Patent GB-952835-A (GB)
Title: Phase and frequency comparator
IPC: H04N5/12
Filed: 1960-04-01
Abstract: 952,835. Automatic frequency and phase control systems; frequency and phase detectors. STANDARD TELEPHONES & CABLES Ltd. April 1, 1960 [April 4, 1959; March 11, 1960], No. 11589/60. Heading H3A. A phase and frequency comparator comprises a series connected storage capacitor and a bidirectionally conductive non-linear element to which series combination is applied a pair of voltages of nominally equal frequency but forming an asymmetry of positive and negative peak voltages in the event of any phase error. Further, the storage capacitor has a long discharge time so that above a certain frequency error, an output is derived whose polarity indicates the sense of the frequency error. In Fig. 1 differentiated sync. pulses are superimposed on the steep flank of a recurrent sawtooth wave produced by applying flyback pulses, e.g. from a television receiver timebase generator to a capacitor 6, and the composite wave applied to a series connected capacitor 12 and a non-linear bidirectionally conductive resistor 13, to derive a D.C. voltage across capacitor 15 of polarity and magnitude indicating the sense and magnitude of the phase difference or the sense of any frequency error above a certain minimum. Alternatively, resistor 13 and the sawtooth forming circuit 5, 6 may be connected as in Fig. 2. In Fig. 16 an oscillator 29 to be frequency and phase controlled applies a bi-polar pulse to one side of non-linear resistor 28, and a differentiated (therefore bi-polar) sync. pulse, of twice the duration of the first, applied to the other side, the D.C. control voltage denoting phase difference being derived across capacitor 15, and its polarity denoting the sense of the phase error. In other arrangements if the two pulsed inputs to any type of phase comparator differ in frequency the output is a series of single wave cycles, of recurrence frequency equal to the difference frequency, and of polarity, i.e. phase sense, denoting the sense of the frequency error. This may be applied to an integrating circuit to form pulses with the polarity of the first halfcycle of the single wave cycles, i.e. indicating the sense of the frequency error. Those pulses may then be rectified by another circuit having a bidirectional conducting element (Fig. 15, not shown) to produce a D.C. output indicating the sense of the frequency error, which, added to the phase comparator output, controls the timebase generator in phase and, above the lock-in frequency range, in frequency also. In all cases the non-linear resistor may alternatively be a glow discharge, Zener diode or pair of Zener diodes connected back to back, or a bi-stable circuit. In phase comparing continuous sine waves one may be doubled in frequency before adding to the other to produce a composite wave of asymmetric peak values denoting phase shift, which is then applied to the non-linear resistor. In an F.M. discriminator a limited F.M. wave may be applied to two tuned circuits, one of different Q and tuned to twice the mean frequency, both connected in series and shunting the F.M. input, and the output applied to a shunt non-linear resistor, so that it follows phase and therefore input frequency variations.