CA1108683A - Electron gun exhibiting reduced flare - Google Patents
Electron gun exhibiting reduced flareInfo
- Publication number
- CA1108683A CA1108683A CA313,898A CA313898A CA1108683A CA 1108683 A CA1108683 A CA 1108683A CA 313898 A CA313898 A CA 313898A CA 1108683 A CA1108683 A CA 1108683A
- Authority
- CA
- Canada
- Prior art keywords
- accelerating
- screen
- gun
- electron gun
- electron
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
- H01J29/46—Arrangements of electrodes and associated parts for generating or controlling the ray or beam, e.g. electron-optical arrangement
- H01J29/48—Electron guns
- H01J29/488—Schematic arrangements of the electrodes for beam forming; Place and form of the elecrodes
Abstract
RCA 72,430 ELECTRON GUN EXHIBITING REDUCED FLARE
Abstract A bipotential type of electron gun comprising,in the order named, a cathode aligned with a plurality of apertured electrodes including a control grid, a screen grid, a first accelerating and focusing electrode, and a second accelerating and focusing electrode. The first accelerating and focusing electrode comprises a hollow member with aligned apertures at opposite ends thereof. The hollow member comprises a first axial portion of magnetic material adjacent to the screen grid and a second axial portion of nonmagnetic material adjacent to the second accelerating and focusing electrode. The electron gun is especially adapted for use in a cathode ray tube operated with a magnetic deflection yoke which includes a pair of toroidal type vertical deflection coils which generate a magnetic fringe field within the region of the first accelerating and focusing electrode.
Abstract A bipotential type of electron gun comprising,in the order named, a cathode aligned with a plurality of apertured electrodes including a control grid, a screen grid, a first accelerating and focusing electrode, and a second accelerating and focusing electrode. The first accelerating and focusing electrode comprises a hollow member with aligned apertures at opposite ends thereof. The hollow member comprises a first axial portion of magnetic material adjacent to the screen grid and a second axial portion of nonmagnetic material adjacent to the second accelerating and focusing electrode. The electron gun is especially adapted for use in a cathode ray tube operated with a magnetic deflection yoke which includes a pair of toroidal type vertical deflection coils which generate a magnetic fringe field within the region of the first accelerating and focusing electrode.
Description
11~8683 RCA 72,430 This invention relates to cathode ray tubes using magnetic deflection yokesjand particularly to color picture tubes for television and to the electron guns therefor.
The invention is particularly applicable to color picture tubes of the shadow mask type having a line type color phosphor screen which is excited by an electron gun producing three co-planar electron beams. One design of electron gun especially useful in such tubes is disclosed in U. S. Patent 3,772,554, issued to ~lughes on November 13, 1973.
For illustrative purposes the present invention will be described as embodied in such a gun. However, the invention may be used in other types of electron guns for other types of cathode ray tubes.
One type of performance degradation encountered in cathode ray tubes is that referred to as flare. This term is used to describe the condition wherein the electron beam, instead of having a sharply defined cross-sectional boundary, has a main or core portion of high density and a peripheral portion of significantly less density. The result is an electron beam spot on the screen which appears out of focus and which produces images of poor definition.
Flare of the electron beam spot can be attributable to a variety of causes. For example, the yoke field itself used to scan the electron beam over the screen may be nonuniform and distort the cross-sectional shape of the beam.
Or, the apertures of the focusing electrodes may be out-of-round and thereby create lens aberrations which produce a distortion of the beam spot. Or, the electrode apertures in the beam-forming region of the gun can be misaligned and
The invention is particularly applicable to color picture tubes of the shadow mask type having a line type color phosphor screen which is excited by an electron gun producing three co-planar electron beams. One design of electron gun especially useful in such tubes is disclosed in U. S. Patent 3,772,554, issued to ~lughes on November 13, 1973.
For illustrative purposes the present invention will be described as embodied in such a gun. However, the invention may be used in other types of electron guns for other types of cathode ray tubes.
One type of performance degradation encountered in cathode ray tubes is that referred to as flare. This term is used to describe the condition wherein the electron beam, instead of having a sharply defined cross-sectional boundary, has a main or core portion of high density and a peripheral portion of significantly less density. The result is an electron beam spot on the screen which appears out of focus and which produces images of poor definition.
Flare of the electron beam spot can be attributable to a variety of causes. For example, the yoke field itself used to scan the electron beam over the screen may be nonuniform and distort the cross-sectional shape of the beam.
Or, the apertures of the focusing electrodes may be out-of-round and thereby create lens aberrations which produce a distortion of the beam spot. Or, the electrode apertures in the beam-forming region of the gun can be misaligned and
-2-RCA 72,430 thereby produce distortions of the beam spot.
