Amplitude Modulation

1-11 is referred to as the MODULATED WAVE and is the waveform that is transmitted through space. When the modulated wave is received and demodulated, the original component waves (carrier and modulating waves) are reproduced with their respective frequencies, phases, and amplitudes unchanged. Modulation of a carrier can be achieved by any of several methods. Generally, the methods are named for the sine-wave characteristic that is altered by the modulation process. In this module, you will study AMPLITUDE MODULATION, which includes CONTINUOUS-WAVE MODULATION. You will also learn about two forms of ANGLE MODULATION (FREQUENCY MODULATION and PHASE MODULATION). A special type of modulation, known as PULSE MODULATION, will also be discussed. Before we present the methods involved in developing modulation, you need to study a process that is essential to the modulation of a carrier, known as heterodyning.

To help you understand the operation of heterodyning circuits, we will begin with a discussion of LINEAR and NONLINEAR devices. In linear devices, the output rises and falls directly with the input. In nonlinear devices, the output does not rise and fall directly with the input.

Linear Impedance

Whether the impedance of a device is linear or nonlinear can be determined by comparing the change in current through the device to the change in voltage applied to the device. The simple circuit shown in view (A) of figure 1-4 is used to explain this process. Figure 1-4A.—Circuit with one linear impedance. First, the current through the device must be measured as the voltage is varied. Then the current and voltage values can be plotted on a graph, such as the one shown in view (B), to determine the impedance of the device. For example, assume the voltage is varied from 0 to 200 volts in 50-volt steps, as shown in view (B). At the first 50-volt point, the ammeter reads 0.5 ampere. These ordinates are plotted as point a in view (B). With 100 volts applied, the ammeter reads 1 ampere; this value is plotted as point b. As these steps are continued, the values are plotted as points c and d. These points are connected with a straight line to show the linear relationship between current and voltage. For every change in voltage applied to the device, a proportional change occurs in the current through the device. When the change in current is proportional to the change in applied voltage, the impedance of the device is linear and a straight line is developed in the graph.

1-12 Figure 1-4B.—Circuit with one linear impedance. The principle of linear impedance can be extended by connecting two impedance devices in series, as shown in figure 1-5, view (A). The characteristics of both individual impedances are determined as explained in the preceding section. For example, assume voltmeter V1 shows 50 volts and the ammeter shows 0.5 ampere. Point a in view (B) represents this ordinate. In the same manner, increasing the voltage in increments of 50 volts gives points b, c, and d. Lines Z1 and Z2 show the characteristics of the two impedances. The total voltage of the series combination can be determined by adding the voltages across Z1 and Z2. For example, at 0.5 ampere, point a (50 volts) plus point e (75 volts) produces point i (125 volts). Also, at 1 ampere, point b plus point f produces point j. Line Z1 + Z2 represents the combined voltage-current characteristics of the two devices.

Figure 1-5A.—Circuit with two linear impedances. 1-13 Figure 1-5B.—Circuit with two linear impedances. View (A) of figure 1-6 shows two impedances in parallel. View (B) plots the impedances both individually (Z1 and Z2) and combined (Z1 x Z2)/(Z1 + Z2). Note that Z1 and Z2 are not equal. At 100 volts, Z1 has 1 ampere of current plotted at point b and Z2 has 0.5 ampere plotted at point f. The coordinates of the equivalent impedance of the parallel combination are found by adding the current through Z1 to the current through Z2. For example, at 100 volts, point b is added to point f to determine point j (1.5 amperes).

1-14 Figure 1-6.—Circuit with parallel linear impedances. Positive or negative voltage values can be used to plot the voltage-current graph. Figure 1-7 shows an example of this situation. First, the voltage versus current is plotted with the battery polarity as shown in view (A). Then the battery polarity is reversed and the remaining voltage versus current points are plotted. As a result, the line shown in view (C) is obtained. Figure 1-7A.—Linear impedance circuit. 1-15 Figure 1-7B.—Linear impedance circuit. Figure 1-7C.—Linear impedance circuit The battery in view (A) could be replaced with an ac generator, as shown in view (B), to plot the characteristic chart. The same linear voltage-current chart would result. Current flow in either direction is directly proportional to the change in voltage.

In conclusion, when dc or sine-wave voltages are applied to a linear impedance, the current through the impedance will vary directly with a change in the voltage. The device could be a resistor, an air-core inductor, a capacitor, or any other linear device. In other words, if a sine-wave generator output is applied to a combination of linear impedances, the resultant current will be a sine wave which is directly proportional to the change in voltage of the generator. The linear impedances do not alter the waveform of the sine wave. The amplitude of the voltage developed across each linear component may vary, or the phase of the wave may shift, but the shape of the wave will remain the same.

1-16

Nonlinear Impedance

You have studied that a linear impedance is one in which the resulting current is directly proportional to a change in the applied voltage. A nonlinear impedance is one in which the resulting current is not directly proportional to the change in the applied voltage. View (A) of figure 1-8 illustrates a circuit which contains a nonlinear impedance (Z), and view (B) shows its voltage-current curve. Figure 1-8A.—Nonlinear impedance circuit. Figure 1-8B.—Nonlinear impedance circuit. As the applied voltage is varied, ammeter readings which correspond with the various voltages can be recorded. For example, assume that 50 volts yields 0.4 milliampere (point a), 100 volts produces 1 milliampere (point b), and 150 volts causes 2.2 milliamperes (point c). Current through the nonlinear impedance does not vary proportionally with the voltage; the chart is not a straight line. Therefore, Z is a nonlinear impedance; that is, the current through the impedance does not faithfully follow the change in voltage. Various combinations of voltage and current for this particular nonlinear impedance may be obtained by use of this voltage-current curve.

1-17

Combined Linear And Nonlinear Impedances

The series combination of a linear and a nonlinear impedance is illustrated in view (A) of figure 1-9. The voltage-current charts of Z1 and Z2 are shown in view (B). A chart of the combined impedance can be plotted by adding the amount of voltage required to produce a particular current through linear impedance Z1 to the amount of voltage required to produce the same amount of current through nonlinear impedance Z2. The total will be the amount of voltage required to produce that particular current through the series combination. For example, point a (25 volts) is added to point c (50 volts) which yields point e (75 volts); and point b (50 volts) is added to point d (100 volts) which yields point f (150 volts).

Intermediate points may be determined in the same manner and the resultant characteristic curve (Z1 + Z2) is obtained for the series combination. Figure 1-9A.—Combined linear and nonlinear impedances. Figure 1-9B.—Combined linear and nonlinear impedances. 1-18 You should see from this graphic analysis that when a linear impedance is combined with a nonlinear impedance, the resulting characteristic curve is nonlinear. Some examples of nonlinear impedances are crystal diodes, transistors, iron-core transformers, and electron tubes.

Ac Applied To Linear And Nonlinear Impedances

Figure 1-10 illustrates an ac sine-wave generator applied to a circuit containing several linear impedances. A sine-wave voltage applied to linear impedances will cause a sine wave of current through them. The wave shape across each linear impedance will be identical to the applied waveform. Figure 1-10.—Sine wave generator applied to several impedances. The amplitude, on the other hand, may differ from the amplitude of the applied voltage. Furthermore, the phase of the voltage developed by any of the impedances may not be identical to the phase of the voltage across any of the other impedances or the phase of the applied voltage. If an impedance is a reactive component (coil or capacitor), voltage or current may lead or lag, but the wave shape will remain the same. In a linear circuit, the output of the generator is not distorted. The frequency remains the same throughout the entire circuit and no new frequencies are generated.

View (A) of figure 1-11 illustrates a circuit that contains a combination of linear and nonlinear impedances with a sine wave of voltage applied. Impedances Z2, Z3, and Z4 are linear; and Z1 is nonlinear. The result of a linear and nonlinear combination of impedances is a nonlinear waveform. The curve Z, shown in view (B), is the nonlinear curve for the circuit of view (A). Because of the nonlinear impedance, current can flow in the circuit only during the positive alternation of the sine-wave generator. If an oscilloscope is connected, as shown in view (A), the waveform across Z3 will not be a sine wave.

Figure 1-12, view (A), illustrates the sine wave from the generator and view (B) shows the waveform across the linear impedance Z3. Notice that the nonlinear impedance Z1 has eliminated the negative half cycles. 1-19 Figure 1-11A.—Circuit with nonlinear impedances. Figure 1-11B.—Circuit with nonlinear impedances. Figure 1-12A.—Waveform in a circuit with nonlinear impedances. 1-20 Figure 1-12B.—Waveform in a circuit with nonlinear impedances. The waveform in view (B) is no longer identical to that of view (A) and the nonlinear impedance network has generated HARMONIC FREQUENCIES. The waveform now consists of the fundamental frequency and its harmonics. (Harmonics were discussed in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits.)

Two Sine Wave Generators In Linear Circuits

A circuit composed of two sine-wave generators, G1 and G2, and two linear impedances, Z1 and Z2, is shown in figure 1-13. The voltage applied to Z1 and Z2 will be the vector sum of the generator voltages. The sum of the individual instantaneous voltages across each impedance will equal the applied voltages. Figure 1-13.—Two sine-wave generators with linear impedances. If the two generator outputs are of the same frequency, then the waveform across Z1 and Z2 will be a sine wave, as shown in figure 1-14, views (A) and (B). No new frequencies will be created. Relative amplitude and phase will be determined by the relative values and types of the impedances.

