosive devices • Creates sparks large enough to ignite flammable mixtures or materials that must be handled in the affected areas These hazardous situations can be caused by a transmitter or antenna installation. These generate electromagnetic radiation in the vicinity of personnel, ordnance, or fueling operations in excess of established safe levels. Sometimes the existing electromagnetic radiation levels increase to a hazardous level. When personnel, ordnance, or fueling evolutions are located in an area that can be illuminated by electromagnetic radiation, hazardous situations may occur.

Electromagnetic radiation is hazardous to personnel in two ways. It can cause rf burns; and it can cause biological, thermal, and neurological effects to personnel (RADHAZ). Because of the differences in characteristics and safety precautions required for each of the two types, they will be discussed separately. An rf burn hazard is a hazardous condition caused by the existence of radio frequency (rf) voltages in places where they are not intended to be. Any ship with high-power hf transmitters is susceptible. Potentially hazardous voltages have been found in many areas. Some of these areas are lifelines, vertical ladders, ASROC launchers, gun mounts, rigging for underway replenishment, and boat davits. Another of these areas is on aircraft tied down on carrier and helicopter flight decks.

Whether or not an induced voltage creates an rf burn hazard depends on whether personnel will come into contact with the object being energized. Generally, only the voltage between an object and the deck is important. The rf burn occurs when a person comes into contact with a source of rf voltage in a 3-41 manner that allows rf current to flow through the area of contact. Resistance of the skin to the current flow at the areas of contact causes heat. The effect of the heat on a person at the point of contact ranges from noticeable warmth to a painful burn.

The most useful and widespread technique in the reduction of rf burn hazards is the proper bonding and grounding of all metallic objects in the rf radiation field. In some cases, the rf burn hazard can be eliminated only through the use of restrictive operating procedures. These procedures govern the simultaneous use of transmitting and cargo equipment. Techniques such as operation of transmitters at reduced power and the prohibition of simultaneous use of certain combinations of antennas, frequencies, and cargo handling equipment are used.

Figure 3-35 shows typical rf radiation hazard warning signs. 3-42 Figure 3-35.—Typical rf radiation hazard warning signs. 3-43 Most studies on the subject of radiation hazards (RADHAZ) have emphasized the impact of electromagnetic radiation on man. Man is singled out because of the biological, thermal, and neurological effects that occur in human organs and other biological tissues. Certain organs of the body are considered to be more susceptible than others to the effects of electromagnetic radiation. Presently available information and experience indicate that the eyes and testes are the most vulnerable body organs. The overwhelming danger to date appears to be the hazard from thermal effects, which are a function of intensity of radiation and frequency. This is particularly true in the range of 1 to 3 gigahertz. Thermal effects appear to taper off in severity outside this range.

When the body is irradiated by energy from a point source, the total body surface is usually not exposed. The larger the area exposed and the larger the radiation power density, the higher the body temperature rise and the greater the hazard. Microwave radiation from a radar source will "cook" you internally, just as a microwave oven cooks a chicken. An injury of great concern is that to the lens of the eye. Exposure of the lens to high-intensity microwaves may cause cataracts. Current medical evidence indicates that a significant temperature elevation of the lens is required for cataract formation. If exposure is limited to 10 milliwatts per centimeter squared, the lens temperature is not elevated to levels at which cataracts occur.

In addition to thermal effects, nonionizing radiation is known to produce nonthermal effects. An association of a biological hazard with the nonthermal effects has not been demonstrated. A peculiar effect experienced by some personnel is the sensation of sound when they are exposed to pulsed microwave fields. This occurs at levels below stated hazard limits and is not, by itself, considered dangerous. Q38. Electromagnetic radiation is hazardous to personnel in what two ways? Q39. What is the most useful and widespread technique to reduce rf burn hazards?

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 basic systems equipment, its principles, terms, and typical circuitry required for you to understand this concept. A RADIO SET CONTROL UNIT is used to remotely control certain transmitter and receiver functions. TRANSMITTER TRANSFER SWITCHBOARDS selectively transfer remote control station functions and signals to transmitters. RECEIVER TRANSFER SWITCHBOARDS transfer receiver audio outputs to remote control station audio circuits.

A TRANSMITTER generates an rf carrier, modulates it with intelligence, amplifies it, and applies it to an antenna. An ANTENNA COUPLER is a device used for impedance matching between an antenna and a transmitter or receiver. A RECEIVER receives electromagnetic energy (rf) and may convert it to a visible or audible form. 3-44 MULTICOUPLERS patch several receivers or transmitters to one antenna. They also filter out harmonics and spurious responses, and provide impedance matching. MARKING is when a circuit is closed and current flows in teletypewriter operation.

SPACING is when a circuit is open and no current flows in teletypewriter operation. INTELLIGENCE is any signal that conveys information (voice, teletypewriter, facsimile). A START unit is the first unit of a teletypewriter signal. It is always a space. A STOP unit is the last unit of a teletypewriter signal. It is always a mark. A TRANSITION is the time it takes to shift from a mark to a space condition or from a space to a mark condition. A CODE in teletypewriter operation is a combination of mark and space conditions representing symbols, figures, or letters.

