Patent GB-168967-A (GB)
Title: Improved method of signalling in systems generating high-frequency oscillations
IPC: H03C1/16
Filed: 1920-06-07
Abstract: 168,967. Western Electric Co., Ltd., (Western Electric Co., Inc.). June 7, 1920. Thermionic modulating systems and apparatus. -In an electric signalling system having an oscillatory circuit associated with an oscillating- device, a high-frequency impedance circuit is connected in shunt with a portion of said oscillating circuit, the input circuit elements of said oscillating device being connected to points in the impedance circuit, and the relative values of the portion of the impedance between said points and the portion outside said points being variable by signalling means. The oscillatory circuit, Fig. 2, shown as an aerial system 23, 24, 25, has in parallel with part of it a high impedance circuit having a constant part comprising a resistance 26 and condensers 27 and a variable part comprising the anode-filament circuit of a thermionic valve 28, the grid-filament circuit of which is coupled to a source of signalling current 55. To the variable part of this high impedance circuit is connected the input circuit of an oscillating device comprising one or more thermionic valves 36 arranged in parallel, the anode circuits of which are supplied by a source 32 through a choke 33 and are connected to the aerial system. A high resistance 38 acts as a grid leak. Signalling currents in the circuit 55 cause changes in the potential applied to the grids of the valves 36, resulting in changes in amplitude of the oscillations without change in frequency. As shown in Fig. 4, the thermionic impedance element 76 is connected indirectly to the grid-filament circuit of the oscillating valve 60. The impedance circuit comprises a constant portion containing an inductance 69, condenser 70, and high resistance leak 71, and a variable portion comprising a condenser 68 and an inductance coil 67 forming the secondary of a transformer, the primary 84 of which is in the anode-filament circuit of the valve- 76. A source 64 supplies the space current to the valves 60 and 76 through a low-frequency choke 65 and high-frequency retards 66, 82. A source of signalling current, e.g. a microphone circuit 72, is coupled to the grid circuit of the valve 76, the coil 75 being shunted by a high resistance 81. Variations in current in the circuit 72 thus cause alterations in impedance in the coil 67 coupled to the coil 84. The circuit 83, 84 may be approximately tuned to the frequency of the oscillations generated, and a shunt circuit 85, 86, used when the generator 64 is of large inductance, may be similarly tuned. The anode 62 is connected to the oscillatory circuit 57, 58, 59 through a large capacity 88 and a variable connection 89. Instead of the thermionic devices 28 or 76, the variable part of the high impedance circuit may comprise a condenser transmitter having a resistance element in shunt with it, such loop circuit comprising transmitter and resistance being tuned to a frequency slightly different from that of the oscillations generated, so as to have large joint reactance which changes rapidly with slight change in the reactance of either. The input circuit of the oscillating device may be connected to either the variable portion of the impedance circuit as shown in Fig. 2, or to the constant portion as shown in Fig. 4.