Patent GB-318273-A (GB)
Title: Improvements in electric wave signalling systems
IPC: H03C1/00
Filed: 1928-05-31
Abstract: 318,273. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). May 31, 1928. Addition to 269,251. Thermionic generators, detectors, and modulators.-Comprises modifications of self-oscillating valve circuits, and particularly push-pull oscillatory modulators and demodulators of the kind described in the parent Specification, designed to give the input and output circuits advantageous values of impedance at the several frequencies present in those circuits. Fig. 1 shows a backcoupled oscillation with two plate circuit branches, one including a battery 6 and a choke 5, and the other including a closed circuit 10 tuned to carrier frequency, and three or more parallel circuits 7, 8, 9. The circuit 10 is coupled to the grid coil 2 and to a load circuit 11, and the grid circuit includes a high resistance 3 and condenser 4 designed to bias the grid with such a negative voltage that plate current passes only at the peaks of grid voltage. To prevent irregular or unstable operation of the oscillator, the plate circuit is given high impedance at low frequencies, by the use of a choke 5 of very high inductance, say 200 henries. The branch circuits 7, 8, 9 are tuned to the carrier and to its respective harmonics, so that the plate circuit has almost zero impedance at the harmonic frequencies. At the carrier frequency the plate circuit impedance is equal to that of the load circuit. In some cases, a single condenser may replace the tuned branch circuits. Omission of the branch circuits 8, 9 gives the internal platefilament path a high impedance, and the falling amplitude of the fundamental oscillations may be compensated by increasing the ratio of the feedback coupling 2. In Fig. 2, two valves are connected in push-pull rotation, and generate oscillations in the manner described with reference to Fig. 1. They also function as modulators or demodulators according to the nature of the input and output circuits employed. For modulating, speech currents from a line LL are applied through a low-pass filter LF and transformers 22 to the grids of the valves, and the high-frequency line HL transmits one or both of the side-bands from the plate-circuits through transformers 23 and a band filter BF. The input circuit impedances are matched at speech frequencies and at the frequency of the carrier and its harmonics, whilst the output circuit impedances are matched at the frequency of the desired side-band or bands. For grid-current modulation, the input circuit impedance should be infinite for the frequency of the desired sideband, and should be zero for all frequencies other than the carrier, its harmonies, speech frequencies, and the desired side-band. The output circuit impedance should be infinite for all frequencies other than the desired side-band. For plate-current modulation, the input circuit impedance should be zero at all frequencies other than speech, the carrier and its harmonics, whilst the output circuit impedance should be zero at all frequencies other than the desired side-band. With the object of approximating to these conditions, the filters LF and BF terminate (adjacent the modulator input and output circuits) in shunt elements as shown in Fig. 3 in the case of plate current modulation. For grid-current modulation, these filters terminate in series elements.