GB442938A - Improvements in or relating to generators of intermittent electrical oscillations - Google Patents

Abstract

442,938. Valve generating circuits ; television. HARDWICK, J., 21, Drayton Park Avenue, West Drayton, and BLYTHEN, F., Heathcote, Keith Road, Hayes, both in Middlesex. May 18, 1934, No. 15132. [Classes 40 (iii) and 40 (v)] A blocking-oscillator, particularly of the " squegger type, in which a back-coupled valve oscillates intermittently to produce impulses of the frequency of intermittence, is provided with means for controlling the duration of oscillation, or the duration of quiescence, or both, preferably leaving their sum, i.e. the impulse period, constant. A combination of such impulse generators is used in a television system for synchronizing purposes. Fig. 4 shows the modified oscillator, comprising a tetrode valve 24 with its outer grid back-coupled at 27 to its inner grid, and with its anode circuit connected to output terminals 34. A grid condenser 30 and leak 31 are provided, and in addition a resistance 35 shunted by a high-frequency byepass condenser 36 and by a rectifier 37 is connected on the cathode side of the grid coupling-coil. During quiescent periods, the condenser 30 is discharging through resistance 31 and the rectifier 37, which latter substantially short-circuits the resistance 35. During oscillatory periods, current flows through the resistance 35 to charge the condenser to the point at which the grid voltage blocks the valve. Increasing the value of resistance 35 increases the period of oscillation, without substantially affecting the period of quiescence ; the latter can be reduced, so as to keep the total period constant, by adjustment of the leak 31. Fluctuations in the frequency of the blocking oscillators may be obviated by trigging them into oscillation by means of current derived as harmonics of the mains supply frequency ; or one oscillator may be synchronized with another by coupling the anode circuits to each other or to a common circuit. Fig. 2 shows the form of impulses generated by the blocking oscillators and the way in which the impulses are combined for synchronizing in the television system shown in Fig. 1. One oscillator 18 generates the short steep-fronted impulses shown at (a) Fig. 2, whilst another oscillator 19 operating at the same frequency-the line frequency -generates longer impulses (b). A third oscillator 20 operating at frame frequency generates impulses (c) of slightly greater amplitude, the duration of which covers several complete periods of the other oscillators. The three sets of impulses are superposed, giving the form shown at (d), and are then fed to a limiter valve 21, or pair of valves in cascade, which cuts off the upper and lower portions resulting in the wave-form (e). These impulses are fed to the saw-tooth generators 10, 11 controlling the scanning beam in the television transmitter 1, and are also fed to the device 5 where they are combined with the picture signals and fed to the modulator 6. At the receiver, the synchronizing signals are separated by a device 23, which may be of the kind described in Specification 423,685, and are supplied to the scanning oscillators 16, 17 for the receiver tube 8. For the line frequency oscillators 10, 16 each impulse is effective, whilst for the frame frequency oscillators 11, 17 the impulses are integrated. The manner in which the impulses are combined for television purposes forms the subjectmatter of Specification 443,012.