1-21 Figure 1-14A.—Waveforms across two nonlinear impedances. Figure 1-14B.—Waveforms across two nonlinear impedances. If the two sine wave generators are of different frequencies, then the sum of the instantaneous values will appear as a complex wave across the impedances, as shown in figure 1-15, views (A) and (B). To determine the wave shape across each individual impedance, assume only one generator is connected at a 1-22 time and compute the sine-wave voltage developed across each impedance for that generator input. Then, combine the instantaneous voltages (caused by each generator input) to obtain the complex waveform across each impedance. The nature of the impedance (resistive or reactive) will determine the shape of the complex waveform. Because the complex waveform is the sum of two individual sine waves, the composite waveform contains only the two original frequencies.

Figure 1-15A.—Sine-wave generators with different frequencies and linear impedances. Figure 1-15B.—Sine-wave generators with different frequencies and linear impedances. Linear impedances may alter complex waveforms, but they do not produce new frequencies. The output of one generator does not influence the output of the other generator.

Two Sine Wave Generators And A Combination Of Linear And Nonlinear

Impedances

Figure 1-16 illustrates a circuit that contains two sine-wave generators (G1 and G2), linear impedance Z1, and nonlinear impedance Z2, in series. When a single sine-wave voltage is applied to a 1-23 combined linear and nonlinear impedance circuit, the voltages developed across the impedances are complex waveforms. Figure 1-16.—Sine-wave generators with a combination of impedances. When two sine wave voltages are applied to a circuit, as in figure 1-16, nonlinear impedance Z2 reshapes the two sine-wave inputs and their harmonics, resulting in a very complex waveform.

Assume that nonlinear impedance Z2 will allow current to flow only when the sum of the two sine-wave generators (G1 and G2) has the polarity indicated. The waveforms present across the linear impedance will appear as a varying waveform. This will be a complex waveform consisting of: • a dc level • the two fundamental sine wave frequencies • the harmonics of the two fundamental frequencies • the sum of the fundamental frequencies • the difference between frequencies The sum and difference frequencies occur because the phase angles of the two fundamentals are constantly changing. If generator G1 produces a 10-hertz voltage and generator G2 produces an 11-hertz voltage, the waveforms produced because of the nonlinear impedance will be as shown in the following list: • a 10-hertz voltage • an 11-hertz voltage • harmonics of 10 hertz and 11 hertz (the higher the harmonic, the lower its strength) • the sum of 10 hertz and 11 hertz (21 hertz) • the difference between 10 hertz and 11 hertz (1 hertz) 1-24 Figure 1-17 illustrates the relationship between the two frequencies (10 and 11 hertz). Since the waveforms are not of the same frequency, the 10 hertz of view (B) and the 11 hertz of view (A) will be in phase at some points and out of phase at other points. You can see this by closely observing the two waveforms at different instants of time. The result of the differences in phase of the two sine waves is shown in view (C). View (D) shows the waveform that results from the nonlinearity in the circuit.

Figure 1-17A.—Frequency relationships. Figure 1-17B.—Frequency relationships. Figure 1-17C.—Frequency relationships. Figure 1-17D.—Frequency relationships. The most important point to remember is that when varying voltages are applied to a circuit which contains a nonlinear impedance, the resultant waveform contains frequencies which are not present at the input source. The process of combining two or more frequencies in a nonlinear impedance results in the production of new frequencies. This process is referred to as heterodyning.

1-25

Spectrum Analysis

The heterodyning process can be analyzed by using SPECTRUM ANALYSIS (the display of electromagnetic energy arranged according to wavelength or frequency). As shown in figure 1-18, spectrum analysis is an effective way of viewing the energy in electronic circuits. It clearly shows the relationships between the two fundamental frequencies (10 and 11 hertz) and their sum (21 hertz) and difference (1 hertz) frequencies. It also allows you to view the BANDWIDTH (the amount of the frequency spectrum that signals occupy) of the signal you are studying.

Figure 1-18.—Spectrum analysis of heterodyned signal.

Typical Heterodyning Circuit

Two conditions must be met in a circuit for heterodyning to occur. First, at least two different frequencies must be applied to the circuit. Second, these signals must be applied to a nonlinear impedance. These two conditions will result in new frequencies (sum and difference) being produced. Any one of the frequencies can be selected by placing a frequency-selective device (such as a tuned tank circuit) in series with the nonlinear impedance in the circuit. Figure 1-19 illustrates a basic heterodyning circuit. The diode D1 serves as the nonlinear impedance in the circuit. Generators G1 and G2 are signal sources of different frequencies. The primary of T1, with its associated capacitance, serves as the frequency-selective device.

Figure 1-19.—Typical heterodyning circuit. 1-26 The principles of this circuit are similar to those of the block diagram circuit of figure 1-16. Notice in figure 1-19 that the two generators are connected in series. Therefore, the resultant waveform of their combined frequencies will determine when the cathode of D1 will be negative with respect to the anode, thereby controlling the conduction of the diode. The new frequencies that are generated by applying these signals to nonlinear impedance D1 are the sum and difference of the two original frequencies. The frequency-selective device T1 may be tuned to whichever frequency is desired for use in later circuit stages. Heterodyning action takes place, intentionally or not, whenever these conditions exist.

Heterodyning (MIXING) circuits are found in most electronic transmitters and receivers. These transmitter and receiver circuits will be explained in detail later in this module. Q-14. Define the heterodyne principle. Q-15. What is a nonlinear impedance? Q-16. What is spectrum analysis? Q-17. What two conditions are necessary for heterodyning to take place?

Amplitude-Modulated Systems

Amplitude modulation refers to any method of varying the amplitude of an electromagnetic carrier frequency in accordance with the intelligence to be transmitted by the carrier. The CARRIER frequency is a radio-frequency wave suitable for modulation by the intelligence to be transmitted. One form of this method of modulation is simply to interrupt the carrier in accordance with a prearranged code.

Continuous Wave (Cw)

The "on-off" KEYING of a continuous wave (cw) carrier frequency was the principal method of modulating a carrier in the early days of electrical communications. The intervals of time when a carrier either was present or absent conveyed the desired intelligence. This is still used in modern communications. When applied to a continuously oscillating radio-frequency source, on-off keying is referred to as cw signaling. This type of communication is sometimes referred to as an interrupted continuous wave (icw). Development The use of a cw transmitter can be very simple. All that is required for the transmitter to work properly is a device to generate the oscillations, a method of keying the oscillations on and off, and an antenna to radiate the energy. Continuous wave was the first type of modulation used. It is still extensively used for long-range communications. When Marconi and others were attempting the transfer of intelligence between two points, without reliance on a conducting path, they employed the use of a practical coding system known as Morse code. You probably know that Morse code is a system of on-off keying developed for telegraph that is capable of passing intelligence over wire at an acceptable rate.

Morse code consists only of periods of signal and no-signal. Figure 1-20 is the International Morse code used with telegraphy and cw modulation. Each character in the code is made up of a series of elements referred to as DOTS or DASHES. These are short (dot) and long (dash) bursts of signal separated by intervals of no signal. The dot is the basic time element of the code. The dash has three times the duration of a dot interval. The waveforms for both are shown in figure 1-21. The elements within each character are separated by intervals of no signal with a time duration of one dot. The characters are separated by a no-signal interval equal in duration to one dash. Each interval 1-27 during which signal is present is called the MARKING interval, and the period of no signal is called the SPACING interval. Figure 1-22 shows the relationships between the rf carrier view (A), the on-off keying waveform view (B), and the resultant carrier wave view (C).

Figure 1-20.—International Morse code. Figure 1-21.—Dot and dash in radiotelegraph code. 1-28 Figure 1-22A.—Essential elements of ON-OFF keying. Figure 1-22B.—Essential elements of ON-OFF keying. Figure 1-22C.—Essential elements of ON-OFF keying. 1-29 Keying Methods Keying a transmitter causes an rf signal to be radiated only when the key contacts are closed. When the key contacts are open, the transmitter does not radiate energy. Keying is accomplished in either the oscillator or amplifier stage of a transmitter. A number of different keying systems are used in Navy transmitters.

In most Navy transmitters, the hand telegraph key is at a low-voltage potential with respect to ground. A keying bar is usually grounded to protect the operator. Generally, a keying relay, with its contacts in the center-tap lead of the filament transformer, is used to key the equipment. Because one or more stages use the same filament transformer, these stages are also keyed. A class C final amplifier, when operated with fixed bias, is usually not keyed. This is because no output occurs when no excitation is applied in class C operation. Keying the final amplifier along with the other stages is not necessary in this case.

OSCILLATOR KEYING.—Two methods of OSCILLATOR KEYING are shown in figure 1-23. In view (A) the grid circuit is closed at all times. The key (K) opens and closes the negative side of the plate circuit. This system is called PLATE KEYING. When the key is open, no plate current can flow and the circuit does not oscillate. In view (B), the cathode circuit is open when the key is open and neither grid current nor plate current can flow. Both circuits are closed when the key is closed. This system is called CATHODE KEYING. Although the circuits of figure 1-23 may be used to key amplifiers, other keying methods are generally employed because of the high values of plate current and voltage encountered.

Figure 1-23A.—Oscillator keying. Figure 1-23B.—Oscillator keying. 1-30 BLOCKED-GRID KEYING.—Two methods of BLOCKED-GRID KEYING are shown in figure 1-24. The key in view (A) shorts cathode resistor R1 allowing normal plate current to flow. With the key open, reduced plate current flows up through resistor R1 making the end connected to grid resistor Rg negative. If R1 has a high enough value, the bias developed is sufficient to cause cutoff of plate current. Depressing the key short-circuits R1. This increases the bias above cutoff and allows the normal flow of plate current. Grid resistor Rg is the usual grid-leak resistor for normal biasing.

