Sensitive frequency demodulator (LM565, LM111)

As shown in the figure, this is a sensitive frequency demodulation circuit. This circuit is composed of a phase-locked integrated circuit LM565, which can track frequency deviation in a wide frequency range. However, for signals with small frequency deviation, due to the small demodulation output signal, the drift of the voltage-controlled oscillator will cause the drift of the comparator working point and produce errors. In order to overcome the above shortcomings, this circuit has a DC regeneration part in the circuit, so it can also work stably for signals with small frequency deviation. In the figure, the demodulation circuit composed of LM565 and comparator LM111 is basically the same as the general frequency demodulation circuit. The difference is that the input signal of A2 pin ③ in this circuit is not taken from LM565 pin ⑥, but from the peak holding circuit A1. In the interval of the FM input signal, the output of A1 is low, so the holding capacitor C1 discharges through the low output impedance of A1 and maintains the signal peak. Using this DC regeneration method, although the voltage-controlled oscillator has drift, the output of the peak holding circuit can be tracked, so the comparator can work normally.