GB2046511A

GB2046511A - Electron gun having a low capacitance cathode and grid assembly

Info

Publication number: GB2046511A
Application number: GB8002559A
Authority: GB
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1979-04-09
Filing date: 1980-01-25
Publication date: 1980-11-12
Also published as: FR2454177A1; DE3010807A1; JPS55139745A; NL8000814A; FR2454177B1; DE3010807C2; CA1149003A; GB2046511B; JPS5919620B2

Abstract

A cathode 52 and grid assembly 20, 36 includes a first grid 20 secured to a first plane surface 16 of a ceramic annulus 14 to extend over the central aperture 18 of the latter, and a second grid having an outer plane section 34 secured to a second plane surface 28 of the annulus, and an inner plane section 40 held within the central aperture, and parallel to the first grid, by a frusto-conical intermediate grid portion 38. The beam passage apertures 22 and 40 of the two grids may be formed after their assembly on the ceramic annulus. <IMAGE>

Description

SPECIFICATION Electron gun having a low capacitance cathode and grid assembly In high frequency electron discharge devices such as cathode ray tubes, the capacitance between the control grid and the accelerating grid of an electron gun must present a small area to the other electrodes in the electron gun in orderto reduce the input capacitance to enable the high frequency signal to be impressed on the control grid for high frequency operation.

U.S. Patent No.2,458,962 disclosed a grid assembly for cathode ray tubes which comprises first and second grid members in the form of a frustum of a cone. The first grid member is closely positioned next to the cathode and the second grid member has an inner section disposed within the first grid member but spaced therefrom. This arrangement presents a large area thereby increasing the capacitance and is not suitable for high frequency cathode ray tubes.

The grid assembly in U.S. Patent No. 2,540,621 is of similar construction to that of U.S. Patent No.

2,458,962 except that the first grid member does not have a frustum of a cone configuration; however, the metal areas of the first and second grid members opposite each other present a large area such that increased capacitance results thereby rendering this grid assembly unsuitable for high frequency operation.

The grid assembly in U.S. Patent No.2,443,916 includes a small first grid member which is welded to an eyelet that secu res the fi rst grid mem ber to a ceramic mounting member onto which the second grid member is also mounted and spaced from the first grid member. This arrangement provides a large surface area that results in a large input capacitance which is a serious disadvantage to high frequency operation.

The present invention relates to electron discharge devices and more particularly to an electron gun structure having an improved low capacitance grid assembly for high frequency cathode ray tubes.

The present invention is realized by securing a small diameter control grid to an inner planar surface of an annular ceramic member and an outer planar section of a focussing grid to an outer planar surface of the ceramic member. Another outer surface in the form of a frustum of a cone is provided as part of the ceramic member and the accelerating grid has an inner planar section disposed adjacent to the control grid with the outer and inner planar sections of the accelerating grid being connected by a frustum of a cone section extending along the frustum of cone surface of the ceramic member. The area presented by this grid assembly results in a small input capacitance to enable high frequency signals to be impressed onto the control grid for high frequency operation.The spacing and alignment of the grids relative to each other and to the cathode is very accurate and the apertures in the grids are formed therein after the grids have been secured to the ceramic member so that their align- ment is very accurate. The cathode and grid assembly is mounted via the accelerating grid being secured to a mounting wafer.

An object of the present invention is to provide a low capacitance grid assembly as part of an electron gun for a cathode ray tube.

Another object of the present invention is the provision of a low capacitance grid assembly which presents a small area to reduce the input capacitance.

A further object of the present invention is to provide a low capacitance grid assembly wherein the grid members are accurately positioned relative to each other.

An additional object of the present invention is the provision of a cathode and grid assembly wherein the grid members are accurately spaced and aligned relative to each other and the cathode is accurately positioned and aligned relative to the grid members.

A still further object of the present invention is to provide a lów capacitance cathode and grid assembly which is mounted in position in a cathode ray tube as part of the electron gun via the accelerating grid member.

Still an additional object of the present invention is the provision of forming the apertures in the grids of a low capacitance grid assembly after the grids have been secured onto a ceramic member.

Other objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof and from the attached drawing of which: Fig.1 is a perspective view of the cathode and grid assembly of the present invention; and Fig.2 is a cross-sectional view taken along the line 2-2OfFig.1.

As shown in Figs.1 and 2, a cathode and grid assembly 10 includes an annular ceramic member 12 having its upper surface secured to an inner planar surface 16 of another ceramic member 14 by a conventional brazing practice. An opening 18 is located in the center of ceramic member 14 and it has a control grid member 20 slightly larger in diameter than opening 18 tightly secured to inner surface 16. Grid member 20 is preferrably brazed to inner surface 16 with braze along the entire periphery of grid member 20 or at spaced locations therealong. Grid member 20 is stainless steel that has a thickness of 0.005 inches and an aperture 22 therethrough having a diameter of 0.022 inches.

Recesses 24 are located in the upper surface of ceramic member 12thatis brazedto surface 16to enable gríd wire 26 to extend to the outside of assembly 10 from its connection with control grid member 20. Wire 26 is a thin wire and has a diameter of 0.008 inches thereby providing a high inductance and low capacitance.

Ceramic member 14 has an outer planar surface 28 and an outer surface 30 in the form of a frustum of a cone. An accelerating grid member 32 has an outer planar section 34, an inner planar section 36 and a frustum of a cone section 38 connecting sections 34 and 36 together. Section 34 is brazed to planar surface 28 in the same manner that grid member 20 is secured to inner surface 16 to secure grid member 32 in position on ceramic member 14. Section 38 conforms substantially to the configuration of a frustum of a cone surface 30 and section 38 precisely positions and aligns section 36 relative to grid member 20 so that the grid members 20 and section 36 of grid member 32 are parallel and 0.02 inches apart. Section 36 has an aperture 40 therethrough having a diameter of 0.022 inches and being accu rately aligned with aperture 22.Section 36 has a diameter smaller than that of grid member 20.

