GB2085698A

GB2085698A - Stigmator for cathode ray tube

Info

Publication number: GB2085698A
Application number: GB8129616A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1980-10-02
Filing date: 1981-10-01
Publication date: 1982-04-28
Also published as: GB2085698B

Abstract

To improve focus and reduce distortion near the periphery of the screen of a CRT, particularly with high luminance, a stigmator providing either one or two quadruple fields is placed between cathode and deflector. The field (element 20-23) may be proportional to the sum of even powers of horizontal and vertical deflection signals or (elements 24-27) proportional to the sum of beams formed from the products of odd powers of the deflection signals. If both fields are present they may be at 45 DEG as shown. The quadruples may produce magnetic fields as shown or electric fields. <IMAGE>

Description

SPECIFICATION Improvements in or relating to cathode ray tubes This invention relates to cathode ray tubes and in particular to apparatus for reducing astigmatism therein, such an apparatus being hereinafter referred to as a stigmator.

In a cathode ray tube (CRT) a cathode produces an electron beam which may be electrostatically or electromagnetically focused before controlled deflection onto a phosphored face to produce an image. In conventional CRTsthe beam deflection is usually produced by an energizing signal in a coil assembly mounted on a yoke on the CRT neck to establish a beam deflecting field. A major cause of astigmatism in such CRTs is non-uniformity of the deflecting field and a fringing field produced by the yoke. The effects of th is non-uniformity is particularly severe in high luminance CRTs in which the electron beam diameter is large.Unfortunately high luminance CRTs are generally required to produce an accurate display, such as for example in the display of information in aircraft, and the astigmatism introduced by the deflection system can set a limit on the brightness and luminance which can be achieved. The current trend towards smaller CRT neck diameters, which is particularly desirable in CRTs for aviation use to reduce both weight and power consumption in the deflection system, tends to increase the deflection aberrations which effectively impose a limit on the savings which can be achieved.

According to the present invention a stigmator for a cathode ray tube having a cathode for producing an electron beam and deflection means for controllably deflecting the beam to a position defined by horizontal and vertical components includes means for establishing a field in the region between cathode and deflection means, the field having a component energized in proportion to the sum of even powers or the product of odd powers of the horizontal and vertical components.

Preferably the field has a first field component energized in proportion to the sum of the even powers of the horizontal and vertical components and a second field component energized in proportion to the product of the odd powers of the horizontal and vertical components.

The field established in the region between cathode and deflection means may be an electrostatic field or an electromagnetic field. An electromagnetic field may advantageously be established by a quadrupole comprising opposing coil pairs serially connected and arranged on normal axes.

Preferably a first quadrupole is energized by a current IQ such that: Ia = k1(X2 + Y2) + k2(X4 + Y4) + .... + kn(X2n + y2n) where X and Y representthe horizontal and vertical components respectively and n is chosen accord ingly to the degree of correction required and a second quadrupole is energized by a current p + K1XY - K~X3Y3 d . . + where X and Y are as previously identified and i is chosen in accordance with the degree of correction required. It will be appreciated that the values of the constants of proportionk.... k,, and K1... K, together with their sign is a function of CRT geometry.An electrostatic field may be similarly established between opposing poles, which in this case may advantageously be internally mounted.

In a preferred arrangement of the presentinven- tion a stigmator for use with an electrostatically focused CRT having a conventional focusing anode arranged to act as a focusing lens has co-planar first and second quadrupoles symmetrically arranged around the CRT neck over the cylindrical lens.

In a CRT where deflection is controlled in accordance with a conventional raster scanning arrangemenu the horizontal and vertical components may be derived from the line scan and field scan signals respectively. In some high luminance CRTs dynamic focus correction is employed to vary the focus signal as the electron beam traverses the CRT face to correct focus aberrations and in such a CRT the horizontal and vertical components may be derived from the dynamic focusing circuitry.

