GB600052A - Improvements in or relating to demodulators for time modulation electric pulse communication systems - Google Patents

Abstract

600,052. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. Oct. 16, 1945, No. 27089. Convention date, Nov. 13, 1944. [Class 40 (v)] [Also in Group XL (a)] Time modulated pulses are demodulated by causing an electron beam to sweep through a cyclic movement in synchronism with the unmodulated pulse timing, so that coincidence of the pulses and a given part of the beam movement varies in accordance with the pulse timing and causes a flow of energy proportional in amplitude to the degree of coincidence. In one embodiment, the detected pulses from radio receiver 1, 2, Fig. 1, are passed to synchronizing circuit 14 which controls a sinusoidal or saw-tooth generator 13, whose frequency is that of the pulse repetition, in the absence of modulation, or a frequency related thereto. The output of generator 13 is fed to the deflecting plates 7, 8 of a cathode-ray tube 3, through a phase adjuster 15. In the position of the switch 19 shown, the pulses are also passed to the biassed-back grid 5 of the tube to deblock the beam during each pulse, the deflection plates 9, 10 being earthed. The time during which the pulsed beam strikes the target electrode 12 is proportional to the modulated time displacement of the received pulses, so that the target current consists of amplitude modulated pulses, which are applied to a lowpass filter 17 and signal reproducer 18. An alternative method of operation is provided by the other position of the switch 19. In this position appropriate biasses are applied to the grid 5 and deflecting plates 9, 10 so that the beam continuously scans in the vicinity of the target 12. The pulses are applied to the deflecting plates 9, 10 and deflect the beam on to the target for a time proportional to their modulated time-displacement. In a second embodiment, the tube has no target electrode, but is provided with a fluorescent screen 53, Fig. 5, and a mask 54, in front of an aperture in which is a selenium cell 56 energized when the beam falls on the portion of the screen behind the aperture. Alternatively, an area of the fluorescent screen may be focused on a photo cell by a lens system, Fig. 6 (not shown). In a further modification, the target of the first embodiment is replaced by a collector of the secondary emission type comprising a dynode 64, Fig. 7, aligned with the aperture 66 in a barrier plate 65. With the switch connections shown, the signal pulses are applied to unblock the grid 5 and the electrode 64 is at a lower positive potential than the barrier plate 65. The emission of secondary electrons from electrode 64 provides a greater signal amplitude thereat, than in the case of a plain collector electrode. When switches 20, 23, 70, 71 are changed to their alternative positions, the system operates by beam deflection as previously described. If all the switches are placed on their left-hand contacts the sensitivity of the secondary-emission electrode is varied by the signal pulses applied. thereto, that electrode being biassed to cut-off in the absence of pulses.