Patent GB-848324-A (GB)
Title: Synchronising electronic time bases
Inventor: SMITH ROBERT BRUCE
IPC: H03L7/107
Filed: 1955-10-11
Abstract: 848,324. Automatic phase control systems. MURPHY RADIO Ltd. Jan. 3, 1957 [Oct. 11, 1955], No. 28976/55. Class 40(5). In a circuit for synchronizing an oscillator with periodical received signals, for colour synchronizing in a colour television system, the output of a phase discriminator which compares the phase of the synchronizing signal with the oscillator output is applied to the oscillator frequency control means through a circuit which can be switched automatically from a condition in which it gives good pull-in over a wide range to a condition in which it gives high stability against interference and a narrow pull-in range. As shown in Fig. 1, a phase discriminator 1 is supplied through a gating circuit 2 with the received synchronizing bursts and also with a signal derived from the oscillator 3 to be controlled. The output of the discriminator is fed via a smoothing circuit 5, 6, to the frequency controlling means of the oscillator, and an unsmoothed portion of the output is fed via capacitor 8 to a diode 9 having a load resistor 11. By this means the output of the discriminator, consisting of the beat of the synchronizing frequency and the oscillator frequency, is rectified and applied to the control grid of a normally cut-off valve 15 to render it conducting. The unsmoothed output of the discriminator 1 is also fed via a capacitor 19 and resistor 21 to the control grid of the valve 15 so that when the valve 15 conducts a corresponding potential appears across the resistor 7 which is added to the relatively small part of the output passed by the smoothing circuit 5, 6. The frequency of the oscillator is thus modulated by an increasingly unsymmetrical beat oscillation and thereby the output of the discriminator is reduced in frequency until it can pass the smoothing circuit at a sufficient amplitude to control the frequency. As the oscillator comes into synchronism the valve 15 ceases to conduct, and the oscillator is left under the control of a narrow range, interference immune circuit. Instead of feeding the valve 15 through circuit 19, 21, a resistor may be included in series with the capacitor 13 and the resistor 14, together with components 19, 21, dispensed with. In an alternative arrangement, Fig. 3, the diode 9 is reversed and provides a negative voltage which biases a valve 25, connected as a Miller integrator, to cut-off with the result that only the actual valve of the capacitor 23 is effective in the smoothing circuit and the oscillator can be pulled in from a wider difference from the synchronous frequency. As the oscillator nears synchronism the outputs of the discriminator 1 and rectifier 9 fall off until the valve 25 conducts and introduces a much greater capacitance into the smoothing circuit. In a modification, Fig. 4 (not shown) a double triode replaces the valve 25, one portion acting as a switch valve and the remaining portion as a Miller integrator. In this case the diode 9 is connected as in Fig. 1. In a further modification, Fig. 5 (not shown), the diode is arranged to control a valve which varies a resistance in series with the smoothing capacitor in the output of the discriminator.