We have discovered that flare may also result from - the electron beam being deflected off axis in the focus lens by stray magnetic fields, such that the beam passes through a peripheral portion of the lens where the electrostatic field lines are strongly curved, thus producing distortions ; in the beam spot on one side of the beam only. We have fur-ther discovered that such magnetic fields often occur as the backward fringing portions of the main deflection field from the deflection yoke itself, and that this is particularly true of yokes having toroidal windings which inherently generate a farther ranging fringe field than do, for example, saddle type yoke windings.
In accordance with the invention, an electron gun has a plurality of electrodes for producing an electro-static focusing lens, one of the electrodes having a portion thereof comprised of magnetic material to shield the electron beam passing therethrough from the fringe field of the deflection yoke. In a preferred embodiment, the electron gun is of the bipotential type comprising a cathode, a control grid, a screen grid, a first accelerating electrode, and a second accelerating electrode. The main focus lens is provided between the first and second accelerating electrodes. The first accelerating electrode has its rear half adjacent the 2 screen grid composed of magnetic material and its front half composed of nonmagnetic material.
In the drawings:
FIGURE l (Sheet l) is a plan view, partly in axial section, of a shadow mask color picture tube incorporating an example of the present invention;
RCA 72,430 36~33 r ~` 1 FIGURE 2 (Sheet 1) is a front end view of the tube of FIGURE 1 showing its rectangular shape;
~` FIGURE 3 (Sheet 2) is an axial section view of the electron gun shown in dotted lines in FIGURE l;
FIGURE 4 (She~t 2) is an axial section view of the :; electron gun taken along the line 4-4 of FIGURE 3; and FIGURE 5 (Sheet 1) is an axial section view of the first accelerating electrode of the electron gun, and schematically depicts hypothetical beam paths for the purpose of explaining the present invention.
FIGURE 1 is a plan view of, for example, a 17V-90 rectangula.r color picture tube having a glass envelope 1 ; made up of a rectangular faceplate panel or cap 3 and a tubular neck 5 connected by a rectangular funnel 7. The panel 3 comprises a viewing faceplate 9 and a peripheral flange or side wall 11 which is sealed to the funnel 7, as shown in FIGURE 2. A mosaic three-color phosphor screen 13 is carried by the inner surface of the faceplate 9. The screen is preferably a line screen with the phosphor lines extending substantially parallel to the minor axis Y-Y of the tube (normal to the plane of FIGURE 1). A multi-apertured color selection electrode or shadow mask 15 is removably mounted, by conventional means, in predetermined spaced relation to the screen 13. An improved bipotential type in-line electron gun 19, shown schematically by dotted lines in FIGURE 1, iS centrally mounted within the neck 5 to generate and direct three electron beams 20 along co-planar convergent paths through the mask 15 to the screen 13.
The tube of FIGURE 1 is designed to be used with an RCA 72,430 6~3 - ..
1 external magnetic deflection yoke, such as the yoke 21 schematically shown surrounding the neck 5 and funnel 7, in ` the neighborhood of their junction. The yoke 21 subjects the three beams 20 to vertical and horizontal magnetic flux, to scan theb-eams horiæontally and vertically in a rectangular raster over the screen 13. The initial plane of deflection (at zero deflection) is shown by the line P-P in FIGURE 1 at about the middle of the yoke 21. The yoke 21 is preferably a self-converging yoke having a pair of toroidal type vertical ; 10 deflection coils and a pair of saddle-type horizontal deflection coils. Because of fringe fields, the zone of deflection of the tube extends axially, from the yoke 21, into the region of the gun 19. Since toroidal coils produce relatively severe fringing fields, the vertical deflection field is especially strong in the gun region. For simplicity, the actual curvature of the deflected beams 20 in the deflection zone is not shown in FIGURE 1.
The in-line gun 19 is designed to generate and direct three equally-spaced co-planar beams along initially-parallel paths to a convergence plane C-C, and then along convergent paths through the deflection plane to the screen 13.
The details of the improved gun 19 are shown in FIGURES 3 and 4. The gun comprises two glass support rods 23 on which the various electrodes are mounted in alignment.
These electrodes comprise three equally-spaced co-planar cathodes 25, one for each beam, and apertured electrodes including a control grid electrode 27, a screen grid electrode 29, a first accelerating and focusing electrode 31, a second accelerating and focusing electrode 33, and a shield cup 35, spaced along the glass rods 23 in the order named.
RCA 72,320 ~08683 .
1 Each cathode 25 comprises a cathode sleeve 37, closed at the forward end by a cap 39 having an end coating 41 of electron emissive material,and a cathode support tube 43. The tubes 43 are supported on the rods 23 by four straps 45 and 47. Each cathode 25 is indirectly heated by a heater coil 49 positioned within the sleeve 37 and having legs 51 welded to heater straps 53 and 55 mounted by studs 57 on the rods 23. The control and screen grid electrodes 27 and 29 are two closely-spaced flat plates having three pairs of small aligned apertures 59 centered with the cathode coatings 41 to initiate three equally spaced co-planar beams along paths, including a central path 20a and two outer paths 20b, extending toward the screen 13. Prefereably, the paths 20a and 20b initially are substantially parallel with the middle path 20a coincident with the central axis A-A.
