US3045141A - Electron beam tube - Google Patents

Description

- July 17, 1962 D. w. EPsTElN ET AL 3,045,141

ELECTRON BEAM TUBE Filed April 15, 1957 States This invention relates to electron beam tubes, and particularly to improvements in electron gun structures for such tubes which utilize an apertured electrode to which signal voltages may be applied to modulate or vary the intensity of an electron beam.

Cathode ray tubes of the kind in use in Oscilloscopes and television are provided with an electron gun for generating a ne beam of electrons which is focused on a target surface, or fluorescent screen, and means for deflecting the beam so that it may reach or scan the entire area of the screen. The electron gun comprises a plurality of electrodes aligned along an axis and including a cathode provided with a surface adapted to emit electrons when heated. The electron gun also includes a control electrode or grid haw'ng a llat apertured portion mounted closely adjacent to the electron emissive surface. The electric potential applied to the control grid may be varied through a range of potentials from a negative value, known as the cut off voltage, at which no electrons are permitted to pass through the control grid aperture, to a less negative potential at which an intense stream of electrons is permitted to pass through the aperture.

One or more accelerating electrodes are disposed intermediate the control grid and the phosphor screen and are operated at fixed positive potentials to produce one or more electron lens fields which operate on the electron stream to focus the electrons into a fine beam and acce1- erate the electrons so that they arrive at the screen with suiiicient energies to produce light emission therefrom.

In a television receiver, the incoming electric signals, which may represent variations inlight and dark areas of a scene or picture to be reproduced, are received in highly attenuated condition. Therefore, they must pass through a number of amplifier stages to raise the signals to levels which, when the signals are applied to the control grid of the cathode ray tubes, are effective to cause variations in the intensity of the electron beam and hence the intensity of the light emission from the phosphor screen. One characteristic of a cathode ray tube, Which is a measure of its ability to translate signal voltages into visible light output, is its transconductance. The transconductance may be defined as the ratio of the rate of change of useful beam current (i.e. current reaching the phosphor screen) to the rate of change of voltage applied to the control grid. The higher the transconductance the less is the signal voltage required to drive the control grid from the cut off condition to the condition of maximum useful beam current.

Generally, in cathode ray tubes of the type under discussion, the control grid or modulating electrode has a single aperture of circular shape. A circular shaped aperture has the disadvantage that a relatively large signal voltage range or large grid drive of 75 volts is required for the whole range of -beam modulation from beam cutoff to maximumbeam current.V Stated in other words the transconductance is not as high as might be desired.

A low grid drive or high transconductance is desired in order to reduce the number of amplifier stages necessary to raise the incoming signals to the levels required to modulate the control grid of the cathode ray tube. This would be advantageous in portable television receivers to reduce power requirements and weight, for example.

The grid drive may be reduced by employing a multiapertured electrode, such as one provided with a fine Patented July I7, 1962/ are` subjected to heating and thus they tend to expand or sag. Furthermore, the provision of multi-apertured grid structures entails -an additional cost.

Accordingly, an object of this invention is to reduce the grid drive requirements of `a cathode ray tube at a minimum of cost;

Another object is to provide the control grid of a cathode ray tube with an aperture of such shape as will permit a substantial reduction in grid drive and will give good resolution without necessitating close grid to cathode spacing.

The foregoing and other objects are achieved in accordance with the invention by providing the control electrode with an aperture, the-boundary of which is delinedk at least in part by a lplurality of projecting portions extending inwardly towards the center of 4the aperture. The projecting portions add a considerable length to the boundary of the aperture so that the control grid may exercise a greater control over the electrons passing therethrough. In one embodiment the aperture is shaped like a three leaf clover. A collar is provided ,around the central grid aperture, in accordance with the invention, the length of the collar lbeing sufficient to cause bending of the field lines adjacent thereto so as to reduce beam divergence and reduce beam spot size.

