Circuit for generating a dynamic focusing voltage for a picture tube

Abstract

In a television receiver, a dynamic, parabolic correction voltage for focusing is generated by way of a transformer whose primary winding is fed with the line deflection current or a line-frequency voltage and to whose secondary winding a capacitor is connected, which capacitor integrates the sawtooth-waveform current in the secondary winding into a parabolic voltage. New types of picture tubes require a correction voltage which does not have a parabolic profile, rather whose form corresponds approximately to the cross section through the center of a bathtub. It is an object to provide a simple circuit for generating such a bathtub-waveform correction voltage. This is achieved by virtue of the fact that a frequency-dependent network, in particular a series resonant circuit comprising an inductance and a capacitance, is connected in parallel with the capacitor, which network is tuned in such a way that the network effects an additional sinusoidal current with a period duration approximately equal to the line trace period. The circuit is particularly simple, requires no active elements, effects a low power loss, can be used with a current transformer and furthermore enables blanking of the correction voltage during the vertical blanking interval.