Dynamic focusing circuit for a cathode ray tube

Abstract

There is disclosed herein a dynamic focusing circuit for the minor deflection circuit, hereinafter referred to as to the vertical deflection circuit, for a cathode ray tube to compensate for defocusing characteristics thereof. Such a cathode ray tube employes a major deflection circuit, hereinafter referred to as the horizontal deflection circuit, for deflecting the beam along a horizontal axis at a relatively high raster scan frequency, such as on the order of 28 KHz. Additionally, such a cathode ray tube has associated therewith a minor deflection circuit for deflecting the beam along the vertical axis at a second substantially lower scan frequency and which may be on the order of 50 Hz. A focusing circuit normally receives a steady state DC focusing signal for focusing the beam as it is scanned along the respective horizontal and vertical axes. A nonresonant focus correction circuit supplies a correction signal to the focusing circuit for compensating for the defocusing of the beam as it is scanned along the vertical axis. The nonresonant focus correction circuit employes circuitry for providing a sinusoidal signal having a frequency on the order of one-half that of and synchronized to that of the vertical scan frequency together with circuitry that responds to the sinusoidal signal for providing therefrom a correction signal having a generally parabolic shape.