GB1188281A - Improvements in or relating to Detection Means for Frequency Modulated Signals - Google Patents

Abstract

1,188,281. Frequency discriminators; data transmission systems. SERCK CONTROLS Ltd. 18 April, 1967 [21 Jan., 1966], No. 2821/66. Headings H3A and H4P. A receiver for frequency shift binary data transmission comprises means for comparing the period of an input signal with the period of a monostable circuit to determine whether the input is at the higher or the lower frequency. In Fig. 1 the input is converted in units 1 and 2 into short strobe pulses which trigger the first of three monostable delay circuits 3-5 arranged in cascade, and which are also fed to output gates 6 and 7. Gate 6 is open if a strobe pulse occurs after the end of the C pulse from unit 5 and before the next B pulse from unit 4, i.e. if the input is at low frequency. Gate 7 is open if a strobe pulse occurs during a C pulse from unit 5, i.e. if the gate is at high frequency. The output from gate 7 inhibits gate 6 and feeds back to its own input to maintain the output for the duration of a strobe pulse; the output also feeds a store 9 which feeds back to unit 5 to lengthen the C pulse when the current state of the store indicates that high frequency is being received. This latter arrangement makes the receiver less sensitive to distortion and noise. As an alternative, Fig. 3 (not shown), there may be two C pulse generators of different pulse lengths, the input being supplied to one or the other according to the state of the output store. As indicated in Fig. I by unit 10, the input may be inverted and fed to a similar channel so that half a cycle of the input waveform is required to indicate a change instead of a full cycle.