FM and PM demodulator circuits

Frequency modulation and phase modulation are examples of angle modulation. Figure 27-20 shows this action graphically. The audio signai causes the frequency (or phase) to shift plus and minus from the quiescent value, which exists when there is no modulation present. Frequency and phase modulation are different but similar enough to make the demodulation schemes the same. The difference between FM and PM is that the phase modulation needs no preemphasis curve t o the audio waveform (it does it naturally) while the frequency modulation transmitter is preemphasi2ed for noise abatement.

Another difference between FM and PM transntitters is the location of the reactance modulator used to generate the modulated signal. In the FM transmitter the

RF CARRIER FREQUENCY

RF CARRIER FREQUENCY

frequency

AUDIO

modulating waveform

27-20 FM modulation.

reactance modulator is part of the frequency determining circuitry, whereas in the PM transmitter it follows that cireuitry.

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