The phase-locked loop

If an FM signal can be injected into a phase-locked loop (PLL) circuit, the loop will generate an error voltage that is a duplicate of the modulating waveform. The frequency changes might be very fast for PLL to lock onto, but error voltage will appear still. Many modern receivers take benefit from of this effect to achieve FM detection.

A circuit called as limiter can be placed ahead of PLL such that the receiver doesn’t respond to amplitude modulation. Therefore, one of the major advantages of FM over AM can be realized. Atmospheric noise and ignition noise cause less disruption of good FM receiver than to AM, CW, or SSB receivers, given that the signal is strong enough. Weak signals appear and disappear, instead of fading, in an FM receiver which employs limiting.