GB908798A - Improvements in or relating to cathode ray tube deflection circuits - Google Patents

Abstract

908,798. Sawtooth generator circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. Dec. 16, 1958 [Dec. 19, 1957], No. 40536/58. Class 40 (6). In a sawtooth current deflection circuit for the indicator tube of a radar receiver, in which the scanning period is only a small portion of the interval between radar pulses, the deflector coil is fed in push-pull by being connected between the anode of two tubes to which sawtooth control voltages are applied. A tapping on the coil is connected to the anode supply voltage via an inductor and to the anode of a normally conductive discharge tube which is provided with negative voltage feed back, a negative pulse being applied to its grid during the application of the sawtooth voltage to the push-pull pair of discharge tubes. In this way, even if the rest position of the cathode-ray tube beam lies off the display screen, the dissipation in the push-pull connected valves is considerably reduced. Fig. 1 shows a sawtooth current deflecting circuit of a radar indicator (not shown), in which tubes B1, B2 are fed from a sawtooth generator ZG synchronized by pulses applied at terminal K to feed in push-pull deflector coil S. A synchronizing pulse at terminal K renders sawtooth generator ZG operative to start the sweep period of the current flow in coil S. The synchronizing pulse also triggers mono-stable trigger circuit M to produce a high value negative pulse at the grid of tube B3 so that the tube would be cut off. However, this is prevented by the negative voltage feedback via circuit C, R1. The sum of the currents, through tubes B1 and B2 during the scanning period is equal to the sum of the standing current and the current through coil L cannot be varied instanteously, so that the current through tube B 3 must also vary continuously and immediately after the start of a scanning period, the total current must be about equal to their rest value in spite of the negative voltage applied to the grid of tube B3. Thus since the grid voltage of tube B3 must also initially remain constant the voltage drop which would be produced by the output of the trigger circuit M must be compensated for by a voltage rise at point A. Thus this voltage rise must be equal to R1/R2 times the variation of the output voltage of circuit M. During the scanning period the current through tube B3 falls so that the voltage at point A is maintained at high value. Because of the voltage increasing effect of coil L which compensates the voltage decrease at the anode of tube B2 by the inductance of the coil S, the anode voltage Vb can have a low value which is sufficient to supply the currents during the rest period. The recovery time of trigger M is about equal to the effective deflection time. During the rest period the current through coil L is again increased to the initial value under control of tube B3.