Figure 1-24A.—Blocked-grid keying. Figure 1-24B.—Blocked-grid keying. The blocked-grid keying method in view (B) affords a complete cutoff of plate current. It is one of the best methods for keying amplifier stages in transmitters. In the voltage divider with the key open, two- thirds of 1,000 volts, or 667 volts, is developed across the 200-kilohm resistor; one-third of 1,000 volts, or 333 volts, is developed across the 100-kilohm resistor. The grid bias is the sum of 100 volts and 333 volts, or 433 volts. This sum is below cutoff and no plate current flows. The plate voltage is 667 volts.

With the key closed, the 200-kilohm resistor drops 1,000 volts. The plate voltage becomes 1,000 volts at the same time the grid bias becomes 100 volts. Grid bias is reduced enough so that the triode amplifier will conduct only on the peaks of the drive signal. 1-31 When greater frequency stability is required, the oscillator should not be keyed, but should remain in continuous operation; other transmitter circuits may be keyed. This procedure keeps the oscillator tube at a normal operating temperature and offers less chance for frequency variations to occur each time the key is closed.

KEYING RELAYS.—In transmitters using a crystal-controlled oscillator, the keying is almost always in a circuit stage following the oscillator. In large transmitters (75 watts or higher), the ordinary hand key cannot accommodate the plate current without excessive arcing. WARNING

Because Of The High Plate Potentials Used, Operating A Hand

Key In The Plate Circuit Is Dangerous. A Slight Slip Of The Hand

Below The Key Knob Could Result In Severe Shock Or, In The

Case Of Defective Rf Plate Chokes, A Severe Rf Burn.

In larger transmitters, some local low-voltage supply, such as a battery, is used with the hand key to open and close a circuit through the coils of a KEYING RELAY. The relay contacts open and close the keying circuit of the amplifier. A schematic diagram of a typical relay-operated keying system is shown in figure 1-25. The hand key closes the circuit from the low-voltage supply through the coil (L) of the keying relay. The relay armature closes the relay contacts as a result of the magnetic pull exerted on the armature. The armature moves against the tension of a spring. When the hand key is opened, the relay coil is deenergized and the spring opens the relay contacts.

Figure 1-25.—Relay-operated keying system. KEY CLICKS.—Ideally, cw keying a transmitter should instantly start and stop radiation of the carrier completely. However, the sudden application and removal of power causes rapid surges of current which may cause interference in nearby receivers. Even though such receivers are tuned to frequencies far removed from that of the transmitter, interference may be present in the form of "clicks" or "thumps." KEY-CLICK FILTERS are used in the keying systems of radio transmitters to prevent such interference.

Two types of key-click filters are shown in figure 1-26. 1-32 Figure 1-26A.—Key-click filters. Figure 1-26B.—Key-click filters. The capacitors and rf chokes in figure 1-26, views (A) and (B), prevent surges of current. In view (B), the choke coil causes a lag in the current when the key is closed, and the current builds up gradually instead of instantly. The capacitor charges as the key is opened and slowly releases the energy stored in the magnetic field of the inductor. The resistor controls the rate of charge of the capacitor and also prevents sparking at the key contacts by the sudden discharge of the capacitor when the key is closed.

MACHINE KEYING.—The speed with which information can be transmitted using a hand key depends on the keying ability of the operator. Early communicators turned to mechanized methods of keying the transmitters to speed transmissions. More information could be passed in a given time by replacing the hand-operated key with a keying device capable of reading information from punched tape. Using this method, several operators could prepare tapes at their normal operating speed. The tapes could then be read through the keying device at a higher rate of speed and more information could be transmitted in a given amount of time.

Spectrum Analysis Continuous-wave transmission has the disadvantage of being a relatively slow transmission method. Still, it has several advantages. Some of the advantages of cw transmission are a high degree of clarity under severe noise conditions, long-range operation, and narrow bandwidth. A highly skilled operator can pick out and read a cw signal even though it has a high degree of background noise or interference. Since only a single-carrier frequency is being transmitted, all of the transmitter power can be concentrated in the intelligence. This concentration of power gives the transmission a greater range. The use of spectrum 1-33 analysis (figure 1-27) illustrates the transmitted frequency characteristics of a cw signal. Because the cw signal is a pure sine wave, it occupies only a single frequency in the rf spectrum and the system is relatively simple.

Figure 1-27.—Carrier-wave signal spectrum analysis. Q-18. What is amplitude modulation? Q-19. What are the three requirements for cw transmission? Q-20. Name two methods of oscillator keying. Q-21. State the method used to increase the speed of keying in a cw transmitter. Q-22. Name three advantages of cw transmission. Single-Stage Transmitters A simple, single-tube cw transmitter can be made by coupling the output of an oscillator directly to an antenna (figure 1-28). The primary purpose of the oscillator is to develop an rf voltage which has a constant frequency and is immune to outside factors which may cause its frequency to shift. The output of this simple transmitter is controlled by placing a telegraph key at point K in series with the voltage supply. Since the plate supply is interrupted when the key is open, the circuit oscillates only as long as the key is closed. Although the transmitter shown uses a Colpitts oscillator, any of the oscillators previously described in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits can be used.

Figure 1-28.—Simple electron-tube transmitter. 1-34 Capacitors C2 and C3 can be GANGED (mechanically linked together) to simplify tuning. Capacitor C1 is used to tune (resonate) the antenna to the transmitter frequency. CA is the effective capacitance existing between the antenna and ground. This antenna-to-ground capacitance is in parallel with the tuning capacitors, C2 and C3. Since the antenna has capacitance, any change in its length or position, such as that caused by swaying of the antenna, changes the value of CA and causes the oscillator to change frequency. Because these frequency changes are undesirable for reliable communications, the multistage transmitter was developed to increase reliability.

Multistage Transmitters The simple, single-tube transmitter, shown in figure 1-28, is rarely used in practical equipment. Most of the transmitters you will see use a number of tubes or stages. The number used depends on the frequency, power, and application of the equipment. For your study, the following three categories of cw transmitters are discussed: (1) master oscillator power amplifier (mopa) transmitters, (2) multistage, high-power transmitters, (3) high- and very-high frequency transmitters. The mopa is both an oscillator and a power amplifier. Power-amplifying stages and frequency- multiplying stages must be used to increase power and raise the frequency from those achievable in a mopa. The main difference between many low- and high-power transmitters is in the number of power- amplifying stages that are used. Similarly, the main difference between many high- and very-high frequency transmitters is in the number of frequency-multiplying stages used.

MASTER OSCILLATOR POWER AMPLIFIER.—For a transmitter to be stable, its oscillator must not be LOADED DOWN. This means that its antenna (which can present a varying impedance) must not be connected directly to the oscillator circuit. The rf oscillations must be sent through another circuit before they are fed to the antenna for good frequency stability to be obtained. That additional circuit is an rf power amplifier. Its purpose is to raise the amplitude of rf oscillations to the required output power level and isolate the oscillator from the antenna. Any transmitter consisting of an oscillator and a single-amplifier stage is called a master oscillator power amplifier transmitter (mopa), as shown in figure 1-29.

Figure 1-29.—Block diagram of a master oscillator power amplifier transmitter (mopa). Most mopa transmitters have only one tube amplifier in the power-amplifier stage. However, the oscillator may not produce sufficient power to drive a power-amplifier tube to the power output level required for the antenna. In such cases, the power-amplifier stages are designed to use two or more amplifiers which can be driven by the oscillator. Two or more amplifiers can be connected in parallel (with similar elements of each tube connected) or in a push-pull arrangement. In a push-pull amplifier, the grids are fed equal rf voltages that are 180 degrees out of phase.

The main advantage of a mopa transmitter is that the power-amplifier stage isolates the oscillator from the antenna. This prevents changes in antenna-to-ground capacitance from affecting the oscillator 1-35 frequency. A second advantage is that the rf power amplifier is operated so that a small change in the voltage applied to its grid circuit will produce a large change in the power developed in its plate circuit. Rf power amplifiers require that a specific amount of power be fed into the grid circuit. Only in this way can the tube deliver an amplified power output. However, the stable oscillator can produce only limited amounts of power. Therefore, the mopa transmitter is limited in the amount of power it can develop. This is one of the disadvantages of the mopa transmitter. Another disadvantage is that it often is impractical for use at very- and ultra-high frequencies. The reason is that the stability of self-excited oscillators decreases rapidly as the operating frequency increases. Circuit tuning capacitances are small at high frequencies and stray capacitances adversely affect frequency stability.

MULTISTAGE HIGH-POWER TRANSMITTERS.—The power amplifier of a high-power transmitter may require far more driving power than can be supplied by an oscillator. Therefore, one or more low-power intermediate amplifiers may be inserted between the oscillator and the final power amplifier to boost power to the antenna. In some types of equipment, a VOLTAGE AMPLIFIER, called a BUFFER is used between the oscillator and the first intermediate amplifier. The ideal buffer is operated class A and is biased negatively to prevent grid current flow during the excitation cycle. Therefore, it does not require driving power from, nor does it load down, the oscillator. The purpose of the buffer is to isolate the oscillator from the following stages and to minimize changes in oscillator frequency that occur with changes in loading. A buffer is required when keying takes place in an intermediate or final amplifier operating at comparatively high power. Look at the block diagrams of several medium-frequency transmitters in figure 1-30. The input and output powers are given for each stage. You should be able to see that the power output rating of a transmitter can be increased by adding amplifier tubes capable of delivering the power required.

