GB191390A - Improvements in and relating to high frequency signalling systems - Google Patents

Abstract

191,390. British Thomson - Houston Co., Ltd., (Assignees of Baker, W. R. G.). Jan. 5, 1922, [Convention date]. Thermionic oscillation generators; keying and modulating apparatus.-The grid circuit of a thermionic oscillation generator comprises a leak resistance shunted by a condenser, and a tuned oscillatory circuit in which oscillations are generated independently of any retro-active coupling. Oscillation is normally prevented by a series condenser which interrupts the leak path and allows the grid to attain a considerable negative potential. Signals are sent by shortcircuiting this condenser, thus, allowing the valve to oscillate. The operation of signalling also closes the plate supply circuit. Alternating current is used for heating the filaments and for supplying rectifiers which energize the plate circuit. A pair of valves and a pair of rectifiers may be used, and the failure of the filaments of either is indicated by a lamp or an audible signal. The Figure shows a pair of oscillating valves 1, 2 with their filaments 3 supplied in parallel from a transformer secondary 6, earthed at its mid-point. The plates 4 are connected in parallel through a high-frequency choke coil 32 to two rectifying-valves 19, 20 connected as a full-wave rectifier to the transformer secondary 21. The grid circuits include separate coils 17, 18, a common grid leak 16 shunted by a condenser 15, a tuned oscillatory circuit 13, 14, and a series condenser 12. The operation of the signal key 37 closes a switch 38 in the primary circuit of the plate supply transformer 21, and also closes a switch 35 in a low-resistance circuit across the condenser 12. The negative charge on the grids is thus allowed to leak away until its negative potential is sufficiently reduced for the valve to oscillate. The oscillations are produced in the tuned grid circuit 13, 14 by virtue of the negative resistance accompanying the secondary emission from the grids. Amplified oscillations are transmitted to the aerial 30, an adjustable part of the aerial tuning inductance 29 being connected between the plates 4 and earthed filaments 3. A condenser 31 prevents short-circuiting of the plate supply current. The continuous waves produced may be interrupted by a rotary switch 34 in the leak path. Failure of the filaments of the oscillating valves 1, 2 or the rectifying valves 19, 20 is indicated by a lamp 41 or audible signal device 44 connected across points in the filament leads which are at equal potentials so long as both filaments are intact, but are unbalanced when either filament is broken. The tuning switches 47, 48, 49 are preferably operated jointly.