Patent GB-718269-A (GB)
Title: Improvements in or relating to electrical pulse modulators
Inventor: FLOOD JOHN EDWARD
IPC: H03K7/00
Filed: 1951-01-19
Abstract: 718,269. Multiplex pulse signalling; gating circuits. POSTMASTER GENERAL. Feb. 29, 1952 [Jan. 19, 1951], No. 1546/51. Classes 40 (5) and 40 (6)  A pulse amplitude modulator or pulse train selector, comprises a capacitance-shunted signal source in series with a rectifier, a second rectifier connected to the other end of the first rectifier, bias means to render the rectifiers norma'ly non- conducting and conducting respectively, and one or more sources of pulse trains the presence of a pulse or coincident pulses from which reverse the conductivity of the rectifiers. A p'urality of modulators may be connected to the same output channel to provide interlaced trains of modulated pulses for a multiplex system. In one embodiment, pulses whose amplitude is determined by the instantaneous amplitude of a signal from transformer T1 are produced when coincident pulses are applied to termina's C, D, and an A.C. hold signal of the type described in Specification 718,262, [Group XXXIX], is present at transformer T2. In the quiescent condition, currents of 7mA. flow through each of the rectifiers W15, W16. from source -50V, +50V, the current flow through rectifier W12 being 9mA. and through resistor R16 being 5mA. The potential at the junction of rectifiers W12, W13, W15, W16 is then slightly below earth potential and rectifier W 13 is non-conducting due to the bias source +5V. When a positive-going 10V. pulse is applied to one of terminals C or D, the current through its associated rectifier W15 or W 16 is cut off and the current through rectifier W12 drops to 2mA. so that the potential of the junction point drops by about 0.5V. When pu'ses are applied simultaneously to points C and D the currents through rectifiers W15, W16 are cut off and no current flows through rectifier W12. The current through resistor R16 then flows through rectifier W31 in the absence of a hold signal in transformer T2. When, however, there is an A.C. current in transformer T2, which is rectified by rectifier W14 to charge condenser C12 positively, W31 is non-conducting and the 5mA. current from R16 charges the stray capacity CL of the load impedance. The potential of point X and the rectifier junction rises until rectifier W13 begins to conduct and for the remainder of the duration of the coincident pulses, current flows through W13 to charge condenser C11 across the signal secondary of transformer T1. The potential of the upper plate of condenser C11 differs from +5V. by the instantaneous signal voltage across the signal secondary and rectifier W13 begins to conduct when the potential at the rectifier junction equals this potential. The output pulses at point X are there'ore amplitude modulated. Rectifier W17 is provided so that a plurality of channel modulators may be connected to point X Rectifier W11, connected between earth and a winding of transformer T1 connected to the signal secondary, acts as a voltage limiter so that the modulated pulses at point X never have an amplitude of less than 2.5 volts. The output pulses are fed to terminal F through a cathode-follower stage V1. At the end of the coincident pulses at terminals C, D, a negative-going 10V pulse is applied to terminal E to render the self-biasing rectifier W18 conducting, to discharge the stray capacity CL of the load impedance. Rectifier W19 prevents the point X from dropping below earth potential. Specification 707,379 also is referred to.