Figure 1-30.—Block diagram of several medium-frequency transmitters. 1-36 HF AND VHF TRANSMITTERS.—Oscillators are too unstable for direct frequency control in very- and ultra-high frequency transmitters. Therefore, these transmitters have oscillators operating at comparatively low frequencies, sometimes as low as 1/100 of the output frequency. The oscillator frequency is raised to the required output frequency by passing it through one or more FREQUENCY MULTIPLIERS. Frequency multipliers are special rf power amplifiers which multiply the input frequency. In practice, the MULTIPLICATION FACTOR (number of times the input frequency is multiplied) is seldom larger than five in any one stage. The block diagram of a typical VHF transmitter, designed for continuous tuning between 256 and 288 megahertz, is shown in figure 1-31.

Figure 1-31.—Block diagram of a vhf transmitter. The stages which multiply the frequency by two are DOUBLERS; those which multiply by four are QUADRUPLERS. The oscillator is tunable from 4 to 4.5 megahertz. The multiplier stages increase the frequency by multiplying successively by 4, 4, 2, and 2, for a total factor of 64. In high-power, high- frequency transmitters, one or more intermediate amplifiers may be used between the last frequency multiplier and the power amplifier. Q-23. Name a disadvantage of a single-stage cw transmitter.

Q-24. What is the purpose of the power-amplifier stage in a master oscillator power amplifier cw transmitter? Q-25. What is the purpose of frequency-multiplier stages in a VHF transmitter?

Amplitude Modulation

The telegraph and radiotelegraph improved man's ability to communicate by allowing speedy passage of information between two distant points. However, it failed to satisfy one of man's other communications needs; that is, the ability to hear and be heard, by voice, at a great distance. In an effort to improve on the telegraph, Alexander Graham Bell developed the principles on which modern communications are built. He developed the modulation of an electric current by complex waveforms, the demodulation of the resulting wave, and recovery of the original waveform. This section will examine the process of varying an electric current in amplitude at an audio frequency.

1-37 Microphones If an rf carrier is to convey intelligence, some feature of the carrier must be varied in accordance with the information to be transmitted. In the case of speech intelligence, sound waves must be converted to electrical energy. A MICROPHONE is an energy converter that changes sound energy into electrical energy. A diaphragm in the microphone moves in and out in accordance with the compression and rarefaction of the atmosphere caused by sound waves. The diaphragm is connected to a device that causes current flow in proportion to the instantaneous pressure delivered to it. Many devices can perform this function. The particular device used in a given application depends on the characteristics desired, such as sensitivity, frequency response, impedance matching, power requirements, and ruggedness.

The SENSITIVITY or EFFICIENCY of a microphone is usually expressed in terms of the electrical power level which the microphone delivers to a matched-impedance load compared to the sound level being converted. The sensitivity is rated in dB and must be as high as possible. A high microphone output requires less gain in the amplifiers used with the microphone. This keeps the effects of thermal noise, amplifier hum, and noise pickup at a minimum. For good quality sound reproduction, the electrical signal from the microphone must correspond in frequency content to the original sound waves. The microphone response should be uniform, or flat, within its frequency range and free from the electrical or mechanical generation of new frequencies.

The impedance of a microphone is important in that it must be matched to the microphone cable and to the amplifier input as well as to the amplifier input load. Exact impedance matching is not always possible, especially in the case where the impedance of the microphone increases with an increase in frequency. A long microphone cable tends to seriously attenuate the high frequencies if the microphone impedance is high. This attenuation is caused by the increased capacitive action of the line at higher frequencies. If the microphone has a low impedance, a lower voltage is developed in the microphone, and more voltage is available at the load. Because many microphone lines used aboard ship are long, low- impedance microphones must be used to preserve a sufficiently high voltage level- over the required frequency range.

The symbol used to represent a microphone in a schematic diagram is shown in figure 1-32. The schematic symbol identifies neither the type of microphone used nor its characteristics. Figure 1-32.—Microphone schematic symbol. CARBON MICROPHONE.—Operation of the SINGLE-BUTTON CARBON MICROPHONE figure 1-33, view (A) is based on varying the resistance of a pile of carbon granules located within the microphone. An insulated cup, referred to as the button, holds the loosely piled granules. It is so mounted that it is in constant contact with the thin metal diaphragm. Sound waves striking the diaphragm vary the pressure on the button which varies the pressure on the pile of carbon granules. The dc resistance of the carbon granule pile is varied by this pressure. This varying resistance is in series with a battery and the 1-38 primary of a transformer. The changing resistance of the carbon pile produces a corresponding change in the current of the circuit. The varying current in the transformer primary produces an alternating voltage in the secondary. The transformer steps up the voltage and matches the low impedance of the microphone to the high impedance of the first amplifier. The voltage across the secondary may be as high as 25 volts peak. The impedance of this type of microphone varies from 50 to 200 ohms. This effect is caused by the pressure of compression and rarefaction of sound waves, discussed in chapter 1 of NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas.

Figure 1-33A.—Carbon microphones. SINGLE-BUTTON CARBON MICROPHONE. The DOUBLE-BUTTON CARBON MICROPHONE is shown in figure 1-33, view (B). Here, one button is positioned on each side of the diaphragm so that an increase in resistance on one side is accompanied by a simultaneous decrease in resistance on the other. Each button is in series with the battery and one-half of the transformer primary. The decreasing current in one-half of the primary and the increasing current in the other half produces an output voltage in the secondary winding. The output voltage is proportional to the sum of the primary winding signal components. This action is similar to that of push-pull amplifiers.

Figure 1-33B.—Carbon microphones. DOUBLE-BUTTON CARBON MICROPHONE. 1-39 One disadvantage of carbon microphones is that of a constant BACKGROUND HISS (hissing noise) which results from random changes in the resistance between individual carbon granules. Other disadvantages are reduced sensitivity and distortion that may result from the granules packing or sticking together. The carbon microphone also has a limited frequency response. Still another disadvantage is a requirement for an external voltage source. The disadvantages, however, are offset by advantages that make its use in military applications widespread. It is lightweight, rugged, and can produce an extremely high output.

CRYSTAL MICROPHONE.—The CRYSTAL MICROPHONE uses the PIEZOELECTRIC EFFECT of Rochelle salt, quartz, or other crystalline materials. This means that when mechanical stress is placed upon the material, a voltage electromagnetic force (EMF) is generated. Since Rochelle salt has the largest voltage output for a given mechanical stress, it is the most commonly used crystal in microphones. View (A) of figure 1-34 is a crystal microphone in which the crystal is mounted so that the sound waves strike it directly. View (B) has a diaphragm that is mechanically linked to the crystal so that the sound waves are indirectly coupled to the crystal.

Figure 1-34A.—Crystal microphones. Figure 1-34B.—Crystal microphones. 1-40 A crystal microphone has a high impedance and does not require an external voltage source. It can be connected directly into the input circuit of a high-gain amplifier. However, because its output is low, several stages of high-gain amplification are required. Crystal microphones are delicate and must be handled with care. Exposure to temperatures above 52 degrees Celsius (125 degrees Fahrenheit) may permanently damage the crystal unit. Crystals are also soluble in water and other liquids and must be protected from moisture and excessive humidity.

DYNAMIC MICROPHONE.—A cross section of the DYNAMIC or MOVING-COIL MICROPHONE is shown in figure 1-35. A coil of fine wire is mounted on the back of the diaphragm and located in the magnetic field of a permanent magnet. When sound waves strike the diaphragm, the coil moves back and forth cutting the magnetic lines of force. This induces a voltage into the coil that is an electrical reproduction of the sound waves. Figure 1-35.—Dynamic microphone. The sensitivity of the dynamic microphone is almost as high as that of the carbon type. It is lightweight and requires no external voltage. The dynamic microphone is rugged and can withstand the effects of vibration, temperature, and moisture. This microphone has a uniform response over a frequency range that extends from 40 to 15,000 hertz. The impedance is very low (generally 50 ohms or less). A transformer is required to match its impedance to that of the input of an af amplifier.

MAGNETIC MICROPHONE.—The MAGNETIC or MOVING-ARMATURE MICROPHONE (figure 1-36) consists of a coil wound on an armature that is mechanically connected to the diaphragm with a driver rod. The coil is located between the pole pieces of the permanent magnet. Any vibration of the diaphragm vibrates the armature at the same rate. This varies the magnetic flux in the armature and through the coil. 1-41 Figure 1-36.—Magnetic microphone action. When the armature is in its resting position (midway between the two poles), the magnetic flux is established across the air gap. However, no resultant flux is established in the armature. When a compression wave strikes the diaphragm, the armature is deflected to the right. Most of the flux continues to move in the direction of the arrows. However, some flux now flows from the north pole of the magnet across the reduced gap at the upper right, down through the armature, and around to the south pole of the magnet.

When a rarefaction wave occurs at the diaphragm, the armature is deflected to the left. Some flux is now directed from the north pole of the magnet, up through the armature, through the reduced gap at the upper left, and back to the south pole. The vibrations of the diaphragm cause an alternating flux in the armature which induces an alternating voltage in the coil. This voltage has the same waveform as that of the sound waves striking the diaphragm. The magnetic microphone is very similar to the dynamic microphone in terms of impedance, sensitivity, and frequency response. However, it is more resistant to vibration, shock, and rough handling than other types of microphones.

Changing sound waves into electrical impulses is the first step in voice communications. It is common to all the transmission media you will study in the remainder of this chapter. We will discuss the various types of modulation that arc used to transfer this information to a transmission medium in the following sections. Q-26. What is a microphone? Q-27. What special electromechanical effect is the basis for carbon microphone operation? Q-28. What is a major disadvantage of a carbon microphone? Q-29. What property of a crystalline material is used in a crystal microphone?

