Automatic gain controlled frequency discriminator for use in a phase locked loop

摘要

A frequency discriminator having a wide capture band is shown to comprise, in addition to a conventional phase detector having two channels fed by a signal whose frequency is to be determined (the first one of the channels containing a tuned circuit operative to shift the phase of the signal in accordance with the difference between the frequency of the signal and the center frequency of the tuned circuit and the second one of the channels containing a phase shifter operative to shift the phase of the signal by 90 DEG regardless of the frequency of the signal), a compensating circuit operative substantially to equalize the amplitudes of the signals applied to the phase detector, the compensating circuit including an amplifier in the first one of the channels to amplify the signal out of the tuned circuit, the gain of the amplifier being controlled by a signal indicative of the difference between the amplitudes of the signals fed to the phase detector.