Patent GB-497071-A (GB)
Title: Synchronisation generator for television systems
IPC: H04N5/06
Filed: 1937-06-11
Abstract: 497,071. Automatic control systems; television ; converting. RADIOAKT.- GES. D. S. LOEWE. June 11, 1937, No. 16242. Convention date, June 13, 1936. [Classes 38 (ii) and 38 (iv)] [Also in Group XL] Oscillation frequency.-Line and frame impulses in a television transmitter are generated by a cascade of sinusoidal frequency-reducing valve circuits in which the slowest oscillation is equal to that of the supply mains in frequency, the valves having each two grids, of which one is back coupled to the anode and the other excited from the anode of the preceding stage. The phase of the last stage is compared with that of the mains and the frequency of the first stage is automatically adjusted accordingly. The primary oscillator is a back-coupled threepole tube 1 tuned to twice the line frequency by elements 2, 3, resistances 4, 5 limiting the currents and eliminating largely the influence of tube properties ; automatic grid bias is furnished by resistance 6. The valve 7 provides the line frequency and is preferably a pentode or mixing hexode. Its oscillatory circuit 8, 9 is used for back coupling anode and first grid, an intercepting grid 13 being coupled to the grid of generator 1 to effect a 2 : 1 reduction of frequency. Resistances 10, 11 are included in grid and anode circuits. Successive stages, each giving a maximum of 9 : 1 reduction, are provided by pentodes 15, with automatic grid bias, in which the first grid is reactively coupled with the anode by an oscillating circuit 62, 20, and the intercepting grid 15 is connected to the cathode by resistance 19, and to the anode of the preceding stage by a large condenser 18 and a resistance 17, which should exceed the impedance of the preceding generator to prevent interference. Fine tuning in each stage is obtained by adjustable resistances 14 of 200-500 ohms in series with iron cored coils. Two impulse tubes 46, 47 provide respectively image and line frequencies. The anode of the last frequency reducer is coupled through a filter system, comprising a stepdown transformer 58, producing pure sine oscillations at the condenser 50 which are applied to the inner grid of the pentode 46, of which the outer grid is excited in opposite phase from the last tube 63, through a phase shifter 51, 52 by which the impulse duration may be adjusted ; anode current is limited by a low potential anode battery 54. The line impulse tube is connected to a common anode resistance 53, and its two grids are excited from the anode and grid circuits respectively of the line frequency stage 7, phase shifters being included in each circuit. For frequency control, Fig. 2 shows one means of phase comparison by a tube 21 with two cascade grids 22, 23 normally so biassed as, jointly, just to interrupt the electron stream. To these grids are respectively supplied impulses in opposite phase from the compared generators 30, 31 so that the anode current completely ceases when their frequencies are identical. A relative change in phase produces anode current impulses, the amplitude of which increases with the error and charges a shunted condenser 33. The potential of this is smoothed by a filter 35a, 35b of suitable time constant and controls a resistance amplifier 34 with anode resistance 37 greater than the oscillation resistance of the circuit 2, 3 of the primary oscillator, Fig. 1, to which it is connected by a condenser 38 so as to act as a variable shunt. Frequency correction in both senses can be obtained by a push-pull arrangement of two such devices, for example Fig. 5, wherein the grids of two amplifers 34, 34<1> are supplied with potential 90‹ phase advanced and retarded respectively in regard to the potential of the tuned circuit 2, 3 so that one acts as a capacity and the other an inductance when its grid bias is raised by the output of the frequency sensitive tube 21 or 21<1>. In a further arrangement for frequency control, the output of two frequency meters, each of which indicates by a frequencyproportional direct current, are opposed in the grid circuit of an amplifier 68, Fig. 4, wherein a resistance 45 in the grid circuit is connected between pairs of detectors respectively receiving the output of tuned circuits fed from the compared sources 30, 31.