US3712998A

US3712998A - Cathode ray tube for producing variable sized displays

Info

Publication number: US3712998A
Application number: US00082628A
Authority: US
Inventors: M Mauck
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1970-10-21
Filing date: 1970-10-21
Publication date: 1973-01-23
Anticipated expiration: 1990-01-23

Abstract

A CATHODE RAY TUBE IS PROVIDED WITH FIRST AND SECOND APERTURED ELECTRODES BETWEEN THE TUBE''S DEFLECTION APPARATUS AND THE PHOSPHOR SCREEN. THE ELECTRODE CLOSEST TO THE TUBE''S DEFLECTION SYSTEM IS SWITCHED BETWEEN THE VOLTAGE OF THE SECOND ELECTRODE AND A LOWER VOLTAGE FOR "GEAR SHIFTING" BETWEEN DIFFERENT SIZED DISPLAYS.

Description

J 23, 1973 M. s. MAUCK 3,712,998

CATHODE RAY TUBE FOR PRODUCING VARIABLE SIZED DISPLAYS Filed Oct. 21, 1970 MICHAEL S. MAUCK INVENTOR BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYS United States Patent.

U.S. Cl. 315-17 Claims ABSTRACT OF THE DISCLOSURE A cathode ray tube is provided with first and second apertured electrodes between the tubes deflection apparatus and the phosphor screen. The electrode closest to the tubes deflection system is switched between the voltage of the second electrode and a lower voltage for gear shifting between difierent sized displays.

BACKGROUND OF THE INVENTION It is sometimes desirable to change the size of a display presented by a cathode ray tube, i.e. to provide gear shifting between display sizes. Although the largest display attainable is of optimum advantage in many cases, nevertheless an increased beam writing rate is frequently of greater advantage. In the latter case, a smaller sized display and a smaller spot size are preferred. A smaller sized display raster allows the electron beam to be deflected more rapidly for covering more of a raster in a given time, while still providing a visible trace. A more concentrated spot also permits speed-up in electron beam movement without the trace becoming too dim. A change in the voltages applied to the conventional tubes electron gun is, of course, capable of producing changes in the size of the display produced. Thus, increasing and decreasing the overall amplitude of the deflection voltages will change the size of the display. However, change in amplification provided in the deflection circuitry is not always convenient, and moreover a change in deflection signal amplitude will not change the image spot size to the degree desired. Altering the accelerating voltages in the conventional electron gun will also affect the size of the display, but the demands made on the electron gun are thereby changed to an undesirable extent.

Furthermore, according to modern practice, an appreciable part of the total acceleration is imparted to the electron beam after the same has passed through the tubes deflection apparatus, this simplifying deflection, allowing deflection to occur at low voltage levels, etc. Attempts have been made to build a post deflection acceleration cathode ray tube, capable of change in display size, employing a helix around the inside thereof located between the electron gun and the screen of the tube. To provide a change in display size, a high axial field is produced by a helix in the case of at least one of the display sizes selected. Unfortunately, this results in flare from the helix, or undesired multiplying of electrons from the sur face of the helix.

SUMMARY OF THE INVENTION According to the present invention, a tube employing post deflection acceleration includes an apertured electrode means located between the tubes deflection apparatus and screen. A switching means is provided for selectively changing the voltage applied to such electrode means for changing the lensing action produced thereby, to contract or expand the whole display as well as the spot size of the electron beam. When contracted in size, the smaller display and smaller electron 'beam spot size permit recording of an image at more rapid scanning speeds.

