Patent GB-707379-A (GB)
Title: Improvements in or relating to electrical pulse modulators
Inventor: FLOWERS THOMAS HAROLD
IPC: H03K7/02
Filed: 1949-11-01
Abstract: 707,379. Multiplex pulse signalling; pulse coincidence circuits. POSTMASTER GENERAL. Oct. 6, 1950 [Nov. 1, 1949], No. 27999/49. Classes 40 (5) and 40 (6) A circuit acting as a pulse modulator, or channel gating circuit in a pulse multiplex system, comprises two rectifiers in series, and of the same polarity in the series connection, and means supplying current to the junction from a plurality of sources, including a pulse source, the resultant current having one of the two polarities only in the presence of a pulse from the pulse source. In some applications such an output is obtained only when a predetermined combination of pulses from more than one source is present. In others it is obtained only when a predetermined combination of pulses and a predetermined signal from a signal source are present. The generalized circuit comprises a shunt arm comprising, connected in parallel, a rectifier W1, Fig. 1, in series with a bias source V, an impedance Zb in series with a bias source B, and an impedance Zp in series with a pulse source P and a series arm comprising a rectifier W2. One side of the shunt arm is connected to the common lead 1 and the other to the series arm, the rectifiers, in the case shown, being connected to offer low resistance in the direction from the shunt to the series arm. Shunt and series channels C1, 1 and M1, 1 are terminated by impedances Zx, Zy respectively, in series with sources of e.m.f. X, Y respectively. These impedances may be the image impedances of the channels if they are electrically long. The bias e.m.f. V and the e.m.f. of one of the sources X, Y may be zero and the bias and pulse sources B, P, may be in series with one another and with a common impedance. If the source X is a signal voltage varying slowly relative to the pulse period, the output pulses in the series channel are modulated in amplitude by the source X. Alternatively, amplitude modulated pulses may be produced in the shunt channel by connecting a signal source at Y. Constant-amplitude duration-modulated pulses may be produced by using triangulartopped input pulses and a limiter across the transmitting channel. The circuit may be used as part of a distributer to select modulated pulses from a pulse train, the input control pulses being synchronized with the pulses to be selected which may be those of one or several communication channels. Thus a plurality of similar circuits using a common channel M1, 1 may be connected to the common junction points E, F, to act as the channel modulators of a multiplex transmitter or the channel distributer gating circuits of a receiver. The conditions for linear modulation and absence of cross talk are defined. In order to reduce the shunt capacitance presented to a multiplex channel the circuits may be grouped, for example ten at a time, each group being connected to the multiplex channel through a rectifier, Fig. 3 (not shown). Wider channel pulses may be used in the modulators by connecting them to the multiplex channel through a common gate circuit supplied with pulses of shorter duration, Fig. 4 (not shown), and such circuits may be grouped as to the primary channel modulators and the common gate circuits, Fig. 5 (not shown). In a practical embodiment of transmitting modulator, for example for use in the system of Specification 694,092, [Group XXXIX], in which depending on a switching operation a multiplex channel pulse is modulated or suppressed, the modulating voltage X is applied through a transformer T, Fig. 7, and resistor Rx to the shunt channel C1. A pulse generator is connected at P to the junction of the shunt resistors Rp, Rp1 and a fixed common bias source B, through resistor Rb. The shunt rectifier W1 is biased by a resistor-capacitance combination Rv, Qv. The channel is switched on and off by controlling the grid voltage of a triode Cs so that when it is conducting the current through Rs biases the modulator to a cut-off condition, while when it is non- conducting the anode is clamped to earth potential by a rectifier Wcs. In an arrangement for demodulating an incoming amplitude-modulated pulse channel, and incorporating channel switching and a re-modulator, the incoming pulses on conductor C1, Fig. 10, are applied to a peak detector Dd, Rd, Qd, buffer valve VT1 and lowpass filter LPF to recover the modulating signal. The latter is fed over transformer T2, conductors N, N1, and transformer T to the shunt conductor C1<SP>1</SP> of the retransmitting modulator. The amplitude of the incoming pulses on conductor C1 drops to the value of the bias source V4 when the channel is "off", this condition causing rectifier W4 to cease conducting and the triode Cs to conduct. Its anode current, flowing in conductors N, N1, and resistor Rs<SP>1</SP>, biases the transmitting modulator to cut off the output pulses.