GB498895A - Improvements in alternating current modulation and demodulation systems - Google Patents

Abstract

498,895. Valve modulating circuits; valve circuits for wireless reception. WILLIAMS, F. C. July 15, 1937, No. 19648. Addition to 469,472. [Class 40 (v)] The parent invention, which is concerned with a modulation system comprising a pair of peak rectifiers passing carrier current on the same half-cycles, is modified by arranging that the rectifiers pass current alternately. In the Figure the carrier voltage is applied across circuit A, the ends of which are connected through diodes D1, D2, and their associated resistance-capacity load circuits to the point j. The output circuit O is connected from j to the centre point of circuit A. Modulation from transformer M is applied in series with a steady bias Eb in the same direction to both diodes. The diode load resistances r1, r2 are unequal so that the current pulses which flow on alternate half cycles, in the same direction, to the output have different values and a resultant modulated output appears in circuit O. In Fig. 2 (not shown), the output circuit is in two portions one of which is reversely connected and the modulation is applied in opposition to the diodes. In this case the diode loads may be equal. In Fig. 3 (not shown), the output circuit is connected in series with the rectifier circuits and receives the difference of their outputs ; again resistances r1 and r2 may be equal. These circuits are reversible to effect demodulation by feeding the signal to the circuit O, a locally generated carrier to circuit A and taking off the modulation from transformer M. Fig. 4 (not shown) is a modulation arrangement using triodes in place of diodes. Carrier and modulation are applied in opposite phase to the grids and the output is taken from a common circuit in the anodes in series with both branches. A steady biassing voltage is applied in opposition to the grids to ensure that the current pulses are unbalanced, or the secondary of the modulating transformer may be unbalanced for the same purpose. The load resistances may be equal or unequal. Fig. 5 (not shown) is an arrangement corresponding to that of Fig. 4 adapted for demodulation in which the circuits O and M are interchanged.