In the drawings:

FIG. l is a side view, partly in section, of a cathode ray tube embodying the invention;

FIG. 2 is an enlarged sectional view of a portion of the tube of FIG. l and illustrating a space potential distribution between some o-f the electrodes of the tube;

FIG. 3 is an end view taken along lines 3 3 of FIG. 2 and illustrating a control grid having an aperture shapedv in accordance with the invention; f V

FIG. 4 is a chart illustrating the grid drive required to produce a range of useful beam current in one conventional tube in comparison with the grid drive required to produce useful beam current in two tubes of the invention under similar conditions; and

|FIGS. 5 and 6 are end views similarly taken to FIG. 3 but rwith portions removed, and showing different aperture shapes. f

yReference is made to FIG. l where there is shown a side elevational yview, partly in section, of a cathode ray tube 10 according to the invention and comprising an evacuated envelope 12 made of an insulating material such as glass. One part of the envelope includes a tubular neck portion 14 having a closed end enclosed by a base 16' with a plurality of pins 18 yfor providing electrical access to 'some of the elements within the tube 10; the other part of the envelope includes a llared or bulbous portion 20 closed by a transparent end-wall 22. The transparent end-wall carries on the inside surface thereof a target element in the form of a fluorescent screen 24, the screen becoming luminescent on being bombarded by electrons. v

An electron gun 26 is disposed within the neck portion 14 to provide a beam of electrons directed toward the screen 24. The electron gun 26 includes a cathode 28 having a heater filament therein (not shown), a grid or control electrode 30, a vfirst accelerating electrode 32, and la second accelerating electrode 34. The electrodes, which are aligned along an axis A-A, are fixed in spaced relationship by being joined to insulating rods 36 as as shown, or may be inclined to the tube axis, as is conventional in some types of ion trap guns. A third accelerating electrode is provided in the form of a conductive coating 40 on the inner surface of the neck 14 and extending over the inner surface of the envelope ared portion 20 to a point adjacent to the fluorescent screen 24. This coating 40 prevents the glass portions of the envelope 12 from becoming indiscriminately charged during tube operation, which would adversely affect beam focusing and deiiection. In addition, the third accelerating electrode or coating 40 cooperates with the second accelerating electrode 34 to produce an electron lens which focuses the `beam on the screen 24.

Means are provided for scanning the electron beam over the screen 24 to form a raster. This means may include a deliection yoke 44 disposed around the part of the tube neck portion 14 adjacent to the iiared portion 20. The yoke 44 comprises a plurality of coils forming magnetic deflecting fields in the beam path for providing scansion of the electron beam over the screen 24.

`In operation, fixed electric potentials are applied to the electrodes, as is customary, to produce a focused beam of electrons.V In the example shown, all values of potentials are given with respect to the cathode 28, shown grounded. The control grid 30 is maintained somewhat above its cut-olf potential, for example -20 volts for a tube having a cut-off -30 volts, although the exact value may vary considerably in different tubes. The lirst accelerating electrode 32 is maintained at a moderately high positive potential, for example 200 volts. The second and third accelerating electrodes 34 and 40 are maintained at progressively higher positive potentials, `for example 4000 and 16000 volts respectively.

Signal information is applied to the control grid 30 from a signal source 4S.

In FIG. 2 there is shown the approximate shape of an electron beam, shown in `the drawing bounded by lines 46, and an electrostatic iield, illustrated by equipotential lines 48, in relation to the cathode 28, control grid 30, and rst accelerating electrode 32 of FIG. 1. The control grid 30 is tubular, formed from sheet metal, and has a flat end portion provided with a single aperture 50 therethrough in a direction along the gun axis A-A.

In accordance with one embodiment of the invention, the control grid aperture 50 is shaped like a three leaf clover, as shown in FIG. 3, having its center O lying on an axis of symmetry coinciding with the gun axis A--A. The boundary of the aperture 50 is defined at least in part by three projecting edge or pointed portions 52, which are cuspidal in shape and are directed towards the center O of the aperture 50. The radial distances of these projecting edge lportions from the gun axis A-A,

or in the symmetrical arrangement shown, from the center O lyingon the taxis of symmetry of the aperture, form local minima. By local minimum is meant a point whichis closer to the gun axis than its immediate neighboring point. For instance, the cuspid or point X on a projection 52 is a local minimum because its distance O'-X from the center O lying on theI axis A-A is less than the distance O-Y of a neighboring point Y, or of Iany point intermediate X and Y, from the center 0.

