Patent GB-635474-A (GB)
Title: Improvements in or relating to thermionic multivibrators for use in multi-channel electrical pulse communication systems
IPC: B29C48/92
Filed: 1944-10-03
Abstract: 635,474. Multivibrator oscillators; pulse modulation. STANDARD TELEPHONES & CABLES, Ltd., and LEVY, M. M. Oct. 3, 1944, No. 18898. [Classes 40 (v) and 40 (vi)] In a complex multivibrator a group of normally non-conducting valves are connected in parallel, means being provided for conditioning a selected valve to operate in conjunctionwith a further valve, separate from the group, to generate a pulse, the other valves of the group remaining in a non-conducting condition. The circuit is described as applied to a multichannel pulse communication system in which time-phase modulated pulses are employed to convey the intelligence. The group of channel valves 9A, 9D, 9G, Figs. 1 and 2, connected in parallel at the cathodes and anodes, are held in a normally non-conducting state due to the potential drop across the cathode resistance 11 developed by the anode current of the output valve 8, resistances 14, 15 maintaining the grid sufficiently positive to draw grid current. The grids of the channel valves are normally held at earth potential through resistances and the modulating source (not shown). Upon one of the channel valves 9A being rendered conducting, it operates in conjunction with the output valve 8 as a cathode-coupled multivibrator pair cutting off the output valve 8 which generates a positive pulse in the anode circuit. The circuit restores to the normal condition, with valve 8 conducting, after a time determined by condenser 17 discharging over resistances 14, 15, or by valve 9A being cut off by an external signal. The pulse transmitter, Figs. 1 and 2, comprises a pulse generator 1 such as a multivibrator feeding a delay line distributer 2 from which pulses are tapped off at intervals A ... T, the voltage at the end of the delay line being fed back to synchronize the generator 1. The pulses are arranged to be slightly narrower than the .time allowed for each channel during transmission. The positive pulse from A, applied to the grid of valve 23A, produces at the cathode a trapezoidal gating pulse, due to the condenser 25A and a resistance 26A, which is applied over a coupling condenser to the grid of channel valve 9A. The amplitude of the trapezoidal pulse is such that at a point half-way along the pulse valve 9A conducts, causing output valve 8 to cut-off due to the multivibrator action, thereby producing a positive pulse in the anode circuit. This pulse is differentiated to produce a short positive pulse which is fed to a mixing circuit 28. In a similar manner, pulses from the points B, C on the distributer 2 feed through valves 23B, 23C complex multivibrators 6, 7, the outputs of which are also applied to the mixing circuit 28 together with a synchronizing pulse from the distributor 2. By applying a modulating signal at the terminal 19A the trapezoidal pulse is raised or lowered in potential, thereby advancing or retarding the short pulse fed to the mixing circuit 28. The modulating signal may be limited to prevent operation of the channel valve 9A in the absence of the trapezoidal gating pulse and to avoid a large negative modulating signal from preventing operation of valve 9A, a short positive pulse may be added to the trailing edge of the trapezoidal pulse. By employing three complex multivibrators 5, 6, 7, each output valve 8 is allowed the time of two channels to restore to its normal condition, thereby minimizing cross-talk. The pulse receiver, Fig. 5, operates in a similar manner to the transmitter, input valve 8 now receiving the negative signal pulses from terminal 43 after passage through the limiter 45. The pulse generator 1 is synchronized from the selector circuit 44, a pulse from point A on the distributer being applied to an amplifying valve 23A to produce a gating pulse on the cathode of channel valve 9A, thereby reducing the bias. Upon a negative pulse being applied to the grid, a positive pulse from the anode of valve 8 is fed to the cathode follower valve 31 which in turn raises the potential of the control grid of channel valve 9A and the other channel valves (not shown). Since the cathode bias of channel valve 9A has alone been reduced, the signal pulse causes valve 9A to conduct but is ineffective in operating the other channel valves. The fall in anode potential of valve 9A is fed back over the coupling condenser 17 to drive the grid of the input valve 8 negative, thereby cutting it off. The circuit remains in this condition until the pulse from point A ceases, thereby cutting off channel valve 9A, which causes valve 8 to conduct. The time-phase modulated signals cause the leading edge of the negative pulse on the screen grid and anode of channel valve 9A to vary, the trailing edge being fixed by the cessation of the pulse from the distributer. In this manner the signals are converted to time-duration modulated pulses, the pulses from the screen grid being fed to the low-pass filter 37A to give the modulating signals at the output terminals. Specifications 587,939, 587,940, 635,472, 635,473, 635,475, 635,477 and 635,509 are referred to.