US3873878A

US3873878A - Electron gun with auxilliary anode nearer to grid than to normal anode

Info

Publication number: US3873878A
Application number: US321137A
Authority: US
Inventors: Conrad J Odenthal
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1970-07-31
Filing date: 1973-01-05
Publication date: 1975-03-25
Anticipated expiration: 1992-03-25

Abstract

A cathode ray tube is provided with an electron gun structure including a cathode, a small apertured grid, and a small apertured anode on the remote side of the grid from the cathode. In addition, a further electrode is located between the aforementioned grid and the anode. A grid electrode, disposed adjacent and preferably mounted on the grid, is provided with an aperture coaxial with the grid aperture, but this aperture has a diameter effectively several times the diameter of the grid aperture. An anode electrode is also located between the grid and the aforementioned anode and is suitably connected to approximately the same voltage as the anode. The anode electrode is similarly provided with an aperture coaxial with the other apertures, but this aperture is also several times greater in diameter than the small aperture in the grid. The anode electrode acts to strengthen the electric field between such anode electrode and the grid whereby to provide a more concentrated electron beam. The region between the anode electrode and the anode provides a divergent lensing action for producing a small electron beam spot size at the screen end of the cathode ray tube.

Description

United States Patent [191 Odenthal [451 Mar. 25, 1975 ELECTRON GUN WITH AUXILLIARY Assig Inventor:

Filed:

Appl.

Conrad J. Odenthal, Beaverton,

Oreg.

nee:

Tektronix, lnc., Beaverton, Oreg. Jan. 5, 1973 Related U.S. Application Data Continuation of Ser. No. 64,938, July 31, I970,

abandoned, which is a division of Ser. No. 720,035, April 10, 1968, Pat. No. 3,567,991.

U.S. Cl. 313/449, 313/460 Int. Cl H0lj 29/02, l-lOlj 29/00 Field of Search 313/83, 86 US, 86 X, 85 R References Cited UNITED STATES PATENTS Primary ExamingrRohertSegal Attorney, Agent, or Firm-Klarquist, Sparkman, Campbell, Leigh, Hall & Whinston ABSTRACT A cathode ray tube is provided with an electron gun structure including a cathode, a small apertured grid, and a small apertured anode on the remote side of the grid from the cathode. In addition, a further electrode is located between the aforementioned grid and the anode. A grid electrode, disposed adjacent and preferably mounted on the grid, is provided with an aperture coaxial with the grid aperture, but this aperture has a diameter effectively several times the diameter of the grid aperture. An anode electrode is also located between the grid and the aforementioned anode and is suitably connected to approximately the same voltage as the anode. The anode electrode is similarly provided with an aperture coaxial with the other apertures, but this aperture is also several times greater in diameter than the small aperture in the grid. The anode electrode acts to strengthen the electric field between such anode electrode and the grid whereby to provide a more concentrated electron beam. The region between the anode electrode and the anode provides a divergent lensing action for producing a small electron beam spot size at the screen end of the cathode ray tube.

3 Claims, 5 Drawing Figures ELECTRON GUN WITH AUXILLIARY ANODE NEARER TO GRID THAN TO NORMAL ANODE BACKGROUND OF THE INVENTION This is a continuation of application Ser. No. 64,938 filed July 31, 1970, now abandoned which is a division of application Ser. No. 720,035 filed 4/10/68.

In a popular form of electron gun for cathode ray tubes, an apertured grid electrode cooperates with an anode to produce a crossover of the tubes electron beam near the aperture in the grid. Such crossover is then imaged on the cathode ray tube screen by the lensing action of remaining gun electrodes. It is. of course, desired that the electron gun operate with optimum officiency whereby as large a proportion as possible of the electrons omitted at the cathode reach the cathode ray tube screen. Moreover, it is frequently desired that the image of the electron beam crossover at the cathodev ray tube screen, that is, the spot size, be as small as possible while at the same time representing a large beam current for producing a bright spot.

Frequently a converging electron lens is located immediately on the screen side of the cathode ray tube guns grid. This lens tends to concentrate the beam somewhat but it does not ordinarily have the property of producing a virtual image smaller than the crossover, but rather the virtual image is sometimes larger than such crossover. Because of the use of beam limiting apertures in the gun, it should be noted that requirements of high beam current and small spot size are usually contradictory. Thus, a converging lens may produce higher beam current through the stopping aperture, but often has an undesired effect on spot size, while a diverging lens tends to decrease spot size but at the same time widens the beam which has more of its current stopped out by the limiting apeerture.