NONSYNCHRONOUS teletypewriter operation is when both transmitter and receiver do not operate continuously. SYNCHRONOUS teletypewriter operation is when both transmitter and receiver operate continuously. WORDS-PER-MINUTE is an approximate rate of speed. It means the number of five letter words with a space between them that can be transmitted or received in a one-minute period. BAUD is a measurement of speed based on the number of code elements or units per second. BITS-PER-SECOND is an acronym of the words binary digit. One bit is equal to one signal unit or element.

NEUTRAL teletypewriter operation is where current flow represents a mark and no flow represents a space. POLAR teletypewriter operation is where current flow of one polarity represents a mark and current of the opposite polarity is a space. RUNNING OPEN is the teletypewriter condition where the type hammer constantly strikes the type box but does not print or move across the page. A KEYER is a device that changes dc pulses to mark and space modulation for teletypewriter transmissions. A CONVERTER changes an audio signal back to dc pulses during teletypewriter reception.

AUDIO FREQUENCY TONE SHIFT systems use amplitude modulation to change dc mark and space impulses into audio impulses. RADIO FREQUENCY CARRIER SHIFT systems use a keyer to shift a radio frequency signal above or below an assigned frequency. These shifts correspond to marks and spaces. A TELETYPEWRITER is a machine that can transmit and or receive letters, numbers, or symbols. It may have a keyboard similar to a typewriter. 3-45 A PERFORATOR is a device that stores a teletypewriter message on a paper tape by punching Baudot coded messages into it.

TRANSMITTER DISTRIBUTOR is a device that reads Baudot code from paper tape and allows a message to be sent or a message to be printed on a page printer. A REPERFORATOR stores an incoming tty signal on paper tape. A PAGE PRINTER prints teletypewriter characters one at a time in a full-page format. This is usually a high-speed printer. RED is the reference color of equipment that passes classified information. It normally refers to patch panels. BLACK is the reference color of equipment that passes unclassified information. It normally refers to patch panels.

A PATCH PANEL is used to tie a receiver or transmitter to its associated equipment. A COMPARATOR compares incoming signals and selects the strongest to be fed to a teletypewriter through a patch panel. This is used in diversity operation A LISSAJOUS PATTERN is a combined, simultaneous display of the amplitude and phase relationships of two input signals on a crt. A TONE-TERMINAL set converts tty dc pulses into audio tones for modulation of a transmitter in audio-frequency tone shift transmissions. MULTIPLEXING is the process of transmitting a number of intelligence signals simultaneously over a single rf carrier.

TIME-DIVISION multiplexing is the process that periodically samples several intelligence signals. This can be a received signal or one to be transmitted. FREQUENCY-DIVISION multiplexing transmits and receives the full 360 degrees of each sine wave. FACSIMILE is the method for transmitting and receiving still images. These images can be maps, photographs, and handwritten or printed text. SCANNING is the process of subdividing a picture in an orderly manner into segments. This is used in facsimile transmission. FRAMING is the process of synchronizing a facsimile receiver to a transmitter. This allows proper picture reproduction.

TEMPEST is a term normally used to describe compromising emanations. These emanations are unintentionally radiated signals that could disclose classified information. ELECTROMAGNETIC INTERFERENCE is a term used to describe the degradation of a receiver or system by externally produced rf energy. 3-46

Answers To Questions Q1. Through Q39.

A1. To convert energy electrical/acoustic to acoustic/electrical and to key/unkey a transmitter. Also it mutes a receiver when transmitting. A2. Transferring remote control functions and signals to transmitters. A3. Transfers receiver audio outputs to remote control stations. A4. 800 watts. A5. Automatic, semiautomatic, and manual. A6. It matches the impedance of an antenna to that of a transmission line at any desired frequency. A7. To aid in heat transfer and prevent corona and arcing. A8. Lsb, usb, isb, AM, cw, fsk.

A9. Digital. A10. To connect an antenna/transmission line to a receiver/transmitter. A11. Patching and filtering and permits the multiple use of receivers and/or transmitters on a single antenna. A12. Space and mark. A13. Intelligence (5), start (1), stop (1). A14. Shift signals. A15. Synchronous and nonsynchronous. A16. A unit of modulation rate. A17. Binary digit. A18. Neutral and polar. A19. Converts dc to corresponding mark and space modulation. A20. Converts the audio signal to dc pulses. A21. Uses AM to change dc to audio.

A22. A keyer provides rf excitation, which can be shifted above or below the assigned frequency. A23. 60, 75, or 100 wpm. A24. Page-size copy paper and perforated tape. A25. It handles classified information. A26. To code or decode messages. 3-47 A27. The comparator compares the signal strengths from the receivers and the converter converts the frequency-shift rf signal into a tty set dc loop control signal. A28. It converts dc to audio or vice versa. A29. Time-division and frequency-division. A30. It allows simultaneous transmission of multiple signals on a single transmission path.

A31. Facsimile. A32. Compromising emanations. A33. Reliability, security, and speed. A34. Reliability. A35. To ensure continuous, optimum performance of communications systems. A36. Electromagnetic interference. A37. Conduction and radiation. A38. Rf burns and biological, thermal, and neurological effects. A39. Proper bonding and grounding.