3,712,998 Patented Jan. 23, 1973 The apparatus according to the present invention desirably includes a second apertured electrode located between the first mentioned electrode and the cathode ray tubes screen. The aperture of the second electrode is larger than that of the first. This second electrode is suitably connected to the tubes post deflection acceleration voltage, while the first electrode is switchable between the post deflection acceleration voltage and a voltage near the voltage level which would have occurred in the intervening field at the location of the first electrode in the absence thereof. With the first electrode switched to the post deflection acceleration voltage, this first electrode is effective for providing a diverging, or display-expanding, field. With the switch in the second position whereby the first electrode is connected to a potential near a potential which would have occurred at the same location in the absence of the electrode, the second electrode provides lensing action, but less divergence, resulting in a smaller display and smaller spot size, since the second lens aperture is larger in diameter and farther from the electron gun.

It is therefore an object of the present invention to provide a cathode ray tube with improved gear shifting apparatus for changing display size, while preserving correct display geometry.

It is another object of the present invention to provide an improved cathode ray tube having a selectable scanning rate.

It is a further object of the present invention to provide an improved cathode ray tube having a selectable display size and electron beam spot size.

It is another object of the present invention to provide an improved cathode ray tube having means for gear shifting between display sizes, such means being simple in construction and compatible with conventional construction techniques for cathode ray tubes.

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 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. 1 is a side cross-sectional view of a cathode ray tube constructed in accordance with the present invention;

FIG. 2 is a transverse cross section of the FIG. 1 tube taken approximately at 22 in FIG. 1;

FIG. 3 is a schematic representation of tube operation according to the present invention in a first mode thereof;

FIG. '4 is a schematic representation illustrating lensing action occurring in the apparatus according to the present invention; and

FIG. 5 is a schematic representation of tube operation according to the present invention in a second mode thereof.

DETAILED DESCRIPTION Referring to the drawings, and particularly to FIG. 1 thereof, a cathode ray tube 10 constructed according to the present invention comprises an elongated envelope 11, suitably formed of insulating material such as glass. The envelope has an electron gun in a first or narrower end thereof, said electron gun including a cathode 14 heated by filament 12, a grid 16, a first anode 18, a focusing electrode 20 and a second anode 22. The first and second anodes are desirably connected to a source of high voltage relative to the cathode, such voltage in the instance of a particular example being 3 kilovolts. The

electron gun provides an electron beam 28 accelerated by the anode voltage toward phosphor screen 30 supported by glass faceplate 32. It is understood the Particular construction of the electron gun is not critical to the present invention.

The cathode ray tube is further provided with deflection means, here comprising vertical deflection plates 24 and horizontal deflection plates 26, for deflecting the electron beam 28 in orthogonal directions to establish a conventional presentation on screen 30 in the usual manner. An electron beamtransparent electrode, preferably comprising a planar grid or mesh 38 mounted on a cylinder 40, is located just on the screen side of deflection plates 26. Post deflection acceleration is employed in the present tube, and mesh 38 provides a planar field equipotential, or at least a field equipotential which is not materially rounded toward the electron gun, for avoiding an electric field configuration which would have an excessive converging efiect on an electron beam presentation. The mesh desirably cooperates according to the structure of the present invention for selectively producing the opposite or a diverging effect as hereinafter more fully described. The mesh 38 is suitably formed of nickel and may be mounted on a stainless steel ring 40 connected to the electron gun anode voltage, e.g. 3 kilovolts. The mesh is desirably a fine screen structure having from 500 to 1,000 screen lines per inch. Thus, a structure in the middle of this range, i.e. having 750 lines per inch, would provide (750) or 562,500 openings per square inch. Such a structure is suitably manufactured according to conventional photo etching methods.

The tube is also suitably provided with a conductive coating 34 or aquadag disposed around the interior of the larger end of the tube. This coating comprised a very thin layer of silver in a particular instance. The coating is effectively extended between phosphor screen 30 and faceplate 32 by means of the transparent conductive layer 36 which is suitably tin oxide. The conductive coating is connected to a high voltage, appreciably higher than the highest accelerating voltage in the electron gun, this voltage being 24 kilovolts in the case of the present example. The coating 34 and the layer 36 may cooperate to provide post deflection acceleration in the tube accord ing to the invention, but with that portion of the interior length of the tube upon which coating 34 is applied being substantially field free. The coating 34 prevents charge build-up on the glass envelope and resultant unwanted lateral deflection of the electron beam.

