Patent GB-599747-A (GB)
Title: Improvements in or relating to circuit arrangements embodying cathode ray tubes having mosaic screens
IPC: H01J29/02
Filed: 1945-07-20
Abstract: 599,747. Television ; electron-image tubes. McGEE, J. D. July 20, 1945, No. 18589. [Classes 39 (i) and 40 (iii)] In a circuit arrangement embodying a cathode-ray tube having a target electrode (mosaic screen) capable of storing charges and arranged to be scanned by a cathode-ray beam. means are provided to vary the velocity of the scanning beam so that elemental areas of the target electrode are restored to an equilibrium potential which is more nearly the same as the potential of the areas on which the scanning beam impinges normally. Fig. 2 shows a circuit arrangement comprising a cathode-ray tube 1 having a double-sided mosaic screen 2, the left-hand side of which is scanned by a cathode-ray beam from a cathode 3 'associated with a modulating electrode 5, first and second anodes 6, 7 and ring electrodes 9 to 12 maintained at suitable potentials so as to set up a decelerating field for the electron beam. The variation in potential of the elements of the mosaic screen for one frame scan is shown in Fig. 1 in which the line A indicates cathode potential. This variation which is a result of oblique incidence of the electron beam is corrected by the application of a suitable modulating voltage v to the steady voltage accelerating the beam such that v=V tan<2>#, where # is the angle of incidence of the beam and V the resolved energy of the beam normal to the screen. The correcting waveform, the exact shape of which depends on the dimensions of the tube employed, may be obtained from the " bend " correcting signals by amplitude modulating the " frame bend " signals by the " line bend " signals or it may be derived from a separate tube such as 1, Fig. 3, scanned in a similar manner to the tube of Fig. 1 and identical in geometry and operating voltages. A flat conducting target electrode 17 of the same size as the mosaic screen 2 of Fig. 2 is connected through condenser 18 to the grid of valve 19, the anode of which is connected to the modulating electrode 5 and through resistor 20 to a variable source of potential 21 and a fixed source 22. Some of the voltage fed to 5 is applied to the cathode 3 via resistor 16 and when the potential of 3 is increased negatively by two or three volts, by varying 21, contact potentials between 3 and 17 are overcome and 17 collects most of the current when the beam is undeflected and its potential is raised to a small negative value with respect to earth. When the electron beam is defected, part or all of it fails to reach 17, the decrease in negative potential of 17 causes the valve 19 to pass a larger anode current, thus driving the anode negatively and this negative potential applied to the cathode 3 increases the velocity of the electron beam so that it again just reaches 17. A similar automatic correction is applied when the target collects more of the electron beam. In operation, the scanning of both tubes is synchronized and the correcting potential across 16 (Fig. 3) is applied across resistor 16, Fig. 2. The variable voltage source 21 may be eliminated by adjusting resistor 24 to bias 19 such that the potential of 3 is sufficiently negative to enable the beam to reach 17. Distortion due to repulsion between the beam and 17 stabilized at cathode potential is reduced since the velocity of the beam is higher in the region where distortion occurs. An advantage is obtained by applying the correcting voltage with a greater amplitude than that given by V tan<2># so that the outer parts of the mosaic screen are driven more negatively than the centre part, thus subjecting the beam to a field tending to bend it towards the centre of the screen. The transverse energy of the beam would be then reduced with a consequent reduction in the liberation of secondary electrons and compensation for distortion of the scanned path. Specification 499,642 is referred to.