Patent GB-420019-A (GB)
Title: Process and apparatus for the electric transmission of signals by high frequency waves
IPC: H03C1/24
Filed: 1933-05-24
Abstract: 420,019. Valve modulating circuits. SOC. INDEPENDANTE DE TELEGRAPHIE SANS FIL, 75, Boulevard Haussmann, Paris. May 24, 1933, Nos. 15069, 15070, 15071, and 15072. Convention dates, May 24, 1932, Oct. 21, 1932, Nov. 2, 1932, and March 2, 1933. [Class 40 (v)] In a radio or like transmitting system, the modulating voltage is applied in the same phase to a number of carrier-wave amplifiers of increasing power-handling capacity connected in cascade. Modulation may be effected solely in the initial stage, the later stages acting merely as amplifiers with increased efficiency ; or the depth of modulation may be increased at each successive stage. To avoid distortion due to the flow of grid current at peak values of the carrier wave, the modulation voltage may be applied to diode or triode damping- valves which put a variable load on the inter-valve circuits in such manner as to compensate for the load introduced by grid current in the succeeding amplifier. Fig. 1 shows a system diagrammatically, in which only one stage 2 is used for modulation. The carrier is applied thereto at 1, 1<1> and the modulating voltage at 3, 3<1>, the modulated wave proceeding to the amplifier 4 and thence to the aerial 7. The modulating voltage is also applied through a transfer device 3a to the input of the amplifier 4, with the object of keeping the power consumption in this amplifier in proportion to the momentary amplitude of the carrier. Fig. 3 shows a valve circuit in which modulation is effected at each stage. Four stages in cascade are shown, operating in push-pull at each stage as regards the carrier, whilst the modulating-voltage is amplified at 14, 14<1> and applied through transformer secondaries 17 ... 20 to each successive stage. Each grid pair is separately biassed by sources 21 .... 24, and the modulating voltage for each stage is adjustable by potentiometers. In a modification (Fig. 4 not shown), the modulating voltage is applied in phase opposition to the grid pairs, and the chain of valves is arranged to pass only the side-bands, the carrier being amplified separately and combined with the side-bands in the aerial. Fig. 8 shows one form of the grid-current compensator. A pair of diode valves 46, 46a are connected back-to-back in a circuit tapped across the tuned circuit 44. The plate voltage and therewith the conductance of the valves is varied by the modulating-voltage applied to a transformer 50. When, at peak voltages of the carrier, grid current in the valves 4<11>, 4<11>a puts an added load on the tuned circuit 44, the load presented by the diodes 46, 46a is reduced. In a modification (Fig. 9 not shown) the diodes are in an inductively-coupled circuit, and their plate voltages are varied by a resistance traversed by the modulating-current. Triode valves may be used instead of diodes, the modulating voltage being applied to their grids. (Fig. 10 not shown). The compensating-device of Figs. 8-10 may be employed to distort the modulating-curve of a transmitter in such a way as to neutralize the curvature of the demodulation curve of a normal broadcast receiver.