SUMMARY OF THE INVENTION In accordance with the present invention, a cathode ray tube includes an electron gun structure having the usual cathode, grid, and anode elements. The grid and anode are provided with small apertures through which the electron beam passes. A beam crossover, or circle of least confusion, is produced near the grid aperture, while the anode accelerates an electron beam on toward thecathode ray tubes screen where such crossover is focused. The present invention further includes a grid electrode means and an anode electrode means located between the aforementioned grid and anode. The grid electrode means is located adjacent and is preferably attached to the grid electrode, and such grid electrode means is provided with an aperture coaxial with the grid aperture but having a diameter several times the diameter of the small aperture in the grid. The anode electrode means, which is located closer to the anode than is the grid electrode means, is also provided with an aperture coaxial with the other apertures, but having a diameter several times that of the grid aperture. The anode electrode means aperture is also preferably larger than the grid electrode means aperture. The anode electrode means is connected to a potential nearer the potential of the anode electrode than the potential of the grid electrode, and acts to strengthen the electric field between such anode electrode means and the grid electrode. As a result, the strong field between the grid and the anode electrode means rapidly concentrates the electron beam while resulting in substantially no magnification thereof. Then, a divergent lens action between the anode electrode means and the anode produces a smaller virtual image of the beam crossover for focusing upon the cathode ray tube screen. It has been found that this structure produces an appreciable increase in beam current with a reduction in spot size, while cathode loading remains substantially the same.

It is accordingly an object of the present invention to provide a cathode ray tube with an electron gun exhibiting improved performance, i.e. providing a smaller spot size with increased beam current and without substantial increase in cathode loading.

It is a further object of the present invention to provide a cathode ray tube with an improved electron gun producing a stronger, more concentrated electron beam, wherein the effects vof space-charge spreading of the electron beam are minimized.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention. however, both as to organization and method of operation, together with I further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements.

DRAWINGS FIG. I is a cross-sectional view of a portion of a prior art electron gun;

FIG. 2 is a side elevational representation of internal structure ofa cathode ray tube according to the present invention;

FIG. 3 is a cross-sectional view of a portion of an electron gun according to the present invention;

FIG. 4 is a plan view, partially brokenaway, of the gun portion illustrated in FIG. 3; and

FIG. 5 is a cross-sectional view of an electron gun structure according to an alternative embodiment of the present invention.

DETAILED DESCRIPTION Referring to FIG. 2, the internal construction of a cathode ray tube according to the present invention includes a cathode l0 surrounded by grid 12 in the form of a grid cup having a small aperture 14 through which electron beam 16 is emitted. The electron beam travels from aperture 14 through central aperture 18 at one end of anode barrel or first anode 20. The opposite end of first anode 20 is open and coaxial with focus ring 22 which in turn faces the open end of second anode 24. Second anode 24 is provided with an aperture stop 26 at the end thereof facing the screen 31 at the screen end of the cathode ray tube. After passing through second anode 24, beam 16 passes between a first pair of deflection plates 28 and a second set of orthogonally oriented deflection plates 30, the function of which is to move the electron beam horizontally and vertically. Typical voltages for the various electrodes are indicated on the drawing. Thus, the first anode 20 is at a high positive voltage with respect to the cathode and grid, and therefore strongly attracts electron beam 16 which passes through aperture 18. Grid 12, which may be biased at a voltage somewhat lower cathode 10, not only regulates the amount of current in beam 16, but also provides a lensing action at aperture 14 for directly determining the size and position of an electron beam crossover. In order to produce a small beam spot at the screen of the cathode ray tube, this crossover is desirably as small as possible, i.e. the electrons are ideally converged to a sharply defined point. However, since the electrons emitted from cathode are nonparallel, but come off the cathode in random directions with varying velocities, a sharp point is not established. However, the electrons do converge into a smaller area or circle of least confusion normally called the crossover. This crossover is imaged on the screen of the cathode ray tube by the electron optics of the system.

FIG. 1 illustrates a prior art omitting portion of a commonly employed electron gun structure, wherein like elements are referred to by the same reference numerals used in FIG. 2. Here the electron beam crossover is indicated at 32. Between the grid 12, which may be at zero volts or at a somewhat negative voltage, and anode 20, there is established an electron field 34 indicated by lines substantially parallel to grid 12 and anode in FIG. 1. This field may be described as a converging field inasmuch as the electrons of beam 16 are accelerated thereby, and tend more in an axial direction 36 as they pass through this field. The convergent field therefore has the property of concentrating the electron beam after crossover 32 so that at least a substantial portion thereof may pass through

apertures

18 and 26. The converging field of this type ordinarily has little magnifying or demagnifying effect so far as the virtual image of the crossover is concerned, but does place the virtual image further behind the cathode surface, and, to the focusing section 22 of the electron gun, the electrons appear to be coming from this virtual image. Because of the ratio of screen distance to focus section 22 over the distance of focus 22 to the virtual image position, the virtual image is projected onto the screen 31 magnified by the ratio of these two distances. And since in the usual CRT the distance from the focus section 22 to the screen 31 is several times larger than the distance from focus 22 to virtual image, the spot on the screen is usually several times larger than the crossover itself.

