This FSK demodulator convert the FSK signal to serial digital signal. FSK modulation is used to transmit digital serial data, and we need to demodulate it to get back the digital data at the receiver. Here is the schematic diagram of the circuit:

fsk demodulator circuit circuit schematic

This demodulator circuit uses 2225/2025  Hz for  mark/space coding, and this frequency is the answering frequency of BELL 103, 108, and 113 standards. [Schematic diagram source: National Semiconductor Application Notes]