GB1368419A - Crystal controlled frequency discriminator - Google Patents

Abstract

1368419 Frequency demodulator; FSK demodulator SINGER CO 11 June 1973 27712/73 Headings H3A and H4P A frequency demodulator comprises a shift register 18 responsive to a crystal controlled clock source 20, 22 for providing a constant delay to a frequency modulated input signal, a gate 28 coupled to receive the direct as well as the delayed input signal for producing constant energy output pulses whose mark to space ratio is dependent on the phase difference of the two applied signals. The constant delay is normally arranged to be “ the time period of the centre frequency of the incoming modulated signal (equivalent to a phase shift of 90 degrees) and is determined by the number of stages in the shift register 18 as well as by the clock frequency. In operation, the incoming modulated signal is limited at 12 and applied to an exclusive OR circuit 28 directly as well as via the shift register 18 which produces varying phase shift upon frequency deviations from the centre frequency. This results in output pulses from 28 having a mark to space ratio dependent on the frequency deviations from the centre frequency. These output pulses are integrated at 32 to obtain the demodulated output. The demodulator described above may be used as a FSK demodulator to obtain a two level output from integrator 32 which may be further applied to a differential amplifier 38 to obtain a two level output with well defined and sharp edges suitable for application to a computer or a teletype.