Theory of Operation and Design of an All-Digital FM Discriminator

The theory of operation and the design of an all-digital FM discriminator is presented in this paper. It is shown that the digital discriminator is equivalent to the Foster-Seely demodulator, i.e., a discriminator that converts the FM signal into an AM signal and then recovers the AM by using envelope detection. The output signal-to-noise ratio is found analytically above and below threshold and the results are verified experimentally. The theoretical analysis includes the effect of quantization noise, frequency jitter in the internal oscillator, and thermal noise. The system can be integrated and with the use of readily available emitter-coupled logic (ECL) can operate at intermediate frequencies suitable for many space, industrial, and commercial applications.