What is fsk demodulator?

Answer 1

Frequency Shift Keying (FSK) modulation is the process of modulating a digital signal onto a carrier that is composed of one of two frequencies, one frequency for high - and the other frequency for low.

FSK demodulation is the process of recovering the original signal by detecting the frequencies involved in the original modulation. Typically, this is done with a bandpass amplifier tuned to one of the two frequencies, followed by a amplitude demodulator. The output is the original signal. It is possible, though often unecessary, to use two bandpass ampliers, one for each frequency, but this is redundant. It is also possible to use a digital signal processing technique to perform a fourier transform on the input signal, but that can be complex and costly.

This is the method used in very early modems, up to around 1200 baud.

It is also possible, using FSK, to send multiple signals across one line. Simply pick suitable frequencies for each modulation state, and mix the outputs into one consolidated analog signal. On the other end, you have multiple bandpass filters running at the same time and, as long as the chosen frequencies are appropriately spaced apart, they will not interfere.

It is also possible to send analog signals with FSK. One particular system I used took six analog signals, converted them to variable duty cycle pulse trains at 10 Hz, FSK modulated them, mixed them all together, sent them over a leased line, reversed the process, and retrieved the six analog signals.