GB2114806A - Electron beam deflector and a display tube including the deflector - Google Patents

Abstract

An electron beam deflector (10) for frame scanning in a flat display tube comprises a plurality of parallel, equally-spaced electrodes (12, 14, 16,) inclined to the incident path (17) of the low energy electron beam (19). In operation the first electrode (12) is maintained at a voltage V corresponding to the voltage of the incident beam. The voltage of the second electrode (14) is steadily increased from zero to V/2, causing the exit beam path (18) to move in the frame scanning direction (Fig. 2(b)) until the beam trajectory is tangential to the second electrode (Fig. 2(c)). The latter's voltage is then abruptly increased to V, causing the beam to pass into the region between the second and third electrodes, but with an unchanged exit path (Fig. 2(d)). The procedure is repeated for successive electrodes to produce the complete frame scan. <IMAGE>

Description

SPECIFICATION Electron beam deflector and a display tube including the deflector The present invention related to an electron beam deflector and more particularly to a modular quadrant beam deflector which is able to bend an electron beam through an angle of 900 whilst simultaneously carrying out frame scanning.

British Patent Application 8007494 (PHB 32699) discloses a quadrant flat display tube in which a low energy electron beam is bent through 900 whilst simultaneously undergoing frame scanning. After being bent through 900 the electron beam is accelerated in a double electron lens before undergoing line deflection and impingement on a display screen. Whilst this tube works satisfactorily, it has been found that the ratio of the surface area for actual display to the remainder of the surface area is rather unfavourable although if the tube is made larger in order to have a greater display area, then the ratio of display area to non-display area becomes more favourable because of the geometry of the electrode arrangement for bending the electron beam and effecting the frame scan.Also in the operation of a display tube of the type described in British Patent Application 8007494, the deflection voltage swing increases with increasing deflection, i.e. with increasing picture height or length.

It is an object of the present invention to deflect an electron beam through an angle, of say 900, by an electron beam deflector which is more favourable in relation to the ratio of display area to non-display area compared with that obtained with Application 8007494 and also requires a substantially constant voltage swing independent of the extent of the deflection.

According to the present invention there is provided an electron beam deflection comprising means for providing a succession of spatially adjacent electric fields inclined to the path of an incident electron beam, the strength of each electric field being varied with respect to time in a predetermined manner whereby the provision of each electric field and its variation in strength causes the incident electron beam to be bent through a predetermined angle and exit from the deflector along a succession of adjacent parallel paths.

Conveniently the means for providing a succession of spatially adjacent electric fields comprises a plurality of parallel, spaced-apart electrodes inclined relative to the path of an incident electron beam and arranged to provide a transmission path for the electron beam therethrough and means for providing a predetermined voltage to each electrode.

If desired the predetermined voltage providing means may include control means which are adapted to provide: to a first electrode in the path of the incident beam a substantially fixed voltage V relative to the remainder of the electrodes; to a second electrode, adjacent the first electrode, a voltage which increases in a predetermined manner to form a first decreasing electric field in the space between the first and second electrodes, the voltage on reaching a predetermined value, for example V/2, being substantially instantaneously increased to V; thereafter to a third electrode, adjacent the second electrode, a voltage which increases in a predetermined manner to form a second decreasing electric field in the space between the second and third electrode, and so on until the voltage on the last electrode reaches said predetermined value, whereafter the voltages on the second to the last electrodes are reduced to zero and the cycle is repeated.

The electrodes may comprise parallel, spacedapart electrically conductive stripes provided on a pair of insulating plates, the spaces between adjacent stripes having a resistive layer thereon.

Alternatively the insulating plates may have many more stripes thereon, which stripes are closer together than in the previously-mentioned arrangement. The stripes are interconnected by a resistive track adjacent one end, remote from the paths of the electron beam, which track acts as a potential divider for the voltages applied to selected ones of the stripes, the intervening stripes serving to equalise the field.

In a further design the electrodes may comprise slotted, electrically-conductive strips arranged as a stack. In the case of the electrodes comprising slotted strips then tabs may be provided on at least one end of each slotted strip, the tabs pointing towards the preceding electrode in the stack and serving to maintain substantially parallel equipotential lines between adjacent pairs of strips for substantially the entire length thereof.