In the region of the crossover, an appreciable spacecharge effect takes place, caused by the close bunching of the electrons in this region. This space-charge produces spreading of the electron beam 16 in addition to the optical spreading as may take place in the electron beam beyond the crossover or in the direction of arrow 36, thereby tending to reduce beam current passing through the aperture stop 26.

Enlargement of the beam by space-charge effects or the like can be reduced somewhat by bringing the anode 20 closer to grid 12 whereby the field gradient of field 34 is increased, However, the grid to anode spacing in a given tube is ordinarily determined by the cathode loading and cut-off voltage permissible or desired. Just bringing the anode closer to the grid increases the cut-off voltage of the tube and increases the cathode loading. Thus, for a given tube, the position of anode 20 relative to grid 12 may be relatively fixed, depending on the cathode loading or anode voltage required.

According to the present invention. electron beam current is increased, and the spot size produced by the beam is reduced without changing the cathode loading. Thus, for a given tube, and a given amount of cathode loading, the gun performance is enhanced. The emitting end of the cathode ray tube electron gun in accordance with the present invention is illustrated in FIG. 3, and also the additional elements thereof are included in the tube structure of FIG. 2. In addition to the conventional cathode 10, grid 12, and anode 20, the structure further includes a grid electrode means 38 disposed adjacent grid 12 between grid 12 and anode 20. The grid electrode means 38 is maintained at a potential nearer to the potential of-the grid 12 than to the potential of anode 20. The potential of the grid electrode means is at least between the potential of the grid 12 and the potential which would correspond to the position of said grid electrode means in a uniform field between the first anode and the grid, assuming such uniform field existed. The grid electrode means 38 is suitably a thin conducting metal wafer or disc of conducting material and is provided with a central aperture 40 coaxially aligned with aperture 14 and with electron beam 16. However, the diameter of aperture 40 is several times the small diameter of aperture 14. The thickness of the wafer comprising the grid electrode means is suitably comparable with the diameter of aperture 14 in the grid electrode.

In the illustrated embodiment, and preferably in actual practice, grid electrode means 38 is mounted immediately upon grid 12 whereby the grid electrode means 38 forms an annular shoulder portion surrounding aperture 14 but spaced radially therefrom. Thus, in this case, grid electrode means 38 resides at the same potential as grid 12, i.e. as selected by potentiometer 44 connected between ground and a minus volts. Aperture 40 is suitably cylindrical and extends axially in the direction of arrow 42 to the extent of the width of the wafer or disc.

Additionally, an anode electrode means 40 is supported between grid 12 and anode 20. and at least a portion thereof is desirably closer to anode 20 than is any portion of grid electrode means 38. Anode electrode means 46 suitably comprises a thin conducting metal disc or wafer disposed between grid electrode means 38 and anode 20. Anode electrode means 46 is provided with a central aperture 48 coaxially aligned with the apertures of the other electrodes and with the electron beam 16', and has an inside diameter larger than the inside diameter of aperture 14, or of aperture 18 which is comparable in size with aperture 14. The inside diameter of aperture 48 is also preferably greater than the inside diameter of aperture 40 of grid electrode means 38. Aperture 48 is also suitably cylindrical in the direction of the electron beam and arrow 42 through the width of anode electrode means 46, wherein such width is comparable to the diameter of aperture 14. Although cylindrical apertures in electrode means 38 and 46 achieve the best results, it is possible that these apertures be other than uniformly cylindrical, so long as the effective diameters of these apertures are appreciably larger than the diameter of

apertures

14 and 18.

Anode electrode means 46 is connected to a potential nearer to the potential of the first anode 20 than the potential of the grid 12. The potential of anode electrode means is at least between the potential of the anode and the potential which would correspond to the position of the anode electrode means in a uniform field between the anode and the grid, assuming such uniform field existed. Under these circumstances. the anode electrode means 46 strengthens the electric field between itself and grid 12, compressing the equipotontials therebetween, as can be seen in FIG. 3. The anode electrode means 46 is most advantageously connected to anode so as to reside at the same potential therewith.

