Frequency discriminator and frequency modulator incorporating such discriminator

摘要

An input signal is mixed with a reference frequency and with a frequency 90 DEG phase-displaced from the reference frequency, so as to produce resultant waveforms whose slopes are dependent on the respective frequency differences detected. Both waveforms are differentiated to produce respective slope signals. Two comparisons are then carried out: one comparison compares the sign of one slope signal with the sign of the waveform producing the other slope signal, and the second comparison compares the sign of the other slope signal with the sign of the waveform producing the first slope signal. The two comparisons produce nominally identical intermediate signals having values dependent on the difference between the input and reference frequencies. Control means operative in dependence on the relative magnitudes of the two waveforms then selects the intermediate signals alternately, each selected intermediate signal constituting the output signal of the circuit. Each intermediate signal is selected as the output signal for a respective period of time for which there are no transitions in the signals from which the respective comparison produced it.