The present invention also relates to a flat display tube including, within an envelope, means for generating an electron beam, a screen displaced laterally of the electron gun and an electron beam deflector made in accordance with the present invention for turning the electron beam towards the screen and simultaneously carrying out frame deflection of the electron beam.

In one embodiment of a flat display tube the electron beam is bent through substantially 900.

In another embodiment the electron beam is bent through 1 800 in two stages. In a first stage the beam is bent through 900 by means of a constant electric field formed between two parallel spacedapart electrodes. In a second stage the beam is deflected through 900 and simultaneously undergoes frame scanning by an electron beam deflector made in accordance with the present invention.

By incorporating the beam deflector in accordance with the present invention into a tube of the type described in Application 8007494 in place of the described beam deflector, then the lateral width of the beam deflector is substantially constant and if the screen size is increased in height then a larger number of spatially adjacent electric fields, that is more electrodes, are necessary, but this has no substantial effect on the lateral width of the deflector. Accordingly the ratio of useful display area to non-useful display area improves with increasing screen size to a much greater extent than is the case in Application 8007494. Furthermore by creating a plurality of spatially adjacent fields, the voltage swing is substantially constant and is substantially independent of the extent of deflection.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of one embodiment of an electron beam deflector comprising a stack of equally-spaced, slotted electrodes, Figure 2 illustrates diagrammatically in four stages referenced (a) to (d), the bending of a beam through 900 and its displacement in the frame direction from an initial beam path by a height d, Figure 3 illustrates the voltage waveforms which are applied to the various electrodes shown in Figure 2 in order to obtain the necessary frame displacement, Figures 4 to 9 illustrate computer plots of a electron beam which has undergone frame scanning using the waveforms shown in Figure 3, Figure 10 is a diagrammatic elevational view of a flat, quadrant display tube incorporating the deflection arrangement in accordance with the present invention, Figure 11 is a sketch drawing illustrating how the screen size can be varied whilst the width of the electron beam deflector remains substantially unchanged, Figure 12 is a diagrammatic view of a flat display tube in which an electron beam is bent through 1 800, and Figure 13 illustrates diagrammatically another embodiment of an electron beam deflector comprising conductive electrodes on a resistive substrate.

In the drawings corresponding reference numerals have been used to indicate the same features.

Figure 1 shows, by way of example, an electron beam deflector 10 comprising three equallyspaced apart slotted electrodes 12, 14, 1 6 which are arranged one above the other as a stack with their major surfaces being inclined relative to an entrant path 1 7 to, and an exit path 18 from, the electron beam deflector 10. The width of the slots is of the order of 3 mm. A low-energy electron beam 19 is derived from the schematically illustrated electron gun 20.In the case of making the electrodes from separate pieces of metal, it has been found desirable to provide turned-down tabs 22, 24 at each end in order to try and ensure substantially parallel, equipotential lines between the adjacent electrodes 12, 14, 1 6. Without these tabs 22, 24 distortion which occurs at the ends of the electrodes 12, 14, 16 would become more pronounced and this would have an adverse affect on the bending of the beam. In the case of bending an electron beam 19 through an angle of 900 then the slotted electrodes 12, 14, 1 6 are disposed at 450 to both entrant and exit beam paths 1 7, 1 8, respectively.