The first accelerating and focusing electrode 31 comprises a hollow member with aligned apertures at opposite ends thereof,and includes first and second cup-shaped members 61 and 63, respectively, joined together at their open ends.
The first cup-shaped member 61 has three medium-sized apertures 65 close to grid electrode 29 and aligned respectively with the three beam paths 20a and 20b, as shown in FIGURE 4. The second cup-shaped member 63 has three large apertures 67 also aligned with the three beam paths. As hereinafter described in greater detail, the first cup-shaped member 61 is made of magnetic material and is substantially non-overlapping with the second cup-shaped member 63 which is made of non-magnetic material.
The second accelerating and focusing electrode 33 is also cup-shaped and comprises a base plate portion 69 ~ RCA 72,430 . ~
~ 8683 ':
,:
1 positioned close to electrode 31,and a side wall or flange 71 extending forward toward the tube screen. The base portion 69 is formed with three apertures 73, which are preferably slightly larger than the adjacent apertures 67 of electrode 31. The middle aperture 73a is aligned with the adjacent middle aperture 67a (and middle heam path 20a) to provide a substantially symmetrical beam focusing electric field between apertures 67a and 73a when electrodes 31 and 33 are energized at different voltages. The two outer apertures 73b are sllghtly offset outwardly with respect to the corresponding outer apertures 67b, to provide an asymmetrical electric field between each pair of outer apertures when ~-electrodes 31 and 33 are energized, to individually focus each outer beam 20b near the screen and also to deflect each beam, toward the middle beam, to a common point of convergence with the middle beam near the screen.
The focusing apertures 67 and 73 are made as large as possible to minimize spherical aberration, and as close together as possible to obtain a desirable small spacing between beam paths. As a result, the fringe portions of adjacent fields interact to produce some astigmatic distortion of the focusing fields, which produces some ellipticity of the normally-circular focused beam spots on the screen. In a three-beam in-line gun, this distortion is greater for the middle beam than for the two outer beams, because both sides of the middle beam field are affected. In order to compensate for this effect, and minimize the elliptical distortion of the beam spots, the wall 69, or at least the surface thereof facing the electrode 31, is curved substantially cylindrically concave to electrode 31, in the direction RCA 73,430 1~8683 ,~:
1 normal to the plane of the three beams, as shown at 79 in FIGURE 3. Preferably, this curvature is greater for the middle beam path than for the outer beam pathsi hence, the wall 69 may be made barrel-shaped, with the curvature 79 terminating at the outer edges of the outer apertures 73b.
The shield cup 35 comprises a base portion 81 attached to tne open end of the flange 71 of electrode 33, and a tubular wall 83 surrounding the three beam paths 20.
The base portion 81 is formed with a large middle beam aperture 85 and two smaller outer beam apertures 87 aligned, respectively, with the three initial beam paths 20a and 20b.
In order to compensate for the distortion wherein the sizes of the rasters scanned on the screen by the external magnetic deflection yoke are different for the middle and outer beams of the three-beam gun, due to the eccentricity of the outer beams in the yoke field, the electron gun is provided with two shield rings 89 of high magnetic permeability (e.g.!an alloy of 52 percent nickel and 48 percent iron, known as "52 metal"~ which are attached to the base 81 with each ring concentrically surrounding one of the outer apertures 87. These magnetic shields 89 by-pass a small portion of the fringe deflection fields in the path of the outer beams, thereby making a slight reduction in the rasters scanned by the outer beams on the screen.
A further correction for this coma distortion is made by mounting two small discs 91 of magnetic material (e.g., that referred to above~ on each side of the middle beam path 20a. These discs 91 enhance the magnetic flux on the middle ; beam transverse to the plane of the three beamsfand decrease the flux in that plane.
RCA 72,430 1~8683 .
1 Each of the electrodes 27, 29, 31 and 33 is mounted on the two glass rods 23 by edge portions embedded in the glass. The two rods 23 extend forwardly beyond the mounting portion of electrode 33, as shown in FIGURE 3. In order to shield the exposed ends 93 of the glass rods 23 from the electron heams, the shield cup 35 is formed with inwardly-extending recess portions 95 into which the rod ends 93 extend. The electron gun 19 is mounted in the neck 5 at one end, via the leads (not shown) from the various electrodes to the stem terminals 97& and at the other end by conventional metal bulb spacers (not shown) which also connect the final electrode 33 to the usual conducting coating on the inner wall of the funnel 7.
FIGURE S showns the first accelerating and focusing electrode 31 together with a schematic representation of the equipotential lines of the focus lens 101 which exist under normal operation of the tube 1. As shown, the focusing lens field dips into the aperture 67,and equipotential lines cross the intended beam path 20a in a perpendicular relationship 20 thereto and then curve sharply so that off axis the .!
equipotential lines 101 are sharply sloped.
