GB2237680A

GB2237680A - Convergence correction in cathode ray tubes

Info

Publication number: GB2237680A
Application number: GB9021373A
Authority: GB
Inventors: Noboru Tominaga; Katsuyo Iwasaki; Koichi Sugahara; Toshio Kuramoto; Kenji Uozumi
Original assignee: Matsushita Electronics Corp
Current assignee: Panasonic Holdings Corp
Priority date: 1989-10-03
Filing date: 1990-10-02
Publication date: 1991-05-08
Anticipated expiration: 2010-10-02
Also published as: CN1018967B; CN1051640A; GB2237680B; JP2945688B2; JPH03120987A; GB9021373D0

Abstract

In a color cathode ray tube equipped with an in-line type electron gun and a deflection yoke designed to generate a uniform deflection magnetic field in which misconvergence is corrected by a dynamic convergence system, the in-line type electron gun has a focusing grid 15 which accommodates three pole pieces 17, 21, 22 of magnetic bodies, and a magnetic field generator 23, in the form of a ring shaped core 24 with four poles 24a, 24b, 24d, 24e each having coils 25a-f wound thereon, is disposed outside the cathode ray tube so as to apply 4-pole dynamic magnetic fields to the pole pieces, thus constituting a dynamic convergence system capable of correcting any misconvergence of the three electron beams in a vertical direction or a horizontal direction. As shown, the outer surfaces of the outer pole pieces 21, 22 are curved similar to the curved side of the oval hollow body of the focussing grid 15. The central pole piece 17 is a rectangular shaped body that is centred inside the hollow body so as to enclose the central electron beam 13 and the two outer electron beams 19, 20 pass between the spaced pole pieces. <IMAGE>

Description

BACKGROUND OF THE INVENTION 1. Field of the invention: The present invention relates generally to a color cathode ray tube, and more particularly to a color cathode ray tube equipped with an in-line type electron gun and a deflection yoke which is designed to generate a uniform deflection magnetic field, thus constituting a dynamic convergence system capable of correcting a misconvergence of three electron beams in a vertical direction or a horizontal direction.

2. Description of the prior art: In general, a deflection yoke used in color cathode ray tubes equipped with an in-line type electron gun produces a pin-cushion shape horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field. This type of deflection yoke is advantageous in eliminating the necessity of correcting deflections by use of a dynamic convergence system, thereby requiring no complex convergence circuit. However, the non-uniform deflection magnetic field is likely to distort the beam spots owing to deflection, thereby failing to achieve high resolution particularly in a peripheral area of a picture image.

It is known that a cathode ray tube equipped with a delta type electron gun is provided with a deflection yoke capable of generating a uniform magnetic field. To overcome the problem pointed out above, the same method as applied to a color cathode ray tube equipped with an in-line type electron gun should-be provided with such a deflection yoke so as to consti tute a dynamic convergence system capable of correcting a misconvergence of three electron beams.

Referring to Figure 5, Japanese Laid-Open Patent Publication No. 64-62993 discloses a color cathode ray tube having a convergence unit 1 provided at the head of an anode which acts as a final accelerating grid, and a pair of mutually opposing magnetic field generators 2 and 3 mounted on the periphery of a neck portion 4 of the cathode ray tube. The ccnver- gence unit 1 includes two pairs of magnetic pole pieces 7a, 7b and 8a, 8b in a cup-shaped housing of non-magnetic metal. The pole pieces 7a, 7b and 8a, 8b are disposed with electron beam paths 5 and 6 being interposed between each pair. In addition, a center beam path 9 is disposed between a pair of shield plates 9a and 9b.The magnetic field generators 2 and 3 include U-shaped cores 2a and 3a around which coils 2b and 3b are wound, and are supported so that their leg portions are magnetically connected to the outsides of the pole pieces 7a, 7b, and 8a, 8b, respectively.

