FSK or Frequency-Shift Keying is a form of frequency modulation in which the modulating signal shifts the output freq. between predetermined values. If problems arise in demodulating FSK signals, it is helpful to use the FSK filter circuit presented here.
The Frequency-Shift Keying filter is very simple and it can be built inside a phone plug. The diagram shows a low pass filter made of R1 and C2. This filter has a cutoff freq. of about 1.6 KHz. Since the “0” and “1” signals of the FSK are sent as tones of 1.2 KHz and 2.4 KHz respectively, the filter removes all unwanted corners fron the signal.
Both D1 and D2 ensure that the FSK signal is limited to about +/- 600 mV amplitude. It can be attached to a loudspeaker or headphone output.
C1 = 0.0082
C2 = 0.018
R1 = 5.6kΩ
D1 = D2 = 1N4148
Sent by Paul Michey, AU.