mixer and lowpass circuit

Hi,
I'm struggling with this incoherent demodulator f_s_k



I would like to understand how this circuit works and the mathematical theory that is the basis.
I tried to look a bit on the net but i did not find much ...
Is there a book / tutorial that explains this type of circuits?
Could you help me? thanks

The schematic shows a COHERENT FSK demodulator, not incoherent. In principle it is a synchronous detector or analog multiplier which, by using the analog switch, multiplies the"FSK" input signal with the rectangular reference signal (carrier) which must be obtained elsewhere (not shown). If both signals are in-phase, after each sampling switch the integrator capacitor charges up to a maximum voltage. If they are out-of-phase, the capacitor remains low .
The low-pass filters reject the RF spectrum allowing only modulation spectrum at the output(s).
The unity-gain opamps before the filters isolate the integrators from the LP filters.

The important part is not shown: the FSK carrier retrieval circuit.

The mathematical theory is simple : the switch is a multiplier and the RC circuits after it is the integrator.
Try a book on FSK demodulation or on synchronous detection. Or build and test the circuit.

The only way you can get V+0-90 degs output is for the two carriers sources into the DG403 to be in quadrature.
FSK is just a pair of frequencies that alternate, so I can't see this circuit being of much use unless the carriers into the DG jump around as well. However the 90 deg output can be used to drive a phase lock loop to generate the DG carriers as its output goes + or - depending how accurately it sits wrt the 0 degs output. So two of these with their own carrier oscillators ( 4 X actual frequency to get the 90 degs offset).
Where did the circuit come from?
Frank