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Detection of FM signals 507
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27-6 Slope detection lets an AM receiver demodulate FM.
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The phase-locked loop
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If an FM signal is injected into a phase-locked loop (PLL) circuit, the loop will produce an error voltage that is a duplicate of the modulating waveform. The frequency changes might be too fast for the PLL to lock onto, but the error voltage will still appear. Many modern receivers take advantage of this effect to achieve FM detection. A circuit called a limiter can be placed ahead of the PLL so that the receiver does not respond to amplitude modulation. Thus, one of the major advantages of FM over AM is realized. Atmospheric noise and ignition noise cause much less disruption of a good FM receiver than to AM, CW, or SSB receivers, provided that the signal is strong enough. Weak signals tend to appear and disappear, rather than fading, in an FM receiver that employs limiting.
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The discriminator
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A discriminator produces an output voltage that depends on the instantaneous signal frequency. When the signal is at the center of the discriminator passband, the output voltage is zero. If the instantaneous signal frequency decreases, a momentary phase shift results and the instantaneous output voltage becomes positive. If the frequency rises above center, the output becomes negative. The instantaneous voltage level (positive or
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508 Data reception negative) is directly proportional to the instantaneous frequency. Therefore, the output voltage is a duplicate of the modulating waveform. A discriminator is sensitive to amplitude variations in the signal, but this problem can be overcome by the use of a limiter.
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The ratio detector
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A discriminator with a built-in limiting effect is known as a ratio detector. This type of FM detector was developed by RCA and is used in high-fidefity receivers and in the audio portions of TV receivers. A simple ratio detector circuit is shown in Fig. 27-7. A transformer splits the signal into two components.
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27-7 A ratio detector for demodulating FM.
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A change in signal amplitude causes equal level changes in both halves of the circuit. These effects cancel because they are always 180 degrees out of phase. This nullifies amplitude variations on the signal. A change in signal frequency causes a phase shift in the circuit. This unbalances it, so that the outputs in the two halves of the circuit become different, which produces an output in direct proportion to the instantaneous phase shift. The output signal is a duplication of the modulating waveform on the FM signal.
Detection of PM signals
Pulse modulation (PM) operates at a low duty cycle. The pulses are far shorter in duration than the intervals between them. A PM signal is mostly empty space. The ratio of average signal power to peak signal power is low, often much less than 1 percent. When the amplitude or duration of the pulses changes, the average transmitter power also changes. Stronger or longer pulses increase the effective signal amplitude;
Digital-to-analog conversion 509 weaker or shorter pulses result in decreased amplitude. Because of this, PM can be detected in the same way as AM. A major advantage of PM is that it s mostly empty space. With pulse amplitude modulation (PAM), pulse duration modulation (PDM), or pulse code modulation (PCM), the time interval is constant between pulse centers. Even at maximum modulation, the ratio of on time to off time is low. Therefore, two or more signals can be intertwined on a single carrier (Fig. 27-8). A PM receiver can pick out one of these signals and detect it, ignoring the others. This is known as time-division multiplexing.
27-8 Time-division multiplexing of two different signals (A and B) on a singlepulsed carrier.
A time-division-multiplex communications circuit requires that the receiver be synchronized with the transmitter. This is easy to do if the pulse frequency is constant. The receiver and transmitter can be clocked from a single, independent, primary time standard such as the broadcasts of radio station WWV. The receiver detector is blocked off during intervals between transmitter pulses and opens up only during windows lasting as long as the longest transmitter pulses. The received data is selected by adjusting the windows to correspond with the desired pulse train. Because the duty cycle of any single signal is so low, it is possible to multiplex dozens or even hundreds of signals on one transmitted carrier.
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