In normal operation of the tube, the vertical deflection flux from the yoke 21 fringes rearwardly into the region of the first accelerating and focusing electrode 31.
If the electrode 31 is of nonmagnetic material so as not to shunt the magnetic flux, as was the case with prior art electron guns, the flux will cause the electron beam passing through the electrode 31 to be deflected off axis. This is shown schematically by the exaggerated hypothetical beam path 30 103. As shown, the path 103 will begin to RCA 72,430 36~3 1 depart from the intended beam path 20a upon entry into the electrode 31 and will follow an arcuate path in an ever increasing deflection away from the axial path 20a. If the first cup member 61 of the electrode 31 is made of magnetic material, asdisclosed herein, the fringing flux from the deflection yoke 21 will be shunted around the beam path thus shielding the beam within the first cup 61 from the flux. As a result, the electron beam path will continue along its intended axial direction 20a throughout the region of the first cup member 61 and will begin to depart from that intended path along the schematically shown path 105 only in the region of the second cup member 63. Since the deflected beam paths 103 and 105 conform substantially to arcs of a circle, the amount of total deflection by the time the beam reaches the aperture 15 67 will be significantly greater in the case of the path 103 than with the path 105. Thus, making only the first cup member 61 of magnetic material will substantially reduce the deflection which the fringing flux field from the yoke 21 would otherwise produce.
Although a complete avoidance of a deflected beam path in the electrode 31 can be obtained by making both cup members 61 and 63 of magnetic material, to do so causes an undesirable loss of deflection sensitivity by unnecessarily shunting too much of the deflection field. ~e have found that to make only the first cup member of magnetic material produces a loss of only about 2% in the total strength of the deflection field from the yoke 21, whereas to make the second cup 63 as well of magnetic material would increase this loss to an excessive undesirable amount. Thus, since undesirable beam deflection is not linearly related to the amount of `
`:
RCA 72,430 ~ 3683 1 shielding, optimum conditions are achieved when only the first cup member 61 is made of magnetic material.
While the invention has been shown as embodied ~r a~
electron gun in which the first cup member 61 of a two_piece first accelerating and focusing electrode 31 is made of magnetic material, equivalent results can be achieved by making the entire electrode 31 of nonmagnetic material and providing a separate insert or surrounding sleeve (not shown) of magnetic material which is disposed within or around the first cup member 61. Reference herein to an electrode being of magnetic material is intended to cover the functionally equivalent structure of a nonma~netic electrode with a magnetic part attached thereto. Also, although the two cup members 61 and 63 are shown as substantially equal in axial length, this is not essential to the practice of the invention. The critical feature of the invention is that a rearward portion only of the electrode 31 be magnetic and that the electrode not be substantially all magnetic. It thus follows that the two cups 61 and 63 of the novel electrode should not be such that they axially overlap so as to effectively render the whole, or substantially whole, ~ electrode magnetic along the beam path. It is preferred that ; the two cups 61 and 63 be totally non-overlapping as shown in the drawings.
In the embodiment of the invention illustrated in the accompanying drawings the electron gun 19 may have as typical dimensions those set forth in the Table below.
However, these dimensions are not critical to a practice of 30 the invention. In some electron gun designs intended RCA 73,430 o ~:
1 as substitutes for the gun 19, the first accelerating and . .
focusing electrode is made to be significantly longer than that shown herein. Such an electrode may in fact exhibit a more severe problem of flare due to the presence of fringing flux from the deflection yoke 21 working on the beam over a greater distance of beam travel.
Table .
Dimension Inches Millimeters .
Aperture 65 0.060 1.524 10 APerture 67 0.160 4.064 Aperture 73 0.172 4.369 Axial length of electrode 310.424 10.770 Width of electrode 31 0,790 20.066 Height of electrode 31 0.380 9.652 15 Spacing between electrodes 31 and 330.060 1.524 Spacing between electron beams 0.200 5.080 Radius Rl 8.00 203.200 Radius R2 2.28 57.912 .
Volts 20 Operating potential cathode 25 0 to 100+
Operating potential electrode 27 0 Operating potential electrode 29 370 Operating potential electrode 31 4800 Operating potential 33 25000 ~ -
We have discovered that flare may also result from - the electron beam being deflected off axis in the focus lens by stray magnetic fields, such that the beam passes through a peripheral portion of the lens where the electrostatic field lines are strongly curved, thus producing distortions ; in the beam spot on one side of the beam only. We have fur-ther discovered that such magnetic fields often occur as the backward fringing portions of the main deflection field from the deflection yoke itself, and that this is particularly true of yokes having toroidal windings which inherently generate a farther ranging fringe field than do, for example, saddle type yoke windings.