Q-30. What is the difference between a dynamic microphone and a magnetic microphone? 1-42

Am Transmitter Principles

In this section we will describe the methods used to apply voice signals (intelligence) to a carrier wave by the process of amplitude modulation (AM). An AM transmitter can be divided into two major sections according to the frequencies at which they operate, radio-frequency (rf) and audio-frequency (af) units. The rf unit is the section of the transmitter used to generate the rf carrier wave. As illustrated in figure 1-37, the carrier originates in the master oscillator stage where it is generated as a constant-amplitude, constant-frequency sine wave. The carrier is not of sufficient amplitude and must be amplified in one or more stages before it attains the high power required by the antenna. With the exception of the last stage, the amplifiers between the oscillator and the antenna are called INTERMEDIATE POWER AMPLIFIERS (ipa). The final stage, which connects to the antenna, is called the FINAL POWER AMPLIFIER (fpa).

Figure 1-37.—Block diagram of an AM transmitter. The second section of the transmitter contains the audio circuitry. This section of the transmitter takes the small signal from the microphone and increases its amplitude to the amount necessary to fully modulate the carrier. The last audio stage is the MODULATOR. It applies its signal to the carrier in the final power amplifier. In this way, intelligence is included in the radiated rf waveform. The Modulated Wave The frequencies present in a signal can be conveniently represented by a graph of the frequency spectrum, shown in figure 1-38. In this graph, each individual frequency is portrayed as a vertical line.

The position of the line along the horizontal axis indicates the frequency of the signal. The height of the frequency line is proportional to the amplitude of the signal. The rf spectrum in figure 1-38 shows the frequencies present when heterodyning occurs between frequencies of 5 and 100 kilohertz. 1-43 Figure 1-38.—Radio-frequency spectrum. Radiating energy at audio frequencies (discussed earlier in this chapter) is not practical. The heterodyning principle, however, makes possible the conversion of an af signal (intelligence) to an rf signal (with af intelligence) which can be radiated or transmitted through space.

Look again at figure 1-38. The sum and difference frequencies are located very near the rf signal (100 kilohertz), while the audio signal (5 kilohertz) is spaced a considerable distance away. Because of this frequency separation, the audio frequency can be easily removed by filter circuits, leaving just three radio frequencies of 95, 100, and 105 kilohertz. These three radio frequencies are radiated through space to the receiving station. At the receiver, the process is reversed. The frequency of 95 kilohertz, for example, is heterodyned with the frequency of 100 kilohertz and the sum and difference frequencies are again produced. (A similar process occurs between the frequencies of 100 and 105 kilohertz.) Of the resultant frequencies (95, 100, 105, and 5 kilohertz), all are filtered out except the 5 kilohertz difference frequency. This frequency, which is identical to the original 5 kilohertz audio applied at the transmitter, is retained and amplified. Thus, the 5 kilohertz audio tone appears to have been radiated through space from the transmitter to the receiver.

In the process just described, the 100 kilohertz frequency is referred to as the CARRIER FREQUENCY, and the sum and difference frequencies are referred to as SIDE FREQUENCIES. Since the sum frequency appears above the carrier frequency, it is referred to as the UPPER SIDE FREQUENCY. The difference frequency appears below the carrier and is referred to as the LOWER

Side Frequency.

When a carrier is modulated by voice or music signals, a large number of sum and difference frequencies are produced. All of the sum frequencies above the carrier are spoken of collectively as the UPPER SIDEBAND. All the difference frequencies below the carrier, also considered as a group, are called the LOWER SIDEBAND. If the carrier and the modulating signal are constant in amplitude, the sum and difference frequencies will also be constant in amplitude. However, when the carrier and sidebands are combined in a single impedance and viewed simultaneously with an oscilloscope, the resultant waveform appears as shown in figure 1-39. This resultant wave is called the MODULATION ENVELOPE. The modulation envelope has the same frequency as the carrier. However, it rises and falls in amplitude with the continual phase shift between the carrier and sidebands. This causes these signals to first aid and then oppose one another.

These cyclic variations in the amplitude of the envelope have the same frequency as the audio-modulating 1-44 voltage. The audio intelligence is actually contained in the spacing or difference between the carrier and sideband frequencies. Figure 1-39.—Formation of the modulation envelope. BANDWIDTH OF AN AM WAVE.—An ideal carrier wave contains a single frequency and occupies very little of the frequency spectrum. When the carrier is amplitude modulated, sideband frequencies are created both above and below the carrier frequency. This causes the signal to use up a greater portion of the frequency spectrum. The amount of space in the frequency spectrum required by the signal is called the BANDWIDTH of the signal.

The bandwidth of a modulated wave is a function of the frequencies contained in the modulating signal. For example, when a 100-kilohertz carrier is modulated by a 5-kilohertz audio tone, sideband frequencies are created at 95 and 105 kilohertz. This signal requires 10 kilohertz of space in the spectrum. If the same 100-kilohertz carrier is modulated by a 10-kilohertz audio tone, sideband frequencies will appear at 90 and 110 kilohertz and the signal will have a bandwidth of 20 kilohertz. Notice that as the modulating signal becomes higher in frequency, the bandwidth required also becomes greater. As illustrated by the above examples, the bandwidth of an amplitude-modulated wave at any instant is two times the highest modulating frequency applied at that time. Thus, if a 400-kilohertz carrier is modulated with 3, 5, and 8 kilohertz simultaneously, sideband frequencies will appear at 392, 395, 397, 403, 405, and 408 kilohertz. This signal extends from 392 to 408 kilohertz and has a bandwidth of 16 kilohertz, twice the highest modulating frequency of 8 kilohertz.

Musical instruments produce complex sound waves containing a great number of frequencies. The frequencies produced by a piano, for example, range from approximately 27 to 4,200 hertz with harmonic frequencies extending beyond 10 kilohertz. Modulating frequencies of up to 15 kilohertz must be included in the signal to transmit a musical passage with a high degree of fidelity. This requires a bandwidth of at least 30 kilohertz to prevent attenuation of higher-order harmonic frequencies. If the signal to be transmitted contains voice frequencies only, and fidelity is of minor importance, the bandwidth requirement is much smaller. A baritone voice includes frequencies of approximately 100 to 350 hertz, or 250 hertz. Intelligible voice communications can be carried out as long as the communications system retains audio frequencies up to several thousand hertz. Comparing the conditions 1-45 for transmitting voice signals with those for transmitting music reveals that much less spectrum space is required for voice communications.

Radio stations in the standard broadcast band are assigned carrier frequencies by the Federal Communications Commission (FCC). When two stations are located near each other, their carriers must be spaced some minimum distance apart in the radio spectrum. Otherwise, the sideband frequencies of one station will interfere with sideband frequencies of the other station. The standard AM broadcast band starts at 535 kilohertz and ends at 1,605 kilohertz. Carrier assignments start at 540 kilohertz and continue in a succession of 10-kilohertz increments until the upper limit of the broadcast band is reached. This adds up to a total of 107 carrier assignments, or CHANNELS, over the entire broadcast band. If stations were assigned to all 107 channels (in a given geographical area), each station would be allotted a channel width of 10 kilohertz. This leaves 5 kilohertz on each side of each carrier for sidebands. Since interference between such closely spaced stations would be nearly impossible to prevent, the FCC avoids assigning adjacent channels to stations in the same area. As a consequence of this policy, one or more vacant channels normally exist between stations in the broadcast band. In the interest of better fidelity, the stations are permitted to use modulating frequencies higher than 5 kilohertz as long as no interference with other stations is produced.

Q-31. What are the two major sections of a typical AM transmitter? Q-32. When 100 kilohertz and 5 kilohertz are heterodyned, what frequencies are present? Q-33. What is the upper sideband of an AM transmission? Q-34. Where is the intelligence in an AM transmission located? Q-35. What determines the bandwidth of an AM transmission? ANALYSIS OF AN AM WAVE.—A significant amount of information concerning the basic principles of amplitude modulation can be obtained from a study of the properties of the modulation envelope.

A carrier wave which has been modulated by voice or music signals is accompanied by two sidebands; each sideband contains individual frequencies that vary continuously. Since a wave of this nature is nearly impossible to analyze, you can assume in the following sections that the modulating signal, unless otherwise qualified, is a single-frequency, constant-amplitude sine wave. PERCENT OF MODULATION IN AN AM WAVE.—The degree of modulation is defined in terms of the maximum permissible amount of modulation. Thus, a fully modulated wave is said to be 100-PERCENT MODULATED. The modulation envelope in figure 1-40, view (A), shows the conditions for 100-percent sine-wave modulation. For this degree of modulation, the peak audio voltage must be equal to the dc supply voltage to the final power amplifier. Under these conditions, the rf output voltage will reach 0 on the negative peak of the modulating signal; on the positive peak, it will rise to twice the amplitude of the unmodulated carrier.

1-46 Figure 1-40A.—Conditions for 100-percent modulation. When analyzed, the modulation envelope consists of the unmodulated rf carrier voltage plus the combined voltage of the two sidebands. The combined sideband voltages are approximately equal to the rf carrier voltage since each sideband frequency contains one-half the carrier voltage, as shown in view (B). This condition is known as 100-percent modulation and the maximum modulated rf voltage is twice the carrier voltage. The audio-modulating voltage can be increased beyond the amount required to produce 100-percent modulation. When this happens, the negative peak of the modulating signal becomes larger in amplitude than the dc plate-supply voltage to the final power amplifier. This causes the final plate voltage to be negative for a short period of time near the negative peak of the modulating signal. For the duration of the negative plate voltage, no rf energy is developed across the plate tank circuit and the rf output voltage remains at 0, as shown in figure 1-41, view (A).