In order that features and advantages of the present invention may be fully appreciated an embodiment will now be described, by way of example only, with reference to the accompanying diagrammatic drawings of which: Figure 1 represents a stigmator in accordance with the present invention, and Figure 2 represents a CRT including the stigmator of Fig 1.

Where parts shown in the figures are the same, they have been given common reference numerals.

A stigmator 10 (Fig 2) is mounted on the neck 11 of a CRT 12. The CRT 12 has a cathode 14 for producing an electron beam 18 in co-operation with a conventional heater 15, grid 16, and anode 19. Deflection coils are mounted on a deflection yoke 17 on the neck 11 of the CRT 12. The stigmator 10 includes coils 20,21,22,23 arranged in a first quadrupole and 24,25,26,27 arranged in a second quadrupole (Fig 1) for establishing electromagnetic fields in the region between the cathode 14 and the deflection yoke 17 in accordance with the present invention. The coils are arranged in a co-planar configuration on a coil former 28 placed around the CRT neck 11. The former 28 is arranged such that the coils are situated in opposing pairs, such as coil pair 24 and 25, acting in the same sense, so that the field is set up between the opposing poles produced thereby.Coil pairs 24, 25 and 26,27 are arranged to produce field components acting normally to each other and are serially connected and energized by a current Ip produced at output 30 of drive amplifier 29. Coil pairs 20, 21 and 22, 23 are similarly connected and energised by a current 1a produced at output 31 of drive amplifier 32.

A signal processor33 receives signals X and Y at inputs 34 and 35 respectively. Signals X and Y rep resents the horizontal and vertical components of the deflection control respectively and may be straightforwardly derived from CRT drive circuitry in accordance with known principles. In a conventional raster scanning control regime signal X and Y are the line and field drive signals respectively, although it will be appreciated that a stigmator in accordance with the present invention may be used with a CRT wherein deflection is controlled by other regimes.

Signal processor 33 and drive amplifiers 29 and 32 are arranged to produce output currents Ip and 1a in accordance with the following: IQ = KQ(X2 + Y2) Ip = Kp(X3Y3) where KQ and KP are constants chosen for the particular embodiment. The degree of correction obtained with this arrangement has been found to be adequate for many applications. 1a may advantageously be derived from a conventional dynamic focus correction signal.

The CRT 12 (referring again to Fig 2) has an electrostatic focusing arrangement including focusing anode 36 which acts as a cylindrical lens. The stigmator 10 is placed over the cylindrical lens. It will be appreciated by those skilled in the artthatthe stigmator 10 provides rotation and amplitude adjust menttothe performance of the cylindrical lens to dynamically correct for astigmatism introduced by the deflection system.

Claims

CLAIMS The matter for which the applicant seeks protection is:

1. A stigmator for a cathode ray tube having a cathode for producing an electron beam and deflection means forcontrollably deflecting the beam to a position defined by horizontal and vertical components including means for establishing a field in the region between the cathode and the deflection means, the field having a component energised in proportion to the sum of even powers or the product of odd powers of the horizontal and vertical components.

2. A stigmator as claimed in claim 1 wherein the field has a first field component energised in proportion to the sum of even powers of the horizontal and vertical components and a second field component energised in proportion to the product of odd powers of the horizontal and vertical components.

3. A stigmator as claimed in claim 1 or claim 2 and wherein the means for establishing a field provides an electrostatic field.

4. A stigmator as claimed in claim 1 or claim 2 and wherein the means for establishing a field provides an electromagnetic field.

5. A stigmator as claimed in claim 4 wherein the means for establishing a field includes a quadrupole.

6. A stigmator as claimed in claim 5 wherein the first field component is provided by a first quadrupole, and the second field component is provided by a second quadrupole.

7. A stigmator as claimed in claim 5 or claim 6 and wherein in use a quadrupole is mounted externally on the cathode ray tube neck.

8. A stigmator substantially as herein described with reference to the accompanying drawings.

9. A cathode ray tube including a stigmator as claimed in any preceding claim.