GB290642A - Improvements in radio telephone signalling - Google Patents

Abstract

290,642. Armstrong, E. H. May 18, 1937, [Convention date]. Transmitting and receiving systems; thermionic modulating systems ; fading, preventing. -In order to eliminate fading and reduce the effect of static interference, signalling is effected by varying the frequency but not the amplitude of the carrier wave. In reception, different impedance branches are used to convert the frequency variations back into corresponding voltage variations, whilst limiting devices are interposed at the transmitting and receiving ends for maintaining the amplitude constant. In transmitting, Fig. 1, the frequency of the tuned oscillator circuit 1, 3 is varied at speechfrequency by the changing reactance of the closely-coupled valve circuit 6, 7 under the influence of the microphone 10. The resulting oscillations are fed through a coupling-coil 12 to an amplifier 13, and then pass through a limiting valve 16 of the screened-grid type in which any existing amplitude-variations are smoothed out. Before the outgoing signals reach the aerial 25 they are passed through a band filter 20 and power amplifier 23. At the receiving end, Fig. 2, the incoming signals are passed through band filters 32, 35, amplifier 33, and a currentlimiter 34 to eliminate amplitude variations. They are then applied to an input circuit 39, 40, 41, which is so adjusted that the portion BC, across the amplifier 42, is non-reactive for the lowest frequency of the signal band, whilst the portion AC across the amplifier 43 is non- reactive for the highest received frequency. The circuit as a whole is made substantially aperiodic by adjusting the resistance 37, so as to balance out any residual amplitude-variations. In these circumstances the output from the detectors 48, 49 will vary in a straight-line law with the input frequency-variations, but will be indifferent to amplitude variations and will therefore reproduce the original signal in the phones 54. At the same time the effect of static interference is reduced. Instead of the arrangement shown in Fig. 1, frequency modulation may be effected by coupling a push-pull valve amplifier across capacitative and inductive reactions arranged on the input circuit of the oscillator valve. In order to reduce damping in the receiver circuits, a valve 63, Fig. 4, may be applied across a resistance 62 in the input branches A B C so as to introduce a negativeresistance effect through a tapping from the plate resistance 64.