Module 12—Modulation Principles
Pages i - ix
1-1 to 1-10
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1-61 to 1-70
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2-1 to 2-10
,2-11 to 2-20
2-21 to 2-30
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AI-1 to AI-6, Index-1 to 2, Assignment 1 , 2
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?
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.
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
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
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.
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.
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
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):
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 e0 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.
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 e0, we can substitute 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 e0 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 e0 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
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