GB2097577A - Electron gun with improved beam forming region and cathode-ray tube and television receiver including same - Google Patents

Abstract

An electron gun (26) for use in a CRT, e.g. for TV, includes beam forming electrodes (34, 36, 38, 39) and beam focusing electrodes (40, 42). To reduce beam flare, the beam forming electrodes include a cathode (34), a control grid (36) adjacent to the cathode, and two screen grids (38, 39). The first screen grid (38) is located adjacent to the control grid. The second screen grid (39) is located between the first screen grid and the beam focusing electrodes. The beam passes through a slot shaped aperture in the second screen grid. The screen grids are excited at different potentials (V2, V2). In one embodiment, an aperture (55) through which the beam passes in the first screen grid is round, and the control grid (36) is grounded. <IMAGE>

Description

SPECIFICATION Electron gun with improved beam forming region and cathode-ray tube and television receiver including same This invention relates to electron guns, such as used in cathode ray tubes, and particularly to an improved beam forming region for such electron guns. The invention may be incorporated into many different types of cathode ray tubes, which in turn may be incorporated into many different types of television receivers. The invention also may be incorporated into many different types of electron guns; however, in the following description, the invention is described with respect to an in-line electron gun which is used in a slit-mask line-screen cathode ray tube having a self-converging deflection yoke, which in turn as used in a television receiver.

An in-line electron gun is one designed to generate at least two, and preferably three, electron beams in a common plane and to direct the beams along convergent paths to a small area spot on the screen. A self-converging yoke is one designed with specific field nonuniformities which automatically maintain the beams converged throughout the raster scan without the need for convergence means other than the yoke itself.

The performance of an electron gun is indicated by the spot diameter of the area of a screen excited by an electron beam from the gun. It is known that such performance is degraded by spherical aberrations and space charge effects. These effects are present in various parts of an electron gun including the beam forming and beam focusing regions of the gun.

In one recently developed electron gun described in U.S. Patent 4,234,814, issued to Chen et al., November 18, 1980, the beam forming region of an electron gun is improved by incorporation of a thick 20 mil (0.508 mm), versus 5 mil (0.127 mm), G2 screen grid electrode. Although this thick G2 electron gun produces an electron beam with a smaller spot diameter, further improvement in spot size is very desirable.

In accordance with the present invention, an electron gun for use in a cathode ray tube which may be incorporated in a television receiver includes an improved beam forming region and a beam focusing region. The beam forming region comprises beam forming electrodes including a cathode, a control grid adjacent to the cathode, and two screen grids. A first screen grid is located adjacent to the control grid, and a second screen grid is located between the first screen grid and the beam focusing region. An aperture in the second screen grid is slot-shaped. Each screen grid is electrically excited with a different electrical potential than the other screen grid.

In the drawings: Figure lisa schematic plan view of a cathode ray tube embodying an electron gun according to the invention.

Figure 2 is a longitudinal elevation, partly in section, of one embodiment of the electron gun of Figure 1, showing a G2' screen grid electrode.

Figure 3 is an elevation view of the G2 screen grid electrode of the gun of Figure 2.

Figure 4 is an elevation view of the G2' screen grid electrode of the gun of Figure 2.

Figure 1 illustrates a cathode ray tube 10 having a glass envelope comprising a rectangular faceplate panel 12 and a tubular neck 14 connected by a rectangular funnel 16. The panel 12 comprises a viewing faceplate 18 and a peripheral sidewall 20. A mosaic three-color phosphor screen 22 is disposed on the inner surface of the faceplate 18. The screen is preferably a line screen with the phosphor lines extending perpendicular to the intended direction of high frequency scanning. A multi-apertured slit-type color selection shadow mask electrode 24 is removably mounted by conventional means in predetermined spaced relation to the screen 22.An in-line electron gun 26 according to the invention (and shown schematically by dashed lines) is centrally mounted within the neck 14to generate and direct three electron beams 28 along coplanar convergent paths through the mask 24 to the screen 22.

The tube of Figure 1 is designed to be used with an external magnetic deflection yoke 30 disposed around the neck 14 and funnel 16 in the neighborhood of their junction, for scanning the three electron beams 28 horizontally and vertically in a rectangular raster over the screen 22. The yoke is preferably self-converging.

Except for the improvements described below, the electron gun 26 may be of the three-beam in-line type similar to that described in U.S. Patent 3,772,554, issued to Hughes on November 1973, or in U.S. Patent 4,234,814, issued to Chen et al. on November 1980.

Thetube 10 maybe used inatelevision receiver such as disclosed in RCA Television Service Data, File 1981, C-7, Chassis CTC 101 Series, published by RCA Corporation, Consumer Electronics, in 1981.

Any modifications to the chassis described in this publication, to obtain the excitations described below, are well within the capabilities of those skilled in the art.

