Patent GB-186069-A (GB)
Title: Improvements in and relating to high frequency signalling systems
IPC: H04B1/04
Filed: 1922-09-15
Abstract: 186,069. British Thomson - Houston Co., Ltd., (Assignees of Baker, W. R. G.). Sept. 17, 1921, [Convention date]. Thermionic oscillation generators and modulating systems; combined transmitting and receiving arrangements.-In transmitting systems in which separate modulating and generating valves are used for telephony, means are provided by which the modulators may be switched into parallel with the generators so as to assist in the production of oscillations when the system is to be used for telegraphy. A make-and-break switch is provided in the grid circuit of the generating- valves for the production of tonic train when desired. The filament heating current is reduced by the disconnection of the aerial from the transmitting-apparatus. The energy supplied to the plate circuits of the valves by a dyanmo may be varied, as by a variable resistance in series with the shunt field coil of the dynamo, and the grid potentials may be simultaneously varied. A lock may be provided for preventing unauthorized use of the set at full power. The diagram shows a set of five valves 1 - - 5, and the switches 41, 42 are shown in position for telephony. The speech currents in the microphone circuit 36 are applied to the grid of an amplifier 5, the plate circuit of which is supplied by a source 38 and is coupled by a condenser 40 to the grid circuit of the modulating- valves 3, 4 connected in parallel. The plate circuits of these valves are supplied by a dynamo 18 through a low-frequency choke coil 19 and a high-frequency choke coil 33, the negative pole of the dynamo being grounded, as are also all the filaments. The plate circuits of the generator valves 1, 2 are supplied by the dynamo 18 through the choke-coil 19 and aerial coupling-coil 20. The connection of the generator and modulator plates in parallel relation to the dynamo 18 and choke 19 results in a modulated potential on the generator plates and a modulated oscillatory current through the coil 20 and the aerial. For variation of the wave length the aerial circuit may be completed through a variable loading- coil 29, and through any one of the loading-coils 28 and series condensers 27. The reaction coupling for the generator valves 1, 2 is obtained by connecting the grids through the appropriate condenser 31 to the aerial circuit. The grid circuit of the generator valves comprises a variable leak resistance 12 shunted by a condenser 11, and a series arrangement of a choke coil 44, rotary interrupter 14 and key 15 all shunted by a condenser 13. The choke coil may be assisted in rejecting high-frequency current from the key circuit by the provision of a condenser 61 so as to form a tuned blocking-circuit. When the apparatus is employed for telephony the key 15 is kept closed and the interrupter so arranged as not to break the grid leak path. To convert the apparatus for telegraphy the switches 41, 42 are swung over so as to connect the plates of the modulators 3, 4 directly to the generator plates, and the modulator grids directly to the generator grids, the four valves 1 - - 4 then serving as generators in parallel. The amplifier valve 5 may if desired be similarly connected in parallel. Signals may be sent by the key 15 with or without the tonic train provided by the interrupter 14, any interruption of the key circuit causing the grids to become negatively charged and stopping the oscillations. A battery 45 and adjusting switch 49 may be provided for giving a normal negative potential to the grids of all the valves. The voltage supplied to the plate circuits of the valves 1 - - 4 may be varied by switch 48 moving over a resistance 46 in series with the shunt field coil 47 of the dynamo 18. Switches 48, 49 may be simultaneously operated. Unauthorized operation of the set at full power may be prevented by a switch 50, connected in. the lead for short-circuiting the whole resistance 46, and held open by a lock 52. When the aerial is disconnected from the transmitter by a switch 23 and is connected to the receiver terminal 25, the filaments are automatically dimmed by opening a short-circuiting contact 54 connected across a resistance 53 in the circuit supplying current to the filaments through the transformer 8, 9. Condensers 57 are connected between the grids and filaments of the valves 3, 4, 5 to prevent the production of high-frequency potentials on the grids when the valves are working as modulators or amplifiers. The adjustments necessary for varying the wave length are affected by a single handle connected to the elements 26, 32, 60. Tonic train telegraphy may be effected by a buzzer replacing the microphone 34, and the key may be placed in the buzzer circuit or in the position shown at 15. In the latter case, in order to cut down the plate current of the valves 3, 4, 5 when the key is open, their grids may be rendered negative by connecting the lead 59 to the positive terminal of the battery 45 instead of grounding the latter.