The dynamic convergence correction is effected by passing a parabolic current or any other sawtooth-shaped waveform dynamic current through the coils 2b and 3b, wherein the dynamic current increases proportionally as the degree of beam deflection becomes large. The electron beams are deflected in the opposite direction, as indicated by the large arrows 10a and lOb, owing to the two-pole magnetic fields generated from the legs of the cores 2a and 3a to the pole pieces 7a, 7b and 8a, 8b.

An advantage of the dynamic convergence correction is that the corrected magnetic field is intensified by the two pairs of pole pieces, thereby ensuring an optimum convergence correction with high sensitivity. The disadvantage is that the dynamic convergence correction is only effected in a horizontal direction and cannot be effected in a vertical and a horizontal direction at the same time.

SUMMARY OF THE INVENTION The color cathode ray tube of the present invention, comprises an in-line type electron gun, a deflection yoke capable of generating a uniform magnetic field so as to correct a misconvergence of three electron beams by the dynamic convergence system, wherein the in-line type electron gun comprises a focusing grid with an oval hollow body having a horizontal major axis and a vertical minor axis, the focusing grid accommodating three pole pieces, the first of which is a rectangular shaped body made of a magnetic material that is centered inside the oval hollow body so as to enclose a central electron beam path, the second and third pole pieces being situated to the left and right but separated from the first pole piece to provide a second,side beam, path and a third,side beam, path therebetween, the second and third pole pieces each having an inside surface and an outside surface curved similar to the curved side of the oval hollow body of the focusing grid, a magnetic field generator disposed around the neck portion of the cathode ray tube, the magnetic field generator comprising a ring-shaped core having four poles each having coils, the ring-shaped core surrounding the neck portion having two poles diametrically situated opposed to the top and bottom sides of the first pole piece and the other diametrically situated two poles being opposed to the sides of the second and third pole piece so that a dynamic current can be made to flow through the coils.

In a preferred embodiment, the second pole piece and the third pole piece have a dome-shaped hollow body with a domed roof portion and a flat bottom portion.

In a preferred embodiment, the second pole piece and the third pole piece are disposed with their flat bottom portions being parallel to the respective sides cf the first rectangular pole piece.

Thus, the invention described herein makes possible the objectives of (1) providing a color cathode ray tube capable of effecting vertical dynamic convergence correction without negating the capability of effecting horizontal dynamic convergence correction by means of the conventional convergence unit, and (2) poviding a color cathode ray tube capable of achieving high resolution over picture images throughout the screen without the necessity of prolonging the length of an electron gun.

BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a vertical side view showing a main portion of a color cathode ray tube according to the present invention; Figure 2 is a perspective view showing a relationship among a focusing grid, and a first, a second and a third magnetic pole; Figures 3 and 4 are schematic views showing a magnetic field appearing in the horizontal cross-section of the cathode ray tube of the invention; and Figure 5 is a schematic view showing a magnetic field appearing in the horizontal cross-section of a known color cathode ray tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, a color cathode ray tube made of glass accommodates an in-line type electron gun 11 in its neck portion 4. The electron gun 11 includes three cathodes 12a, 12b, and 12c, a control grid 13, an accelerating grid 14, a focusing grid 15, and an anode 16 acting as a final accelerating grid.

The focusing grid 15 and the anode 16 have cylindrical bodies of metal whose horizontal axis is larger than its vertical axis. The anode 16 is provided with a convergence unit 1 at the head thereof, the convergence unit 1 having the same structure as that shown in Figure 5. More specifically, the convergence unit 1 has two pairs of pole pieces 7a, 7b, and 8a, 8b. Two pairs of two-pole magnetic field generators 2 and 3 are mounted around a periphery of the neck portion 4 at opposite positions, so that they are magnetically connected to the pole pieces 7a, 7b, and 8a, 8b of Ushaped cores 2 and 3 having leg portions around which coils 2a and 2b are wound.

