Patent GB-958794-A (GB)
Title: Phase-and frequency-comparison circuit comprising two rectifying sections
IPC: H04N5/12
Filed: 1960-06-03
Abstract: 958,794. Automatic frequency and phase control systems. STANDARD TELEPHONES & CABLES Ltd. (Standard Elektrik Lorenz A.G.). June 3, 1960, No. 19712/60. Heading H3A. In an automatic frequency and phase-control system the phase detector comprises two back biased or self-biased rectifiers serially connected with like polarity and across which a sync. pulse input is applied in push-pull and a reference input pulse or sawtooth wave of equal amplitude then added to render one or other of the rectifiers conductive when the sum voltage exceeds a value greater than that of the sync. or reference pulse (or sawtooth). Moreover, the discriminator output circuit is of time constant such that a D.C. output is produced of polarity indicating the sense of any frequency difference between the two inputs. In Fig. 7, differentiated flyback pulses (sine-shaped) from. a television timebase generator are applied to the junction of diodes 11, 12 biased off at 13 by three times the pulse amplitudes and equal amplitude sync. pulses in push-pull to their other electrodes, to derive from long time constant output circuit 20-24 pulses of polarity indicating the sense of the phase difference, and if there is any frequency drift, a D.C. output polarity persists as an indication of the sense of the frequency drift until, after a period equal to the difference (or drift) frequency, coincidence of the two pulse inputs again occurs. Alternatively the rectifiers 11, 12 may be parallel connected with the sync. push-pull input and the output circuit, and the differential pulse applied to their common connection. Alternatively, a conventional phase discriminator may be used whose output varies in sinewave form during frequency drift and, coincidence of the input pulses and the initial part of the sinewave being of polarity dependent on the sign of the frequency drift, this polarity may be detected by an " integrating," e.g. shunt capacitive circuit, followed by a peak rectifier giving alternative polarity outputs, e.g. a non-linear resistor or pair of Zener diodes connected back-to-back and having a long time constant output circuit. The output of this frequency drift sensing circuit is then combined with that of the phase comparator, to give an A.F.C. voltage of large pull-in range. In Fig. 19, timebase sawtooth waves are integrated across capacitor 7 to form parabolic pulses applied with incoming sync. pulses to a gating valve 1 which applies at each peak coincidence a pulse to a phase discriminator 9, 10 comparing the steep portions of the sawteeth with the pulses. The rectifiers are gas tubes 9, 10 whose striking voltage forms the back bias. The time constant circuit (capacitor 11) producing an output indicating the sense of any frequency error during frequency drift of the inputs to the phase comparator may have its efficiency increased, especially for television vertical timebases, by a high input impedance D.C. amplifier 12 between the discriminator and blocking oscillator timebase generator 19. Alternatively, flyback pulses may be integrated to form the sawtooth waves and the pulses applied directly to the gating valve 1. Transistors may replace the rectifiers and gating valve, and in Fig. 7, the sync. pulses may be applied to the bases of an NPN and PNP transistor, the emitter collector paths being the rectifiers. Specification 952,835 is referred to.