JPH0253904B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in electron guns for cathode ray tubes.

The electron gun of a cathode ray tube focuses and accelerates electrons emitted from the cathode into a beam. Then, this electron beam
This stimulates a small luminescent spot called a beam spot on the screen. Since the beam spot becomes the picture element of the displayed image, its characteristics are extremely important in terms of the performance of the cathode ray tube. Generally, the size of the beam spot has an optimum value determined by the size of the screen and the purpose, and it is not preferable to be larger or smaller than the optimum value.
In the case of television cathode ray tubes, if the beam spot is too small, the striped pattern of the scanning lines will be too noticeable and the screen will not be good, and if the beam spot is too large, it will not be possible to clearly display fine parts of the image.
Although it is ideal to always maintain the size of the beam spot at such an optimum value, it is extremely difficult in practice. This is because when the beam current is modulated to modulate the brightness of the screen, the beam spot diameter tends to increase as the beam current increases. This tendency is particularly noticeable in color cathode ray tubes that require high brightness. In color cathode ray tubes, the beam current is modulated within a range from a small current value of several tens of μA to a large current value of several thousand μA, so even if an optimally sized beam spot is obtained at low current, it is difficult to increase the beam current. The beam spot diameter increases accordingly. If the beam spot diameter is increased to near the maximum value in order to display bright parts of the screen, the beam spot diameter will increase to about 3 to 4 times the optimum value, and the sharpness of the bright parts will deteriorate significantly.

Therefore, in terms of improving the image quality of cathode ray tubes, it is important to realize an electron gun that can prevent the beam spot diameter from becoming excessively large when a large current is applied and can always obtain an optimally large beam spot.

SUMMARY OF THE INVENTION An object of the present invention is to provide an electron gun that can reduce the beam spot diameter when using a large current, and to improve the image quality of a high-brightness screen.

The present invention will be explained below with reference to the drawings.

Figure 1 is a conceptual diagram explaining the operation of an electron gun used in a cathode ray tube. Electrons emitted from the cathode 1 are distributed between the G ₁ electrode (first grid electrode) 2 and the G ₂ electrode (second grid electrode). By the focusing action of the cathode lens 4 formed by the beams 3 and 3, the beams cross each other, producing a minimum point 5 of the beam diameter called a crossover.
The electron beam from the crossover advances while diverging, but is preliminary focused by a prefocus lens 7 formed between the G ₂ electrode 3 and the G ₃ electrode (third grid electrode) 6. Next, a main lens 9 is formed between the G ₃ electrode 6 and the G ₄ electrode (anode) 8.
is finally focused to form a beam spot 11 on the screen 10. In other words, the crossover 5 corresponds to an electron lens system object, and since this is imaged on the screen 10 by the prefocus lens 7 and the main lens 9, the beam spot 11 is focused on the image of the crossover 5. Equivalent to.
This relationship holds fairly accurately when the current is small, and the beam spot diameter on the screen 10 is given by the product of the crossover diameter and the imaging magnification of the lens system. The crossover diameter at small current is
This is mainly determined by the initial velocity distribution of electrons from the cathode, but it is usually sufficiently small and as long as an electron lens with appropriate imaging magnification is used, it is easy to obtain a beam spot of the optimum size required. However, when the beam current is increased to obtain a brighter screen, the beam spot diameter on the screen 10 tends to increase significantly, and this is mainly due to the following three reasons.

The first is that when the beam current is increased, the diameter of the crossover 5 itself increases, and the second is that the divergence angle 12 of the electron beam from the crossover 5 increases, so the lens system, especially the main lens 9. The third reason is that aberrations appear strongly.The third reason is that the current density near the screen increases, so that the repulsion between electrons becomes stronger.

Among these, the third type, mutual repulsion between electrons, is a phenomenon essentially accompanying the electron beam, so it is difficult to fundamentally solve this problem. Therefore, in order to suppress the increase in the beam spot diameter at the time of large current, it is necessary to simultaneously reduce the diameter of the crossover 5 and the beam divergence angle 12.

