The discriminator

A discriminator produces output voltage which depends on instantaneous signal frequency. When signal is at the center of discriminator passband, the output voltage is zero. If instantaneous signal frequency decreases, a momentary phase shift results and instantaneous output voltage and becomes positive. If frequency rises above center, the output becomes negative. The immediate voltage level is directly proportional to instantaneous frequency. Thus, the output voltage is a duplicate of modulating waveform.

A discriminator is quite sensitive to amplitude variations in signal, but this problem can be overcome by use of a limiter.