FSK Frequency-Shift Keying is a form of frequency modulation in which the modulating signal changes the output frequency. between predetermined values. If problems arise in FSK demodulation signal is shown using this filter. The Frequency-Shift Keying filter is very simple and can be built into a phone plug. Filter FSK Circuit Diagram The diagram shows a low-pass filter made of R1 and C2. This filter has a cutoff frequency. about 1.6 KHz. Once the "0" and "1" signals from FSK tones are sent as 1.2 kHz and 2.4 kHz respectively filter removes all the undesired signals. Both D1 and D2 ensure that the FSK signal is limited to about + / - 600 mV amplitude. It can be connected to a speaker or headphone output. components C1 = 0.0082 C2 = 0.018 R1 = 5.6kΩ D1 = D2 = 1N4148