GB427508A - Improvements in and relating to carrier wave signalling systems - Google Patents

Abstract

427,508. Wireless transmitting systems. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London. May 15, 1934, No. 14661. Convention date, May 19, 1933. [Class 40 (v)] Relates to modulating systems of the kind in which the mean carrier intensity varies with the signal volume, and consists in separately rectifying the lower-frequency and the higher-frequency components of the signal to obtain the bias for regulating the carrier wave amplifier ; and in providing means for maintaining that bias constant when signals exceed a predetermined volume. The carrier wave from a source 1 is applied through an amplifier 2 to the input circuit of a push-pull amplifier 3 from which it passes to a load circuit 4. The signal from a microphone 18 passes through an amplifier 19 to a valve 20 which by means of a choke 21 in its plate circuit modulates the output of the valve 2. The bias on the grids of the valve pair 3 is supplied through a line 15 from a battery 13 and a resistance 16. Signal voltage from the amplifier 20 is again amplified by the valve 20<1> and its higherfrequency components are passed through a condenser 25 to a full-wave rectifier 27 the output from which is smoothed and passed through the resistance 16. The lowerfrequency signal components pass through an inductance 30 to a frequency-doubling valve pair 32, which feeds a full-wave rectifier 36, and the resulting direct current is smoothed and passes through the same resistance 16. When no signal is present, the bias provided by the battery 13 as indicated by the line 42, Fig. 3, is such that the valves 3 either just fail to pass any plate current, or pass a carrier of low steady amplitude indicated by the line 50. When signals are present, the bias is reduced to an extent dependent on the envelope 52 of the signal current 43 as shown at 45. The effect is that the troughs of the modulated wave all fall to the zero line, or to the line 50. When signals exceed a certain peak valve, the circuit may be arranged so that no further reduction of grid bias is effected as indicated at 53, and this results in the carrier wave falling below the level 50 as shown at 58. This limitation is produced by connecting a discharge tube 49 between the bias lead 15 and a point on the battery 13, which tube becomes conductive when the signal amplitude reaches the predetermined peak valve.