Figure 2 is an elevation, in partial central longitudinal section, ofthethree-beam electron gun 26, in a plane perpendicular to the plane of the coplanar beams. As such, structure pertaining to but a single one of the three beams is illustrated in the drawing. The electron gun 26 is of the bipotential type and includes two glass support rods 32 on which the various electrodes are mounted. The electrodes comprise two regions, a beam forming region and beam focusing region. The electrodes in the beam forming region include three equally-spaced coplanar cathodes 34 (one shown), a control grid (G1) electrode 36, and a two-part screen grid comprising a first electrode (G2) 38 and a second electrode (G2') 39. The electrodes at the beam focusing region comprise a first lens or focusing (G3) electrode 40, and a second lens or focusing (G4) electrode 42.An electrical shield cup 44 is attached to the G4 electrode. All of these electrodes are aligned on a central beam axis A-A and mounted in spaced relation along the glass rods 32 in the order named. The focusing electrodes G3 and G4 also serve as accelerating electrodes in the bipotential gun 26.

Also shown in the electron gun 26 are a plurality of magnetic members 46 mounted on the floor of the shield cup 44forthe purpose of coma correction of the raster produced by the electron beams as they are scanned over the screen 22. The coma correction magnetic members 46 may be, for example, those described in the above-cited U.S. Patent 3,772,554.

The tubular cathode 34 of the electron gun 26 includes a planar emitting surface 48 on an end wall thereof.

The G1, G2, and G2' electrodes comprise transverse plates which have aligned apertures 54, 55, and 56 (and 41), respectively, therein. The G3 comprises two elongated rectangular cup-shaped members attached at their open ends. A first of these G3 members has a transverse wall 58, adjacent to the G2', with an aperture 60 therein. The G4, like the G3, comprises two rectangular cup-shaped members attached at their open ends.

Both the G3 and G4 electrodes have apertures 62 and 64, respectively, attheirfacing ends between which the main focusing lens of the electron gun is established.

In one embodiment of the inventive gun 26, the dimensions presented in Table I were used.

TABLE I mils mm Cathode - G1 spacing (hot) 3 0.076 G1 thickness 5 0.127 G1 aperture diameter 25 0.635 G1-G2 spacing 11 0.279 G2 thickness 15 0.391 G2-G2' spacing 5 0.127 G2' thickness outer portion 20 0.508 G2' thickness central portion 4 0.102 G2 aperture 55 diameter 25 0.635 G2' aperture 56 length 84 2.134 G2' aperture 56 width 28 0.686 G2'-G3 spacing 29 0.737 G3 aperture 60 diameter 60 1.524 G3 length 925 23.495 G3 lens diameter 214 5.436 G4 lens diameter 227 5.766 G3-G4 spacing 50 1.270 Figure 3 illustrates further detail of the G2 electrode 38. The G2 is shown as a flat plate, but may include various embosses for added strength. The G2 electrode plate has three preferably circular in-line apertures 55, 55' and 55", which are aligned with the electron beam path.The plate also includes two claw portions 38' which are normally embedded in the two glass support rods 32.

Further detail of the G2' electrode 39 is shown in Figure 4. The G2' is shown comprising a flat first plate 50 having large central apertures 41,41' and 41" over which a thinner second plate 43 is attached. The second plate 43 has slot-shaped apertures 56, 56' and 56" therein which are aligned with the apertures in the G2 and with the electron beam paths. The plate 50 also includes two claw portions 39' which are normally embedded in the two glass support rods 32.

In a preferred embodiment of the electron gun 26, electrical potentials are applied to the gun electrodes such as are presented in Table II.

TABLE II VK 47.5 V V, O V2 570 V V2- 900 V V3 7.2 kV V4 25 kV It has been found that the beam forming region of the electron gun 26 provides a decided improvement in electron beam flare which is a function of deflection angle. Such improvement is made without significantly degrading the electron beam spot at the center of a tube. It is believed that the improvement is caused by increasing the strength of a quadrupole lens in the beam forming region by placement of a separately excited slotted G2' grid between the G2 and G3 electrodes. By changing the voltage on the G2' grid, beam astigmatism introduced by the G2' grid is varied. This variation of astigmatism can be used to compensate for astigmatism introduced by the yoke deflection field and thereby improve tube overall performance.

Generally, variation of the G2' grid voltage permits a tradeoffto be made between electron beam spot shape at the center of the screen and electron beam flare at the corners of the screen. However, if the voltage applied to the G2' grid is modulated, it is possible to achieve reduced flare at the screen corners and maintain a circular beam spot at the center of the screen.

It should be noted that, in designing the gun embodiments described herein, many tradeoffs may be made in design. For example, grid spacing may be varied for variations in grid thickness or for variations in aperture diameter, or vice-versa. Most of these tradeoffs, where they do not relate to the present invention, are within the knowledge of those skilled in the art.

Claims (6)

1. An electron gun for use in a cathode ray tube, said gun including a beam forming region and a beam focusing region of which said beam forming region includes a cathode, a control grid adjacent to said cathode, and two screen grids, a first of said screen grids being adjacent to a side of said control grid opposite said cathode and a second screen grid being between said first screen grid and said beam focusing region, said second screen grid including a slot-shaped aperture aligned with an electron beam path, and said first and second screen grids each being electrically excited at different electrical potentials.

2. An electron gun as defined in claim 1, wherein said first screen grid includes a round aperture aligned with said beam path.

3. An electron gun as defined in claim 2, wherein said control grid is electrically grounded.

4. A cathode ray tube electron gun substantially as hereinbefore described with reference to Figures 2, 3 and 4 of the accompanying drawings.

5. A cathode ray tube having an electron gun as defined by any of claims 1-4.

6. A television receiver including a cathode ray tube as defined in claim 5.