Patent GB-571214-A (GB)
Title: Improvements in and relating to high frequency communication systems
IPC: H04B3/20
Filed: 1943-07-09
Abstract: 571,214. Wireless transmitting and receiving systems. BRITISH THOMSONHOUSTON CO., Ltd. July 9, 1943, No. 11150. Convention date, July 11, 1942. [Class 40 (v)] A two-way high-frequency communication system operating both ways on the same carrier frequency, is provided with automatic switching means to prevent the transmitter and receiver at one end of the line being both operative at the same time. The complete system connected to either end of the line 10 comprises the highfrequency oscillator valves 15, 16, the modulator 19, 20, 21, 22, 119, the receiving valves 25, 26, 27, 28, 29, the switching and controlling valves 32, 281, 33, 34, 30, 31, the relay 237, 260 controlling the switches 238, 261, 262, 263, 264, and the handset 23. The high-frequency oscillator comprises a master-oscillator 15 whose cathode potential is controlled from the line 40, and a plate modulated R.F. amplifier 16. The modulating amplifier includes an input transformer 70, a pre-amplifier stage 19, the phase-splitting stage 20, a push-pull output stage 21, 22 and the output transformer 66. Negative feedback is taken via condenser 111 and resistance 112 from one end of the transformer 66 to the cathode 89 of the phase-splitting valve 20. A diode 119 is connected to the transformer 66 in such a way that signals of greater strength than the delay bias derived from battery 47 are rectified and applied via resistances 122, 78 and condenser 82 to the grids 72, 76 of the amplifier 19, thus reducing the overall gain and preventing over-modulation of the transmitter. The input to the receiver includes a fixed potentiometer 130, the series limiting resistance 131 and gas-discharge tube 132, provided to limit the R.F. voltage appearing at the receiver when the transmitter is operative, and the tuned filter 134. The mixer-oscillator 25, whose mixing and oscillating grids 294, 299 are controlled from the line 540, is followed by an I.F. amplifier 26 and the double-diode rectifier 27. The anode 151 of this supplies delayed A.V.C. to the amplifier 26 and grid 135 of the mixer 25, while the second anode 153 and cathode 152 feed audio-frequencies to the push-pull output valves 28, 29 via the phase-splitting network comprising resistances 162, 163, 164, 165. The grid bias of the valves 28, 29 is controlled by valve 32. The handset 23 is connected to the input 70 of the modulator and output transformer 183 of the receiver, and the transmitter output and receiver input are connected together and to the line 10 via the series tuned circuit comprising condenser 18 and inductance 17, so that to prevent oscillation of the whole system, either the transmitter or the receiver must be rendered inoperative at any one instant. The switching and controlling means comprise a D.C. amplifier 32 operated by the D.C. output from the rectifier 27, and controlling the receiver output valves 28, 29, and a diode 33 which rectifies I.F. signal appearing on the line 440, and which controls the D.C. amplifier 34 which in turn controls the bias on grid 237 of the gas, discharge tube 31. In addition, grid 230 of the tube 31 is controlled by audio signals from the line 640 which are amplified by the valve 30, and the anode current of the tube 31 is caused to energize a relay 237 which operates a switch 238 provided with contacts 263 controlling the potential on the line 40 and the operation of the transmitter, and controlling further a D.C. amplifier 281 which determines the potential on the line 540 and the operation of the receiver. Further contacts 261 on the switch 238 close circuit 260, 265 which delays opening of the switch when tube 31 ceases to pass anode current, while contacts 262 modify the bias on grid 230 of tube 31 when closed, and contacts 264 apply A.C. from a source 268 to the anode 236 of the tube 31. In the normal stand-by position, the grid potential of the valve 32 is only slightly negative, so that it is conductive, and the potential developed across the resistance 178 over-biasses the valves 28, 29, so as to make them inoperative. At the same time the absence of a signal on the diode 33 causes only a small potential drop to occur across resistance 205, so that the anode 250 of the D.C. amplifier 34 is non-conductive. Thus, there is no potential drop across the resistance 251 biassing the grid 237 of the gas-discharge tube 31. The grid 230 of this tube is biassed negatively from the tap 235 on the buttery, so that its anode current is zero and the relay 237 stands in the position shown ; thus, contact 263 causes the positive potential of the anode 236 of the tube 31 to be applied via the line 40 to the cathode 42 of the oscillator valve 15, rendering it inoperative. At the same time the positive potential applied by the contacts 263 to the grid 282 of the D.C. amplifier 281 causes the second anode 291 to be non-conductive, so that the line 540 is at low potential, and the receiver mixing valve 25 is operative. When a signal commences in the handset 23, the amplified signal produced by the valve 30 produces positive peaks on the grid 230 of the gas-discharge tube 31, whose anode thereupon becomes conductive. The reduced potential drop across resistance 280 applied to the D.C. amplifier 281 causes its second anode 291 to become conductive, and the negative potential developed across the resistance 292 renders the receiver mixing valve 25 inoperative. The increased current flow to the anode 236 of the tube 31 causes the relay 237 to operate, and the reversal of the switch contact 262 grounds the line 40 and causes the oscillator 15 to start functioning immediately after the receiver valve 25 has been rendered inoperative. The closing of the switch contact 262 reduces the fixed bias on the grid 230 of the tube 31, and closing of the contact 261 completes the circuit through the relay winding 260 and the resistance 265, which delays opening of the relay when the tube 31 ceases to conduct. The contact 264 applies A.C. from a source 268 to the anode 236 of the gas-discharge tube 31, providing recurring periods when the potential of the anode 236 is low and the grids 230, 237 can take charge of operation of the tube if suitably biassed. Series resistance 239 and shtmt resistance 280 improve the sensitivity and speed of operation of the relay 237. Upon cessation of the signal in the line 640 from the handset 23, the grid 230 of the gas-discharge tube 31 takes up a steady negative potential, and the anode 236 ceases to conduct. After a time interval determined by the circuit 265, 269, the relay contacts reverse, the line 40-is biassed positively by contact 263, and the transmitter ceases to function. The positive potential applied to the grid 282 of the D.C. amplifier 281 causes the anode 291 to cease conducting, time constants being so chosen that the potential across the resistance 292 falls to zero and the receiver mixer valve 25 commences functioning immediately after the transmitter valve 15 has ceased operation. Upon reception of a signal from the line 10, the I.F. signal appearing in the line 440 is rectified by the diode 33 and produces a negative potential across the resistance 205 which is applied'to the grid 240 of the D.C. amplifier 34, thus causing the anode 250 to conduct. The resultant potential across the resistance 251 biasses the grid 237 of the gas-discharge tube 31 negative, so-that it will not conduct, even if the grid 230 becomes positive. The negative potential appearing on the grid 191 of the valve 32 causes its anode 194 to become non-conducting, and the time constants are so chosen that the potential drop across the resistance 178 falls to zero and the output valves 28, 29 become operative immediately after the tube 31 has been blocked by the D.C. amplifier 34. Upon cessation of a signal from the line 10, the valve 32 becomes conductive, and the output stages 28, 29 inoperative, followed by removal of the bias on grid 237 of the tube 31, and the apparatus reverts to the standby position. The grid 237 of the tube 31 may be connected to an adjustable tap on the resistance 252, and the negative bias on the grid 237 when a signal is being received is so adjusted that an intense signal on the grid 230, such as that caused by whistling into the handset 23, will cause the anode 236 to become conducting and " break-in " transmission to be commenced.