Patent GB-679368-A (GB)
Title: Improvements in and relating to electric oscillation generators
IPC: H03K4/43
Filed: 1949-03-01
Abstract: 679,368. Valve modulating and saw-tooth generating circuits. WARNER, A. R., and WARD, D. A. March 1, 1950 [March 1, 1949], No. 5583/49. Class 40 (v) and 40 (vi). [Also in Group XXXV] An electric oscillation generator comprises means for producing high frequency oscillations, means for developing a saw-tooth waveform voltage, and means for controlling the power output of high-frequency oscillations from the generator in accordance with the saw-tooth waveform. The generator may be used to operate a gas discharge tube for advertising purposes, in which the discharge in the tube is caused to increase gradually along the tube, then to be extinguished and to repeat the operation cyclically. As shown, the stage 1 comprising valves V3 and V4 arranged as a Class C push-pull oscillator, supplies the gasdischarge tube 8 via transformer T2, the screen grids of valves V3 and V4 being connected together and their potential controlled by a saw-tooth generator stage 2, to give a steadily rising oscillator output which is cut off sharply at its maximum. The valve V2 is arranged as a blocking oscillator having a transformer T1 tightly coupled between its grid and cathode circuits, its anode being connected to the grid of valve V1 and capacitor C1, and through the resistor R3 to the cathode of V1. The grid resistor R3 has a value substantially higher than the resistors R1 and R2 and the ratio R1/R2 is less than the ratio C1/C2. When a pulse of anode current is produced in the valve V2 the grid of the valve V1, is made highly negative and the anode current of V1 suppressed for a period depending on the circuit constants, the cycle being repeated when the valve V2 again passes a pulse of anode current. In this way a saw-tooth voltage is developed across resistors R1 and R2 in the cathode circuit of V1, which is applied through a non- linear resistor M2 to the screen grids of valves V3, V4, the resistor M2 shunted by a capacitor C5 providing a negative bias on the screen grids to reduce the oscillator output to zero. With some valves M2 and C5 may be omitted. When the valve V1 is passing anode current its cathode is at a high potential, and with the component values given the potential across capacitor C1 is about one-third that across C2. When the valve V2 passes anode current it presents a low impedance across capacitors C1 and C2, and the upper plate of C1 and the grid of valve V are reduced rapidly to zero potential V1 being biased to beyond cut-off since its cathode potential is positive. The junction point of C1 and C2 at this time is at a moderately high positive potential and capacitor C1 receives a charge from C2 in the reverse direction to that previously applied, thus ensuring that the grid of valve V1 is biased well.beyond cut-off. Capacitor C1 then begins to discharge through resistors R1 and R3, and capacitor C2 through resistor R2. The time constant of the circuit C2, R2, is relatively small and the discharge of C2 has no effect on the anode current of VI, but the time constant of the circuit C1, R1, R3, is relatively high and the negative bias developed across R3 gradually decreases until anode current again begins to flow in valve V and the current through R1 reverses, C1 discharging through valve V1 and commencing to recharge with opposite polarity, together with C2, until the potentials across C1 and C2 reach the limiting values determined by resistors R1 and R2 and the cycle of operations is repeated. The ratio R1/R2 controls the " off " period of the gas-discharge tube 8, and the resistor R3 controls the speed of the extension of the discharge in the tube, the total cycle time being controllable by the time constant of the grid bias circuit of valve V2. The resistor R1 may be shunted by a non- linear resistor M1 in series with a resistor R4 to linearize the charging voltage of the capacitor C1. A multivibrator or thyratron may be used instead of a blocking oscillator for cyclically discharging capacitors Cl and C2, or alternatively a pair of contacts operated by an electromagnetic relay in dependence, upon the power output from the oscillator may be used. To operate several discharge tubes each with its own oscillator, one blocking oscillator may control all the saw-tooth generators. Alternatively each discharge tube may have its own oscillator, saw-tooth generator, and blocking oscillator, and one blocking oscillator may be used to trigger the remainder so that the operation of all the tubes is synchronized. Specifications 399,566 and 423,362 are referred to.