Patent GB-524588-A (GB)
Title: Improvements in and relating to thermionic valve apparatus
IPC: H03F1/06
Filed: 1939-02-01
Abstract: 524,588. Modulation systems. SANDEMAN, E. K., and KIRKE, H. L. Feb. 1, 1939, No. 3427. [Class 40 (v)] In apparatus of the kind in which two thermionic valves are coupled to a common load, one directly and the other through an impedance-inverting network, the two valves being operated under different conditions and cooperating to drive a current corresponding to a modulated carrier through a load, one valve is driven with a modulated carrier of such wave form that it supplies power to the load during the whole cycle of modulation. The anode voltage swing of this valve is limited in one direction to a value substantially equal to that necessary to supply an oscillation of amplitude equal to that of the unmodulated carrier to the load, while the other valve is so arranged and. driven that its power output adds to that of the first valve during the half of the cycle where the output of the first valve is limited, the second valve being so biassed that its drive is operative to determine the wave form of the current in the load during at least a part of the remainder of the modulation cycle. As shown in Fig. 1, the audio-frequency output from a transmitter AF is fed in parallel to modulating amplifiers AM, BM which feed modulated amplifiers AMA, BMA, each of which is supplied with carrier-frequency oscillations from a common source CF, if desired through separating amplifiers S1, S2. Modulated oscillations from stage AMA are fed to a carrier-frequency amplifier CS, one output of which is fed through a quarter-wave impedancetransforming network N to a load, which may be an aerial coupling circuit CC feeding power to an aerial A. The output of stage BMA is fed to an' amplifier PS coupled directly to the load. Intermediate stages S3, S4 are interposed as shown ; one of these includes a phase-shifting network which introduces a phase' shift of about 90 degrees. The carrier stage CS is driven with a normally modulated carrier, and supplies power to the load throughout the whole .cycle of modulation, but is driven into limitation during the part of the period when the stage PS is delivering power to the load. During a portion of the remaining part of the period, the stage PS absorbs power from the load. A series arrangement for the valves AM, AMA or BM, BMA is shown in Fig. 4, in which M is the modulating valve and MA is the modulated valve. The valve M is driven with audio frequency by means of the transformer T and is biassed by a voltage source S. The valve MA is driven with a radio frequency voltage from RF, a leaky condenser C1 and resistance R1 being arranged to provide bias from the grid current taken by MA. The filament of the valve MA is insulated from ground at audio frequency and is' effectively earthed at radio frequency by a circuit C2a, L2 tuned to the carrier frequency. A high-tension source on both valves is applied through a choke CH, and a conventional type of output circuit with neutralization is provided by inductance L1, and condensers C2-C5, the modulated output being taken from terminal O. A rejector circuit L3, C6 prevents radio frequency from reaching the valve M. In a modification, Fig. 5, the output of the modulating amplifier M2 is applied to the grid of the modulated amplifier MA2, through a condenser C7 and an inductance L4, to which the radio-frequency drive from a valve RF2 is coupled as shown. The valve M2 is fed with high-tension through a resistance R2, and the valve MA2 is fed through a choke CH2. Specification 471,423 is referred to.