In order to make certain that apertures 22 and 40 are accurately aligned, they are formed in grid members 20 and 32 after these grid members have been brazed to ceramic member 14. Apertures 22 and 40 are formed in the grid members by the conventional practice of electron discharge machining.

In this way, apertures 22 and 40 are assured of being accurately aligned.

A cathode support 42 has an inturned flange 44 brazed to the bottom surface of ceramic member 12.

A ceramic disc 46 has a hole 48 in which is secured a cathode tube 50 on the end of which is secured a cathode cap 52. Tube 50 is tightly secured in ceramic disc 46 via the formation of beadings 54 in tube 50.

Beadings 54 are tightly pressed against the surfaces of ceramic disc 46 to tightly lock tube 50 thereto.

Ceramic disc 46 is positioned against flange 44 and it is secured thereagainst by flange 56 of retainer member 58 that is secured to support 42 via a press fit or welding. In this way, cathode cap 52 is accu rately positioned and aligned relative to grid member 20 and aperture 22. The spacing between grid member 20 and cathode cap 52 is 0.0025 inches.

The cathode cap 52 has a small diameter and tube 50 is sufficiently long to reduce capacitance. A cathode spacer 60 in the form of a metal ring of proper thick ness can be placed between ceramic disc 46 and flange 44 to compensate for spacing problems due to brazing thickness and tolerances of flange 44, ceramic disc 46 and beadings 54 to make certain that cathode cap 52 is accurately positioned and aligned relative to grid member 20.

Another ceramic disc 62 has a flange 64 that abuts against the bottom of support42 and it is secured thereagainst by tabs 66 extending outwardly from support 42 and bent against disc 62. A heater filament 68 extends along tube 50 and has its ends connected respectively to leads 70 and 72 which are secured to ceramic disc 62 via rivets 74 that are located in holes 76.

A wafer member 78 is secured onto section 34 of grid member 32 and it has tabs 80 that are secured into glass rods for mounting the cathode and grid assembly 10 thereon as part of the electron gun.

The small diameter of grid member 20, the small diameter of section 36 of grid member 32, the small diameter cathode and small diameter of grid lead 26 all combine to substantially reduce capacitance so that input signals impressed onto control grid member 20 via lead 26 will enable electron beam operation at a very high frequency without excessive loading to the grid.

Where grid member 32 is secured to surface 28 of ceramic member 14 and grid member 20 is secured to surface 16 will cause reduction in capacitance due to the distance therebetween. Accelerating grid member 32 is connected to 200-400 volts. The accurate spacing and alignment of grid members 20 and 32 relative to each other and to the cathode enables high resolution to be realized. If desired, another high inductance grid lead 26a of the same size as lead 26 can be connected to grid member 20, and, together with lead 26, this arrangement can form a transmission line system for even higher frequency response.

It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above-described preferred embodiment of the present invention without departing from the spirit of the invention. For example, ceramic members 12 and 14 can be one piece. Therefore, the scope of the present invention is to be determined by the scope of the claims.

Claims

1. A cathode and grid assembly for use in an electron discharge device, said assembly comprising ceramic means having an annular section and another section, said another section including a planar inner surface, a planar outer surface, an outer frustum of a cone surface and a central opening; a first grid secured to said planar inner surface over said central opening, said first grid being slightly larger in diameter than said central opening and having an aperture extending therethrough;; a second grid having an outer section, an inner section and a frustum of a cone section connecting said outer and inner sections together, said outer section being secured to said outer planar surface while said inner section is positioned within said central opening parallel with respect to said first grid and said frustum of a cone section extends along said frustum of a cone surface, said inner section having another aperture therethrough in alignment with said first grid aperture; and cathode means secured to said ceramic means and being accurately positioned relative to said first grid.

2. A cathode and grid assembly according to claim 1 wherein mounting means is mounted to said outer section of said second grid for mounting said cathode and grid assembly as part of an electron gun.

3. A cathode and grid assembly according to claim 1 wherein said ceramic means has hole means therethrough and lead means of high inductance is connected to said first grid and extends through said hole means.

4. A cathode and grid assembly according to claim 3 wherein other lead means of high inductance is connected to said first grid and extends through said hole means.

5. A cathode and grid assembly according to claim 1 wherein said inner section of said second grid has a smaller diameter than that of said first grid.

6. A method of forming a grid structure for use as part of an electron gun assembly for an electron discharge device, said method comprising the steps of: securing a first grid member to an inner planar surface of a ceramic means and over a central opening of said ceramic means; securing an outer planar section of a second grid member to an outer planar surface of said ceramic means so that an inner planar section of said second grid member is positioned within said central opening parallel with respect to said first grid member; and forming apertures of the same diameter in said first grid member and said inner planar section of said second grid member.

7. A method according to claim 6 wherein said securing steps comprise brazing said first grid member to said inner planar surface and said outer planar section to said outer planar surface of said ceramic means.

8. A method according to claim 6 wherein said forming step comprises electron discharge machining.

9. The cathode and grid assembly for use in an electron discharge device, substantially as hereinbefore described with reference to the accompanying drawings.

10. A method of forming a grid structure for use as part of an electron gun assembly for electron discharge device, substantially as hereinbefore described.