Patent GB-168056-A (GB)
Title: Improvements in thermionic-valve modulating-systems for signalling purposes
IPC: H03C1/20
Filed: 1921-08-12
Abstract: 168,056. Western Electric Co., Ltd., (Assignees of Heising, R. A.). Aug. 17, 1920, [Convention date]. Addition to 133,366. Thermionic modulating systems a n d apparatus; multiplex signalling. - The parent Specification describes a system of modulating a high-frequency carrier-wave, derived from a thermionic generator, by microphonic or speech frequency alternations imposed upon the grid and filament of a second thermionic valve, the latter valve being shunted across the high-tension supply and a choke coil in the plate circuit of the generator valve so as to provide a shunt path having a resistance that varies according to the superposed speech, thereby giving rise through the action of the choke-coil to corresponding .variations in the voltage applied to the plate of the generator, and sympathetic amplitude-variations in the energy radiated. The characteristic feature of the present invention lies in the use of one or more thermionic amplifiers applied to the modulated carrier wave and connected in parallel with the oscillation-generator across its high-tension source and a choke coil so that the energy that passes through these amplifiers is further modulated by the similar diversion of current through the variable-resistance shunt path of the microphonically-controlled valve. Means are also described for controlling the degree of modulation in each amplifying-unit in order to minimize distortion. The filaments are heated by alternating current, the connections being so arranged as to eliminate from the radiated energy any disturbing frequencies arising from the heating-alternator. Fig. 1 shows the invention applied to a multiplex system transmitting three conversations on carrier weaves of different frequencies simultaneously, by one aerial a from the aerials T, T<1>, T<2>. Speech from the line L is imposed upon the grid-filament circuit of the valve V, which is shunted across the hightension 1 and choke coil 2, both of these being in the plate circuit of the generating-valve O. The modulated output from the valve O is passed through high-frequency amplifiers HA, HA<1>, the plates of which, when the switch S is on contact 3, are in circuit with the choke-coil 2 and in parallel across the source 1 with the valves V and O, and consequently suffer the effects of the current-diversion through the valve V as the resistance of this valve is varied by the applied speech. Variable condensers 9 and resistances 10 in the grid circuits of the units HA, HA<1> control the degree of amplification so as to avoid distortion. By means of shunt circuits 18, 19, 20 and 21, 22, 23 the aerial may be tuned to the frequency of either of the transmitters T, T<1>, T<2> filter circuits 16, 17 further preventing one unit from reacting on the other. In a modified arrangement, a low-frequency amplifier is inserted between the line L and the valve V, and the speech modulation is effected in the highfrequency amplifiers only, and not in the generating-valve. The high-tension is derived from a dynamo, and a battery is provided in order to adjust the value of the grid potentials. The filament connections to the heating-alternator are balanced by means of a third neutral main.