Fsk radio-frequency demodulators

A demodulator for a digital radio receiver, the demodulator comprising:

• a frequency discriminator;
• and
• a Viterbi decoder,
• wherein the frequency discriminator comprises:
• an input for receiving a series digital signal samples representative of a frequency-shift-key (FSK)-modulated signal;
• and
• digital logic for performing frequency discrimination on the digital signal samples to generate a series of frequency samples, wherein each frequency sample is representative of an instantaneous frequency of the FSK-modulated signal in a respective frequency-sample period, and wherein there are an integer oversampling factor, N, greater than one, of frequency-sample periods in each symbol period of the FSK-modulated signal, and
• wherein the Viterbi decoder is arranged to:
• receive the series of frequency samples from the frequency discriminator;
• determine a plurality of branch metrics for each symbol period of the FSK-modulated signal by determining a plurality of distances between a vector of N successive received frequency samples and each of a plurality of reference waveform vectors, wherein each reference waveform vector comprises N elements;
• use the determined branch metrics in a Viterbi process to determine a maximum-likelihood decoding of the FSK-modulated signal;
• and
• output a series of demodulated symbol values corresponding to the maximum-likelihood decoding of the FSK-modulated signal.