Patent GB-799542-A (GB)
Title: Oscillator for generating a saw-tooth wave
Inventor: JEPMOND HOWARD KRISTIAN VAN
IPC: H03K4/43
Filed: 1956-08-17
Abstract: 799,542. Sawtooth generator circuits. HAZELTINE CORPORATION. Aug. 17, 1956, No. 25248/56. Class 40 (6). A sawtooth generator comprises a network coupling the anode, screen grid and cathode of a discharge tube so that the system oscillates due to positive feedback and an additional negative feedback network coupled to the control grid so that during the rising portion of the sawtooth wave the positive feedback is balanced out. Fig. 1 shows the frame sawtooth generating circuit of a television receiver. The anode and screen grids of tube 10 are coupled in positive feedback circuit by transformer 14 and the anode is also coupled in a Miller circuit to the control grid via capacitors 24, 26, the tube being normally biased so that it is conductive. The combined negative-going line and frame synchronizing signals from source 36 are applied to the screen grid by transformer winding 14b, the frame pulses being ineffective because of the integrating action of winding 14b in conjunction with capacitors 48 and 50. The negative line pulses supplied to the screen grid cause the anode current to fall thus increasing the anode voltage which due to the action of transformer 14 causes the screen-grid voltage to fall still further. This action is cumulative so that the screen-grid voltage is driven heavily in a negative direction. The positive voltage pulse at the anode is,applied to the control grid so that the control grid and,anode attain approximately the same potential. Capacitors 24, 26 charge through the control grid to cathode circuit, the time constant for the charging circuit being short. As the anode swings positively and the screen grid negatively, control is lost by the screen grid and a half cycle of oscillation is generated at the natural frequency of the circuit which is short compared with that of the sawtooth wave required to be generated. As the anode voltage swings sufficiently in the negative direction near the end of the oscillatory half cycle the control grid partakes of the negative swing, the charging cycle of capacitors 24, 26 is discontinued and the control electrode moves rapidly to a large negative voltage VG, Fig. 2, the screen voltage VSG, Fig. 2, flattens out so that the negative feedback to the control grid counterbalances the positive feedback during the rising sweep portion of the sawtooth waves. At the end of the negative anode swing the charge on capacitors 24, 26 leaks through resistors 28, 30 and 32 thus causing the control-grid voltage to increase slowly, until it has risen to a value so that when the next negative synchronizing pulse is applied to the screen grid from source 36, .the screen grid takes over control of the anode current of the tube. The potentiometer 32 serves as a hold control and resistor 28 affords .control of the linearity of the rising current wave at the anode of the tube. The resistor 18 may be made variable, thus sparing the amplitude of the generated wave, the capacitors 48 and 50 form a voltage divider which feeds voltage to the blanking circuit 52 of the cathode-ray tube. The presence of winding 14b in the synchronizing circuit prevents oscillations from being sent back to the source 36. Potentiometer 56 controls the brightness of the cathode-ray tube picture so that in conjunction with resistance 54 the bias potential at the screen grid of tube 10 is varied in accordance with the brightness of the picture, thus varying the extent of the beam deflection current with the intensity of the beam since, as variation of potentiometer 56 decreases the picture brightness, it simultaneously increases the amplitude of the current applied to the deflecting coils 22. In a modification (Fig. 3, not shown) a diode is connected between the control grid of the tube 10 and earth providing a charging path for capacitors 24, 26, so that the positive swing of the control grid of tube 10 may be limited.