GB592835A - Improvements in or relating to rectifying or demodulating circuits - Google Patents

Abstract

592,835. Valve circuits for radio reception. JOHNSON,E.A. June 2, 1945, No. 13929. [Class 40 (v)] [Also in Group XXXVIII] In a modulating circuit, a capacitor is changed to a potential proportional to the instantaneous amplitude of the voltage to be demodulated through an electronic switch periodically closed by a reference oscillation Fig. 1 shows an arrangement in which the reference and the modulated potentials (V and v respectively) appear across the secondaries of transformers 2 and 1. Leaky grid valve 4 and hence valve 3 conducts on positive peaks of V so that capacitor 7 is charged or discharged to a potential proportional to the instantaneous sum of V and v, once per reference cycle. In the arrangement of Fig. 3, v and V are applied to transformers 11 and 10. The effect of v is to alter the otherwise equal potentials to which capacitors 14 and 15 are charged on alternate peaks of V, so that a voltage appears at the junction 19 of resistors 17 and 18 with respect to earth, which is adjusted once per cycle of V to the instantaneous value and sign of v when V has its peak. The time constant C16 (R17+R18) is made long compared with the periodicity of V to maintain the output constant between switching periods. To prevent paralysis of the circuit if v exceeds V a relay may be connected across transformer 11 to place a discharge resistor across C 16 if v exceeds a predetermined valve, alternatively C16 may be omitted and the time constant 2C14.C15 (R17+R18)/C14+C15 made of the order of one period of V. A full wave version of Fig. 3 (Fig. 4, not shown) is also given.' The output voltage being adjusted every half cycle of V. A biassed output may be obtained in either case by making R17 and R18 unequal.