Mixed signal decoder for audio frequency applications

Frequency shift keying (FSK) finds applications in low frequency transmission over band limited audio frequency channels. One such application is calling number identification, a service currently offered by telephone service providers. A number of approaches to FSK demodulation are described in the literature. These approaches may work well when the transmitted frequencies are significantly higher than the baud rate. However, when the baud rate is comparable to the transmission frequencies, the conventional approaches are costly to implement. This paper describes a mixed signal approach to FSK demodulation suitable in particular to the decoding of this type of signal, and is based on a finite state machine (FSM). The proposed circuit consists of an analog interface followed by a threshold device, which is composed of a novel comparator circuit.