According to the present invention, the cathode ray tube is provided with a substantially planar metal electrode 44 suitably disposed at right angles to the axis of the tube or to the nondeflected central path of electron beam 28. The electrode 44 is desirably disc shaped and has a central aperture 46 through which electron beam 28 passes. The aperture is preferably circular with the center thereof coinciding with the nondeflected position of electron beam 28. The electrode 44 is also desirably provided with an axial flange ring 42 extending rearwardly toward mesh 38 for the purpose of protecting envelope 11 in this area from unwanted charging by electron bombardment. Ring 42 also may be used for conveniently supporting electrode 44. This electrode 44 is found to contribute in providing a distortionless lensing action, whereby the size of the resultant presentation may be altered without interfering with the proportions of the presentation as initially determined by the tubes deflection means.

Electrode 44 is connected to the movable contact 52 of a single pole double throw switch for gear shifting between one presentation size and another as hereinafter more fully described, said switch having fixed

contacts

54 and 56. In the right-hand position of the switch as shown, wherein movable contact 52 connects with fixed contact 54, electrode 44 is connected to a 24 kilovolt voltage source comprising the source of post deflection acceleration voltage for the tube. The left-hand switch contact 56 connects to movable contact 58 of potentiometer 60 wherein the latter is coupled to a voltage level between the 24 kilovolt post deflection acceleration voltage and a lower voltage, here suitably comprising a 3 kilovolt source connected to the electron gun anodes.

A second substantially planar metal electrode 48 is disposed in spaced parallel juxtaposition with electrode 44 between electrode 44 and screen 30. Electrode 48 is desirably disc shaped, the plane of the disc being perpendicular to the axis of tube 10, or perpendicular to the nondeflected central position of electron beam 28. Moreover, the electrode 48 is provided with a central aperture 50 geometrically similar to the first aperture 46 while suitably being scaled up to have an appropriately longer focal length for preserving distortionless projection geometry. The aperture 50 is desirably circular and coaxial with the nondeflected path of electron beam 28 and the aperture 46 of electrode 44.

Both

electrodes

44 and 48 are relatively close to screen 38, with the latter being in comparatively close spaced relation to the deflection means at the end of the electron gun. Electrode 48 is coupled to the 24 kilovolt post dcflection acceleration voltage of the tube, with electrode 48 (or electrode 44 when similarly connected) thereby contributing to the realization of the post deflection acceleration. Conductive coating 34 suitably extends from the right-hand end of the tube, where it makes contact with conductive layer 36, to a position radially outwardly adjacent electrode 48. The glass envelope surrounding the region between

electrodes

44 and 48 is uncoated. However, electrode 44 has a small enough aperture so that the envelope is protected from electron bombardment in this region by disc electrode 44 itself.

Operation of the present invention, in a first mode thereof, is illustrated in FIG. 3, wherein movable switch contact 52 connects with fixed contact 54 for placing electrode 44 at the high post deflection acceleration potential, to which electrode 48 is also connected. Aperture 46 acts as a lens, while the second aperture, 50, is fieldfree. That is, electrode 48 is connected. to the same post deflection acceleration voltage to which electrode 44 on one side and conductive coating 34 on the other side are connected, whereby the second aperture has substantially no effect upon the electron beam. The first aperture 46 operates as a diverging lens, the operation of which is illustrated by the analogous double concave optical lens depicted in dashed lines at 65. An electron beam 62 appearing to originate at a point 63 in the electron gun and deflected at the angle shown by the tubes deflection means is bent by lens 65 to angular path 64. The resultant deflection from the tube center in the presentation on screen 30 is given as D The electron beam then appears to originate from a point 68 following a path 66. This action is further illustrated in FIG. 4 wherein the electric field equipotentials are shown at 82. As the electron beam 62 proceeds downstream further from mesh 38 and approaches the aperture 46, the beam encounters predominantly downstream bulging equipotential lines, caused to exist at 84 in the present structure, which produce outward bending of the electron beam. The electron beam tends to turn in a direction for crossing the equipotentials at more nearly right angles. The presence of screen 38 is desirable if diverging action is to be produced by electrode 44, in that screen 38 provides a substantially planar field equipotential in line with its surface. Without mesh 38 equipotentials would tend to be depressed toward the electron gun, producing an overall converging, instead of a diverging, action.