In accordance with the invention, an electron gun has a plurality of electrodes for producing an electro-static focusing lens, one of the electrodes having a portion thereof comprised of magnetic material to shield the electron beam passing therethrough from the fringe field of the deflection yoke. In a preferred embodiment, the electron gun is of the bipotential type comprising a cathode, a control grid, a screen grid, a first accelerating electrode, and a second accelerating electrode. The main focus lens is provided between the first and second accelerating electrodes. The first accelerating electrode has its rear half adjacent the 2 screen grid composed of magnetic material and its front half composed of nonmagnetic material.
In the drawings:
FIGURE l (Sheet l) is a plan view, partly in axial section, of a shadow mask color picture tube incorporating an example of the present invention;
RCA 72,430 36~33 r ~` 1 FIGURE 2 (Sheet 1) is a front end view of the tube of FIGURE 1 showing its rectangular shape;
~` FIGURE 3 (Sheet 2) is an axial section view of the electron gun shown in dotted lines in FIGURE l;
FIGURE 4 (She~t 2) is an axial section view of the :; electron gun taken along the line 4-4 of FIGURE 3; and FIGURE 5 (Sheet 1) is an axial section view of the first accelerating electrode of the electron gun, and schematically depicts hypothetical beam paths for the purpose of explaining the present invention.
FIGURE 1 is a plan view of, for example, a 17V-90 rectangula.r color picture tube having a glass envelope 1 ; made up of a rectangular faceplate panel or cap 3 and a tubular neck 5 connected by a rectangular funnel 7. The panel 3 comprises a viewing faceplate 9 and a peripheral flange or side wall 11 which is sealed to the funnel 7, as shown in FIGURE 2. A mosaic three-color phosphor screen 13 is carried by the inner surface of the faceplate 9. The screen is preferably a line screen with the phosphor lines extending substantially parallel to the minor axis Y-Y of the tube (normal to the plane of FIGURE 1). A multi-apertured color selection electrode or shadow mask 15 is removably mounted, by conventional means, in predetermined spaced relation to the screen 13. An improved bipotential type in-line electron gun 19, shown schematically by dotted lines in FIGURE 1, iS centrally mounted within the neck 5 to generate and direct three electron beams 20 along co-planar convergent paths through the mask 15 to the screen 13.
The tube of FIGURE 1 is designed to be used with an RCA 72,430 6~3 - ..
1 external magnetic deflection yoke, such as the yoke 21 schematically shown surrounding the neck 5 and funnel 7, in ` the neighborhood of their junction. The yoke 21 subjects the three beams 20 to vertical and horizontal magnetic flux, to scan theb-eams horiæontally and vertically in a rectangular raster over the screen 13. The initial plane of deflection (at zero deflection) is shown by the line P-P in FIGURE 1 at about the middle of the yoke 21. The yoke 21 is preferably a self-converging yoke having a pair of toroidal type vertical ; 10 deflection coils and a pair of saddle-type horizontal deflection coils. Because of fringe fields, the zone of deflection of the tube extends axially, from the yoke 21, into the region of the gun 19. Since toroidal coils produce relatively severe fringing fields, the vertical deflection field is especially strong in the gun region. For simplicity, the actual curvature of the deflected beams 20 in the deflection zone is not shown in FIGURE 1.
The in-line gun 19 is designed to generate and direct three equally-spaced co-planar beams along initially-parallel paths to a convergence plane C-C, and then along convergent paths through the deflection plane to the screen 13.
The details of the improved gun 19 are shown in FIGURES 3 and 4. The gun comprises two glass support rods 23 on which the various electrodes are mounted in alignment.
These electrodes comprise three equally-spaced co-planar cathodes 25, one for each beam, and apertured electrodes including a control grid electrode 27, a screen grid electrode 29, a first accelerating and focusing electrode 31, a second accelerating and focusing electrode 33, and a shield cup 35, spaced along the glass rods 23 in the order named.
RCA 72,320 ~08683 .
1 Each cathode 25 comprises a cathode sleeve 37, closed at the forward end by a cap 39 having an end coating 41 of electron emissive material,and a cathode support tube 43. The tubes 43 are supported on the rods 23 by four straps 45 and 47. Each cathode 25 is indirectly heated by a heater coil 49 positioned within the sleeve 37 and having legs 51 welded to heater straps 53 and 55 mounted by studs 57 on the rods 23. The control and screen grid electrodes 27 and 29 are two closely-spaced flat plates having three pairs of small aligned apertures 59 centered with the cathode coatings 41 to initiate three equally spaced co-planar beams along paths, including a central path 20a and two outer paths 20b, extending toward the screen 13. Prefereably, the paths 20a and 20b initially are substantially parallel with the middle path 20a coincident with the central axis A-A.
The first accelerating and focusing electrode 31 comprises a hollow member with aligned apertures at opposite ends thereof,and includes first and second cup-shaped members 61 and 63, respectively, joined together at their open ends.
