FOCUSING MEANS FOR CATHODE RAY TUBES

Abstract: A cathode ray tube comprising a cathode (K) for emitting a beam of electrons; a grid (G) for controlling the intensity of the beam; three or more anodes (A1, A3, A4); means for varying the potential of at least one of the anodes (A3) for focusing the beam to form a spot on a screen; and an additional, dynamic focus electrode (A2) maintained generally at a potential close to the cathode/grid potential. Suitable means may be included for applying to said dynamic focus electrode a high frequency signal to provide a rapid focus control means. Said dynamic focus means may comprise a plurality of circumferentially disposed segments and include means for applying a separate high voltage to each segment thus allowing the possibility of creating a quadrupole action in conjunction with at least one adjacent electrode, said segments covered with a deposited high-resistance conducting film to prevent charging effects. Preferably the potential of the first anode (A1) is substancially greater than the controlling voltage range of the grid (G), the potential of the static focus electrode (A3), interposed between the first and final anodes, is substantially equal to or lower than that of the first anode; and the potential of the final anode (A4) is substantially higher than that of the first anode (A1).

Description: FOCUSING MEANS FOR CATHODE RAY TUBES This invention relates to means for focusing a beam of electrons which is generated by the cathode of a cathode ray tube (crt) and controlled by electrodes such as a grid and one or more anodes to form an image ("the spot") on a screen. A conventional means of achieving this is to provide an electrode whose potential, usually at a value of several kilovolts, is to some degree variable to accommodate variations, within manufacturing tolerances, in tube dimensions and positions of the components within the tube and, if need be, drift in power supply outputs which affect the potential applied to other electrodes. Whilst it is a relatively straightforward matter to provide means for such occasional variations in focus electrode potential, problems are encountered in providing means for a rapid and continuous variation in the high value potential, typically several kilovolts, normally applied to the focus electrode. Such a variation would be necessary to provide a "dynamic focus" facility, ie to maintain a sharp focus when the spot is not only at the centre of the screen but also at the periphery of the screen at the extremes of its raster scan. This facility is required especially in view of the trend towards flatter and squarer tubes which although presenting a less distorted image to the viewer present rather greater problems in maintaining spot focus to the corners of the screen, and also in the use of high brightness tubes, such as those required for aircraft cockpit displays and the like, the electron beams in which have a reduced depth of focus. The present invention is aimed at providing a high performance cathode ray tube which includes a low-aberration focusing lens having an electrode maintained at a sufficiently low potential to enable a high frequency dynamic focus signal to be applied to it by readily available and inexpensive means, for example through a high voltage FET. According to this invention a cathode ray tube comprises: a cathode for emitting a beam of electrons; a grid for controlling the intensity of said beam; three or more anodes; and means for varying the potential of at least one of said anodes for focusing said beam to form a spot on a screen; characterised by an additional, dynamic focus electrode maintained generally at a potential close to the cathode/grid potential. By "a potential close to the cathode/grid potential" is meant a voltage, relative to the potential of the grid or of the cathode (one of which will usually, in practice, be maintained at earth potential), which is within the range of, for example, a high-voltage FET, ie within a few hundred volts compared with the typical several thousand volt potentials of the other anodes so that if need be it is a relatively straightforward matter to apply a high-frequency signal, such as that required for dynamic focus control, to the dynamic focus electrode. The dynamic focus electrode preferably takes the form of a ring, having a diameter similar to that of the adjacent anodes and extending axially only a short distance compared with its diameter; if the electrode extends too far in an axial direction then its effect on electron trajectories may prevent the beam focusing at the screen distance, and also the potential within the electron beam may become low enough to result in unacceptable aberration