FSK Decoders An FSK (frequency-shift keyed) signal decoder can be built as shown in Fig. 13.28. The
decoder receives a signal at one of two distinct carrier frequencies, 1270 Hz or 1070 Hz,
representing the RS-232C logic levels or mark (-5 V) or space
(+ 14V), respectively. As the signal appears at the input, the loop locks to the input
frequency and tracks it between two possible frequencies with a corresponding dc shift
at the output.