Figure 2 illustrates one way of operating four slotted electrodes 12, 14, 1 6, 26 in order to provide bending of the beam through 900 as well as deflection, for example, frame deflection, of the electron beam. It will be assumed that the electron beam 1 9 leaving the electron gun (not shown) has an energy of 250 electron volts (eV) and that the voltage applied to the first electrode is also 250 volts thereby creating a field-free space between the electron gun and the first electrode 12. The second and subsequent electrodes are initially held at zero volts.Thus there is a strong, repelling field between the first and second electrodes 12, 14. As shown in diagram (a), the incident electron beam passes through the slot in the first electrode 12 and enters the space between the first and second electrodes 12, 14 whereupon it is repelled by the field due to the second electrode 14 and passes back through the slot in the first electrode 12 at an angle of 900 to its entrant path 17. If the potential of the second electrode 14 is progressively increased, see diagram (b), then the repelling field between the electrodes 12, 14 becomes weaker and that between the electrodes 14, 16 grows.In consequence the bend in the electron beam 19 becomes less tight with the result that the electron beam exits along the path 1 8 parallel to the initial beam path shown in diagram (a) but this new exit path is displaced in height relative thereto. By increasing the voltage applied to the second electrode 14, the bend in the beam moves progressively closer to the second electrode 14, until at 125 volts, i.e. half that of the voltage of the first electrode 12, the tangent to the mid-point of the bend effectively coincides with the plane of the second electrode 14, diagram (c), thereupon the voltage applied to the second electrode 14 is suddenly changed to 250 volts and the bend in the beam passes through the slot in the second electrode 14 as shown in diagram (d) but the exit path remains substantially unchanged. This effect can be explained as follows: by increasing the voltage applied to the second electrode 14 towards 125 volts then the repelling field between the first and second electrodes 12, 14 weakens whilst simultaneously that between the second and third electrodes 14, 16 strengthens. When the voltage applied to the second electrode is 125 volts (or half that of the potential drop between the first and third electrodes) then there are substantially equal fields between the first and second and the second and third electrodes and the bend in the electron beam 1 9 is able to reach the slot in the second electrode 14.Thereafter if the voltage on the second electrode 14 is increased further then the repelling field between the second and third electrodes 14, 16 will become stronger as that between the first and second electrodes 12, 14 weakens with the result that the bend in the electron beam 19 will not penetrate far through the slot in the second electrode 14. However, by switching the voltage applied to the second electrode 14 to 250 volts then a field-free space exists from the electron gun (not shown) to the second electrode 14 and in consequence the electron beam 1 9 follows a straight beam path as far as the second electrode 14. After passing through the slot in the second electrode 14, the electrode beam 19 is influenced by the strong repelling field existing between the third electrode 16 at 0 volts and the second electrode 14 at 250 volts.Under the influence of the strong repelling field the electron beam 19 executes a tight bend in the space between the second and third electrodes 14, 1 6 and leaves the beam deflector 10 along the same or substantially the same exit path as was the case just prior to the voltage on the electrode 14 being raised to 250 volts, this path being displaced by height dfrom the initial exit path shown in diagram (a). It has been found that by switching the voltage applied to the second electrode 14 from 125 volts to 250 volts substantially instantaneously there is no discernable affect on a display in spite of the fact that the path length of the electron beam has changed.

Although four electrodes have been shown in Figure 2, in theory the number of slotted electrodes is limitless but as a practical realisation it is considered that such a means of bending an electron beam and frame deflecting it is suitable for display tubes having diagonals up to 25 cms.

Irrespective of the number of electrodes, full frame deflection is achieved by having a substantially constant voltage swing, i.e. between 0 and 250 volts in the present example.

Figure 3 is a plot of electrode voltages, Vp, against time T and illustrates the waveforms of the voltages V12, V14, V1 6 and V26 applied to each of the electrodes 12, 14, 16 and 26, respectively in Figure 2. As is evident, the first electrode 12 is held continuously at a voltage V1 2 of 250 volts which equals that of the energy of the electron beams from the electron gun.At the commencement of the field period, the remaining electrodes are at zero volts but shortly after the voltage V1 4 increases until it reaches 1 25 volts or half that of the potential of the first electrode, whereat it is substantially instantaneously changed to the voltage of the first electrode 1 2 whilst the potential at the third electrode 1 6 is increased progressively. The period of time taken for the voltage V1 4 to change from 0 volts to 250 volts is termed the sub-frame period Tsf. This cycle continues for all the electrodes being used so that the full frame period is Tsf times the number of modules; this number corresponding to (N-1) electrodes where N is the number of electrodes.The actual shape of the graph illustrating the change of voltage applied to the second and subsequent electrodes is dependent upon the actual frame scan being carried out.