As shown in Figures 2 and 3, the focusing grid 15 of non-magnetic metal has a hollow main body having an oval cross-section having a horizontal major axis and a vertical minor axis, accommodating three pole pieces 17, 21, and 22 therein. The first pole piece 17 is a rectangular shaped body of a magnetic material that is centered inside the main body in such a manner as to enclose an electron beam path 18. The second and third pole pieces 21 and 22 are situated on opposite sids of the first pole piece 17. Each pole piece 21 and 22 is dome-shaped with a curved roof portion and a flat bottom portion, wherein the bottom portion of each pole piece 21 and 22 are located parallel to the centered first pole piece 17.As shown in Figure 3, the electron beam side paths 19 and 20 are disposed between the first and second pole pieces 17 and 21, and between the first and third pole pieces 17 and 22.

As shown in Figure 3, a magnetic field generator 23 is disposed around the periphery of the neck portion 4. The magnetic field generator 23 includes a ring-shaped 4-pole core 24 of ferrite having magnetic poles 24a, 24b, 24c, and 24d, each of which has a coil 25a, 25b, 25c, and 25d. In addition, the poles .4c and 24d have other coil 25e and 25f. The poles 24a and 24b are magnetically connected to the top and bottom sides of the first pole piece 17, and the poles 24c and 24d are magnetically connected to the outsides of the flat bottom portions of the pole pieces 21 and 22, respectively.In Figure 1, the distance L between the centers of the magnetic field generators 2 (3) and 23 is predetermined to be 0.5 times or more the inside diameter D of the neck portion 4 so as to be out of retch of the magnetic field of each generator 2, 3, and 23.

Under the arrangement described above, when a 4-pole correction is to be effected, a dynamic current is flown through the coils 25a to 25d so that the poles 24a and 24b become positive and the poles 24c and 24d become negative, thereby effecting dynamic convergence. As a result, within the focusing grid 15 a magnetic field is generated as indicated by the dotted lines in Figure 3, and the electron beams at each side are vertically urged in opposite directions as indicated by the large arrows. The electron beams at the center are shielded from the magnetic field by the pole piece 17.

When a two-pole correction is to be effected, a dynamic current is allowed to flow through the coils 25e and 25f so that either pole 24c or 24d becomes negative and the other becomes positive, so as to generate a magnetic field in the focusing grid 15. As a result, the electron beams at each side are urged upwards or downwards as indicated by the large arrows.

As is evident from the foregoing, the present invention makes it possible to effect vertical dynamic convergence correction without negating the capability of effecting horizontal dynamic convergence correction by means of the conventional convergence unit. In addition, high resolution is achieved over picture images throughout the screen without the necessity of prolonging the length of an electron gun.

It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

Claims

1. A color cathode ray tube onprising an in-line type electron gun, a deflection yoke capable of generating a uniform magnetic field so as to correct misconvergence of three electron beams by the dynamic convergence system, wherein the in-line type electron gun comprises a focusing grid with an oval hollow body having a horizontal major axis and a vertical minor axis, the focusing grid accommodating three pole pieces, the first of which is a rectangular shaped body made of a magnetic material that is centered inside the oval hollow body so as to enclose a central electron beam path, the second and third pole pieces being situated to the left and right but separated from the first pole piece to provide a second, side beam, path and a third, side beam,path therebetween, the second and third pole pieces each having an inside surface an outside surface curved similar to the curved side of the oval hollow body of the focusing grid, a magnetic field generator disposed around the neck portion of the cathode ray tube, the magnetic field generator comprising a ring-shaped core having four poles each having coils, the ring-shaped core surrounding the neck portion having two poles diametrically situated opposed to the top and bottom sides of the first pole piece and the other diametrically situated two poles being opposed to the sides of the second and third pole piece so that a dynamic current can be made to flow through the coils.

2. A color cathode ray tube according to claim 1, wherein the second pole piece and the third pole piece have a dome-shaped hollow body with a domed roof portion and a flat bottom portion.

3. A color cathode ray tube according to claim 1 or 2, wherein the second pole piece and the third pole piece are disposed with their flat bottom portions parallel to the adjacent sides of the first rectangular pole piece.

4. A color cathode ray tube substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 4 of the accompanying drawings.