GB1262550A - Frequency discriminator - Google Patents

Abstract

1,262,550. Frequency discriminators; suppressing noise modulation. INTERNATIONAL STANDARD ELECTRIC CORP. 14 May, 1969 [18 May, 1968], No. 24544/69. Addition to 1,152,313. Headings H3A and H3R. In a frequency discriminator for frequency modulated signals which are shaped to produce a train of pulses spaced in time in response to the modulation signal, input signals are applied to a semi-conductor device, i.e. transistor 3, which has a time lapse between the application of the signal and its becoming non-conductive in response thereto, the output of the semiconductor device being an inverted input pulse train having pulses whose duration is equal to the spaces of the input-signal plus the time lapse. The input and output signals are added, in AND gate 5 to form pulses having a constant duration and spaced in response to the modulating signal. The added signal is applied to a second semi-conductor device, i.e. transistor 13, having a time lapse between the application of the signal and its becoming non-conductive thereto, which is not less than the shortest time between two pulses in the added signal. The output signal of the second semi-conductor device is integrated by diode 15 and capacitor 16, to produce a demodulated output signal. In a first embodiment (Fig. 1, not shown), the time lapse of the transistor 13 is fixed, a fixed bias being applied via a resistor to its base, and an output is taken across the R.C. filter circuit 16, 17. In the embodiment shown, the filter circuit 16, 17 is followed by an emitter-follower output transistor 18 and the transistor 13 is given a sliding D.C. bias derived from the output of this stage via a smoothing network 19, 20, 21. A resistor may be placed in series with the capacitor 20 so as to give negative feedback at signal frequencies, so as to linearize the detector.