Digital frequency shift demodulator (CA3020, MC723, MC724)

As shown in the figure, this is a digital frequency shift demodulation circuit. This circuit can be used in the frequency range of 1kHz to 10kHz, and can demodulate to a frequency deviation of 1%. The input stage of the circuit is composed of a wideband amplifier CA3020. Its differential output is added to the JK input of the JK trigger. The gate circuit MC724 constitutes a monostable multivibrator. The timing period of the monostable multivibrator is equal to half a period of the center frequency. The input signal is delayed by the monostable multivibrator and then selects the trigger. For example, 2920Hz represents the symbol and 2750Hz represents the space, then the center frequency is 2835Hz, so the timing period of the monostable multivibrator is 175μs. When the input is 2920Hz, the trigger selection is exactly J=1, K=0, so the Q output is "1" (representing the symbol); when the input is 2750Hz, the trigger selection is exactly J=0, K=1, so the Q output is "0" (representing the space). The identification accuracy of this circuit mainly depends on the timing accuracy of the monostable multivibrator.