Patent GB-972098-A (GB)
Title: Improvements in or relating to circuit arrangements for synchronizing a local oscillator by means of a phase-discriminator
IPC: H04N5/12
Filed: 1961-10-24
Abstract: 972,098. Automatic phase control systems; frequency and phase detectors. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Oct. 24, 1961 [Oct. 27, 1960], No. 38054/61. Heading H3A. Sync. pulses 4 and flyback pulses 2 derived from a television or other timeback generator are applied over an adding circuit so that only at coincidence they charge a capacitor 6, thereby across a shunt resistor 7 a beat sawtooth voltage being developed of recurrence frequency equal to the frequency difference between pulses 2, 4 and applied to open a self-biasing gate valve 11 only during the first half of the sawtooth. Further, flyback pulses 2 are integrated over a capacitor 16 and applied as sawtooth waves 14 through the gate valve 11 to a transformer 19, 20 feeding a phase discriminator 25-35, the transformer output also receiving sawtooth waves 14 directly from capacitor 16 through a further transformer 22, 28 of reversed phase and half the amplitude of the first transformer sawtooth output. The resultant sawtooth input to the centre tap 25 of the phase discriminator therefore is reversed in phase during the first and second half of the beat frequency sawtooth applied to the gate valve 11, and sync. pulses 4<SP>1</SP> applied in push-pull 'to the discriminator produce as output from the latter not a pure A.C., depending on the frequency difference during out of synchronism, but a D.C. output component of polarity dependent on whether the sync. pulses 4 are of greater or less frequency than the flyback pulses 2. (The input sync. pulses 27, 29 to the discriminator then respectively are always superimposed on either the positive or negative half portions of the discriminator sawtooth input.) The catching range of the system during out of synchronism is thereby greatly increased. During synchronism the gate valve 11 is inoperative and the phase discriminator output operates to maintain the sync. pulses 4 mid-way of the steep (flyback) slope of the sawtooth valves 14. In Fig. 4 sync. and flyback pulses 4, 2 are applied to different grids of a valve 44 to derive across anode circuit capacitor 6 a beat frequency sawtooth applied with opposite phases to two gate valves 38, 39 so that each is conductive alternately during respective half portions of the sawtooth slope. Flyback pulses 2 are integrated over capacitor 16 to derive sawtooth waves 41, 40 (the latter phase inverted through a transformer 58) applied to suppressor grids of the gate valves 38, 39, so that from anode resistor 42 common to valves 38, 39 sawtooth waves reversing in phase during alternate halves of the beat frequency sawtooth are applied to the phase discriminator, its other input, in push-pull, being sync. pulses derived from the screen grid of valve 44. During exact synchronism of pulses 2, 4, to avoid gate valves 38, 39 both being conductive due to blocking capacitors 54, 56, resulting in zero sawtooth combined output, one valve 38 is rendered non- conducting by its suppressor grid being D.C. coupled by resistor 57 with the negatively, selfbiasing suppressor grid of valve 44. Alternatively, valve 38 may be back biased to cut-off, but rendered conductive by the beat frequency sawtooth. Alternatively, a multi-grid tube acting as an asymmetrical phase discriminator may be used. Specification 941,725 is referred to.