levels. The tube may include means for applying to said dynamic focus electrode a high frequency signal to provide a rapid focus control means. Such a high frequency signal applied to the dynamic focus electrode may be arranged as a function of the position of the spot on the screen. In one application of the invention this may be achieved by means synchronised with or controlled by means for generating coil currents used for controlling a raster scan on the screen. The signal applied to the dynamic focus electrode may also or alternatively be controlled by the means for controlling the grid-to-cathode potential difference (in practice, the means for controlling the grid potential or the cathode potential depending on the device, the other potential usually being fixed) in order to optimise the variation of focus with spot brightness; a signal having a frequency as high as several MHz may be required to accommodate variations in spot brightness both with time and with screen position. The dynamic focus electrode may be in the form of a ring of unitary construction located symmetrically around the axis of the cathode ray tube, or it may be of segmented construction, according to its possible application. Thus, in order to provide a dynamic focus facility, a simple ring or short cylinder is sufficient, this embodiment not requiring precision components and hence being the simplest to construct and assemble. A two-segment split ring may be used to provide a spot-wobble facility. A four-segment split ring may provide a stigmator with pre-aligned orientation, or precision electrostatic centre spot alignment. A particular application of a four-element ring is in connection with a precision in-line (PIL) three-gun colour tube, which has a tendency to produce an astigmatic beam profile with asymmetry in and perpendicular to the plane in which the three guns are located. An eight-segment split ring may provide an electrically-rotatable stigmator. The spaces between the components of the split rings may be covered with a deposited high-resistance conducting film to prevent even and uneven (ie not rotationally symmetrical) charging effects. Alternatively, the ring or a coating thereon may be made of a high-resistance material, further coated with a conducting material to define the ring segments. It will be appreciated that, by suitable connections from pairs or more of segments to common drive circuits, any higher number segmented ring can be made to fulfil the function of a lower number. This effect may also be achieved by electronic mixing of suitable input signals to individual segment voltage drivers. Particularly in tubes employing a four- or an eight-segment split ring electrode, means may be provided for applying to the anode a high-frequency signal for dynamic correction of astigmatism, such a signal being, like the dynamic focus signal referred to above, a function of the position of the spot on the screen. The focus electrodes, and the electrodes on either side of them, may incorporate non-rotationally symmetrical features to provide a quadrupole action, such as described, for example, by P W Hawkes and E Kasper in Principles of Electron Optics, Volume 2, pages 810 - 813 (Academic Press, 1989) and by Klemperer and Barnett in Electron Optics, 3rd Edition, pages 221 - 224 (Cambridge University Press, 1971). By way of example, a number of embodiments of the invention will now be described with reference to the drawings, of which: Figure 1 is a schematic cross-section in an axial plane of the electron gun portion of a cathode ray tube constructed in accordance with the invention; Figures 2, 3 and are schematic diagrams of alternative electron guns within the scope of the invention; Figure 5 is a schematic diagram illustrating in greater detail the electrode structure of the electron gun shown in Figure 2; Figures 6 and 7 are sections, on one half of the axis only, of the electrode structure illustrated in Figure 5 f showing computer- simulated equipotentials and electron trajectories when two different sets of potentials are applied to the electrodes; and Figures 8, and 10 are similar sections to Figures 6 and 7 showing respectively the res-. ts of computer simulations of three further embodiments of the invention. With reference to Figure 1, an electron gun is located within the neck portion 1 of a cathode ray tube for producing and focusing a beam of electrons to form a spot on a screen 2. The tube typically includes coils 3 for generating an electromagnetic field by means of which the position of the spot on the screen is controlled. The gun comprises a cathode K, a grid G, and a focusing lens including a suitably shaped first anode Al, a short, dynamic focus electrode