Patent GB-941725-A (GB)
Title: Improvements in or relating to circuit arrangements for synchronizing a local oscillator
IPC: H04N5/12
Filed: 1960-11-15
Abstract: 941,725. Automatic phase control systems. PHILIPS ELECTRICAL INDUSTRIES Ltd. Nov. 15, 1960 [Nov. 18, 1959], No. 39201/60. Heading H3A. Sync. pulses and a sawtooth reference wave derived from an oscillator are applied to a phase discriminator whose output phase locks the oscillator such that the sawtooth flyback and pulses coincide. Further, during out of synchronism either the sync. pulse or sawtooth input is alternately opened and blocked by a gate circuit during a period beginning whenever a sync. pulse and sawtooth flyback coincide so that an error voltage of polarity indicating the sense of any frequency difference between the sync. pulses and sawtooth wave is produced from the phase discriminator and the lock-in range of the system extended during out of synchronizm. In Fig. 1 a source 1 of sync. pulses and source 2 of sawtooth flyback pulses are series connected with a rectifier 3 to produce across an integrating circuit 4, 5 a sawtooth wave with positive flyback when the pulses from 1, 2 coincide. This sawtooth (of period dependent on the frequency difference between sources 1, 2) during its first part opens a gate valve 8 normally self-biased to cut-off by grid detection so that sync. pulses 1 pass for a time through gate 8 to a balanced phase discriminator 13-18 with sawtooth second input 19 derived from controlled oscillator 21 and are superimposed on either the positive or negative half portions of the sawtooth input 19 depending respectively on whether fs (the sync. pulse frequency) is greater or less than #0 (the oscillator and sawtooth frequency to be controlled) to produce respectively a positive or negative frequency control voltage from the discriminator correcting the frequency f 0  of oscillator 21. During continued synchronizm the gate valve 8 is continuously open and sync. pulses pass uninterruptedly to the phase discriminator to produce fine control of phase. In Fig. 2 the pulse sources 1, 2 are respectively connected to self-biased control and suppressor grids of an otherwise cut-off valve 27 acting also as the gate valve 8. During out of synchronizm each coincidence of pulses 1, 2 produces across an anode connected capacitor 4 a voltage drop which is applied to the grid of a phase inverter valve 34 cutting it off for a time, its resulting increased anode voltage, being applied to the screen grid of valve 27, gating the latter open for a time so that amplified sync. pulses are produced on its screen grid for a time (while valve 34 is blocked) and applied to a phase discriminator with additional sawtooth wave input as in Fig. 1. This continues until the positive going sawtooth wave on capacitor 4 re-opens gate valve 34. During continuous in-synchronizm a negative voltage is developed on suppressor grid connected capacitor 30 which through a resistance 33 cuts-off valve 34 continuously so that the sync. pulse input to the phase discriminator is uninterrupted. In Fig. 5 a monostable circuit 38 is switched so that a normally conducting right hand valve is blocked for a period beginning whenever pulses from sources 1, 2 are coincident. This temporarily opens gate valve 8 to sawtooth waves from source 19 which are then applied to a phase detector valve 43 together with sync. pulses from source 1. The output from valve 43 during any period when sawtooth waves pass to it is dependent on whether the sync. pulses are faster or slower, i.e. are superimposed on the positive or negative half portions of the sawtooth, and produces a frequency control on oscillator 21. The unstable period of monostable circuit 38 being greater than the period between sync. pulses, gate valve 8 is maintained continuously open during in-synchronizm. The systems may be applied to television receiver scanning. Specification 652,122 is referred to.