Digital filter sets for frequency shift keyed modems

An efficient structure for the digital filter pair required in a noncoherent FSK demodulator consists of a common receive filter cascaded with two separate discriminator filters. The authors describe two numerical procedures aimed at achieving approximately matched filter transfer functions for such a structure. The first procedure is a modification of an earlier technique used to equalise the group delays, while providing good first approximations to the desired amplitude responses; the second simultaneously optimises the two amplitude responses so obtained. A design example for a 300 baud FSK system is presented.