The first cup-shaped member 61 has three medium-sized apertures 65 close to grid electrode 29 and aligned respectively with the three beam paths 20a and 20b, as shown in FIGURE 4. The second cup-shaped member 63 has three large apertures 67 also aligned with the three beam paths. As hereinafter described in greater detail, the first cup-shaped member 61 is made of magnetic material and is substantially non-overlapping with the second cup-shaped member 63 which is made of non-magnetic material.
The second accelerating and focusing electrode 33 is also cup-shaped and comprises a base plate portion 69 ~ RCA 72,430 . ~
~ 8683 ':
,:
1 positioned close to electrode 31,and a side wall or flange 71 extending forward toward the tube screen. The base portion 69 is formed with three apertures 73, which are preferably slightly larger than the adjacent apertures 67 of electrode 31. The middle aperture 73a is aligned with the adjacent middle aperture 67a (and middle heam path 20a) to provide a substantially symmetrical beam focusing electric field between apertures 67a and 73a when electrodes 31 and 33 are energized at different voltages. The two outer apertures 73b are sllghtly offset outwardly with respect to the corresponding outer apertures 67b, to provide an asymmetrical electric field between each pair of outer apertures when ~-electrodes 31 and 33 are energized, to individually focus each outer beam 20b near the screen and also to deflect each beam, toward the middle beam, to a common point of convergence with the middle beam near the screen.
The focusing apertures 67 and 73 are made as large as possible to minimize spherical aberration, and as close together as possible to obtain a desirable small spacing between beam paths. As a result, the fringe portions of adjacent fields interact to produce some astigmatic distortion of the focusing fields, which produces some ellipticity of the normally-circular focused beam spots on the screen. In a three-beam in-line gun, this distortion is greater for the middle beam than for the two outer beams, because both sides of the middle beam field are affected. In order to compensate for this effect, and minimize the elliptical distortion of the beam spots, the wall 69, or at least the surface thereof facing the electrode 31, is curved substantially cylindrically concave to electrode 31, in the direction RCA 73,430 1~8683 ,~:
1 normal to the plane of the three beams, as shown at 79 in FIGURE 3. Preferably, this curvature is greater for the middle beam path than for the outer beam pathsi hence, the wall 69 may be made barrel-shaped, with the curvature 79 terminating at the outer edges of the outer apertures 73b.
The shield cup 35 comprises a base portion 81 attached to tne open end of the flange 71 of electrode 33, and a tubular wall 83 surrounding the three beam paths 20.
The base portion 81 is formed with a large middle beam aperture 85 and two smaller outer beam apertures 87 aligned, respectively, with the three initial beam paths 20a and 20b.
In order to compensate for the distortion wherein the sizes of the rasters scanned on the screen by the external magnetic deflection yoke are different for the middle and outer beams of the three-beam gun, due to the eccentricity of the outer beams in the yoke field, the electron gun is provided with two shield rings 89 of high magnetic permeability (e.g.!an alloy of 52 percent nickel and 48 percent iron, known as "52 metal"~ which are attached to the base 81 with each ring concentrically surrounding one of the outer apertures 87. These magnetic shields 89 by-pass a small portion of the fringe deflection fields in the path of the outer beams, thereby making a slight reduction in the rasters scanned by the outer beams on the screen.
A further correction for this coma distortion is made by mounting two small discs 91 of magnetic material (e.g., that referred to above~ on each side of the middle beam path 20a. These discs 91 enhance the magnetic flux on the middle ; beam transverse to the plane of the three beamsfand decrease the flux in that plane.
RCA 72,430 1~8683 .
1 Each of the electrodes 27, 29, 31 and 33 is mounted on the two glass rods 23 by edge portions embedded in the glass. The two rods 23 extend forwardly beyond the mounting portion of electrode 33, as shown in FIGURE 3. In order to shield the exposed ends 93 of the glass rods 23 from the electron heams, the shield cup 35 is formed with inwardly-extending recess portions 95 into which the rod ends 93 extend. The electron gun 19 is mounted in the neck 5 at one end, via the leads (not shown) from the various electrodes to the stem terminals 97& and at the other end by conventional metal bulb spacers (not shown) which also connect the final electrode 33 to the usual conducting coating on the inner wall of the funnel 7.
FIGURE S showns the first accelerating and focusing electrode 31 together with a schematic representation of the equipotential lines of the focus lens 101 which exist under normal operation of the tube 1. As shown, the focusing lens field dips into the aperture 67,and equipotential lines cross the intended beam path 20a in a perpendicular relationship 20 thereto and then curve sharply so that off axis the .!
equipotential lines 101 are sharply sloped.
In normal operation of the tube, the vertical deflection flux from the yoke 21 fringes rearwardly into the region of the first accelerating and focusing electrode 31.
