Patent GB-697692-A (GB)
Title: Improvements in phase-sensitive rectifiers
Inventor: CROFT ROBERT ALAN
IPC: H03D1/10
Filed: 1951-06-13
Abstract: 697,692. Rectifying; automatic control systems. PLESSEY CO., Ltd. June 3, 1952 [June 13, 1951], No. 14036/51. Classes 38 (ii) and 38 (iv). Relates to phase - sensitive rectifiers for use particularly in control and servo systems in which a varying quantity such as a distance, angle, temperature, &c., is represented by a periodic voltage whose amplitude, wave shape, or phase, or a combination of these parameters, is related to the varying quantity. The rectifier enables a fluctuating or quasi-steady voltage, whose magnitude and sign is related to the varying quantity, to be ob-. tained from the periodic wave, by measuring the difference between the instantaneous magnitudes of the wave-form at two selected points during each cycle. Fig. 1 shows a suitable circuit, and Figs. 2 to 5 show waveforms at different points thereof. The input wave, shown in Fig. 2, is applied at m through a capacity C5 to a centre tap d on the secondary coil of a transformer T to whose primary is applied a reference wave, Fig. 3, in the form of square pulses of the same periodicity as the input wave. The ends c, e of the secondary coil are connected to unlike electrodes of two diodes D1, D2 or like unidirectional conductors, the other electrodes of which are connected through two resistance-shunted condensers C1, C2. The leading edge of a positive pulse applied to the transformer gives a positive pip across the secondary and by making the end c negative relative to e passes current through the diodes D1, D2 and charges the condensers C1, C2 so that at the end of the pip the diodes are again blocked. During the brief unblocking of the diodes a conductive path is open between the points g, d which allows the point d to assume a bias voltage provided by a battery B. On this is superposed the input wave of Fig. 2 which therefore assumes the position shown in Fig. 4. When the trailing edge of the pulse, Fig. 3, is applied to the primary of the transformer T, the end c of the secondary is driven positive relatively to the end e and the diodes D3, D4 are rendered momentarily conductive so that the momentary voltage at d is transferred to the point k. As indicated in Fig. 4, this voltage is made up of the bias voltage from the battery B and the difference between the amplitudes of the periodic wave at the two sampling points. There is thus provided at the point k across the condenser C6 a quasi-steady output voltage as indicated in Fig. 5. If the periodic wave is composed of a fundamental and one or more even harmonics, as shown in Fig. 1, the output is independent of these harmonics. The differentiating transformer T may be replaced by one or more pulse sources, and the thermionic diodes D1 ... D4 by crystal diodes. A fraction only of the sample taken at the first selected point may be subtracted from the sample at the second point by applying the input wave as a whole between the terminal m and the bias battery, and a fraction between the terminal g and the bias battery. Either sample may be multiplied by a factor by amplifying or alternating the input waveform.