It will be noted that the boundary of the aperture 50 has a considerably greater length than the circumference of a circular aperture of the same included area. This is the result of the inner projections which exercise'a greater control over the electron beam and hence results in increased transconductance.

In accordance with another feature of the invention, the aperture 50 is surrounded by an annular skirt or collar 54 Whose diameter is considerably greater than the maximum width of the aperture 50 and having a substantial length B. The skirt 54 serves to bend the equipotential eld lines 48 adjacent thereto inwardly towards the aperture 50 to form a strong converging electron lens, so that Athe electrons of the beam will converge at a region C intermediate the control grid 30 and rst accelerating electrode 32 where the beam exhibits a minimum crosssectional area. The focusing field produced by the second and third accelerating electrodes 34 and 40 is then used tto image the beam from this region C at the screen 24.

In Ithe absence of the skirt 54, there is a tendency of the aperture 50, because of its lobular geometry, to produce three divergent electron streams, resulting in increased spot size of the beam. However, the skirt 54 overcomes the tendency towards increased beam divergence andincreased spot size by causing a bending of the field lines as described above. The resolution is thereby improved. In addition, the presence of the skirt 50 is instrumental in further decreasing the'grid drive.

While the following dimensions and spacings may be varied to suit operating conditions, they have been used in the practice of this invention to provide a tube with improved performance as compared to a tube provided with a control electrode having a conventional single circular aperture. In a successfully operated tube of the type described, the control grid 30 to cathode 28 spacing was .O05"; the spacing D between the control grid 30 and the tirst accelerating electrode 32 was .060"; the inside diameter of the skirt 54 was .l25"; the length B of the skirt was .030; and the cloverleaf aperture was formed Iby punching three .0208 diameter mutually tangential holes into the end of the control grid 30.

The grid to cathode spacing of .005 inch used in the practice of this invention is relatively large as compared to the spacing required of prior art mesh-like grid structures, which is approximately .001 inch. Wider absolute tolerances are also permitted in the practice of this inventio-n.

The absolute dimensions given above for the skirt 54 should not be construed as limiting the invention. What is important is that some means be provided to shape the field lines 48 in the manner described above. Variations may be made in the length and diameter of the skirt, other things being equal, so long as the variations produce the desired field configuration. Also, instead of the skirt 54 mounted on the control grid 30, a ring-like electrode may be mounted adjacent to the control grid `but spaced therefrom, for example, to provide the same effective bending of the field lines. In this case, the ring electrodeconld be maintained at the control grid potential, or it could be operated at any suitable potential to provide the desired shaping of the ield lines 48.

In FIG. 4 Ithere is compared the performance of a cathode ray tube of conventional electron gun design, namely a gun having a circular control grid aperture, indicated by the graph line 56, and the performance of two cathode ray tubes each having an aperture shaped in accordance with .the embodiment of the invention shown in FIGS. '2 and 3 and with gun dimensions and spacings as described above. The curve 58 represents a tube provided with a skirt 54 and curve 60 represents a tube without a skirt. The conventional tube had a circular aperture `of .036" diameter, giving a total opening area equivalent to that of the cloverleaf aperture. The performance curves 56, S8, and 60 show the variation in beam current as a function of grid drive, or signal voltage. In each case, the tubes are driven from their respective cut-oi voltages. As indicated in the drawing, both tubes made in accordance with the invention required substantially less grid drive than the standard for the same useful beam The prongs 66 may be formed integral with the cirv cularportions of the aperture 64, as shown. This may be done by chemically etching away the metal of the grid, for example, or by punching out the aperture with a suitably shaped die. Alternatively, the prongs may comprise fine Wires individually welded on the side of the control grid facing the tirs-t accelerating electrode.

As an example of dimensions which may be used, the aperture `64 may be .O38 inch in diameter; the prongs 66 may be .003 inch wide, and they may -be projected .014 inch towards the center of the aperture 64.