In a non-illustrated arrangement which represents a variation on the working voltages already described with reference to Figures 2 and 3, the first electrode 1 2 is maintained at 250 volts, i.e. at a voltage corresponding to that of the electron beam from the electron gun 20, but at start-up the remainder of the electrodes 14, 1 6, 26 are at a negative voltage, say -50 volts. As frame scanning commences the potential of the second electrode 14 is made more positive.At a voltage of + 114 volts, the beam will have been displaced vertically by a distance dfrom its position when the second electrode 14 was at a voltage of -50 volts and, if it is then changed substantially instantaneously to 250 volts, the beam will emerge at this same height and the potential of the third electrode 1 6 is then made more positive starting from its initial value of -50 volts. An advantage of starting at a negative voltage and making the voltage more positive is that in obtaining the maximum shift in height dof the exit beam, the tangent to the bend in the electron beam does not coincide with the plane of the adjacent slotted electrode. It is believed that the finite thickness of the slotted electrodes causes a slight jump in the exit path when going from the situation shown in diagram (c) to that shown in diagram (d) of Figure 2.This slight jump is not noted when using different electrode implementations such as will be described later with reference to Figure 13 and the range of voltages described with reference to Figures 2 and 3.

Figures 4 to 9 illustrate computer plots of frame scanning using an electron beam deflector 10 in accordance with the present invention and also illustrate the equipotential lines. For convenience of description, the deflector 10 will be assumed to have four equally-spaced parallel eiectrodes 12, 14, 16 and 26. Using the exemplary voltages discussed with reference to Figure 2, it will be realised that in Figure 4 the first electrode 12 is at 250 volts (assuming an electron beam energy of 250 eV) whilst the remainder of the electrodes are at 0 volts. By increasing the voltage applied to the second electrode 14 so that it approaches 125 volts then as shown in Figure 5, the bend in the electron beam 1 9 approaches the second electrode 14.At that point, the second electrode voltage is raised to 250 volts such that the beam passes through the slot in the second electrode 14 and enters the space between the second and third electrodes 14, 16, this is shown in Figure 6. A comparison of Figures 5 and 6 will show that the exit path of the electron beam 19 is the same, that is, there has been no vertical displacement of the exit path 1 8.

Figure 7 illustrates the situation when the voltage applied to the third electrode 16 approaches 125 volts and Figure 8 illustrates that when the voltage applied to the third electrode 1 6 is suddenly changed to 250 volts then the electron beam 1 9 passes into the space between the third and fourth electrodes 1 6, 26 but nevertheless leaves on the same exit path 18 as in Figure 7.

Finally, Figure 9 illustrates the situation when the fourth electrode 26 is approaching 125 volts and the maximum displacement of the electron beam has been executed. Thereafter the voltages applied to the second and subsequent electrodes are reduced to zero and the cycle recommences.

It will be noted from Figures 4 to 9 how the bent tabs 22, 24 on the slotted electrodes enable substantially parallel equipotential lines to exist for substantially the entire length in the spaces between adjacent electrodes 12, 14, 1 6 and 26.

For the sake of completeness of description, the equipotential lines denote steps of substantially 20 volts and their degree of closeness represents the relative strengths of the repelling fields. Also it is possible to see from Figures 5 and 7 how the repelling field extends equally across the spaces between three successive electrodes when the middle electrode is at 125 volts.

In order to illustrate the geometry of the bending action between the situations shown in Figures 5 and 6, if it is assumed that the distance between adjacent electrodes h then the distance between the electron beam passing along its entrant path 17 through the slot in the first electrode 12 into the space between the electrodes 12, 14 and exiting through the same slot along its exit path can be shown to be equal to 4h. In the case of Figure 6, the space between the first and second electrodes 12, 14 has become field-free and it can be shown that the distance between the point at which the electron beam 1 9 enters the slot in the second electrode 14 and passes back therethrough is 2h whilst the corresponding distance along the slot of the first electrode remains at 4h and therefore the exit path remains unchanged.The depth of penetration of the bend in the electron beam in Figure 6 is h/2 which corresponds to the 125 V equipotential line. Reverting to Figures 4 and 5, it will be realised that the vertical displacement (d) of the exit path is a function of the distance, h, between the equally spaced electrodes, namely d=hW2.

