UB5UG developed scheme of FM detector on three elements 2I-NOT.
The frequency-modulated if signal is fed to the pulse shaper (inverter DI), and with it - the element of coincidence D3 and the inverter D2. when the output 01 is a logic low, the capacitor C2 is slowly charged via the input impedance of the element D3. and when the high is quickly discharged through D2. Thus, the front of the pulse from the output of D1 is supplied to upper circuit on the input element D3 with some delay relative to the pulse front, which came to the second input of D3.
In this case, the pulse duration at the output of the detector is proportional to the delay of the front, and their constant component -- of the modulating signal.
If the average frequency of the if signal is 500 Hz, the capacitor C2 must have the capacity of 50..,150 PF. With a smaller value of FC use a capacitor with a larger capacity. In any case, it is necessary to choose so that the voltage of the LF was the highest.
In the FM detector can be used, for example, the chip 155.
Author: UB5UG; Publication: N. Bolshakov, rf.atnn.ru