The same tube structure is schematically illustrated in FIG. 5, but with the movable switch contact 52 thrown to the left for making connection with contact 56 and producing desired gear shifting of the display. By means of the switch, electrode 44 is now connected to the movable contact 58 of potentiometer 60, and in accordance with a preferred embodiment movable contact 58 is adjusted so that electrode 44 receives a potential equaling the potential which would exist at the same location in the field between mesh 38 and electrode 48 in the absence of electrode 44. At this potential, there is substantially no lensing effect produced by first electrode 44. Now, the lensing action takes place at aperture 50 of electrode 48. The lens at this location is weaker since electrode 48 is farther from the electron gun while aperture 50 is suitably larger. Therefore, the electron beam 70, originallly deflected at the same angle as electron beam 62 in FIG. 3, is only changed in direction to the path indicated at 72. The electron beam, instead of appearing to originate at point 78, will appear to originate at point 76 following an apparent path 74 between point 76 and aperture 50. The resultant deflection at screen 30 is indicated by D which is less than D The diverging action is analogous to that produced by a weaker double concave optical lens 80, illustrated in dashed lines in FIG. 5.

The deflection exemplified by the angular positions of

electron beams

62 and 70 are, of course, not the only angular positions for which divergence of the electron beam according to the present invention is produced. The particular angular position of

electron beams

62 and 70 may illustrate the deflection at the middle of an edge of a raster or other presentation, resulting respectively in a half-raster width D in the case of FIG. 3, and D in the case of FIG. 5. The expansion produced by the electric fields of

electrodes

44 and 48 will produce proportional changes for smaller initial deflection angles. The effect of gear shifting will be to change the size of the entire presentation or raster from one size to another. Thus, the entire presentation produced in the mode of operation illustrated in FIG. 5 will be proportionally smaller than the presentation produced in the mode according to FIG. 3. Furthermore, the electron beam in the case of the FIG. 5 mode will be more concentrated at 72 in FIG. 5 than is the case at 64 in FIG. 3, resulting in a smaller spot size where the beam intersects screen 30. Since the presentation and spot size are smaller in FIG. 5, a faster writing speed may be employed in the FIG. 5 mode of operation. The electron beam may be deflected more rapidly in the case of the FIG. 5 mode of operation, this being an important reason for desiring the gear shifting function. For example, the electron beam has less ground to cover, and moreover, the spot size is more concentrated and therefore of higher intensity for resulting in an acceptably visible trace with faster scanning. Means for actually speeding up the scanning are well known by those skilled in the art, involving employing faster signals at the deflection plates, and will not be specifically illustrated herein.

In the mode of operation illustrated in FIG. 3, it is seen that expansion of the display is produced notwithstanding the fact that acceleration is applied subsequent to deflection of the electron beam. Therefore, the defiection means of the cathode ray tube deflects a lower velocity electron beam and need not produce as much actual deflection as will subsequently be realized on the screen 30 as a result of the diverging effect of electrode 44. Consequently, requirements on the deflection apparatus are less stringent.