A2, a longer, static focus electrode A3 and a further anode A4. This is a basic form of the invention, and is suitable for use in a miniature tube in which the internal diameter of the anodes can be 5 mm or less. Typical values of electrode potentials suitable for a 5 m internal diameter gun are: Example 1 K = +5v to +80v A2*= 0v G = Ov A3 = +8«5Kv Al = +10Kv A4 = +25Kv Such a structure would be suitable for each of the three guns of a precision in-line ("PIL") colour tube, with a cathode to screen distance of about 250mm. By substantially extending the gap between A3 and A4, the spot can be focused at a similar distance with a lower Al voltage, eg +5Kv. In applications where a smaller cathode-to-screen distance is required (for example a sub-miniature crt having a spot size of about 20 microns, screen size of about 20mm, overall tube length 100mm) , reduced potentials would be used, with appropriate adjustments of the lengths of the electrodes where necessary. The values given in this and the following examples for A2 and A3 are typical values required to focus a screen-centred spot. In practice a range in potential of perhaps a few hundred volts would be provided to enable these electrodes to fulfil their focusing function. An embodiment of the invention which is suitable for use with larger anodes (having a diameter of 12•2mm, for example) as well as for the miniature type, and is also suitable for cathode ray tubes required to operate over a wide range of eht voltage, for example those of the "penetron" type, is shown in Figure 2. The structure can be seen as a development of the Figure 1 structure which, in order to provide a two-stage acceleration following A3, with reduced spherical aberration, includes an additional electrode A5, interposed between A3 and the final anode A-. and strapped to the first anode Al. This structure is capable of good aberration performance over a wide range of electrode potentials and of A4/A5 potential ratios, as is desirable in penetron tubes. Typical electrode potentials for a +25Kv eht potential are as follows: Example 2 K = 0v A2 = Ov G = -20v A3 = +4-3Kv Al = A5 = +5Kv ' A4 = +25Kv For a final potential of +10Kv in the same tube, the electrode potentials would be as follows, with the A3 potential changed to re-focus the spot at the lower eht voltage (A5 is electrically connected to Al and remains at K ) : Exam le K = 0v A2 = 0v G = -20v A3 = +l « 5Kv Al = A5 = +5Kv A4 = +10Kv It should be noted that in all the examples, the abbreviations K, G, Al etc are used to denote electrodes having generally equivalent functions within the electrode structure. A form of gun having reduced aberrations but still suitable for use in a miniature tube is shown in Figure 3- I n this embodiment an additional focus electrode A6 has been interposed between the first anode Al and the dynamic focus electrode A2 and is electrically strapped to the static focus electrode A3; this embodiment also includes the additional anode A5, strapped to Al, interposed between A3 and the final anode A4. Typical potentials are: A2 = Ov A3 = A6 = +6-3Kv A-. = +25Kv It should also be noted that other developments of Example 1 are possible in which, for example, A6 is included and A5 is not, as well as vice versa. A modification of the Figure 3 arrangement is shown in Figure 4. The basic electrode pattern remains, but a large gap is provided between the anode A5 and the final anode A4, the electric field in the gap being controlled by a high resistance conducting film A7 deposited on a rotationally-symmetric ring and electrically connected at its ends to A4 and A5- In an alternative version of this embodiment, a similar high resistance film is deposited on a portion of a support structure for A5 and/or A4, the film being electrically connected at its ends as before to A4 and A5. In the simple case, the voltage within the conducting film varies linearly with axial position resulting in a fairly uniform electric field between A and A , but other arrangements producing a non-uniform field are possible. This feature is especially beneficial where the potential difference between the final anode and the other electrodes is particularly high. Typical values of the electrode potentials of the embodiment illustrated in Figure 4 are as follows: Example 5 K = Ov A2 = Ov G = -20v A3 = A6 = +3-4Kv Al = A5 = +5Kv A4 = +25Kv In this example, the potential of the cathode K is shown as Ov. In all the embodiments illustrated it is essentially a matter of convenience whether the cathode or the grid G is maintained at or around earth. The grid - cathode voltage may in any case be very small compared with the grid - first anode voltage: in preferred embodiments of the invention the