FSK modem (modulator and demodulator)

Description

This circuit implements a simple FSK modem, consisting of both the frequency shift keying modulator a corresponding demodulator. In this example, the demodulator consists of a shift register and a two AND-gates that detect the 0011 or 0101 patterns for the two frequencies generated by the modulator.

The following image shows example waveforms for this circuit:

Naturally, in a typical modem application, the modulator and demodulator cannot use the same clock signal. Instead, additional logic is required in the demodulator block to recover the clock signal from the transmitted data signal. For the simple FSK modulation scheme, it is sufficient to detect signal transitions and to sample the input signal in the middle between two transitions. For more efficient schemes, special encodings like Manchester or NRZI (non-return-to-zero inverted) codes are used to ensure that the clock signal can be recovered reliably.

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