In most cases, it will be desirable to provide a plurality of projections which are symmetrically disposed, as shown in FIGSIS and 5. However, in some tubes it may prove advantageous to utilize a control grid aperture having a single inwardly directed projection, as shown in FIG. 6. As FIG. 5, portions of the grid structure have been removed for the sake of simplicity. In FIG. 6 the grid 68 has an aperture 70, which is generally circular. The aperture 78 is provided with `a single projection 72, which, like the projections v52 in FIG. 3, is cuspidal in shape. Alternatively, a prong-like projection, as in F.IG. 5 may be used. A single projection, or a non-symmetrical array of projections may be used, for instance, in those types of cathode ray tube requiring resolution in only one direction. Suitable use would be found in a line screen tube, or example.

It is now apparent that by means of the invention the grid drive requirements of a cathode ray tube may be reduced lat low cost by means not requiring close grid to catho-de spacing.

What is claimed is:

1. A cathode ray tube comprising a source of electrons, a control electrode adjacent to said `source and provided with an aperture, the boundary of said aperture including at least one projecting portion extending inwardly towards the center of said aperture, and means adjoining said control electrode for forming a converging electron lens for the electrons passing through said aperture.

2. The invention according to claim 1, wherein said means includes an annular member adjoining to said control electrode.

3. An electrode assembly comprising a source of electrons, a control electr-ode adjacent to said source, said control electrode being provided with an aperture, the boundary of said aperture including at least one projecting portion extending inwardly towards the center of said aperture, an accelerating electrode adjacent to said control -electrode, and an annular skirt adjoining on the side of said control electrode adjacent to said accelerating electrode and surrounding said aperture.

4. An electron gun comprising a plurality of electrodes aligned along a common axis, said electrodes including a cathode, a control grid, and an accelerating electrode arranged in that order, said control grid being provided with an aperture, the boundary of said aperture being defined art least in part by a plurality of projections directed inwardly towards said axis, and an annular `skirt adjoinof said control electrode adjacent to said accelerating electrode and surrounding said aperture.

5. In an electron gun, `an electron emitting and control structure comprising a cathode and a control electrode, means energizing said cathode and control electrode, said control electrode consisting of `a cup-shaped electrically conductive member having a circular aperture formed centrally through the base .thereof and a plurality of tangs aixed to and equally spaced around the periphery of said aperture and extending radially inwardly thereof a predetermined distance.

6. An electrode for an electron gun comprising a hollow cylindrical member, ia hat annular base and a plurality of tangs, the outer edge of said base being attached integrally to one end of said cylindrical member, each of said plurality of tangs being aiiixedto Ithe edge of the opening in said ybase at equally spaced points and projecting radially inwardly thereof a predetermined distance, said cylindrical member, said ybase :and said plurality of tangs being fabricated of an electrically conductivematerial.

7. In an electrode for an electron gun having an aperture formed therein, a plurality of tangs alixed to the periphery of said `aperture at equally spaced points and proe jecting radially inwardly of said aperture la predetermined distance, the total cross-sectional area of said .tangs being equal to an area of 5 Ito 15% the -area of said aperture, said electrode and said plurality of tangs being fabricated of an electrically conductive material.

8. In a television picture tube, an electron gun having at least a cathode and a control electrode, said control electrode Ibeing substantially cylindrical and including an end wall, said cathode -being disposed within said control electrode coaxially therewith and including a substantially cylindrical conductive sleeve, said end wall being formed with an elongated vertically extending aperture including two substantially circular parts, .the spacing between'said parts of said elongated aperture being in an amount less than the `sum of the radii of said parts whereby oppositely facing cusps are formed between said parts, the overall length of said elongated aperture beingsubstantially equal to the diameter of said conductive sleeve of said cathode.

9. In an electron gun, -a cathode and a control electrode, said control electrode includ/ing an electrically conductive plate member having a cloverleaf-shaped aperture centrally therethrough, said clovcrleaf aperture comprising a plurality of lobed portions forming Ia plurality of inwardly projecting cusps.

V10. An electrode for `'an electron gun comprising a hollow cylindrical member; Ia fiat annular base; and a plurality of cusped tangs; the outer :edge'of said base being attached to one end of said cylindrical member; each of said plurality of cusped tangs being ,affixed to the edge of the :opening in said base at equally spaced points and projecting radially inwardly ,thereof a predetermined dis'- tance; said cylindrical member, said base, and said plurality of cusped lt-angs being fabricated of an electrically conductive material.

References Cited in the iile of 'this patent UNITED STATES PATENTS France May v13, 1952

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