Patent GB-721791-A (GB)
Title: Improvements in or relating to synchronizing-circuits for electric oscillators
IPC: H04N5/12
Filed: 1952-11-03
Abstract: 721,791. Automatic frequency- and phasecontrol systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Nov. 3, 1952 [Nov. 6, 1951], No. 27614/52. Class 38 (4). [Also in Groups XL (b) and XL (c)] An oscillator wave is synchronized with a pilot wave by mixing in a phase-comparison stage to derive a frequency-control voltage, and a network is interposed between the oscillator and the phase comparer so as to provide a phase-shift which varies with the oscillator frequency, and thus to counteract the phase difference which would otherwise occur between the oscillator wave and the pilot wave when the latter has drifted from its normal frequency. The system is particularly applicable to the synchronization of the line-scanning oscillator of a television receiver in order to prevent lateral shift of the picture when the frequency of the sync pulses drifts. In Fig. 1 the phasecomparer 3 is fed on the one hand by sync pulses 2 and on the other hand by the output of the controlled oscillator 1, and the output of the phase-comparer operates as a frequency-control voltage 5 on the oscillator 1. When the sync pulse frequency drifts, the oscillator frequency is automatically adjusted into agreement by the control voltage 5, but the maintenance of this voltage is dependent on a phase difference between the two inputs to the phase-comparer 3. The invention consists in providing, between the oscillator output and the phase-comparer, a phase-shift network 4 which advances or retards the phase according as the oscillator 1 is operating on one side or the other of its normal frequency. In this way the oscillator is kept in phase with the sync pulses over a considerable but limited range of frequency drift of the sync pulses. In a modified circuit shown in block form in Fig. 2, and in detail in Fig. 3, the sync pulses 6 are applied to the phasecomparer 7 through a network K1 which converts the pulses into sine waves V1 whose phase is dependent on the sync pulse frequency. The controlled scanning oscillator 8, 11 feeds a pulse output 12 through a second network K2 to create a sine wave V2 whose phase is also dependent on frequency to a greater extent than in the case of the network K1 The sine waves V1, V2 are compared in phase in the mixer 7 and the resulting output voltage not only adjusts the frequency of the oscillator 8 to agree with that of the sync pulses, but also brings the fly-back pulses 12 into phase with the sync pulses. In Fig. 3 the complete television signal 14 is applied to the grid of a pentode valve 18a which is biased to eliminate the picture signals and to pass the pulse signals to an anode transformer 20, 21. This differentiates the pulses and applies short positive pulses to the grid of an associated triode valve 18, whose anode is connected to the junction of a coil 27 and a pair of condensers 28, 29 constituting the tuned phase-shifter K1 of Fig. 1. A fraction of the phase-shifted voltage is fed across a resistor 30 to one control grid of a multi-grid phase-comparing valve 31. The other control grid of this valve is connected through a phase-shifter 53, 54 (corresponding to K2, Fig. 2) to a secondary coil 52 on the transformer feeding the line-scanning coils 51. The pulses V14 across the coil 52 are thu sconverted into a sine-wave voltage which is dependent in phase on frequency drift, and is delivered to the phase-comparer 31. Pulses of anode current, whose duration depend on the relative phases of the two inputs, are passed to an integrating network in the anode circuit, and the integrated plate voltage is fed to a multi-vibrator pair 42 to control its frequency. A voltage wave 43 taken from the multi-vibrator is fed to the output valve 46. The phase/frequency characteristic of the network 53, 54 should be steeper than that of the network 27, 28, 29 by the provision of suitable resistors; and at normal frequency the voltages compared in the valve 31 should be in quadrature. Specification 615,886 is referred to.