Patent GB-592585-A (GB)
Title: Improved systems for generating time modulated pulses
IPC: H03K7/08
Filed: 1944-06-08
Abstract: 592,585. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. June 8, 1944, No. 11032. Convention date, Aug. 24, 1942. [Class 40 (v)] A time modulation system comprises a source of foundation waves of fixed frequency, non-linear transmitting means having a discontinuity threshold and connected to transmit all portions of said waves which lie on a given side of said threshold, whereby the transmitted waves have corners corresponding to the instants of intersection of said foundation waves with said threshold, a source of signal waves, means for varying the position of said threshold with respect to said -foundation waves under control of said signal waves, whereby said instants of intersection are displaced in time, and means for doubly differentiating said transmitted waves to derive pulses corresponding to said corners. Figs. 1 and 2 show an arrangement in which a stabilized sine wave oscillator 11 is used to synchronise a saw-tooth oscillator 10 the output from which is passed through a clipper circuit 12 passing only the upper half of the wave, as W, Fig. 2, to the grid of a limiter 14 through a network 19, 15, 20. The grid of 14 is biassed positively by source 21 so that the most negative part of wave W extends only slightly below cut-off Eco, Fig. 2. The anode and screen potentials are adjusted to give a small gap between cut-off and the grid potential Egc, Fig. 2, at which grid current flows. Thus limiter 14 passes only a small section of wave W between these limits as shown by X, Fig. 2. A modulating signal is fed to the grid of 14 through jack 16 from microphone 30 and transformer 32, a battery 34 ensuring that the modulating signals are always negative. The effect of the modulating signals is to move curve W relative to the two threshold levels as W<1> and thus select sections of the saw tooth of different widths, to give width modulated pulses X or X<1>. The pulses X are differentiated at 35 to give pulses Y, and differentiated again at 36 to give pulses Z, which are used to modulate a carrier wave transmitter 37 ... 39. The modulation signal may be applied through jacks 17, 18 to the screen or suppressor grids. This would result in shifting only the grid current threshold Egc relative to wave W, so that only the downward pulses Z would be time modulated. These must be selected for modulating the transmitter. If a symmetrical triangular wave is used as a foundation, both sides of the pulses are shifted by the modulating signal. The clipping function of valve 14 may be obtained by passing the wave through two series connected valves in succession and using only the cut-off threshold of each valve Fig. 5 (not shown). Fig. 6 shows an arrangement in which the foundation wave is a sine wave generated at 111, amplified at 110, and after preliminary clipping at 112 is applied to a main clipping unit comprising opposed rectifiers 114, 115 with their intermediate point M positively biassed by source 119. Modulating signals from 130 are applied through jack 116 and the output comprises width-modulated pulses which are differentiated twice in units 135, 136 and used to modulate transmitter 137 ... 139. If the modulation is applied at jack 117 the upper and lower limits are displaced in opposite directions, and if applied to jack 118, only one limit is displaced. Alternatively the input sine wave may be modulated at 109 before clipping into pulses. A modifica-. tion of this clipping unit giving push-pull operation is also described (Fig. 8, not shown). All the above arrangements may be used in a system where the foundation frequency is made M/2 times as great as normal and only every Mth pair of up-and-down pulses is passed for transmission (Fig. 9, not shown). Specification 592,584 is referred to.