Figure 1-40B.—Conditions for 100-percent modulation. 1-47 Figure 1-41A.—Overmodulation conditions. Look carefully at the modulation envelope in view (A). It shows that the negative peak of the modulating signal has effectively been limited. If the signal were demodulated (detected in the receiver), it would have an appearance somewhat similar to a square wave. This condition, known as OVERMODULATION, causes the signal to sound severely distorted (although this will depend on the degree of overmodulation). Overmodulation will generate unwanted (SPURIOUS) sideband frequencies. This effect can easily be detected by tuning a receiver near, but somewhat outside the desired frequency. You would likely be able to tune to one or more of these undesired sideband frequencies, but the reception would be severely distorted, possibly unintelligible. (Without overmodulation, no such unwanted sideband frequencies would exist and you would be able to tune only to the desired frequency.) These unwanted frequencies will appear for a considerable range both above and below the desired channel. This effect is sometimes called SPLATTER. These spurious frequencies, shown in view (B), cause interference with other stations operating on adjacent channels. You should clearly understand that overmodulation, and its attendant distortion and interference is to be avoided.

Figure 1-41B.—Overmodulation conditions. In addition to the above problems, overmodulation also causes abnormally large voltages and currents to exist at various points within the transmitter. Therefore, sufficient overload protection by 1-48 circuit breakers and fuses should be provided. When this protection is not provided, the excessive voltages can cause arcing between transformer windings and between the plates of capacitors, which will permanently destroy the dielectric material. Excessive currents can also cause overheating of tubes and other components.

Ideally, you will want to operate a transmitter at 100-percent modulation so that you can provide the maximum amount of energy in the sideband. However, because of the large and rapid fluctuations in amplitude that these signals normally contain, this ideal condition is seldom possible. When the modulator is properly adjusted, the loudest parts of the transmission will produce 100-percent modulation. The quieter parts of the signal then produce lesser degrees of modulation. To measure degrees of modulation less than 100 percent, you can use a MODULATION FACTOR (M) to indicate the relative magnitudes of the rf carrier and the audio-modulating signal. Numerically, the modulation factor is: To illustrate this use of the equation, assume that a carrier wave with a peak amplitude of 400 volts is modulated by a 3-kilohertz sine wave with a peak amplitude of 200 volts. The modulation factor is figured as follows: If the modulation factor were multiplied by 100, the resultant quantity would be the PERCENT OF MODULATION (%M): 1-49 By using the correct equation, you can determine the percent of modulation from the modulation envelope pattern. This method is useful when the percent of modulation is to be determined using the pattern on the screen of an oscilloscope. For example, assume that your oscilloscope is connected to the output of a modulator circuit and produces the screen pattern shown in figure 1-42. According to the setting of the calibration control, each large division on the vertical scale is equal to 200 volts. By using this scale, you can see that the peak carrier amplitude (unmodulated portion) is 400 volts. The peak amplitude of the carrier is designated as eo in figure 1-42.

Figure 1-42.—Computing percent of modulation from the modulation envelope. The amplitude of the audio-modulating voltage can also be determined from amplitude variations in the envelope pattern. Notice that the peak-to-peak variations in envelope amplitude (emax − e min) is equal to 400 volts on the scale. Note then that the peak amplitude of the audio voltage is 200 volts. If these rf and audio voltage values are inserted into the equation, the pattern in figure 1-42 is found to represent 50-percent modulation. 1-50 If Em and Ec in the equation are assumed to represent peak-to-peak values, the following formula results: Since the peak-to-peak value of E m in figure 1-42 is emax − emin, we can substitute as follows: Also, since the peak-to-peak value of the carrier Ec is 2 times eo, we can subsititute 2eo for Ec as follows: Linear vertical distance represents voltage on the screen of a cathode-ray tube. Vertical distance units can be used in place of voltage in equations. Thus, if only the percent of modulation is required, the oscilloscope need not be calibrated and the actual circuit voltages are not required. In figure 1-42, emax represents 600 volts (3 large divisions); emin is 200 volts (1 division); and eo is 400 volts (2 divisions).

Using the equation and the dimensions of the screen pattern, you can figure the percent of modulation as follows: When eo of the equation is difficult to measure, an alternative solution can be obtained with the equation below: VECTOR ANALYSIS OF AN AM WAVE.—You studied earlier in this chapter that the modulation envelope results when the instantaneous sums of the carrier and sideband voltages are plotted with respect to time. An attempt to add these three voltages, point-by-point, would prove to be a huge task. The same end result can be obtained by using a rotating vector to represent each of the three 1-51 frequencies in the composite envelope. In the following analysis, vectors will be scaled to indicate the peak voltage value of the frequencies they represent.

The analysis has been simplified further by using a frequency of 8 hertz to represent the carrier frequency. Each cycle of the carrier then requires 1/8 of a second to complete 360 degrees. The carrier will be 100-percent modulated by a sine wave having a frequency of 1 hertz, thereby producing sideband frequencies of 7 and 9 hertz. Envelope Development from Vectors.—The modulating signal, upper sideband, carrier, and lower sideband waveforms are illustrated in views (A) through (D), respectively, in figure 1-43. Notice that the vertical lines passing through the figure divide each waveform into segments of 1/8 of a second each.

These lines also coincide with the starting and ending points of each cycle of the carrier wave. Figure 1-43.—Formation of the modulation envelope by the addition of vectors representing the carrier and sidebands. 1-52 During the first 1/8 of a second (T1 to T2), the carrier wave completes exactly 1 cycle, or 360 degrees, as shown in view (C). The upper sideband, which has a frequency of 9 hertz, will complete each cycle in less than 1/8 of a second. Therefore, during the time required for the carrier to complete 1 cycle of 360 degrees, the upper sideband [view (B)] is able to complete 1 cycle of 360 degrees plus an additional 45 degrees of the next cycle, for a total of 405 degrees.

The lower sideband [view (D)] has a frequency of 7 hertz and cannot complete an entire cycle in 1/8 of a second. During the time interval required for the carrier wave to progress through 360 degrees, the lower sideband frequency of 7 hertz can complete only 315 degrees, 45 degrees short of a full cycle. Keeping these factors in mind, you should be able to see that the phase angles between the two sideband frequencies, and between each sideband frequency and the carrier frequency, will continually shift. At an instant in time (T3), the carrier and sidebands will be in phase [view (E)], causing the envelope amplitude [view (F)] to be twice the amplitude of the carrier. At another instant in time (T7), the sidebands are out of phase with the carrier [view (E)], causing complete cancellation of the rf voltage.

The envelope amplitude will become 0 at this point. You should see that, although the carrier and sideband frequencies have constant amplitudes, the ever-changing phase differences between them causes the modulation envelope to vary continuously in amplitude. The vector analysis of the modulation envelope will be developed with the aid of figure 1-44. In figure 1-44, view (A), a vertical vector (C) has been drawn to represent the carrier wave in figure 1-43. At T1 in figure 1-43, the upper and lower sideband frequencies are of opposite phase with respect to each other, and 90 degrees out of phase with respect to the carrier. This condition is illustrated in figure 1-44, view (A), by sideband vectors U and L drawn in opposite directions along the horizontal axis. Since the upper sideband U is equal in amplitude but opposite in phase to lower sideband L, the two sideband voltages cancel one another; the amplitude of the envelope at T1 is equal to the amplitude of the carrier.

The same vector diagram is shown on a smaller scale in figure 1-43, view (E). Figure 1-44A.—Vector diagrams for T1 and T2. During the 1/8 of a second time interval between T1 and T2, all three vectors rotate in a counterclockwise direction at a velocity determined by their respective frequencies. The vector representing the carrier, for example, has made one complete rotation of 360 degrees and is back in its original position, as shown in figure 1-44, view (B). The upper sideband frequency, however, will complete 405 degrees in this same 1/8 of a second. Notice in view (B) that vector U has made one complete counterclockwise rotation of 360 degrees, plus an additional 45 degrees for a total rotation of 1-53 405 degrees. Vector L, representing the lower sideband, rotates at a velocity less than that of either the carrier or the upper sideband. In 1/8 of a second, vector L completes only 315 degrees, which is 45 degrees short of one complete rotation. At the end of 1/8 of a second, the three vectors have advanced to the positions shown in view (B).

Figure 1-44B.—Vector diagrams for T1 and T2. The resultant vector in view (B) is obtained by adding vector U to vector L. Since each sideband has one-half the amplitude of the carrier, and the two sidebands differ in phase by 90 degrees, the amplitude of the resultant vector can be computed. This computation (not shown) would show the resultant vector to have an amplitude that is approximately 70 percent that of the carrier. Thus, at T2 the amplitude of the modulation envelope is about 1.7 times the amplitude of the carrier. This condition is shown in figure 1-43, view (F).

By a similar procedure, vector diagrams can be constructed for time intervals T3 through T9. This has been done in figure 1-43, view (E). From these nine individual vector diagrams, the complete modulation envelope in figure 1-43, view (F), can be constructed. Notice in particular the vector diagrams for T3 and T7. At T3, all three waves, and therefore all three vectors, are in phase. The modulation envelope at this instant must, therefore, be equal to twice the amplitude of the carrier since each sideband frequency has one-half the amplitude of the carrier.

