Focusing means for cathode ray tubes

Details Focusing means for cathode ray tubes Focusing means for cathode ray tubes

1993-09-09

1996-02-06

Razavi, Michael T.

Electric lamp and discharge devices

Cathode ray tube

Plural beam generating or control

313414, 315 15, 315 16, H01J 2948

Patent

active

054898145

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to means for focusing a beam of electrons which is generated by the cathode of a cathode ray tube (crt) and controlled by electrodes such as a grid and one or more anodes to form an image ("the spot") on a screen.
2. Discussion of Prior Art
A conventional means of achieving this is to provide an electrode whose potential, usually at a value of several kilovolts, is to some degree variable to accommodate variations, within manufacturing tolerances, in tube dimensions and positions of the components within the tube and, if need be, drift in power supply outputs which affect the potential applied to other electrodes.
While it is a relatively straightforward matter to provide means for such occasional variations in focus electrode potential, problems are encountered in providing means for a rapid and continuous variation in the high value potential, typically several kilovolts, normally applied to the focus electrode.
Such a variation would be necessary to provide a "dynamic focus" facility, i.e. to maintain a sharp focus when the spot is not only at the centre of the screen, but also at the periphery of the screen at the extremes of its raster scan. This facility is required especially in view of the trend towards flatter and squarer tubes, which although presenting a less distorted image to the viewer present rather greater problems in maintaining spot focus to the corners of the screen, and also in the use of high brightness tubes, such as those required for aircraft cockpit displays and the like, the electron beams in which have a reduced depth of focus.


SUMMARY OF THE INVENTION

The present invention is aimed at providing a high performance cathode ray tube which includes a low-aberration focusing lens having an electrode maintained at a sufficiently low potential to enable a high frequency dynamic focus signal to be applied to it by readily available and inexpensive means, for example through a high voltage FET.
According to this invention a cathode ray tube comprises: a cathode for emitting a beam of electrons; a grid For controlling the intensity of said beam; three or more anodes; and means for varying the potential of at least one of said anodes for focusing said beam to form a spot on a screen; characterised by an additional, dynamic focus electrode maintained generally at a potential close to the cathode/grid potential.
By "a potential close to the cathode/grid potential" is meant a voltage relative to the potential of the grid or of the cathode (one of which will usually, in practice, be maintained at earth potential), which is within the range of, For example, a high-voltage FET, i.e., within a few hundred volts compared with the typical several thousand volt potentials of the other anodes so that if need be it is a relatively straightforward matter to apply a high-frequency signal, such as that required for dynamic focus control, to the dynamic focus electrode.
The dynamic focus electrode preferably takes the form of a ring, having a diameter similar to that of the adjacent anodes and extending axially only a short distance compared with its diameter; if the electrode extends too far in an axial direction then its effect on electron trajectories may prevent the beam focusing at the screen distance, and also the potential within the electron beam may become low enough to result in unacceptable aberration levels.
The tube may include means for applying to said dynamic focus electrode a high frequency signal to provide a rapid focus control means.
Such a high frequency signal applied to the dynamic focus electrode may be arranged as a function of the position of the spot on the screen. In one application of the invention this may be achieved by means synchronized with or controlled by means for generating coil currents used for controlling a raster scan on the screen.
The signal applied to the dynamic focus electrode may also or alternatively be controlled by the means for controlling the grid-to-cathode potential diffe

REFERENCES:
patent: 4704565 (1987-11-01), Blacker, Jr.
patent: 4897575 (1990-01-01), Shimmoma
patent: 5113112 (1992-05-01), Shimoma
patent: 5142190 (1992-08-01), Koh
patent: 5157301 (1992-10-01), Tominaga

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