grid - first anode voltage is in one case at least twenty times, or in another case at least fifty times, the modulating range of the grid voltage in order to minimise variations in cross-over position with grid modulating voltage. In another arrangement an extra accelerator electrode Aδ, maintained at a potential between that of A and A4, is placed between A5 and A4. Typical potential values of the electrodes in this arrangement are: Example 6 K = Ov A2 = Ov G = -20v A3 = A6 = +4«4Kv Al = A = ÷6 ' 25Kv A4 = +25Kv A8 = +12«5Kv A disadvantage of this arrangement is that the extra electrode needs a separate voltage supply, albeit simply from an additional tapping from the eht transformer or from a potential divider across the supply to A . The detailed dimensions of the various configurations and the performance achieved therewith will nυw be described with reference to Figure 5. which shows in schematic form the electrode structure of Figure 2 in greater detail. Figures 6 and 7 which illustrate the application of the Figure 2 structure to Examples 2 and 3. and Figures 8, 9 and 10 which illustrate embodiments of Examples 4, 5 and 6 respectively. Figures 6 to 10 show the anodes and computer-simulated equipotentials at lKv intervals and electron trajectories for electrons diverging at a number of different, equispaced angles from the emission section (here drawn from a point source and computed to aid lens aberration assessment) , in all cases on one side of the axis only. With reference to Figure 5. an electron gun consists of a cathode K, a grid G located a short distance in front of the emission surface of the cathode, and a first anode Al comprising a main, generally cylindrical body 2 which supports two baffles 3 and 4. The function of the first baffle 3 is to provide the anode potential close to the axis at the desired grid-anode distance and hence define the grid-anode field, and the second baffle 4 provides a small beam-limiting aperture, in accordance with common practice (although baffle 4 would normally be omitted for triple-gun colour tubes). In the example illustrated, Al extends axially for a distance of 10mm beyond the first baffle 3 and its inside diameter, in common with the other anodes, is 12-2mm. The structure also includes four further electrodes, each separated axially by a gap of about l«5mm, which consist in turn of a dynamic focus electrode A2, which is 2mm long, a static focus electrode A3, which is 8mm long, a second anode A5, which is 3om long, and a final anode A4. The final anode A4 includes a baffle the purpose of which is to provide a "spray aperture", ie to reduce the number of stray electrons reaching the screen and so improve contrast. The electrodes may typically be machined cylinders, having a wall thickness in the order of 1mm or, for example in a colour tube, three in-line structures may be made from pre-formed parts in accordance with established practice. Although shown square in the drawings, in practice the electrodes are polished and radiused to reduce field stress at the metal surfaces. The equipotential distribution is controlled primarily by the electrode potentials, their inside diameter and their length as measured between gap centres; the length of the gaps has a second order effect. In Figures 6 and 7. the electrodes illustrated in Figure have been assigned the values listed above for Examples 4 and 5 respectively. The modification on the potential field within the anode structure due to the presence of even the narrow, earthy A2 is clear: the potential between the Al zone and the A3 zone is reduced by A2 forming the initial focusing action of a five-electrode lens. In the examples illustrated in Figures 8 to 10 the same effect is apparent: the presence of a narrow A2 at earth potential between A6 and A3, which are equal in potential and on either side of A2, provides an initial focusing action which is simple to control. In all the examples illustrated, the good aberration performance, at least with regard to spherical aberration for an on-axis spot, is indicated by the nearly uniform spacing and tapering of the electron trajectories close to the right-hand edge of the figures (towards the closest approach to a single point on the screen) . Although the examples illustrated all include a first anode (Al) having a rather high potential, the invention is also applicable to electrode arrangements in which an additional electrode A0 is interposed between the grid G and the first anode Al and maintained at a potential substantially higher than the maximum grid potential and substantially lower than the potential of the first anode, ie as in the first ("pre-focus") stage of a conventional bipotential lens configuration.

