GB565403A - Improvements in and relating to phase modulators - Google Patents

Abstract

565,403. Phase and frequency modulation ; valve generating circuits. MULLARD RADIO VALVE CO., Ltd., and BAILEY, C. E. G. Feb. 5, 1943, No. 1985. [Class 40 (v)] Phase modulation with substantially zero amplitude modulation is effected by differentially modulating two phases of the carrier.wave in quadrature relationship and combining, the outputs, the. quadrature waves being generated by a reciprocally coupled pair of valves. As shown, the triode sections of a pair of triodehexode valves V2, V3 are reciprocally coupled by condensers C, CT of the same order of magnitude as the grid-anode capacity of the valves, and a piezo-electric crystal K or a parallel-resonant circuit is connected between the grid of the valve V2 and earth. The Specification shows that the circuit oscillates at a frequency at which the crystal or resonant circuit presents an inductive reactance, and that the.grid voltages are in quadrature with each other. Each triode grid is connected to a control grid of the associated hexode section, and the anodes of the two hexodes are connected to a common tuned output circuit L1, C4. The amplification of the hexodes is controlled by signal voltages applied in push-pull to their inner grids, the voltages being derived from a microphone M through transformer TR1, valve V1 and transformer TR2. The amplification of the valve V1 is automatically limited by control voltages applied to its grid and derived by diode rectifiers connected across the two halves of the secondary of the transformer TR2. The microphone voltages of higher frequency may be reduced in amplitude by a low-pass filter network in the grid circuit of the valve V1 with the object of giving the modulated output the characteristics of frequency, rather than phase, modulation over this band. The modulated-wave in the circuit L1, C4 is fed to the output feeder through frequency-multiplying stages V4, V5, V6, which may also act as limiters to remove residual amplitude modulation. The Specification shows that the circuit gives pure phase modulation if the radio frequency output of the valves V2, V3 are respectively proportional to #1+ sin x and #1- sin x, where x is the instantaneous modulation voltage. This condition may be approximated by inserting series resistances between the ends of the transformer secondary TR2 and the respective grids.