If the electrode 31 is of nonmagnetic material so as not to shunt the magnetic flux, as was the case with prior art electron guns, the flux will cause the electron beam passing through the electrode 31 to be deflected off axis. This is shown schematically by the exaggerated hypothetical beam path 30 103. As shown, the path 103 will begin to RCA 72,430 36~3 1 depart from the intended beam path 20a upon entry into the electrode 31 and will follow an arcuate path in an ever increasing deflection away from the axial path 20a. If the first cup member 61 of the electrode 31 is made of magnetic material, asdisclosed herein, the fringing flux from the deflection yoke 21 will be shunted around the beam path thus shielding the beam within the first cup 61 from the flux. As a result, the electron beam path will continue along its intended axial direction 20a throughout the region of the first cup member 61 and will begin to depart from that intended path along the schematically shown path 105 only in the region of the second cup member 63. Since the deflected beam paths 103 and 105 conform substantially to arcs of a circle, the amount of total deflection by the time the beam reaches the aperture 15 67 will be significantly greater in the case of the path 103 than with the path 105. Thus, making only the first cup member 61 of magnetic material will substantially reduce the deflection which the fringing flux field from the yoke 21 would otherwise produce.
Although a complete avoidance of a deflected beam path in the electrode 31 can be obtained by making both cup members 61 and 63 of magnetic material, to do so causes an undesirable loss of deflection sensitivity by unnecessarily shunting too much of the deflection field. ~e have found that to make only the first cup member of magnetic material produces a loss of only about 2% in the total strength of the deflection field from the yoke 21, whereas to make the second cup 63 as well of magnetic material would increase this loss to an excessive undesirable amount. Thus, since undesirable beam deflection is not linearly related to the amount of `
`:
RCA 72,430 ~ 3683 1 shielding, optimum conditions are achieved when only the first cup member 61 is made of magnetic material.
While the invention has been shown as embodied ~r a~
electron gun in which the first cup member 61 of a two_piece first accelerating and focusing electrode 31 is made of magnetic material, equivalent results can be achieved by making the entire electrode 31 of nonmagnetic material and providing a separate insert or surrounding sleeve (not shown) of magnetic material which is disposed within or around the first cup member 61. Reference herein to an electrode being of magnetic material is intended to cover the functionally equivalent structure of a nonma~netic electrode with a magnetic part attached thereto. Also, although the two cup members 61 and 63 are shown as substantially equal in axial length, this is not essential to the practice of the invention. The critical feature of the invention is that a rearward portion only of the electrode 31 be magnetic and that the electrode not be substantially all magnetic. It thus follows that the two cups 61 and 63 of the novel electrode should not be such that they axially overlap so as to effectively render the whole, or substantially whole, ~ electrode magnetic along the beam path. It is preferred that ; the two cups 61 and 63 be totally non-overlapping as shown in the drawings.
In the embodiment of the invention illustrated in the accompanying drawings the electron gun 19 may have as typical dimensions those set forth in the Table below.
However, these dimensions are not critical to a practice of 30 the invention. In some electron gun designs intended RCA 73,430 o ~:
1 as substitutes for the gun 19, the first accelerating and . .
focusing electrode is made to be significantly longer than that shown herein. Such an electrode may in fact exhibit a more severe problem of flare due to the presence of fringing flux from the deflection yoke 21 working on the beam over a greater distance of beam travel.
Table .
Dimension Inches Millimeters .
Aperture 65 0.060 1.524 10 APerture 67 0.160 4.064 Aperture 73 0.172 4.369 Axial length of electrode 310.424 10.770 Width of electrode 31 0,790 20.066 Height of electrode 31 0.380 9.652 15 Spacing between electrodes 31 and 330.060 1.524 Spacing between electron beams 0.200 5.080 Radius Rl 8.00 203.200 Radius R2 2.28 57.912 .
Volts 20 Operating potential cathode 25 0 to 100+
Operating potential electrode 27 0 Operating potential electrode 29 370 Operating potential electrode 31 4800 Operating potential 33 25000 ~ -
Claims (7)
1. An electron gun for incorporation in a cathode ray tube adapted to be operated with a magnetic deflection yoke that produces a magnetic deflection field which fringes into the region of said gun, said gun comprising a plurality of electrodes having aligned apertures through which an electron beam of said gun passes to a screen, one of said electrodes comprising a hollow member having aligned apertures at the opposite ends thereof through which said electron beam passes, said member comprising a first tubular portion more remote from said screen consisting of magnetic material and a second tubular portion nearer to said screen consisting of nonmagnetic material, whereby said beam is shielded from said magnetic fringe field within said first portion but not within said second portion.
2. The electron gun of claim 1 wherein said first and second portions are substantially non-overlapping.
3. The electron gun of claim 1 wherein said first and second portions are of approximately equal lengths along the path of said electron beams.
4. The electron gun of claim 1 wherein said electron gun comprises, in the order named, a cathode, control grid, screen grid, first accelerating and focusing electrode and second accelerating and focusing electrode and wherein said hollow electrode is said first accelerating and focusing electrode.
RCA 72,430
RCA 72,430
5. The electron gun of claim 4 wherein said gun is of the bipotential type adapted to have its main focus lens established between the adjacent ends of said first and second accelerating and focusing electrodes.
