GB564045A - Improvements in and relating to frequency modulation apparatus - Google Patents

Abstract

564,045. Phase and frequency modulation. BRITISH THOMSON-HOUSTON CO., Ltd. May 28, 1943, No. 8573. Convention date, May 28, 1942. [Class 40 (v)] Relates to phase modulation systems in which two or more phase-displaced carrier voltages are amplitude modulated and combined vectorially to produce the phase-modulated wave ; and comprises means for obtaining a linear relation between signal voltage and phase deviation by deriving a voltage proportional to the amplitude modulation of the combined wave and applying it as an additional or correcting phase modulation voltage. As shown, the carrier from a source 10 is applied in phase opposition to the inner grids of the valves 11, 13 and in quadrature to the inner grid of the valve 12. The signal from a microphone 16 is applied in phase opposition to the screen grids of valves 11, 13, but the valve 12 is unmodulated,. The anodes of the three valves are connected to each other and to a tuned output circuit 43, 44 feeding a phase-modulated wave to the aerial 15 through an amplifier 14. Amplitude modulation also present in the carrier is rectified by the diode 71, the load resistance 72 being connected in the grid circuit of an amplifier 81, the anode resistance 50 of which is in the screen lead of the valve 12. A vapour discharge device 82 is connected between the cathode of valve 81 and earth. In this way, as the depth of modulation rises, the screen of valve 12 becomes less positive and the strength of its unmodulated carrier output is reduced. This applies a correction of phase angle to the carrier in the circuit 43, 44, and by adjustment of the resistance 50 a linear relation is obtained between the phase deviation and the modulating signal. The valve 81 may be omitted if the diode 71 is inverted and a portion of its load resistance 72 is included in the screen circuit of the valve 12, Fig. 9 (not shown). A condenser-resistance coupling may replace the direct connection to the.grid of the valve 81, and a resistance may be used in-place of the discharge device 82, Fig. 10 (not shown). A low-pass filter may be inserted between the rectifier 71 and the valve 81 for reducing the effect of the higher frequency components of the modulating signal, Fig. 8 (not shown). The system permits a large phase shift of the carrier wave and reduces the amount of frequency multiplication necessary to produce a suitable frequency deviation. Specification 562,432 is referred to.