GB419298A - Improvements in time base arrangements for use in oscillographic observations and the like - Google Patents

Abstract

419,298. Valve generating circuits. COSSOR, Ltd., A. C., and PUCKLE, O. S., Highbury Grove, Highbury, London. May 5, 1933, No. 13152. [Class 40 (v).] In an electrical timebase circuit, for cathode ray television systems and other purposes, in which a condenser is gradually charged and rapidly discharged, the discharge occurs over a thermionic valve, under the control of a second thermionic valve, the valves being reciprocally coupled by resistances connected between their anodes and the positive pole of the voltage source and by connections between the anode of each valve and the grid of the other, so that a rise of anode current in the discharging valve brings about a cumulative rise of potential on its grid, to hasten the discharge. The condenser may alternatively, be charged rapidly by the said valves and discharged gradually over an impedance. The controlling capacity, which is selected from C1-C4 is charged from a rectified A.C. supply over a resistance which, may consist of a tetrode or pentode P1 the anode current of which is adjusted by connection of the control grid or screen to a potentiometer R4 to control the speed of the time base weep. The discharge takes place over a triode T1 which is reciprocally connected to a pentode or tetrode P2, the anode feed resistance R2 of which is adjusted so that the grid of T1 normally has a high negative bias relatively to the cathode. The capacity C1-C4 charges up until the potential of the cathode of T1 drops sufficiently for T1 to become conductive whereupon the resulting drop in potential of the anode of T1 connected by condenser C to the control grid of P2 causes the grid to become negative, reducing the anode current of P2 so that the potential of the anode of P2 increases. The grid of T1 connected to the anode of P2 becomes more positive, hastens the discharge, and decreases the grid potential of P2 still further so that the effect is cumulative. The time constant of C.R. is adjusted so that the grid of P2 remains negative practically to the end of the discharge, when the anode potential of T1 rises to its normal value and the next cycle commences. The adjustable anode resistance R1 of valve T1 controls the discharge rate, the rate of increase of discharge current to maximum and the rapidity of the cut-off, while R2 can be adjusted to control the amplitude of the time-base sweep. The maximum value of the anode current of T1 is limited since grid current in T1 causes a voltage drop across R2. To avoid this and obtain a more rapid discharge, resistance R2 is replaced by a valve V, Fig. 2, the cathode of which is connected by an adjustable biassing resistance Rv to the anode of P2 which is connected to the grid of V. Cumulative increase in the grid potential of T1 occurs as before but the decrease in anode current of P2 over Rv decreases the negative bias on V and therefore the impedance of V so that the rate of discharge is increased and the limiting effect of grid current is minimized. If the controlling capacity is connected in the dotted line position, Fig. 1, it is rapidly charged over valves T1, P2 and relatively slowly discharged over P1. Examining wave forms. The wave is applied over terminals W1, W2 to plate S and gun GN of the oscillograph and synchronizes the time-base sweep. For this purpose, the wave is applied to the grid of triode T2 which, with anode feed resistance R5, forms a potentiometer control for the screen of pentode P2 which is therefore supplied with a copy of the wave form amplified or attenuated according to the adjustment of R3. By adjustment of R2, the frequency of discharge may be made a submultiple of the wave frequency. The wave may be fed directly to the screen of P2 or grid of T1 for synchronizing purposes. For centring the oscillograph spot, the potentials of the plates Pxs, Pys are controlled by potentiometers Q2, Q1, the potential of the gun GN being determined by a fixed potentiometer R6, R7. The gun is connected to a byepass condenser C5 for capacity currents during the fly-back of the spot. Specification 302,585, [Class 37], is referred to.