GB339459A - Improvements in or relating to radio signalling systems - Google Patents

Abstract

339,459. Marconi's Wireless Telegraph Co., Ltd., (Assignees of Beverage, H. H.). Nov. 19, 1928, [Convention date]. Fading, preventing; thermionic generators and modulating systems; thermionic amplifiers.- To eliminate the effects of fading, the transmitter carrier wave is frequencymodulated, the volume at the receiver being varied inversely in accord. ance with received signal intensity. Fig. 1 shows a tone signal transmitter 2 transmitting marking and spacing waves of slightly different fre. quency, effected by keying a portion of the inductance 5 or varying the capacity of condenser 24. The receiving amplifier comprises screened high frequency amplifiers 32, 42, equally responsive to both transmitted frequencies, a detector stage 40 and low pass filter 44 the circuit being completed through a resistance 46 the drop across which adds more or less negative bias to the grids of the valves of amplifier 32, normally biassed by a battery 54, in accordance with the received signal intensity, so that the amplification is a maximum with weak signals. The receiver proper, 60, is preferably connected between the amplifiers 32, 42 and discriminates between marking and spacing frequencies, either by being tuned to one or by having circuits tuned to each and actuating a recording pin in opposite directions. Fig. 2 shows a transmitter in which output from a microphone 102 or code signalling means 112, 114. is fed through the transformer 116 to a pair of oscillators 122, 128, the input circuits 124, 130 of which are respectively tuned to the two extremes of the desired frequency range so that the common forced oscillation frequency, due to the common anode coil 126, lies between the two resonant frequencies, its magnitude depending on the relative potential applied to the oscillator grids. The output from the system is made to modulate a constant frequency crystalcontrolled oscillator 140, the carrier is suppressed by the balanced modulator 150 and one sideband is eliminated in a filter 154, the resultant being amplified at 156 and radiated. Fig. 3 shows a receiver having an amplifier 162, detector 164 and local oscillator 166, and intermediate frequency amplifier 168, part of the output from which is passed through a band pass filter 170 to a detector 172, filter circuit 176 and biassing resistance 178 which acts similarly to the volume control resistance 46 of Fig. 1, the condenser 182 determining the time constant of the control action. The output from amplifier 168 is passed to a detector 190 through a circuit 188 tuned to one extreme of the frequency range thus transforming the frequency modulation into amplitude modulation. The output circuit may feed a loud speaker if speech is transmitted, or filters 196 and utilizers 198 in the case of multiplex code signals.