Detection of FSK signals

Frequency shift keying is detected by using same methods as CW detection. The carrier beats against BFO in mixer, producing an audio tone which alternates between 2 different pitches. The block diagram of the figure given below can therefore apply to FSK reception to CW reception.

With FSK, BFO frequency is always set well above, below, both the mark and the space carrier frequencies. The offset affects audio tone frequencies and is set so that certain standard pitches result. There are many sets of standard tone frequencies, depending on whether communications is amateur, commercial, or military. To obtain the proper pitches, the BFO should be set precisely at the right frequency. There is little tolerance for the error.