Patent GB-269251-A (GB)
Title: Improvements in or appertaining to modulated carrier wave signalling systems
IPC: H03B5/10
Filed: 1926-01-14
Abstract: 269,251. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). Jan. 14, 1926. Thermionic oscillation generators ; modulating systems and detectors.-Relates to self-oscillatory push-pull modulators and de-modulators for carrier wave signalling systems, and comprises arrangements for stabilizing the frequency of the oscillations, and for giving a constant modulator gain, in spite of variations in plate battery voltage, filament current, and modulation input. With this object, high resistances are connected in the grid circuit, and the reaction coupling is increased so that the grids are normally made positive once in each cycle and the flow of grid current through the resistance varies the negative bias on the grids in proportion to the amplitude of the oscillations. When the valve filaments are connected in series, means are provided for compensating the resulting asymmetry in the action of the push-pull pair. Fig. 1 shows push-pull pairs BOM. BODM of oscillating and modulating valves similarly arranged to those disclosed in Specification 130,432 and interposed between an ordinary telephone line 4 and a carrier-wave line 2. Incoming telephone currents in the line 4 are transferred by a balanced transformer 8 to the grid circuits of the valves 9, 10 exciting the grids in phase opposition. The two halves of the grid circuits of the valves 9, 10 exciting the grids in phase opposition. The two halves of the secondary coil are connected by two series condensers 27, 28 between which the connection to the filaments is made. This connection includes a coil 22 which is coupled to a tuned circuit 15 connected in series with a resistance 16 in the common branch of the two plate circuits, this coupling causing the valves to oscillate at the carrier frequency. A resistance 29 shunts the feed-back coupling 15. Plate current is supplied to the valves 9, 10 from a condenser-shunted source 20 through a choke 21; and the modulated carrier wave, minus the pure carrier, is transferred through the plate coils 18, 19 and filters FTB, FHP, to the line 2. Each of the input transformer coils is shunted by a resistance 23, 24 and the inner ends of these coils are connected through very high resistances 25 26 (of the order of four megohms) to the positive legs of the valve filaments, which are seriallv connected and are heated by a battery 12. The resistances 25, 26 which may be shunted by condensers, give a negative bias to the grids owing to the flow of grid current when the grids are rendered periodically positive at the peaks of oscillatory voltage. The resistances 25, 26 are of different values to compensate for the difference in potential of the serially-connected filaments. The arrangement is stated to stabilize the oscillation frequency, in spite of variations in plate and filament supply voltages, and to ensure constancy of modulator gain. The oscillating de-modulator circuits BODM are similar to those already described, the modulated carrier being fed from the line 2 through filter FLP, to the line 4. The four valve filaments are all in series with the battery 12 and resistance 11. In a modification shown in Fig. 4, the circuits are simplified by the omission of the condensers 27, 28 and by the use of a single connection from the mid-point of the secondary coil 8 through the back-coupling coil 22 and a single condenser-shunted resistance 31 to the filaments. The arrangement results in dissymmetry of grid bias in the two circuits, which is compensated for by inserting a resistance in parallel with an inductance 32 in the plate circuit of one of the valves, between the coils 18, 19. This enables the pure carrier wave to be suppressed, in spite of the dissymmetry referred to. The secondary coil of the input transformer 8 is shunted bv a resistance 35 and by a condenser 34, the latter being provided to maintain the grids at substantially the same highfrequency potential, and to prevent grid-current modulation of the kind referred to in Specification 247,626. The modulation effected by the valves is due to the curvature of the plate-current grid-potential characteristic. The plate circuit resistances 16, 29 are omitted, and the oscillation circuit 15 is designed with low inductance and high capacity. The de-modulator circuits are modified by interposing series-tuned circuits 36, 37 between the plates of the valves and the feed-back circuit 15; and by placing a condenser 38 between the junction of these circuits and the mid-point of the output coil. The circuits 36, 37 provide a low-impedance path for the carrier wave, and for the side bands impressed on the grid circuits of the demodulating valves.