GB469472A - Improvements in alternating current modulation and demodulation systems - Google Patents

Abstract

469,472. Valve modulating circuits ; valve circuits for wireless reception. WILLIAMS, F. C. Oct. 24, 1935, No. 29425. [Class 40 (v)] A modulation system comprises a pair of rectifiers, with resistance-capacity load circuits in series, so arranged that they pass carrier current on the same half cycles ; an output circuit receives the difference between the two rectified currents, the difference being varied by differently biassing the two rectifiers by the modulating voltage. In the Figure, the carrier voltage is applied across the circuit A, the ends of which are connected through the load resistances r1 r2 to the diodes D1, D2. The output circuit O is connected to the junction of the diodes. This junction j is connected through the modulating transformer M to a potentiometer for obtaining bias as shown. With no modulation applied and the steady bias Eb of the point j zero, if the diodes and their loads are balanced the voltages across r1 and r2 are equal and cancel the applied voltage 2E and no current is supplied to the output circuit. If the bias Eb is not zero; current flows in the output circuit and current depending upon modulating voltages may be caused to flow in the output by applying modulation at M. By altering the time constants of the rectifier load circuits, the modulation-frequency characteristic may be varied and the system may be used to generate harmonics by appropriate tuning of the output circuit. Any required modulation depth or suppressed carrier effect can be obtained by suitable adjustment of the bias Eb relative to the modulation voltage. Fig. 2 (not shown), is an inversion of Fig. 1, the diodes passing current in the same direction, the output circuit being divided and reversely connected. In Fig. 3 (not shown), triodes or screen grid valves are used as rectifiers, with the carrier applied in parallel. and the modulation in push-pull to the grids. The system may be adapted to act as a demodulator of suppressed carrier transmissions by introducing the local carrier in place of the carrier, the signal into the output circuit and obtaining the output from the modulating transformer. The arrangement of Fig. 3 is not, however, directly reversible ; the modulation and carrier frequency circuits must be interchanged as in Fig. 6 (not shown).