Since the field is strengthened in region 50 between anode electrode means 46 and grid 12, or has a higher gradient therebetween, the electron beam 16' will be more greatly accelerated in region 50 than in the corresponding portion of field 34 in FIG. 1. Therefore, electron beam 16' will be concentrated more densely, resulting in a narrower beam by the time it reaches field region 52. This added concentration of the beam results from two factors. First, the greater field gradient produces a more convergent lensing action resulting in less widening of the beam that would otherwise take place after crossover 32. Second, the added acceleration of the electrons of electron beam 16 in region 50 aids in overcoming space-charge spreading of the beam as would otherwise occur near and subsequent to crossover 32. Therefore, by the time the beam reaches region 52 between anode electrode means 46 and anode 20, the beam is more concentrated whereby substantially more beam current may later pass through aperture 26 and reach the screen end of the cathode ray tube. As the beam passes through region 52, a divergence takes place inasmuch as the curvature of the field in region 52 provides a diverging lensing action. This divergence can more easily take place in this region while the beam is at a lower potential than it sub sequently reaches, e.g. at anode 20 and beyond. This divergence produces a smaller spot size on the screen end of the cathode ray tube. The reason for the smaller spot size is related to the electron optics involved between anode electrode means 46 and anode 20. The diverging property of the field produces a smaller virtual image of the crossover (or of the crossover image already produced by the field in region 50) which is then focused at the screen end of the tube. This smaller virtual image is produced in the same manner that a diverging optical lens produces a smaller virtual image of an object viewed therethrough. Since the beam is well concentrated before entering region 52, this divergence together with the action of aperture stop 26 does not detract materially from the beam current reaching the screen end of the tube or nearly as much as a divergence taking place after the beam has spread further. The field in region 50 is considered a converging field although the field equipotentials are substantially parallel to one another and to grid 12 and anode 20 so far as the smaller diameter electron beam is concerned at that point. This field narrows the electron beam in the sense that the electron beam, because of acceleration, does not spread as much as it otherwise would. The converging field formed of substantially parallel and planar equipotentials does not produce magnification, i.e. a larger virtual image of the crossover. Rather, the virtual image produced by the field in region 50 is approximately the same size as the crossover, but the field in region 52 then produces demagnification thereof.

It should be noted that the increased beam current and decreased spot size provided by the present structure is accomplished without increase in cathode loading. It is postulated that grid electrode means 38, residing at a low potential, i.e. the potential of the grid, holds the compressed electric field in region 50 from being forced closer to aperture 14. Thus, it will be observed that equipotential 54 in FIG. 3 is no closer to aperture 14 than is corresponding equipotential 54 associated with the FIG. 1 structure. Without grid electrode means 38, the field would be compressed closer towards the cathode, resulting in increased cut-off voltage, undesirably high cathode loading, and undesirable electron optical effects. So far as the cathode is coneerned, the field drawing the electrons therefrom appears the same in either the FIG. 3 or the FIG. 1 structures. Therefore, the increased beam current with decreased spot size can be achieved without excessive cathode loading or the like.

The following table illustrates improved advantages obtained in a typical gun structure constructed in accordance with the FIG. 3 embodiment, as compared with one constructed in accordance with the prior art FIG. 1 arrangement:

In this table, data was taken with the first anode 20 at a positive 3.5 kv with respect to the cathode. Cut-off voltage is taken as the value of grid voltage needed to visually extinguish the spot. Spot size in mils was measured by the shrinking raster method. 1,. is the cathode current in milliamperes at zero grid bias. I,, is the corresponding portion of the cathode current which passes through stop aperture 26 to screen 3I. Gun efficiency is the current reaching the screen divided by current leaving the cathode times 100.

In a typical embodiment, the construction of FIG. 3 had the dimensions given in the following table:

TABLE II Spacing between cathode l0 and top The above dimensions are merely given by way of specific example of a typical construction. In general, the aperture 40 in grid electrode means 38 should be approximately 8 to IOtimes largerthan that ofaperture l4. Aperture 48 should in turn be larger than aperture 40, for example, approximately 1.25 to l.5 times the diameter of aperture 40. Anode electrode means 46 should also be as close as possible to the grid electrode means 38 without producing arcing therebetween at the voltages employed. The aperture 18 in the anode is standard and is small, e.g. approximately two or three times the diameter of the grid aperture 14.