While switching of electrode 44 between post deflection acceleration voltage and the voltage representative of physical position of electrode 44 has been discussed, it is understood these voltages may be varied somewhat to produce the desired effects. For instance, for the FIG. 3 mode of operation, the voltage of electrode 44 may be adjusted by further potentiometer means to be a voltage near but less than the post deflection acceleration voltage. Furthermore, in a mode of operation more nearly represented by FIG. 5, the electrode 44 can be adjusted by movable potentiometer tap 58 to a voltage closer to a voltage of electrode 48 than the voltage representative of the position of electrode 44 in the field between electrode 48 and mesh 38. In either case, diverging action would be contributed by both

electrodes

44 and 48. On the other hand, if movable switch contact 52 connects with contact 56 as in FIG. 5, and movable tap 58 of potentiometer 60 is moved to the left such that the voltage applied to electrode 44 is less than the voltage at the location of electrode 44 in the absence of such electrode, electrode 44 will con tribute a converging action, while electrode 48 provides a diverging field. Moving the movable tap 58 to the left in this manner makes the presentation even smaller. It is sometimes desirable in the case of the FIG. 5 mode to change the action of the lens slightly by means of potentiometer 60, e.g. to provide convergence, for the purpose of minor distortion correction, as hereinafter mentioned, when the electrodes are less than ideal.

In general, however, the configuration described employing

fiat disc electrodes

44 and 48 produces distortionfree operation in either gear shifting position of switch contact 52. Thus,.the display geometry is preserved in changing from one proportional display to the other proportional display. Gear shifting is achieved, for example, substantially without encountering barrel or pin-cushion distortion in either switching position. This preservation of display geometry with gear shifting has not been found possible by prior art means. For optimizing preservation of display geometry, it has been found that certain dimensional ratios are desirable. The electrode apertures are suitably circular, and aperture 50 should be larger than aperture 46 for optimum results. Moreover, the ratio of each electrodes aperture diameter to the distance from such aperture to mesh 38 should be about two. Referring to FIG. 2, D (x+y)aD /xE2 for optimum results, wherein D and D are the diameters of

apertures

46 and 50, respectively.

Furthermore, the

electrodes

44 and 48 are suitably spaced from each other by about the same distance as electrode 44 is spaced from mesh electrode 48. In a particular instance, x and y each equaled one-half inch. D thus equaled one inch while D was two inches.

While electrode 48 is here illustrated as a separate discshaped electrode, it will be appreciated that a substantially similar electric field Will be attained by a circular left-hand edge of conductive coating 34, which is here connected to the same potential point. Thus, a separate disc-shaped electrode 48, while desirable, can nevertheless be eliminated with the left-hand circular edge of coating 34 at a substantially similar location performing a similar function. In such case it is desirable that a spacing exist between electrode 44 and the left end of coating 34 about equal to or possibly a little greater than the spacing illustrated here between

electrodes

44 and 48. In the event that the edge of coating 34 is thus employed, some adjustments may be made when the dimensional relationships are less than ideal. Dimensional aspectsof the substitute electrode may be compensated for through adjustment of movable tap 58, as hereinbefore indicated, to alleviate minor distortion effects which may be present when movable switch contact 52 is thrown to the left.

While I have shown and described various 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. I 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 comprising:

an elongated envelope having an electron gun located within a first end portion thereof for producing an electron beam, and a screen located at a second end thereof and adapted to receive said electron beam, said electron gun having a voltage applied thereto for accelerating said electron beam toward said screen,

and post deflection acceleration means having a higher voltage applied thereto,

deflection means located between said electron gun and said screen for deflecting said electron beam in orthogonal directions to provide a presentation on said screen, including means for applying a changing input to said deflection means whereby said presentation is produced by a moving electron beam, first electrode means disposed between said deflection means and said screen, said first electrode means being provided with an aperture through which said electron beam passes,

second electrode means located between said first electrode means and said screen and also having an aperture through which said electron beam passes, means for coupling said second electrode means to a voltage which is positive with respect to the highest voltage applied to said electron gun for accelerating said electron beam, said voltage coupled to said second electrode means being comparable to said voltage applied to said post deflection acceleration means,

and means for providing a voltage to said first electrode means in the range between the voltage to which said second electrode means is coupled and the, high est accelerating voltage applied to said electron gun, and including means for selectably changing the voltage applied to said first electrode means within said range for selectably contracting or expanding said presentation.

