FSK demodulator apparatus in accordance with the teachings of the present invention is provided wherein the input signal to be demodulated is phase compared with a plurality of reference phases of the mark and space frequencies to be detected to bracket the input signal within a particular quadrant for the frequency involved. The results of each phase comparison are then integrated with respect to time and threshold compared in such a manner that one or more phase comparison outputs, each having a duty cycle not equal to 50%, will be produced for the frequency involved, resulting in an output from the threshold comparison. This output may then be employed to establish a logic condition indicative of the frequency then being received.