AFC apparatus with selection between average value mode and keyed mode

Claim 1

An AFC apparatus comprising an FM demodulator, a low-pass filter connected to the output of said demodulator and having a cut-off frequency capable of being switched to control a time constant thereof, a first comparator supplied with a first reference voltage and with an output of said FM demodulator through said low-pass filter for comparison with said first reference voltage, a second comparator supplied with a second reference voltage and with said output of said FM demodulator through said low-pass filter for comparison with said second reference voltage, a first latch circuit for holding a comparison output of said first comparator in response to a key pulse, a second latch circuit for holding a comparison output of said second comparator in response to said key pulse, oscillator means controlled by the outputs of said latch circuits, and a decision circuit for determining the presence or absence of an applied key pulse, said decision circuit controlling the time constant of said low-pass filter by reducing the cut-off frequency of said low-pass filter in the absence of an applied key pulse to smooth an AC component of an output signal of said demodulator, thereby selecting an average value AFC system mode, and increasing the cut-off frequency of said low-pass filter in the presence of an applied key pulse to set a time constant smaller than the key pulse duration, thereby selecting a keyed AFC system mode.