GB619968A - Improvements relating to high frequency transmitting apparatus - Google Patents

Abstract

619,968. Valve modulating circuits. BRITISH THOMSON-HOUSTON CO., Ltd. June 19, 1946, No. 18484. Convention date, June 19, 1945. [Class 40 (v)] 'In a modulating system in which modulation is effected at an intermediate high-frequency stage, and the modulated wave is fed to a power or output stage, the load on the intermediate stage is maintained steady even at peak modulation by a load compensating valve which is brought into action at the modulation peaks. As shown in Fig. 2, the intermediate stage comprises a pentode amplifier 3 following a series of low-power stages 1, and is modulated on its suppressor grid g3 by signals from a source 2. The valve 3 feeds an output stage 6, the grid of which is connected to a grid-current compensating valve 6a. The tuned grid circuit of the stage 6 is shunted by a load-compensating pentode valve 4 which has its control grid connected to the anode of a triode valve 5. This valve is back-biassed so as to pass current only on the positive peaks of the modulation voltage present on its grid. When the valve 5 passes current, the control grid voltage of the valve 4 falls and the load current taken by the compensating valve 4 is reduced, thus producing a linear modulated output from the valve 6. As the voltage on the control grid of the valve 4 falls, the screen voltage rises and results in a rise in the screen voltage of the modulator valve 3, and an increase in the R.F. output at the modulation peak. In a modification (Fig. 5, not shown), the compensating valve 4 is replaced by two triode valves, each having its grid connected to the anode of the valve 5, the anode of one triode being connected to the screen grid of the valve 3 and the other triode being connected across the tuned input circuit of the output stage 6. Overall negative feedback may be provided by connecting the detected output from the stage 6 to the signal input circuit 2.