Design of digital discriminator filters for voiceband FSK data modems

Matched filters, which are optimum for noncoherent FSK receivers when the ratio of carrier frequency to baud rate is large, produce unacceptable levels of isochronous distortion when the baud interval contains only one or two cycles of a signalling tone. The authors show how, when digital filters are used, certain constraints may be applied to the filter coefficients which enable a clean eye diagram to be recovered with little loss of S/N ratio in white noise. The method is illustrated for the case of a 1200 b/s voiceband FSK modem.