GB722108A - Improvements in or relating to synchronous electronic circuits - Google Patents

Abstract

722,108. Phase-sensitive circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Dec. 11, 1952 [Dec. 12, 1951], No. 29142/51. Class 38 (4). [Also in Group XL (c)] A phase-sensitive rectifier or discriminator is fed by a constant-current source so that it presents a very high impedance to a D.C. amplifier of virtual earth type, e.g. due to a high-resistance negative feedback shunt between its output and input terminals. In the half-wave rectifier-discriminator of Fig. 4, diodes 25, 26 both conduct during alternate half cycles of a reference wave applied at 31 so that a signal input to be demodulated applied at the junction 32 of resistances 16, 17 connected across the diodes is blocked from passing through the consequently biased diodes 25, 26 to the output 11, 12. During the other reference half cycles the diodes 25, 26, however, are blocked and the input signal at 32 passes to the output 11, 12, through the diodes 13 or 14 in turn, for portions of the signal input half-cycle which are negative and positive respectively during this time. The difference in D.C. output of the diodes 13, 14 is proportional to the amplitude of the input signal, independent of the amplitude of the reference wave, and varies with the cosine of the difference in phase angle between the reference and signal waves. As shown, the input is fed from a pentode valve 33, i.e. of high impedance and constant-current characteristic, and its impedance is still further increased by applying H.T. to its anode through a series valve 39 and inductor 36 with a retroactive feedback to its grid through a capacitor 40 from the anode of the pentode valve 33, which counteracts changes of anode current through the pentode 33, i.e. further increases its impedance. In a full-wave modification the reference wave is applied also to a further circuit of diodes 13, 14, 25, 26 but of reversed polarity, and both diode circuit outputs applied to opposite ends of an output transformer secondary with the output circuit connected to a centre tap of the secondary. The input signal to be demodulated is then applied (from a pentode valve) to the primary of the output transformer. The rectifier-discriminator output may be amplified by a virtual earth, i.e. low input impedance D.C. amplifier (Fig. 5) having a negative feedback shunt of high resistance R between its output and input terminals and the value of R is made large so that the amplifier is of high sensitivity but yet has negligible voltage drift at its output, due to the drift being proportional to R but inversely proportional to the impedance of the phase-sensitive rectifier. Jour. Inst. Elec. Engrs., Vol. 93, Part IIIA, page 1279 et seq., is referred to. Specification 717,122 is referred to.