GB647711A - Improvements in trace blanking circuits for cathode ray oscilloscopes - Google Patents

Abstract

647,711. Cathode ray oscillographs. WESTERN ELECTRIC CO., Inc. Aug. 12, 1946, No. 23860. Convention date, Oct. 10, 1945. [Class 37] [Also in Group XL (b)] For a cathode-ray oscillograph in which an electron spot varying in intensity in accordance with a control voltage is moved over a screen in accordance with the whole variation of a repetitive voltage, a circuit for blanking the trace comprises means responsive to the repetitive voltage to vary the control voltage to extinguish the spot during an interval of the repetitive voltage and the responsive means is controllable so that the beginning and end of said interval may be adjustably determined. In the circuit arrangement, Fig. 4, a sawtooth controlling voltage 10 from a generator 20 is fed to a valve 30 from the cathode circuit of which is derived a similar sawtooth waveform which is delayed by a condenser 28. This output is fed to the anode 41 of a gas-filled triode 40 whose cathode 42 and grid 45 are appropriately biassed by a battery 43 and by a battery 46 in conjunction with an adjustable tap 48 on a potentiometer 47. The grid voltage determines the point B (Fig. 1) on curve 10 at which the tube 40 strikes and the bias applied by the battery 43 determines the anode voltage at which conduction in the valve 40 is cut off. During the period that the valve 40 is conducting, a positive bias is applied to the grid 51 of a pentode 50 which conducts, or becomes more conducting and provides a fall in the voltage at the anode 53. The decrease in voltage, which is constant whilst the tube 40 is conducting, is applied via condenser 61 to the control grid 62 of the cathode-ray tube 70 and is arranged to be of sufficient magnitude to extinguish the spot. As the voltage derived from the valve 30 and applied to the anode 41 of the tube 40 is delayed relative to the waveform 10, it can be arranged that the tube 40 conducts for a short period after the minimum point of the waveform 10 so that a small initial portion of the forward trace is also blanked in addition to a small final portion of the forward trace and the whole of the return movement of the trace.