Patent GB-637086-A (GB)
Title: An electron discharge device circuit adapted to become conductive on the applicationof a predetermined voltage
IPC: H04J3/04
Filed: 1946-08-06
Abstract: 637,086. Multiplex pulse signaling; valve circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. , Aug. 6, 1946, No. 23333. Convention date, Aug. 4, 1945. [Classes 40 (v) and 40 (vi)] In a pulse multiplex communication system a step wave is generated in which each voltage step is associated with a different one of the channels, each of which comprises a valve stage which is normally cut-off and each of which is rendered conducting at a different output voltage of the step-wave generator. Fig. 1 shows in schematic form the elements of the transmitting end of an eight channel timemodulated pulse multiplex system. A 90 kc/s. crystal-controlled oscillator 10 triggers a pulse generator 12 to give a train of short pulses having an 11.1 Á sec. interval between them. These are applied to the step-wave generator 14 to produce a wave of 100 Á sec. duration comprising nine steps, the sudden drop in voltage at the end being used to trigger the synchronizing pulse generator 30. The step waveform is also applied via the coupling amplifier 16 to the parallel connected inputs of the eight channel selectors 18, which are arranged to become successively conducting at the different voltage step levels. The pulse generated as each channel selector becomes conducting triggers the sawtooth generator 20, the output from which renders the channel pulse generator 22 conducting at a point along the rising stroke of the sawtooth which is dependent upon the amplitude of the audio input applied to that channel via the modulator 26. Thus the output of the channel pulse generator 22 is a pulse modulated in time corresponding to the amplitude of the speech on that channel about a mean position. It is mixed with the pulses from the other channels in the differentiator and clippers 32, 34. In this latter stage the synchronizing pulses are added and the resultant pulse train is amplified and converted into bursts of R.F. oscillations by the magnetron oscillator 40. Circuit details, Figs. 2 and 2a. A conventional blocking oscillator 12 is triggered by the crystal-controlled oscillator 10 to give a train of positive-going pulses which are applied to the step waveform generator comprising capacitors C1, C2 and diodes D1, D2 to generate a step wave across capacitor C2, which is applied via the buffer cathode follower stage 16 to the channel selectors 18. The step waveform is also applied to the valves E, A, B which are normally cut-off. When the charge on capacitor C2 reaches a given level valve A conducts and a sharp positive pulse is applied by transformer T2 to the grid of valve B which is rendered conducting to produce an output pulse which is applied to lead 15 which is of constant amplitude irrespective of the amplitude of the pulse applied to valve A. The positive pulse at the grid of valve B is also applied to render valve E conducting and capacitor C2 is discharged. Valve 60 is normally biassed to cutoff by the cathode resistor 53. On a predetermined step of the voltage across capacitor C2 any given selector valve is rendered conducting and remains conducting for the duration of the step wave. The grid resistor 49 limits the grid current flowing and thus prevents the valves from drawing an increasing anode current as the applied voltage rises above a certain level. The anode decoupling resistor 55 ensures that a constant amplitude output pulse is obtained from every selector irrespective of the value to which its cathode bias resistor has been set. The resistor 57 serves to bleed current from the anode supply and enables the mean value of the voltage at point 56 to be set to the correct operating point for the valve 60. The output pulse from valve 60 is differentiated by network 61, 62 to trigger the sawtooth generator 20. The sawtooth waveform across capacitor 63 is applied to the valve 22 which is normally non-conducting, the point along the rising slope of the sawtooth at which the valve conducts is dependent upon its bias which is dependent upon the amplitude of the speech and rectified ringing current applied to the valve 69. A variation in current of the cathode follower valve 69 in response to the modulating signal varies the voltage across its cathode resistor 70, and hence changes the average anode current of valve 26 and the cathode bias of valve 22. Thus the pulse produced in the output circuit, of valve 22 is time modulated about a mean position, its position depending upon the amplitude of the modulating signal. The resistor 71 which is of very large value prevents a pulse from being omitted if the modulator valve 26 is cut off by excessive modulation. The output pulse from the valve 22 is differentiated by the inductor 66, the damping resistors 67, 67<1> limiting the oscillations in the coil to a single half period for each complete change of current through it. The resulting pulse is clipped and limited in stage 24 and mixed with the pulses from other channels in the differentiators and limiters 32, 34 and amplified in the power amplifier stage 36. Synchronizing pulses are added from the synchronizing pulse generator valve 81 which is triggered by pulses from lead 15 after differentiating by network P, Pl. The negative going pulses at the output 88 of the power amplifier 36 are applied to the magnetron 40 and cause it to produce R.F. pulses having a duration somewhat less than those of the applied pulses. The amplitude of the pulses applied to the magnetron is kept constant by a regulator.