GB1452379A - Demodulators for frequency-shift keyed signals - Google Patents

Abstract

1452379 Data transmission; FSK systems SIEMENS AG 18 April 1974 [30 April 1973] 16935/74 Heading H4P In a demodulator for an incoming approximately rectangular FSK signal, a comparison rectangular signal is formed (e.g. by a counter) equal to or less than a half period of incoming signals to which the comparison signal is compared, the timed difference characterizing the value of the incoming signal, which is then passed through a low pass filter and threshold stage. Two consecutive half periods may be used with a similar comparison using a second counter and preferably the arithmetic mean is determined. In the embodiment only deviations between incoming signal, half waves and the comparison signal are measured hence resulting in high discrimination. In the described arrangements the frequencies are produced as decapitated sinusoids having steep flanks. Incoming signals B1 or B2 representative of separate frequencies are differentiated to produce pulses E1 or E2 at positive flanks, which after a delay t1-t2 opens a gate G1 allowing pulses to be admitted to a counter Z, also to a trigger stage K which emits a 1 for duration L. At time say t11, counter Z is reset also stage K which emits 0. Stage D1Z forms the difference between B1 and L=M1, or B2 and L=M2 which are applied to a summing circuit SU which sums the parts p1, q1 &c. making up M1 &c. and through low pass filter TP and threshold stage SCH feed a teleprinter, processor, or display. In an alternative arrangement Fig. 5 (not shown) through switches S1, S2 each half wave is examined in separate parallel circuits and after summing, the separate signals are led to a store (PS) whose output is dependent on both half periods of incoming waves.