At T7, the two sideband frequencies are in phase with each other but 180 degrees out of phase with the carrier. This causes the combined sideband voltage to cancel the carrier voltage, and the modulation envelope becomes 0 at that instant. Note that for the transmitter output to be 0 at T7, both the carrier and sideband frequencies must be present. If any one of these three frequencies were missing, complete cancellation would not occur and rf energy would be present in the output. Although this vector analysis was made for frequencies of 7, 8, and 9 hertz, the same description could be applied to the frequencies actually present at the output of a transmitter.

Modulation Level of an AM Wave As stated earlier, the modulating signal can be introduced into any active element of a tube. In addition to the various arrangements possible within a single stage, the modulating signal can also be applied to any of the rf stages in the transmitter. For example, the modulating signal could be applied to the control grid or plate of one of the intermediate power amplifiers. A modulator circuit is usually placed into one of two categories, high- or low-level modulation. Circuits are categorized according to the level of the carrier wave at the point in the system where the 1-54 modulation is applied. The FCC defines HIGH-LEVEL MODULATION in the Code of Federal Regulations as "modulation produced in the plate circuit of the last radio stage of the system." This same document defines LOW-LEVEL MODULATION as "modulation produced in an earlier stage than the final." Q-36. What is percent of modulation?

Q-37. With a single modulating tone, what is the amplitude of the sideband frequencies at 100-percent modulation? Q-38. What is the formula for percent of modulation? Q-39. What is high-level modulation?

Modulation Systems

To complete your understanding of AM modulation, we are now going to analyze the operation of a typical plate modulator. Detailed circuit descriptions will be used to give you an understanding of a basic AM plate modulator. In addition, we will cover basic circuit descriptions for cathode and grid electron- tube modulators and for base, emitter, and collector transistor modulators in this chapter. Plate Modulator Figure 1-45 is a basic plate-modulator circuit. Plate modulation permits the transmitter to operate with high efficiency. It is the simplest of the modulators available and is also the easiest to adjust for proper operation. The modulator is coupled to the plate circuit of the final rf amplifier through the modulation transformer. For 100-percent modulation, the modulator must supply enough power to cause the plate voltage of the final rf amplifier to vary between 0 and twice the dc operating plate voltage. The modulator tube (V2) is a power amplifier biased so that it operates class A. The final rf power amplifier (V1) is biased in the nonlinear portion of its operating range (class C). This provides for efficient operation of V1 and produces the necessary heterodyning action between the rf carrier and the af modulating frequencies.

1-55 Figure 1-45.—Plate-modulation circuit. PLATE MODULATOR CIRCUIT OPERATION.—Figure 1-46, views (A) through (E), shows the waveforms associated with the plate-modulator circuit shown in figure 1-45. Refer to these two figures throughout the following discussion. 1-56 Figure 1-46.—Plate-modulator waveforms. The rf power amplifier (V1) acts as a class C amplifier when no modulation is present in the plate circuit. V2 is the modulator which transfers the modulating voltage to the plate circuit of V1. Let's see how this circuit produces a modulated rf output.

View (A) of figure 1-46 shows the plate supply voltage for V1 as a constant dc value (Eb) at time 1 with no modulating signal applied. V1 is biased at cutoff at this time. The incoming rf carrier [view (A)] is applied to the grid of V1 by transformer T1 and causes the plate circuit current to PULSE (SURGE) 1-57 each time the grid is driven positive. These rf pulses are referred to as current pulses and are shown in view (C). The plate tank output circuit (T3) is shocked into oscillation by these current pulses and the rf output waveform shown in view (E) is developed. The rf plate voltage waveform is shown in view (D).

An audio-modulating voltage applied to the grid of V2 is amplified by the modulator and coupled to the plate of V1 by modulation transformer T2. The secondary of T2 is in series with the plate-supply voltage (E) of V1. The modulating voltage will either add to or subtract from the plate voltage of V1. This is shown in view (A) at time 2 and time 3. At time 2 in view (A), the plate supply voltage for V1 increases to twice its normal value and the rf plate current pulses double, as shown in view (C). At time 3 in view (A), the supply voltage is reduced to 0 and the rf plate current decreases to 0, as shown in view (C). These changes in rf plate current cause rf tank T3 voltage to double at time 2 and to decrease to 0 at time 3, as shown in view (E). This action results in the modulation envelope shown in view (E) that represents 100-percent modulation. This is transformer-coupled out of tank circuit T3 to an antenna.

Because of the oscillating action of tank circuit T3, V1 has to be rated to handle at least four times its normal plate supply voltage (Eb), as shown by the plate voltage waveform in view (D). Heterodyning the audio frequency intelligence from the modulator (V2) with the carrier in the plate circuit of the final power amplifier (V1) requires a large amount of audio power. All of the power or voltage that contains the intelligence must come from the modulator stage. This is why plate modulation is called high-level modulation.

The heterodyning action in the plate modulator effectively changes an audio frequency to a different part of the frequency spectrum. This action allows antennas and equipment of practical sizes to be used to transmit the intelligence. Now, let's look at several other typical modulators. Collector-Injection Modulator The COLLECTOR-INJECTION MODULATOR is the transistor equivalent of the electron-tube AM plate modulator. This transistor modulator can be used for low-level or relatively high-level modulation. It is referred to as relatively high-level modulation because, at the present time, transistors are limited in their power-handling capability. As illustrated in figure 1-47, the circuit design for a transistor collector- injection modulator is very similar to that of a plate modulator. The collector-injection modulator is capable of 100-percent modulation with medium power-handling capabilities.

1-58 Figure 1-47.—Collector-injection modulator. In figure 1-47, the rf carrier is applied to the base of modulator Q1. The modulating signal is applied to the collector in series with the collector supply voltage through T3. The output is then taken from the secondary of T2. With no modulating signal, Q1 acts as an rf amplifier for the carrier frequency. When the modulation signal is applied, it adds to or subtracts from the collector supply voltage. This causes the rf current pulses of the collector to vary in amplitude with the collector supply voltage. These collector current pulses cause oscillations in the tank circuit (C4 and the primary of T2). The tank circuit is tuned to the carrier frequency. During periods when the collector current is high, the tank circuit oscillates strongly. At times when the collector current is small, or entirely absent, little or no energy is supplied to the tank and oscillations become weak or die out. Thus, the modulation envelope is developed as it was in a plate modulator.

As transistor technology continues to develop, higher power applications of transistor collector- injection modulation will be employed. Plate and collector-injection modulation are the most commonly used types of modulation because the modulating signal can be applied in the final stages of rf amplification. This allows the majority of the rf amplifier stages to be operated class C for maximum efficiency. The plate and collector-injection modulators also require large amounts of af modulating power since the modulator stage must supply the power contained in the sidebands.

Q-40. For what class of operation is the final rf power amplifier of a plate-modulator circuit biased? Q-41. The modulator is required to be what kind of a circuit stage in a plate modulator? Q-42. How much must the fpa plate current vary to produce 100-percent modulation in a plate modulator? RF INPUT

Nts120147

T1 L1 L2 C2 R3 Q1 C4 L3 AF MOD L5 T3 T2 L4 OUTPUT L6 C3 R2 C1 Vcc + R1 1-59 Q-43. The collector-injection modulator is similar to what ty pe of tube modulator? Control-Grid Modulator In cases when the use of a minimum of af modulator power is desired, a form of low-level modulation is necessary. The CONTROL-GRID MODULATOR is used widely in portable and mobile equipment to reduce size and power requirements. It is also used in extremely high-power, wideband equipment such as television transmitters where high-level or plate modulation is difficult and costly to achieve. Figure 1-48 is a basic schematic for a typical control-grid modulator.

Figure 1-48.—Control-grid modulator. The control-grid modulator uses a variation of grid bias (at the frequency of the modulating signal) to vary the instantaneous plate voltage and current. These variations cause modulation of the carrier frequency. The carrier frequency is introduced through coupling capacitor Cc. The modulating frequency is introduced in series with the grid bias through T1. As the modulating signal increases and decreases (positive and negative), it will add to or subtract from the bias on rf amplifier V1. This change in bias causes a corresponding change in plate voltage and current. These changes in plate voltage and current add vectorially to the carrier frequency and provide a modulation envelope in the same fashion as does the plate modulator. Since changes in the plate circuit of the rf amplifier are controlled by changes in the grid bias, the gain of the tube requires only a low-level modulating signal. Even when the input signals are at these low levels, occasional modulation voltage peaks will occur that will cause V1 to saturate. This creates distortion in the output. Care must be taken to bias the rf amplifier tube for maximum power out 1-60 while maintaining minimum distortion. The power to develop the modulation envelope comes from the rf amplifier. Because the rf amplifier has to be capable of supplying this additional power, it is biased for (and driven by the carrier frequency at) a much lower output level than its rating. This reduced efficiency is necessary during nonmodulated periods to provide the tube with the power to develop the sidebands.

Compared to plate modulation, grid modulation is less efficient, produces more distortion, and requires the rf power amplifier to supply all the power in the output signal. Grid modulation has the advantage of not requiring much power from the modulator. Base-Injection Modulator The BASE-INJECTION MODULATOR is similar to the control-grid modulator in electron-tube circuits. It is used to produce low-level modulation in equipment operating at very low power levels. In figure 1-49, the bias on Q1 is established by the voltage divider R1 and R2. With the rf carrier input at T1, and no modulating signal, the circuit acts as a standard rf amplifier. When a modulating signal is injected through C1, it develops a voltage across R1 that adds to or subtracts from the bias on Q1. This change in bias changes the gain of Q1, causing more or less energy to be supplied to the collector tank circuit. The tank circuit develops the modulation envelope as the rf frequency and af modulating frequency are mixed in the collector circuit. Again, this action is identical to that in the plate modulator.

