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CRT Dynamic Focus Using a Prefocus Electrode

IP.com Disclosure Number: IPCOM000088144D
Original Publication Date: 1977-Apr-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Lovie, RH: AUTHOR [+2]

Abstract

One type of cathode ray tube (CRT) contains a prefocus electrode the function of which is to control the spot size produced by the tube. Dynamic focus can be achieved by applying a signal, derived from the line and field time bases, to the prefocus electrode. This results in simplification of the high voltage supplies and the dynamic focus amplifier and leads to improved spot uniformity.

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CRT Dynamic Focus Using a Prefocus Electrode

One type of cathode ray tube (CRT) contains a prefocus electrode the function of which is to control the spot size produced by the tube. Dynamic focus can be achieved by applying a signal, derived from the line and field time bases, to the prefocus electrode. This results in simplification of the high voltage supplies and the dynamic focus amplifier and leads to improved spot uniformity.

Dynamic focus correction is needed in high-resolution, wide-angle CRTs to maintain the electron beam in focus over the entire screen. The required focus voltage for each point on the screen is a function of its distance from the screen center and can be derived electronically from the X and Y position signals in the display. Conventionally, the dynamic focus signal is fed, together with a DC focus voltage, to the focus electrode in the gun. This leads to some problems:- (1) Typically, a 500V swing is required for a 90 Degrees tube. (2) Since some current is drawn from the focus electrode, the focus supply must have low output impedance. This does not permit the dynamic focus signal to be injected in parallel with the output, so the signal must be injected into the regulator feedback loop, resulting in a high cost focus supply. (3) When dynamic focus is applied in the conventional way, there is some residual spot growth at the edges of the screen due to perturbation introduced by the deflection yoke. In general, the spot growth increase...