2. The apparatus according to claim 1 wherein said first electrode means comprises an apertured disc disposed at substantially right angles to the axial direction of a non-deflected electron beam in said cathode ray tube.

3. The apparatus according to claim 2 wherein said second electrode means also comprises an apertured disc disposed at substantially right angles to said non-deflected electron beam.

4. The apparatus according to claim 1 further including an electron beam transparent electrode between said first electrode means and said deflection means, said electron beam transparent electrode comprising means for producing an equipotential surface substantially at right angles to a nondeflected electron beam in said cathode ray tube.

5. The apparatus according to claim 4 wherein said electron beam transparent electrode comprises a mesh electrode which is connected to substantially the highest voltage in said electron gun employed for accelerating said electron beam.

6. The apparatus according to claim 3 wherein the aperture in said second electrode means is larger in diameter than the aperture in said first electrode means.

7. The apparatus according to claim 6 further including a mesh electrode between said first electrode means and said deflection means, said mesh electrode being connected to a voltage level proximate a voltage level in said electron gun, and wherein the ratio of the aperture diameters to the distance of the same to the said mesh electrode equals approximately two.

8. The apparatus according to claim 4 wherein said second electrode means is connected to the voltage applied to said post deflection acceleration means, ,and wherein said means for selectably changing comprises switch means for switching said first electrode means between the voltage to which said second electrode means is connected and a predetermined voltage between the said voltage to which said second electrode means is connected and the highest voltage applied to said electron gun.

9. The apparatus according to claim 8 wherein said predetermined voltage substantially corresponds to the voltage at the position of said first electrode means in an electric field between said electron beam transparent electrode and said second electrode means in the absence of said first electrode means.

10. The apparatus according to claim 8 wherein said predetermined voltage is between the said highest voltage of said electron gun and the voltage at the position of said first electrode means in an electric field between said electron beam transparent electrode and said second electrode means in the absence of said first electrode means.

11. The apparatus according to claim 1 wherein said second electrode means comprises a portion of said post deflection acceleration means comprising a conductive coating on the inside of the envelope of said cathode ray tube in the portion of said tube between said first electrode means and the screen of said cathode ray tube, said conductive coating providing an aperture at the end thereof toward said first electrode means.

12. A cathode ray tube comprising:

an elongated envelope having an electron gun located within the first end portion thereof for producing an electron beam, and a screen located in the second end thereof and adapted to receive said electron beam,

deflection means located between said electron gun and said screen for deflecting said electron beam in orthogonal directions to provide a presentation on said screen,

a substantially planar mesh electrode disposed across the path of said electron beam between said deflection means and said screen to provide a substantially planar field potential, said mesh electrode having voltage applied thereto commensurate with the accelerating voltage of said electron gun,

a substantially planar electrode between said mesh electrode and said screen and located substantially closer to said mesh electrode than to said screen, said substantially planar electrode having an aper ture therein for passage of said electron beam,

and switching means for selectively changing the voltage applied to said substantially planar electrode between a voltage level substantially higher than the voltage level of said mesh electrode and a voltage level intermediate such higher voltage level and the voltage level of said mesh electrode.

13. The apparatus according to claim 12 wherein said aperture is substantially circular and wherein the ratio of said apertures diameter to the distance between said substantially planar electrode and said mesh electrode is approximately two.

14. The apparatus according to claim 13 further including an additional apertured electrode between said first mentioned substantially planar electrode and the cathode ray tube screen, said additional electrode being connected to a higher voltage than said mesh electrode.

15. The apparatus according to claim 14 wherein said additional apertured electrode has an aperture similar to the aperture in the first electrode but scaled up to have a longer focal length.

References Cited