Patent GB-633827-A (GB)
Title: Improvements in or relating to time marks for the sweep of a cathode ray tube
IPC: H03K3/04
Filed: 1946-06-26
Abstract: 633,827. Cathode-ray oscillographs. SPERRY PRODUCTS, Inc. June 26, 1946, No. 19104. Convention date, June 28, 1945. [Class 37] [Also in Group XL (c)] In a cathode-ray oscilloscope a sweep is generated between one set of deflecting means and a periodic voltage, initiated simultaneously with the sweep, is impressed between the other set to form time marks in the sweep. The horizontal sweep is controlled by a push-pull sweep circuit 20, Fig. 1, controlled by a gas discharge tube 21 which is shunted across a condenser 22 charged through a resistor 23 to a potential of 400 volts from a battery 29. The grid of the tube 21 is normally biassed sufficiently negative by means of a battery 24 acting through a grid leak 25 to render it non- conducting. The tube 21 may be caused to conduct either in response to a signal such as a wave train generator, or it may be caused to discharge periodically. Where the discharge is controlled by the input signal this impresses a positive potential through a condenser 26 on the grid of the tube 21 and causes it to conduct and discharge the condenser 22. The pushpull sweep circuit 20 is thus energized to place the sweep voltage on the horizontal deflection plates 11 of a cathode-ray oscilloscope 10. If no input signal is used for control, the discharge of the tube 21 may be effected periodically by choosing the potential of the battery 24 so that the tube 21 discharges repeatedly as the condenser 22 recharges to a predetermined voltage. Time marks are introduced into the horizontal sweep by means of a multivibrator 30 comprising two valves 31, 32, the plate voltage of which is obtained through resistors 34, 35 from a battery 33 supplying 300 volts. Grid bias is supplied through resistors 36, 37. The valves 31, 32 are connected so that the output of the valve 31 feeds the grid of the valve 32 through a condenser 38 while the output of the valve 32 feeds the grid of the valve 31 through a condenser 39. If the cathodes 40, 41 are earthed the multivibrator operates continuously, the discharge of one valve energizing the other. By connecting the cathode 41 to the vertical deflecting plate 12 of the oscilloscope 10 a vertical component is introduced into the horizontal sweep each time the valve 32 passes current. The cathode of the valve 31 is not connected to the plate 12 and therefore periodic spaced time marks are introduced into the horizontal sweep. A condenser 45 and resistor 46 filter out low frequency components. Breakdown of the tube 21, which initiates the horizontal sweep, is also utilized to render the multivibrator effective, so that the time marks are in fixed predetermined relation to the sweep. The lower ends of resistors 42, 43 in the cathode circuits of the valves 31, 32 are connected to the anode of the tube 21. When the condenser 22 is full charged from the source 29, the cathodes of the valves 31, 32 are at a higher potential than the anodes, since the battery 29 has a voltage of 400 whilst the battery. 33 has a voltage of 300. Hence no current flows through the valves and the multivibrator does not oscillate. On the discharge of the condenser 22 the potential of the cathodes drops to near zero and the multivibrator starts to oscillate. Bv placing a small condenser 50 between the cathode of the valve 31 and earth the action of the valve 31 is delayed due to the voltage which the condenser applies to the cathode and oscillation of the multivibrator always begins with the valve 32 becoming effective as soon as the tube 21 breaks down. The same result can be obtained by connecting the lower end of the resistor 42 directly to earth, making this valve continuously conducting. Vibration will therefore always start with the valve 31. Since the output of the tube 21 is of square wave form, the time mark will always be of square wave form. The time signals are introduced into the sweep in synchronized relation with the sweep so that the time signals always appear at the same points in the sweep. In a modification Fig. 2, the connection between the discharge tube 21 and the multivibrator 30 is made through a control valve 60 instead of through the cathode resistors 42, 43, the lower ends of which are permanently earthed. The anode of the valve 60 is connected to the grid of the valve 31, the grid is earthed through a grid leak 62, and the cathode is connected to the negative end of an earthed battery 65. Normally-the anode of the valve 60 is at a negative potential with respect to earth approximately equal to the negative potential supplied by the battery 65 and this negative potential is applied to the grid of the valve 31, which is therefore biassed beyond cut-off and the multivibrator does not operate. On the discharge of the condenser 22 the grid of the valve 60 receives a negative pulse which carries this valve beyond cut-off, the valve ceases to conduct, and the grid of the valve 31 becomes less negative and permits operation of the multivibrator. Thus the time signals are introduced each time the tube 21 discharges and therefore each time the sweep circuit 20 is energized, so that the time marks are synchronized with the sweep and occur only when the sweep occurs.