Patent GB-600291-A (GB)
Title: Improvements in or relating to demodulators for the modulated electric pulses
IPC: H03K9/04
Filed: 1945-08-30
Abstract: 600,291. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. Aug. 30, 1945, No. 22406. Convention date, Sept. 28, 1942. [Class 40 (v)] A time-modulated pulse train is converted to a train of amplitude-modulated pulses by generating a wave having recurring inclined portions and combining the time-modulated pulse train therewith so that the pulses occur along inclined portions of the wave in accordance with their time displacement. In one embodiment the pulse source 10, Fig. 1, which may be a radio receiver, feeds synchronizing means 14 which controls a sinusoidal or sawtooth wave generator 12. The combined outputs of pulse and wave sources 10, 12 are applied to the grid of triode 16 biassed preferably so that the combined amplitudes of the two inputs give zero output for one extreme of pulse modulation displacement. When used to demodulate push-pull modulated pairs of pulses 21 ... 26, Fig. 2, the sinusoidal wave 30 may have a frequency equal to 1.5 times the mean pulse recurrence frequency and the inclined portion of the wave has the unmodulated superimposed pulse 21a occurring on the axis 29 of the wave. The plate-current grid-voltage characteristic 40 of triode 16 shows that the valve is just cut off when the combined wave and unmodulated pulse 21a are applied to its grid. Superposition of the remaining pulses on the wave produces output anode current pulses 22b ... 26b of amplitudes proportional to the time displacements of the input pulses. Where the wave generator 12 produces a saw-tooth wave such as 42, Fig. 5, the phase may be adjusted so that each pulse is superimposed on the inclined portion 43 of a saw-tooth. Alternate pulses such as 72, 73 are given high and low amplitudes 44, 45 respectively. Thus, alternate pulses exceed the cut-off level 46 of the mixer valve and provide the amplitude envelope required. In another embodiment, a pentode valve 100, Fig. 3, is biassed to cut-off by a resistancecapacitance circuit 102. Time-modulated pulses are applied to the control grid from terminal 106 and connected to the screen grid is a shockexcitable resonant circuit 102 tunable by condenser 104. A high resistance 112 is connected in the suppressor grid circuit. The time constant of the output circuit 115, 116 is of the same order as the pulse period. If the circuit 102 is tuned to a frequency of substantially three times the pulse recurrence frequency, curve c, Fig. 5, illustrates the production of the sinusoidal wave which is mixed with the pulses in pentode 100. Each pulse energizes the screen grid to excite circuit 102 to self oscillation. If the pulses are unmodulated, the wave 120, for example, excited by pulse 70 is crossing its zero axis when pulse 71 excites the next wave 124. Modulated pulses 72 ... 75, however, are received when the inclined portions of the previously excited waves are not passing through zero, so that the output pulses above the clipping level 130 are amplitude modulated. In a further embodiment, Fig. 4 (not shown), the resonant circuit is replaced by a saw-tooth generator connected to the screen grid of the pentode valve, and arranged to be controlled by the pulses so that they terminate the sawtooth being formed at their occurrence. In one example, the saw-tooth period is about twothirds of the pulse period so that the unmodulated pulse 184, Fig. 5, in conjunction with saw-tooth 183 just bring the biassed valve to cut-off level while alternate modulated pulses such as 190, 194 appear amplitude modulated in the anode circuit. The application of the invention to other forms of pulse time modulation is also described. Special pilot pulses may be transmitted, at predetermined intervals and of greater amplitude or duration than the signal pulses, to control the base wave generator at the demodulator. In order to increase the envelope energy of the output from the demodulator valve, the time constant circuit 115, 116, Fig. 3, is provided to transform the output pulses into substantially saw-tooth waves as described in Specification 596,078. Specifications 509,820, 511,222, 521,139 and 523,575 also are referred to.