Phase locked loop FMPM detectors

The phase locked loop (PLL) circuit can be used as an FM demodulator if its control voltage is monitored. Figure 27-27A shows the basic PLL circuit. It consists of a voltage-controlled oscillator (VCO), phase detector, iow-pass Filter, and an amplifier. The FM signal from the IF amplifier is applied to one port of the phase detector, and the output of the VCO is ooiinected to the other port. When the two frequencies are equal, there is no output from the circuit (or, the value is quiescent). When the FM IF signal deviates above or below the frequency of the VCO, there will be an error term generated. This error signal is processed in the low-pass filter and amplifier to control the VCO. Tts purpose is to drive the VCO back on the right frequency. ft's this error signal that becomes the modulated output of the PLL FM demodulator circuit. Figure 27-27B shows a PLL based on the NE-565 PLL integrated circuit. The resonant frequency is set by Ri and C}, which should be the center frequency of the FM signal. As the signal deviates up and down, the error voltage is monitored and becomes the modulated output signal.

27-27 Block diagram of ^oil-less FM detector.

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