Patent GB-479302-A (GB)
Title: Improvements in or relating to modulated carrier wave radio and like transmitters
IPC: H03C1/22
Filed: 1936-08-12
Abstract: 479,302. Modulating systems. TELEFUNKEN GES. FUR DRAHTLOSE TELEGRAPRIE. Aug. 12, 1936, Nos. 22248, 22249, 22250, 22251, and 22252. Convention dates, Aug. 12, 1935, Nov. 21, 1935, Nov. 29, 1935, Nov. 30, 1935, and Dec. 3, 1935. [Class 40 (v)] In an amplitude modulating arrangement in which the modulated resultant is obtained by combining two alternating currents of the same carrier frequency at least one of them being varied by means of a valve, compensation for cathode emission or other variations is effected by rectifying one or more voltages dependent on the component voltages, freeing the rectified resultant from modulating frequencies and applying it to control the gain of at least one of the modulating valves. The carrier frequency is fed, through a transformer 12, in phase opposition to the grids of two screen grid valves 10, 11 having a common tuned anode circuit. The control grid of one of the valves 10 is supplied with the modulating current and that of the other valve 11 is supplied through a bias battery 24 from the output of a rectifier 19 which rectifies and filters the output from a H.F. amplifier 17 connected to the tuned anode output of the two valves 10, 11. The arrangement compensates for variations in the cathode emission of the two valves. In a modification, the stability of the modulator is increased by arranging that the rectifier 19 only rectifies when the output current is in one of the two possible phases. This is arranged by using a grid controlled rectifier the grid of which is supplied with the carrier frequency, if necessary through a phase shifter, the anode being supplied from the H.F. output of the modulator. Alternatively, a hexode working on the lower bend of its characteristic may be used, the inner grid being supplied with carrier frequency and the other signal grid with the modulated wave. In a further modification two hexode valves replace the valves 10, 11, the signal grids being supplied in phase opposition from the carrier source and the inner grids, one through a bias battery and filter and rectifier from a transformer in its own anode lead, and the other through a modulating transformer, bias battery and filter and rectifier from a transformer in its anode lead. By this means variations in each of the valves are compensated separately. Another modification, Fig. 7, comprises the supply of carrier energy through a transformer 58 having two secondaries 61, 62, one of which, 61, supplies one primary 63 of a transformer 59 directly and the other 62 supplies another secondary 64 of the transformer 59 through a valve 57. The input to the valve 57 also includes modulating potentials at 66 and the primaries 63, 64 are opposed so that a differential modulated wave appears in the secondary 65. Valve 57 is compensated for changes in emissivity by potential on an inner grid obtained through a transformer 44 in its anode circuit and a rectifier and filter. In television modulation arrangements according to Fig. 1, the average value of the carrier may be controlled in accordance with " mean brightness by replacing the battery 24 by a photo-cell and shunt resistance and battery, the cell being subjected to the total illumination of the subject. Alternatively, Fig. 9, the rectifier 80 of the modulated energy from the amplifier 78 has, in its grid circuit, adjustable carrier frequency for selection of the phase in which control is effective and a resistance 82 supplied from a photo-cell 83 subjected to the total illumination. The rectified output taken across condensers and resistances 84-87 is applied to the grid of one valve 41 of two opposed parallel connected hexodes 40, 41, carrier frequency being applied to the grids 3 of both valves, modulation signals to the valve 40 and the outputs being opposed in the transformer 74. In a further arrangement, Fig. 11, for modulation by television signals in which the modulating potentials are periodically interrupted, e.g. in the intervals between line or frame scanning periods, two hexode valves 90, 91 are provided. Valve 90 is controlled by the modulating potentials in series with a bias voltage e1 and valve 91 has a fixed bias 94. The output is rectified by a phase discriminating valve circuit 98, 99 the control grid 3 of the valve 98 being supplied with current which blocks the valve during line scanning periods and opens it only during the line or frame intervals. The valve 98 can thus only rectify any output from the modulator which occurs during the interruptions in the line or frame scanning intervals-and is thus due to unbalance between the valves 90, 91. A condenser 102 is charged by the rectified output and controls a valve 103 supplying current to a condenser 107 from which it is applied as bias in the grid circuit of the valve 90. The arrangement is such that inequalities in the valves 90, 91 cause the charging of the condenser 107 in the scanning intervals or interruptions only or permit its discharge through a resistance 106 to compensate for such inequalities. The modulated output voltage e2 is stated to be modulated corresponding to the brightness distribution of the picture. In modifications of the arrangement in Fig. 11, the condenser 107 and resistance 106 may be in the grid circuit of the valve 91 or in the circuits of both valves. Modulation arrangements according to the invention may include superposition of synchronizing signals by making the modulated output H.F. greater than zero for " black " picture points and interrupting the H.F. supply to the bridge at the appropriate times for transmitting the synchronizing impulses. The arrangements may also be used with floating carrier by rectifying the modulating potential and applying it in the compensating circuit.