Demodulators for FM

Figure  shows a block diagram of a general FM demodulator, which is implemented by generating an AM signal whose amplitude is proportional to the instantaneous frequency of the FM signal, and then using an AM demodulator to recover the message signal. Transforming the FM signal into an AM signal can be achieved by passing the FM signal through an LTI system whose frequency response is nearly a straight line in the frequency band of the FM signal.

FM demodulators mainly fall into two categories: frequency discriminators and locked-loop demodulators. While both give the same performance for relatively high signal levels, locked- loop demodulators provide better performance than the discriminator when signal levels are low and noise is a problem. The discriminator produces an output voltage proportional to the frequency variations that occur in the FMsignal at its input. Locked-loop demodulators are more cost-efficient when implemented in IC form.