Figure 2 shows cathode 1 to G ₃ of a conventional electron gun.
Equipotential line 1 during large current in the space up to electrode 6
3 and the general state of the electron orbit 14. The area where electrons are emitted on the cathode 1 is
At the time of large current, it widens to near the projection plane 15 of the hole of the _G1 electrode 2. Further, although the potential of the _G1 electrode 2 is always kept negative with respect to the cathode potential, the potential difference between the two becomes extremely small when a large current is applied. In addition to this, in conventional electron guns, the distance g ₁ between the cathode 1 and the G ₁ electrode 2 is usually
The thickness t ₁ of the G ₁ electrode 2 is made as small as possible. Therefore, the equipotential line 16 immediately in front of the cathode 1 is slightly convex in the direction of the cathode and is approximately parallel to the cathode. Therefore, the effect of the cathode lens at a large current is very weak against the electrons 17 from the center of the cathode, and these electrons 17 intersect with each other at a position 18 quite far away from the cathode 1. On the other hand, in the vicinity of the cathode, since the G ₁ electrode 2 is present nearby, a strongly curved portion 19 is created in the equipotential line 16. The electrons 20 emitted from the cathode periphery are located at a strongly curved part 19 of the equipotential line.
, it receives a strong force in the axial direction, and a crossing point occurs at a point 21 near the cathode 1 .

As described above, in conventional electron guns, the intersection point position of each electron on the central axis differs significantly depending on the emission position of the electron on the cathode surface, and this is due to the large aberration of the cathode lens. means.
The crossover diameter, ie, the minimum diameter of the electron beam, is determined by the diameter 22 of the circle of least confusion created by the collection of each electron beam. Naturally, the diameter 22 of the circle of least confusion increases as the aberration of the cathode lens increases. As described above, the main reason why the beam spot diameter of a conventional electron gun increases significantly when a large current is applied is attributable to the extremely large aberration of the cathode lens.

As described above, the present invention reduces the aberration of the cathode lens to reduce the crossover diameter during large currents, and at the same time maintains the beam divergence angle at an appropriate value. This made it possible to significantly reduce the beam spot diameter.

FIG. 3 is a configuration diagram showing an embodiment of the electron gun of the present invention.

A method for reducing the aberration of the cathode lens will be explained with reference to FIG. In this figure, G ₂ electrode 3 and
The interval g ₃ between the G ₃ electrodes 6 is significantly narrower than in the conventional example. By doing this, G ₁ electrode 2
The potential in the space from _G3 to electrode 6 increases. FIG. 4 shows the axial potential in this space, where the curve 28 is for the conventional example and the curve 29 is for the case where the interval g ₃ in FIG. 3 is made small.

In this way, in the structure of FIG. 3, the axial potential near the G ₂ electrode 3 increases, so that the focusing force applied to the electrons 23 emitted from the center of the cathode 1 becomes stronger. Therefore, the position 24 of the intersection point where these electrons 23 occur on the axis is moved closer to the cathode 1 than in the conventional example shown in FIG. As a result, the deviation from the intersection point 26 formed by the electrons 25 emitted from the periphery of the cathode 1 is reduced. In this way, all the electrons emitted from the cathode 1 will have a crossover point within a narrower range on the axis. This means that the aberration of the cathode lens is reduced,
The crossover diameter 27, which is determined by the minimum confusion of each electron beam, is therefore also reduced.

As a result of electron trajectory calculation analysis, the above effect is clearly observed when the ratio g ₃ of the distance g 3 between the G ₂ electrode 3 and the G ₃ electrode 6 to the pore diameter D ₁ of the G ₁ electrode is g ₃ /
This is limited to cases where D ₁ is set to a small value of 1.5 or less.
However, making g ₃ /D ₁ smaller than necessary is disadvantageous in terms of manufacturing and the breakdown voltage between the electrodes, so it is preferable to set it to 0.4 or more. by the way,
When g ₃ /D ₁ is reduced in this manner, the potential at each of the intersection points 24, 26, etc. increases as described above, so that the beam becomes difficult to converge after the intersection point, and the divergence angle 12 increases. As a result, the beam spreads too much at the main lens, creating a halo around the beam spot 11 on the screen 10, reducing the contrast of the image. Therefore, a sufficient improvement effect cannot be expected simply by reducing g ₃ /D ₁ to a small value of 1.5 or less.