Turning now to Figure 10, the display tube illustrated comprises an evacuated envelope 30 which may be in the form of a dished portion closed by a flat glass face plate on which a screen 32 is provided. A low energy (250 eV) electron beam 19 is produced by an electron gun 20 whose longitudinal axis is parallel to an edge of the screen 32. After leaving the final anode of the electron gun 20, the beam is bent through 900 and simultaneously undergoes frame deflection by an electron beam deflector 10 made in accordance with the present invention. The electron beam after having undergone frame deflection still has the low energy at which it left the electron gun 20. In order to achieve satisfactory electrostatic line deflection of the electron beam and a focussed beam condition at the screen, it has been found that this is best done at a potential approaching that of the screen.

Accordingly, it is necessary to increase the energy of the electron beam over a short distance whilst ensuring that of the beam spot on the screen is not unacceptably large.

In the illustrated embodiment, the energy of the electron beam is increased by means of a double electron lens 34 which comprises a first electron lens formed by first and second parallelarranged, slotted electrodes 36, 38 and a second electron lens formed by third and fourth parallelarranged, slotted electrodes 40, 42. As the second electrode 38 and the third electrode 40 are at the same potential, it is convenient and more compact to combine them into a box-like structure having slots in opposite upstanding walls. During the acceleration of the beam, the first electron lens causes the electron beam to converge so that its image forms the object of the second electron lens which also converges the beam.The accelerated electron beam, on leaving the second electron lens, undergoes line deflection by a line deflector 44 formed by two spaced-apart electrodes which diverge in a direction towards the display region of the tube.

By varying the potential difference between the divergent electrodes, the angle of entry of the electron beam into the display region of the tube is varied. This display region comprises the screen 32 and the spaced-apart, parallel-arranged repeller electrode (not shown) which define between them a trajectory control space. A substantially constant potential difference is maintained between the screen and the repeller electrode. Consequently, by changing the angle of entry of the electron beam into the trajectory control space at line frequency, line scanning of the screen will be produced. Details of the construction and operation of the double electron lens 34 and the line deflector 44 are disclosed in British Patent Application 8007494 (PHB 32699) details of which are incorporated by way of reference.Although the double electron lens 34 has been disclosed as a means for increasing the beam energy, other methods of increasing the energy of the beam emerging from the structure may be used instead.

Figure 11 illustrates that the lateral width W of the beam deflector 10 remains substantially constant irrespective of the increase in the size of the screen 32 and the consequent increase in height of the double electron lens 34 and the line deflector 44. However, as is evident from Figure 11, the number of electrodes of the electron beam deflector 10 is increased in order to cope with the increase in screen size. In theory the number of electrodes used in the deflector 10 is limitless but in practice the differences in path lengths of the electron beam in reaching different parts of the screen 32 impose certain limitations which can be dynamically corrected at the electron gun. It is felt that focusing and brightness problems will limit the screen size to, for example, one having a diagonal of 25 cms.

Figure 12 illustrates diagrammatically another embodiment of a flat display tube. In this display tube the electron beam 1 9 from the electron gun 20 is bent through 900 by a first quadrant beam deflector comprising parallel slotted electrodes 50, 52 arranged at 450 to the electron beam path from the electron gun 20. The electrode 50 is maintained at a steady 250 volts, corresponding to the voltage of the electron beam, and the electrode 52 is at 0 volts. Hence the electron beam always follows the same path through the first quadrant deflector to a second quadrant beam deflector 10, the space between them being field-free. The remainder of the tube both in its construction and operation is much the same as has been described with reference to Figure 10 and accordingly in the interests of brevity the description will not be repeated.

The electron gun 20 used in the embodiment of Figure 12 is preferably a long gun which is suited for a situation in which one has a long path length and wants a small spot size.

Figure 13 illustrates another embodiment of an electron beam deflector. This embodiment comprises two parallel, spaced-apart insulating plates 60, 62, of which the piate 62 has been shown diagrammatically in broken lines in the interests of clarity. The space between the plates is between 2 and 3 mm.The plates 60, 62 may be of an insulating material or may have a resistive layer thereon with a resistance of the order of 1 OMsquare. A plurality of gold line electrodes 64 of the order of 200 pm wide are provided, for example, by thick film printing, on the facing surfaces of the plates 60, 62. These line electrodes 64 which are inclined at 450 to the electron beam path are interconnected at a point remote from the electron beam paths by a laser trimmed resistive track 66 which acts as a potential divider. Selected ones of the line electrodes 64 have connection pads to which external voltages are applied in order to achieve the desired simultaneous beam bending and frame scanning.The selected electrodes have been referenced 12, 14, 16 and 26 since electrically they serve the same function as those shown in Figures 1 to 3. The intervening line electrodes 64 serve as filled equalising electrodes.

