Large-signal transient analysis and effects of lateral charge spreading in television camera tubes

Abstract

The transient response of television camera tubes to changing levels of illumination is examined. When the entire transient occurs within the exponential retarding potential region of the electron beam <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</tex> characteristic and photoconductive lag can be neglected, exact analytical expressions describing the rise and decay of video signal levels are established. The author present separate solutions to the problem based on the assumptions of electron beam scanning and charging by a hypothetical defocused beam (uniform charging case). The results support arguments in favor of target current biasing to minimize lag and turn-on time. We also show the effects of scanning as opposed to continuous dwelling by a flood beam. The decay transient is shown to be universial in the sense that it is independent of the initial signal strength and depends only on physical tube parameters. The validity of the expression describing signal decay is confirmed experimentally for a silicon diode array vidicon and a return beam silicon diode array vidicon. The effect on the resolution of target surface conductivity in conjunction with a finite electron beam impedance is treated for a steady-state per pattern and isolated circular spot illumination. In particular, the strong dependance of resolution, as limited by a conducting target surface, on operating current level is described. Lateral target leakage is shown to offer little advantage in decreasing lag.