The particular construction of FIG. 3 is subject to a number of variations. For example, referring to FIG. 5,

illustrating an alternative embodiment, grid electrode means 38 has an outer diameter not much greater than its inner diameter wherein the inner diameter provides the aperture 40. Grid electrode means 38 is an annular ring mounted on grid 12. Then an anode electrode means 46 is disposed outside the outer diameter of grid electrode means 38. Anode electrode means 46 thus is disposed at least partially in overlapping relation to the grid electrode, but, as will be appreciated, the same will act to compress the electric field close to grid aperture 14 without increasing cathode loading. Also, a divergent field will be provided towards aperture 18 in anode 20. Other variations of the structure according to the present invention will occur to those skilled in the art.

While I have shown and described preferred embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. 1 therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

I claim:

1. A. cathode ray tube in which the improvement comprises an electron gun structure including:

a cathode for emitting an electron beam,

an anode electrode supported in alignment with said cathode having an anode aperture through which at least a portion of said beam passes,

a grid electrode having a grid aperture therethrough supported between said anode and said cathode in axial alignment therewith for passage of said electron beam, said grid electrode having an annular shoulder portion on the side thereof opposite that facing the cathode, the shoulder portion having a 8 substantially cylindrical inner aperture spaced from said grid aperture by a distance several times the diameter of said grid aperture, said inner aperture being substantially coaxial with the path of said electron beam, said anode aperture having a diameter of the same order of magnitude as said grid aperture, the diameter of said anode aperture being smaller than the diameter of said inner aperture of said shoulder, and an intermediate anode electrode supported next adjacent said grid electrode and the shoulder thereof between said grid electrode and said anode electrode in closer spaced relation to said grid electrode than to said anode electrode, said intermediate anode electrode having a large aperture therethrough in axial alignment with the aforementioned grid and anode apertures, the diameter of said large aperture being in the range of 1.25 to approximately 1.5 times larger than the inside diameter of the inner aperture of said annular shoulder portion while also being larger than the anode aperture. 2. A tube in accordance with claim 1 wherein said shoulder portion has an axial dimension of the same order of magnitude as the diameter of said grid aperture, and wherein said intermediate anode electrode overlaps and is at least partially in the same plane with said shoulder, the intermediate anode electrode being disposed at least partially in surrounding relation to said shoulder.

3. A tube according to claim 1 wherein the cylindri cal inner aperture of the annular shoulder portion has a diameter at least approximately 8 to 10 times the di- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,873,878

DATED r h 25, 1975 |NVENTOR(S) 3 .Conrad J. Odenthal M It is certified that error appears in the above-identified patent and that said Letters Patent 5 are hereby corrected as shown below:

In the Title, "AUXILLIARY" should be AUXILIARY-.

Column 1, line 16, "omitted" should be -emitted.

Column 1, line 36, "apeerture" should be -aperture.

Column 2, line 66, after "lower" insert -than-.

Column 3, line 13, "omitting" should be -emitting.

Column 4, line 35, "40" should be 46-.

. Column 5, lines 1-2, "equipotontials should be -'-equipotentials Signed and Scaled this Twelfth D i October 1976 I [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'Parems and Trademarks

Claims

1. A cathode ray tube in which the improvement comprises an electron gun structure including: a cathode for emitting an electron beam, an anode electrode supported in alignment with said cathode having an anode aperture through which at least a portion of said beam passes, a grid electrode having a grid aperture therethrough supported between said anode and said cathode in axial alignment therewith for passage of said electron beam, said grid electrode having an annular shoulder portion on the side thereof opposite that facing the cathode, the shoulder portion having a substantially cylindrical inner aperture spaced from said grid aperture by a distance several times the diameter of said grid aperture, said inner aperture being substantially coaxial with the path of said electron beam, said anode aperture having a diameter of the same order of magnitude as said grid aperture, the diameter of said anode aperture being smaller than the diameter of said inner aperture of said shoulder, and an intermediate anode electrode supported next adjacent said grid electrode and the shoulder thereof between said grid electrode and said anode electrode in closer spaced relation to said grid electrode than to said anode electrode, said intermediate anode electrode having a large aperture therethrough in axial alignment with the aforementioned grid and anode apertures, the diameter of said large aperture being in the range of 1.25 to approximately 1.5 times larger than the inside diameter of the inner aperture of said annular shoulder portion while also being larger than the anode aperture.

2. A tube in accordance with claim 1 wherein said shoulder portion has an axial dimension of the same order of magnitude as the diameter of said grid aperture, and wherein said intermediate anode electrode overlaps and is at least partially in the same plane with said shoulder, the intermediate anode electrode being disposed at least partially in surrounding relation to said shoulder.

3. A tube according to claim 1 wherein the cylindrical inner aperture of the annular shoulder portion has a diameter at least approximately 8 to 10 times the diameter of the grid aperture.