GB453732A - Improvements in carrier wave modulating systems - Google Patents

Abstract

453,732. Modulating systems. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London. Oct. 2, 1935, No. 27202. Convention date, Oct. 4, 1934. [Class 40 (v)] Modulation is effected according to the floating-carrier method, by using Class B amplifiers for the modulating-voltage, and by utilizing the variation in their direct current resistance to alter the grid or platevoltage of the carrier-wave amplifier, and so to raise or lower the carrier level according to signal volume. In the Figure the microphone 18 is connected through an amplifier 20 and transformer 21 to a pair of push-pull Class B amplifiers 23. The carrier source 10 is coupled to an amplifier valve 13 which passes no plate current until its grid is excited. The plate circuits of the valves 23, 13 are connected in series with each other and with the supply source 32. When the microphone is inactive the valves 23 have high D.C. resistance and there is only a small fraction of the voltage of the source 32 applied to the platefilament circuit of the valve 13, which consequently passes only weak carrier pulses to the aerial. The level of carrier wave output in the absence of signals may be adjusted by biassing the valves 23, or by inserting a resistance 42 across the valves 23 in parallel with the condenser 29. As the signal grows in volume the carrier pulses to the aerial circuit are proportionately strengthened. At the same time the carrier output is modulated in the rhythm of the signal on account of the inclusion in the series circuit of the secondary coil 28 of the output transformer of the amplifiers 23. In a modification, the valve 13 has an independent plate supply, and the plate current of the valves 23, together with the signal current arising in the secondary coil 28, flow through a resistance situated in the grid circuit of the carrier amplifier 13.