Claims: CLAIMS 1. A cathode ray tube comprising: a cathode (K) for emitting a beam of electrons; a grid (G) for controlling the intensity of said beam; three or more anodes (Al, A3, A4); and means for varying the potential of at least one of the anodes for focusing said beam to form a spot on a screen; characterised by an additional, dynamic focus electrode (A2) maintained generally at a potential close to the cathode/grid potential. 2. A cathode ray tube according to Claim 1 comprising: a first anode (Al) whose potential is substantially greater than the controlling voltage range of the grid-to-cathode potential difference; a final anode (A4) whose potential is substantially higher than that of the first anode; a static focus electrode (A3) interposed between said first and final anodes, the potential of the static focus electrode being substantially equal to or lower than that of the first anode (Al); and means for varying the potential of the static focus electrode (A3) in order to form a spot on the screen. 3. A cathode ray tube according to Claim 2 in which the dynamic focus electrode (A2) is located between the first anode (Al) and the static focus electrode (A3) and including a first additional anode (A6) having a potential lower than that of the first anode interposed between the first anode and the dynamic focus electrode. 4. A cathode ray tube according to Claim 3 i n which the first additional anode (A6) and the static focus electrode (A3) are maintained at the same potential. 5. A cathode ray tube according to Claim 2, Claim 3 or Claim 4 in which the dynamic focus electrode (A2) is located between the first anode (Al) and the static focus electrode (A3) and a second additional anode (A5) having a potential higher than that of the static focus electrode and lower than that of the final anode (A4) is interposed between the static focus electrode and the final anode. 6. A cathode ray tube according to Claim n which the second additional anode (A5) and the first anode (Al) are maintained at the same potential. 7- A cathode ray tube according to Claim or Claim 6 in which a high-resistance conducting film is deposited on a rotationally-symmetric ring (A7) located in the region of an extended gap between the second additional electrode (A5) and the final anode (A4) , the film being electrically connected at its ends to said electrode and anode respectively. 8. A cathode ray tube according to Claim 5 or Claim 6 in which a high-resistance conducting film is deposited on a portion of a support structure for the second additional electrode and/or the final anode in the region of an extended gap between said electrode and said anode, the film being electrically connected at its ends to said electrode and anode respectively. 9- A cathode ray tube according to any of Claims 2 to 8 including a third additional anode (A0) interposed between the grid and said first anode and maintained at a potential substantially higher than the maximum grid potential and substantially lower than the potential of said first anode. 10. A cathode ray tube according to any preceding claim in which the axial length of the dynamic focus electrode (A2) is substantially less than the axial length of the static focus electrode (A3) . 11. A cathode ray tube according to any preceding claim in which the axial length of the dynamic focus electrode (A2) is substantially less than its radius. 12. A cathode ray tube according to any preceding claim including means for applying to said dynamic focus electrode a high frequency signal to provide a rapid focus control means. 13. A cathode ray tube according to any preceding claim in which the dynamic focus electrode comprises a plurality of circumferentially- disposed segments and including means for applying a separate high frequency signal to each of said segments. 14. A cathode ray tube according to Claim 13 in which the spaces between said segments are covered with a deposited high-resistance conducting film to prevent even and uneven charging effects. 1 . A cathode ray tube according to Claim 13 in which the dynamic focus electrode or a coating thereon is made of a high-resistance material, further coated with a conducting material to define said segments. 16. A cathode ray tube according to any of Claims 2 to 15 including means for varying the high frequency signal applied to the dynamic focus electrode as a function of the position of the spot on the screen. 17. A cathode ray tube according to any of Claims 2 to 16 including means for varying the high frequency signal applied to the dynamic focus electrode as a function of the grid-to-cathode potential difference. 18. A cathode ray tube according to any preceding claim in which the dynamic focus electrode and at least one adjacent electrode incorporate non-rotationally symmetrical features to provide a quadrupole action.

Inventors: BANBURY JOHN RANDOLPH, BANBURY, JOHN, RANDOLPH
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1992
Main classification: H01J29
IPC codes: H01J29