Therefore, the pore diameter D ₂ of G ₂ electrode 3 is the pore diameter of G ₁ electrode 2.
By making D smaller than ₁ , the curvature of the equipotential lines 30, 31, etc. near the hole increases, the focusing effect becomes stronger, and the beam becomes more concentrated after the intersection of the electron beams 24, 26, etc. It will be able to be narrowed down effectively. Also, by making D ₂ smaller than D ₁ , G ₂ to draw the electron flow from the cathode
It becomes possible to lower the potential of the electrode 3. In this case, the potential difference between the G ₂ electrode 3 and the G ₃ electrode 6 increases by that amount, so the lens effect due to the equipotential lines 30, 31, etc. generated near the G ₂ electrode is further emphasized, and the divergence angle 12 is set to an appropriate value. It will be possible to maintain it.

As described above, the present invention provides g ₃ /D ₁ of 1.5 or less, 0.4
By making D ₂ smaller than D ₁ at the same time, it becomes possible to reduce the crossover diameter at high currents and maintain the beam divergence angle at an appropriate value. It becomes possible to significantly improve the spot characteristics of the spot.

A specific example of the present invention is shown below.

G ₁ electrode hole diameter D ₁ ; 0.65mmφ G ₂ electrode hole diameter D ₂ ; 0.3mmφ G ₃ electrode hole diameter D ₃ ; 1.0mmφ Cathode- _{G 1} electrode spacing g ₁ ; 0.1mm G ₁ electrode thickness t ₁ ; 0.1mm G ₁ Electrode - G ₂ electrode spacing g ₂ ; 0.2 mm G ₂ electrode thickness t ₂ ; 0.2 mm G ₂ electrode - G ₃ electrode spacing g ₃ ; 0.65 mm g ₃ /G ₁ = 1 G ₂ electrode potential; 300V G ₃ electrode Potential: 6KV G ₄ electrode potential: 25KV Figure 5 is a characteristic diagram of the beam spot diameter ratio (beam spot diameter before improvement at the center of the screen is 1) with respect to g ₃ /D ₁ (beam current is 3000 μA)
The curve a is for the case where D ₂ /D ₁ =1. In this case, although an improvement effect appears as g ₃ /D ₁ becomes smaller than 1.5, the practical limit of g ₃ /D ₁ = 0.4
Even if you reduce it to 15%, it will only be reduced by 15% at most. Curve b is for D ₂ /D ₁ =0.7, and when g ₃ /D ₁ becomes smaller than 1.5, a significant improvement effect begins to appear. If D ₂ /D ₁ is reduced to 0.46 as in the above embodiment, the beam spot diameter can be reduced by about 30% when g ₃ /D ₁ =1.

In addition, in the design of the electron gun shown in Fig. 2,
The spacing between each grid electrode is determined based on the hole diameter D ₁ of the G ₁ electrode, but the spacing g ₃ between the G ₂ electrode and G ₃ electrode is set to 0.4
By setting the G 2 electrode to satisfy the relationship of ≦g ₃ /D ₁ ≦1.5 and setting the hole diameter D ₂ of the G ₂ electrode to D ₂ <D ₁ , the beam spot diameter at high current is reduced compared to the conventional structure. can be done.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing the configuration of a conventional electron gun and electron trajectories, Figure 3 is an explanatory diagram showing an embodiment of the present invention, and Figure 4 is an on-axis diagram of the present invention and the conventional example. The potential comparison diagram, FIG. 5, is a characteristic diagram of the beam spot diameter ratio with respect to g ₃ /D ₁ . 1...Cathode, 2...G ₁ electrode, 3...G ₂ electrode, 4...Cathode lens, 5...Crossover, 7
...Pre-focus lens, 8...G ₄ electrode, 9
...Main lens, 10...Screen, 11...
...Beam spot, 12...Beam divergence angle, 1
3, 16, 30, 31... Equipotential line, 14, 1
7,20,23,25...electron orbit, 18,2
1, 24, 26...crossing point of each electron, 28, 29
...Axis potential distribution.

Claims (1)

[Claims] 1. A cathode, a first grid electrode having a circular hole for passing an electron beam having a diameter D ₁ in an electrode portion spaced apart from the cathode by a certain distance, and a first grid electrode spaced apart from the first grid electrode by a certain distance. A second grid electrode is provided with a circular hole for passing the electron beam with a diameter D ₂ in the separated electrode part, and a constant DC voltage is applied, and the electron beam is inserted into the electrode part separated from the second grid electrode by a distance g ₃ . An electron gun comprising a third grid electrode provided with a passage hole and to which a constant DC voltage is applied, and satisfying the following relationship: 0.4≦g ₃ /D ₁ ≦1.5 D ₂ <D ₁ .