6. In combination (a) a cathode ray tube comprising a mosaic phosphor screen including an array of different color-emitting phosphor elements and an electron gun which projects a plurality of electron beams towards said screen through a deflection zone and (b) a deflection yoke which generates horizontal and vertical magnetic deflection fields in said zone, said yoke including a pair of coils which produces a fringe field that extends into the region of said electron gun; said gun comprising, in the order named, a cathode, a control grid, a screen grid, a first accelerating and focusing electrode, and a second accelerating and focus-ing electrode, said accelerating and focusing electrodes being adapted to have a main beam focusing lens field estab-lished therebetween, said first accelerating and focusing electrode comprising a hollow member with a first beam passage aperture therein adjacent to said screen grid and a second beam passage aperture therein adjacent to said second accelerating and focusing electrode, said hollow member com-prising a first portion of magnetic material adjacent to said screen grid and a second portion of nonmagnetic material ad-jacent to said second accelerating and focusing electrode, whereby said fringe field of said yoke is shunted from within said first portion but not from within said second portion and said electron beam is shielded from said fringe field as it passes RCA 72,430
6. In combination (a) a cathode ray tube comprising a mosaic phosphor screen including an array of different color-emitting phosphor elements and an electron gun which projects a plurality of electron beams towards said screen through a deflection zone and (b) a deflection yoke which generates horizontal and vertical magnetic deflection fields in said zone, said yoke including a pair of coils which produces a fringe field that extends into the region of said electron gun; said gun comprising, in the order named, a cathode, a control grid, a screen grid, a first accelerating and focusing electrode, and a second accelerating and focus-ing electrode, said accelerating and focusing electrodes being adapted to have a main beam focusing lens field estab-lished therebetween, said first accelerating and focusing electrode comprising a hollow member with a first beam passage aperture therein adjacent to said screen grid and a second beam passage aperture therein adjacent to said second accelerating and focusing electrode, said hollow member com-prising a first portion of magnetic material adjacent to said screen grid and a second portion of nonmagnetic material ad-jacent to said second accelerating and focusing electrode, whereby said fringe field of said yoke is shunted from within said first portion but not from within said second portion and said electron beam is shielded from said fringe field as it passes RCA 72,430
Claim 6 continued through said first portion thereby reducing undesirable deflection of said beam at said main beam focusing lens to thereby reduce flare of the beam spot produced by the impingement of said beam on said screen.
7. The combination of claim 6 wherein said phos-phor elements are lines, said electron beams are co-planar, and said pair of deflection coils are vertical deflection coils of the toroidal type.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US85245077A | 1977-11-17 | 1977-11-17 | |
| US852,450 | 1977-11-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1108683A true CA1108683A (en) | 1981-09-08 |
Family
ID=25313363
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA313,898A Expired CA1108683A (en) | 1977-11-17 | 1978-10-23 | Electron gun exhibiting reduced flare |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1108683A (en) |
| FR (1) | FR2409595A1 (en) |
| IT (1) | IT1099542B (en) |
| MX (1) | MX148052A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4952186A (en) * | 1989-10-24 | 1990-08-28 | Rca Licensing Corporation | Method of making a color picture tube electron gun with reduced convergence drift |
| US5010271A (en) * | 1989-10-24 | 1991-04-23 | Rca Licensing Corporation | Color picture tube having an electron gun with reduced convergence drift |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0821338B2 (en) * | 1987-01-26 | 1996-03-04 | 株式会社日立製作所 | Electron gun for color picture tube |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3020434A (en) * | 1958-12-08 | 1962-02-06 | Philco Corp | Self shielding electron gun and cathode ray tube system including same |
| US3047758A (en) * | 1959-12-01 | 1962-07-31 | Machlett Laboraotries Inc | Cathode ray tubes |
| US3873879A (en) * | 1972-01-14 | 1975-03-25 | Rca Corp | In-line electron gun |
-
1978
- 1978-10-23 CA CA313,898A patent/CA1108683A/en not_active Expired
- 1978-10-26 IT IT29155/78A patent/IT1099542B/en active
- 1978-10-27 MX MX175402A patent/MX148052A/en unknown
- 1978-11-16 FR FR7832333A patent/FR2409595A1/en active Granted
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4952186A (en) * | 1989-10-24 | 1990-08-28 | Rca Licensing Corporation | Method of making a color picture tube electron gun with reduced convergence drift |
| US5010271A (en) * | 1989-10-24 | 1991-04-23 | Rca Licensing Corporation | Color picture tube having an electron gun with reduced convergence drift |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2409595A1 (en) | 1979-06-15 |
| MX148052A (en) | 1983-03-09 |
| IT7829155A0 (en) | 1978-10-26 |
| FR2409595B1 (en) | 1984-02-10 |
| IT1099542B (en) | 1985-09-18 |
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