GB606780A - Frequency modulation system for electric signalling - Google Patents

Abstract

606,780. Frequency modulation. STANDARD TELEPHONES & CABLES, Ltd. Jan. 18, 1946, No. 1828. Convention date, Jan. 23, 1945. [Classes 40 (v) and 40 (vi)] In a frequency-modulation system, the signal voltage b, Fig. 2, is first converted into a 'regular sequence of sawtooth or like voltages c, the slope of which varies according to the instantaneous value of the signal voltage. The saw-tooth voltages are then applied to a device which varies the phase of the carrier wave, and therefore varies the carrier frequency d proportionally to the slope of the saw-teeth. In Fig. 1, the saw-tooth generators comprise condensers 7, 8 charged through resistances 13, 14 and discharged through triode valves 5, 6 when positive pulses a are applied to the grids from a supersonic oscillator 1 and pulse generator 2. The driving-voltage is supplied from the signal source through a transformer 10, and the sawtooth generators are operative alternately, negative voltages on the condensers being inhibited by the diodes 19, 20. The output voltages of the form c, Fig. 2, are applied through a high-pass filter 35 to the grid 36 of a reactance valve 37 connected across the tuned output circuit 40 of an amplifier 39 fed from the crystal-controlled carrier source 45. The phase of the carrier passing to the aerial 50 is consequently varied in dependence on the grid voltage of the valve 37, and the emitted frequency thus varies proportionally with the rate of change of that grid voltage, i.e. with the signal voltage. Quadrature carrier voltage for the grid of the reactance valve is supplied through a condenser and variable resistance 42 from the tuned anode circuit 40 of the valve 39. In a modification, Figs. 3 and 4 (not shown), only one saw-tooth generator is used, and a battery is connected in series with the secondary of the transformer 10. Specification 596,376 is referred to.