In operation the electron beam 1 9 is directed between the facing surfaces of the plates 60 and 62. The voltages shown in Figure 2 can be used in this arrangement because the line electrodes do not extend towards the electron beam path to any great extent, unlike the discrete slotted electrodes of Figure 1.

In a simplified version of the embodiment shown in Figure 13, the intervening electrodes may be omitted.

Although the particular embodiment of the present invention has been described with reference to bending an electron beam through 900, it is possible for the beam to be bent through angles other than 900 by angling the input beam at angles other than 450 with the deflecting electrostatic field. Further, the electron beam deflector 10 may be used in other applications besides the display tube disclosed in British Patent Application 8007494.

Claims (11)

Claims

1. An electron beam deflector comprising means for providing a succession of spatially adjacent electric fields inclined to the path of an incident electron beam, the strength of each electric field being varied with respect to time in a predetermined manner, whereby the provision of each electric field and its variation in strength causes the incident electron beam to be bent through a predetermined angle and exit from the deflector along a succession of adjacent parallel paths.

2. A deflector as claimed in Claim 1, wherein the means for providing a succession of spatially adjacent electric fields comprises a plurality of parallel, spaced-apart electrodes inclined relative to the path of an incident electron beam and arranged to provide a transmission path for the electron beam therethrough and means for providing a predetermined voltage to each electrode.

3. A deflector as claimed in Claim 2, wherein the predetermined voltage providing means includes control means which are adapted to provide: to a first electrode in the path of the incident beam a substantially fixed voltage V relative to the remainder of the electrodes; to a second electrode, adjacent the first electrode, a voltage which increases in a predetermined manner to form a first decreasing electric field in the space between the first and second electrodes, the voltage on reaching a predetermined value being substantially instantaneously increased to V; thereafter to a third electrode, adjacent the second electrode, a voltage which increases in a predetermined manner to form a second decreasing electric field in the space between the second and third electrodes, and so on, until the voltage on the last .electrode reaches said predetermined value, whereafter the voltages on the second to the last electrodes are reduced to zero and the cycle is repeated.

4. A deflector as claimed in Claim 2 or 3, wherein the electrodes comprise parallel, spacedapart electrically conductive stripes provided on a pair of insulating plates, the spaces between adjacent stripes having a resistive layer thereon.

5. A deflector as claimed in Claim 4, wherein field equalising electrodes are disposed in said spaces and wherein the stripes and field equalising electrodes are interconnected by a resistive track.

6. A deflector as claimed in Claim 2 or 3, wherein the electrodes comprise slotted, electrically-conductive strips arranged as a stack.

7. A deflector as claimed in Claim 6, wherein tabs are provided on at least one end of each slotted strip, the tabs pointing towards the preceding electrode in the stack and serving to maintain substantially parallel equipotential lines between adjacent pairs of strips for substantially the entire length thereof.

8. An electron beam deflector constructed and arranged to operate substantially as hereinbefore described with reference to Figures 1 to 9 or Figure 13 of the accompanying drawings.

9. A flat display tube including, within an envelope, means for generating an electron beam, a screen displaced laterally of the electron gun and an electron beam deflector as claimed in any one of Claims 1 to 8 for turning the electron beam towards the screen and simultaneously carrying out frame deflection of the electron beam.

10. A flat display tube as claimed in Claim 9, wherein the electron beam from the beam generating means is bent through 1800 in two stages in order to reach the screen, the first stage comprises means for producing a steady electric field at an angle relative to the electron beam path from the beam generating means whereby the electron beam is deflected towards the second stage which comprises said electron beam deflector.

11. A flat display tube as claimed in Claim 9, substantially as hereinbefore described with reference to Figures 10 and 11 or Figure 12 of the accompanying drawings.