Figure 1-49.—Base-injection modulator. Because of the extremely low-level signals required to produce modulation, the base-injection modulator is well suited for use in small, portable equipment, such as "walkie-talkies," and test equipment. Cathode Modulator Another low-level modulator, the CATHODE MODULATOR, is generally employed where the audio power is limited and distortion of the grid-modulated circuit cannot be allowed. The cathode 1-61 modulator varies the voltage of the cathode to produce the modulation envelope. Since the cathode is in series with the grid and plate circuits, you should be able see that changing the cathode voltage will effectively change the voltage of the other tube elements. By properly controlling the voltages on the tube, you can cause the cathode modulator to operate in a form of plate modulation with high efficiency.

Usually, the cathode modulator is designed to perform about midway between plate and grid modulator levels, using the advantages of each type. When operated between the two levels, the modulator provides a more linear output with moderate efficiency and a modest audio power requirement. In figure 1-50, the rf carrier is applied to the grid of V1 and the modulating signal is applied in series with the cathode through T1. Since the modulating signal is effectively in series with the grid and plate voltage, the level of modulating voltage required will be determined by the relationships of the three voltages. The modulation takes place in the plate circuit with the plate tank developing the modulation envelope, just as it did in the plate modulator.

Figure 1-50.—Cathode modulator. Emitter-Injection Modulator This is the transistor equivalent of the cathode modulator. The EMITTER-INJECTION MODULATOR has the same characteristics as the base-injection modulator discussed earlier. It is an 1-62 extremely low-level modulator that is useful in portable equipment. In emitter-injection modulation, the gain of the rf amplifier is varied by the changing voltage on the emitter. The changing voltage is caused by the injection of the modulating signal into the emitter circuitry of Q1, as shown in figure 1-51. Here the modulating voltage adds to or subtracts from transistor biasing. The change in bias causes a change in collector current and results in a heterodyning action. The modulation envelope is developed across the collector-tank circuit.

Figure 1-51.—Emitter-injection modulator. Q-44. When is a control-grid modulator used? Q-45. What type of modulator is the cathode modulator (low- or high-level)? Q-46. What causes the change in collector current in an emitter-injection modulator? You have studied six methods of amplitude modulation. These are not the only methods available, but they are the most common. All methods of AM modulation use the same theory of heterodyning across a nonlinear device. AM modulation is one of the easiest and least expensive types of modulation to achieve. The primary disadvantages of AM modulation are susceptibility to noise interference and the inefficiency of the transmitter. Power is wasted in the transmission of the carrier frequency because it contains no AM intelligence. In the next chapter, you will study other forms of modulation that have been developed to overcome these disadvantages.

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 amplitude modulation, its basic principles, and typical circuitry used to generate this modulation. The SINE WAVE is the basis for all complex waveforms and is generated by moving a coil through a magnetic field. 1-63 AMPLITUDE (instantaneous voltage) of a coil is found by the formula: PHASE or PHASE ANGLE is the angle that exists between the starting position of a vector generating the sine wave and its position at a given instant.

FREQUENCY is the rate at which the vector rotates. HETERODYNING is the process of mixing two different frequencies across a nonlinear impedance to give the ORIGINAL frequencies, a SUM frequency, and a DIFFERENCE frequency. 1-64 CONTINUOUS-WAVE MODULATION is the basic form of rf communications. It is essentially on-off keying of an rf carrier. HAND-OPERATED and MACHINE KEYING are two types of cw keying. PLATE, CATHODE, and BLOCKED-GRID KEYING are circuits commonly used in hand-operated and machine keying. KEYING RELAYS are used for safety and to handle the current requirements in high-power transmitters.

1-65 KEY-CLICK FILTERS are used to prevent interference in cw transmitters. Although it is a relatively slow transmission method, CW COMMUNICATIONS is highly reliable under severe noise conditions for long-range operation. SINGLE-STAGE CW TRANSMITTERS can be made by coupling the output of an oscillator to an antenna. 1-66 MULTISTAGE CW TRANSMITTERS are used to improve frequency stability and increase output power. A MICROPHONE is an energy converter that changes sound energy into electrical energy. A CARBON MICROPHONE uses carbon granules and an external battery supply to generate af voltages from sound waves.

1-67 A CRYSTAL MICROPHONE uses the piezoelectric effect to generate an output voltage. A DYNAMIC MICROPHONE uses a coil of fine wire mounted on the back of a diaphragm located in the magnetic field of a permanent magnet. 1-68 A MAGNETIC MICROPHONE uses a moving armature in a magnetic field to generate an output. The FREQUENCY SPECTRUM of a modulated wave can be conveniently illustrated in graph form as frequency versus amplitude. 1-69 The MODULATION ENVELOPE is the waveform observed when the CARRIER, UPPER SIDEBAND, and LOWER SIDEBAND are combined in a single impedance and observed as time versus amplitude.

The BANDWIDTH of an rf signal is the amount of space in the frequency spectrum used by the signal. PERCENT OF MODULATION is a measure of the relative magnitudes of the rf carrier and the af modulating signal. 1-70 HIGH-LEVEL MODULATION is modulation produced in the plate circuit of the last radio stage of the system. LOW-LEVEL MODULATION is modulation produced in an earlier stage than the final power amplifier. The PLATE MODULATOR is a high-level modulator. The modulator tube must be capable of varying the plate-supply voltage of the final power amplifier. It must vary the plate voltage so that the plate current pulses will vary between 0 and nearly twice their unmodulated value to achieve 100-percent modulation.

1-71 A COLLECTOR-INJECTION MODULATOR is a transistorized version of the plate modulator. It is classified as a high-level modulator, although present state-of-the-art transistors limit them to medium-power applications. A CONTROL-GRID MODULATOR is a low-level modulator that is used where a minimum of af modulator power is desired. It is less efficient than a plate modulator and produces more distortion.

Unmodulated

RF INPUT FINAL RF

Power Amplifier

Modulated

Rf Output

T1 V1 T3 ECC

Modulator

Modulation

Transformer

FPA POWER SUPPLY Eb MOD. POWER SUPPLY V2 T2 AUDIO

Modulating

SIGNAL NTS12I01 1-72 A BASE-INJECTION MODULATOR is used to produce low-level modulation in equipment operating at very low power levels. It is often used in small portable equipment and test equipment. The CATHODE MODULATOR is a low-level modulator employed where the audio power is limited and the inherent distortion of the grid modulator cannot be tolerated. C V1 RF AMPL V2 AF MOD RF DRIVE IN L AF IN + E bb C1 T1 C R C2RFC 2 RFC 1 MOD 1-73 The EMITTER-INJECTION MODULATOR is an extremely low-level modulator that is useful in portable equipment.

The primary disadvantages of AM modulation are susceptibility to NOISE INTERFERENCE and the INEFFICIENCY of the transmitter. 1-74

Answers To Questions Q1. Through Q46.

A-1. Modulation is the impressing of intelligence on a transmission medium. A-2. May be anything that transmits information, such as light, smoke, sound, wire lines, or radio-frequency waves. A-3. Mixing two frequencies across a nonlinear impedance. A-4. The process of recovering intelligence from a modulated carrier. A-5. The sine wave. A-6. To represent quantities that have both magnitude and direction. A-7. Sine ! = opposite side ÷ hypotenuse. A-8. e = E max sine ! . A-9. The value at any given point on the sine wave.

A-10. Phase or phase angle. A-11. The rate at which the vector which is generating the sine wave is rotating. A-12. The elapsed time from the beginning of cycle to its completion. A-13. Wavelength = rate of travel × period. A-14. Process of combining two signal frequencies in a nonlinear device. A-15. An impedance in which the resulting current is not proportional to the applied voltage. A-16. The display of electromagnetic energy that is arranged according to wavelength or frequency. A-17. At least two different frequencies applied to a nonlinear impedance.

A-18. Any method of modulating an electromagnetic carrier frequency by varying its amplitude in accordance with the intelligence. A-19. A method of generating oscillations, a method of turning the oscillations on and off (keying), and an antenna to radiate the energy. A-20. Plate keying and cathode keying. A-21. Machine keying. A-22. A high degree of clarity even under severe noise conditions, long-range operation, and narrow bandwidth. A-23. Antenna-to-ground capacitance can cause the oscillator frequency to vary.

A-24. To isolate the oscillator from the antenna and increase the amplitude of the rf oscillations to the required output level. A-25. To raise the low frequency of a stable oscillator to the vhf range. 1-75 A-26. An energy converter that changes sound energy into electrical energy. A-27. The changing resistance of carbon granules as pressure is applied to them. A-28. Background hiss resulting from random changes in the resistance between individual carbon granules. A-29. The piezoelectric effect. A-30. A dynamic microphone has a moving coil and the magnetic microphone has a moving armature.

A-31. Rf and af units. A-32. 100 kilohertz, 5 kilohertz, 95 kilohertz, and 105 kilohertz. A-33. All of the sum frequencies above the carrier. A-34. The intelligence is contained in the spacing between the carrier and sideband frequencies. A-35. The highest modulating frequency. A-36. The depth or degree of modulation. A-37. One-half the amplitude of the carrier. A-38. A-39. Modulation produced in the plate circuit of the last radio stage of the system. A-40. Class C. A-41. Power amplifier. A-42. Between 0 and nearly two times its unmodulated value.

A-43. Plate modulator. A-44. In cases when the use of a minimum of af modulator power is desired. A-45. Low-level. A-46. Gain is varied by changing the voltage on the emitter.