Cathode ray tube with low dynamic correction voltage

Details Cathode ray tube with low dynamic correction voltage Cathode ray tube with low dynamic correction voltage

2000-02-08

2001-07-03

Ham, Seungsook (Department: 2878)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Cathode-ray deflections circuits

C315S382100, C313S414000

Reexamination Certificate

active

06255788

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a cathode ray tube having an electron gun equipped with a main lens having a function of controlling a shape of an electron beam spot which is deflected to the peripheral portion of an display screen, to improve a resolution at the peripheral portion of the screen of the cathode ray tube for use in a direct view color television receiver or a color display terminal.
The cathode ray tube which is utilized in color display of a direct view type or projection type television receiver, display terminal device and the like, is composed of a panel portion that is an image screen, a neck portion accommodating an electron gun, and a funnel portion for connecting the panel portion and the neck portion. A deflection yoke is attached to the funnel portion for scanning an electron beam emitted from the electron gun on a phosphor screen that is formed on an inner face of the panel portion.
The electron gun which is accommodated in the neck portion is provided with an electron beam generating unit having a cathode for generating the electron beam and a control electrode for controlling the electron beam, and a main lens unit comprising various electrodes for focusing, accelerating and converging the controlled electron beam.
The electron beam emitted from the cathode is modulated by signals applied on the control electrode or the cathode, and is directed onto the phosphor screen after being formed into a required sectional shape and provided with a required energy by the main lens electrodes.
FIG. 5
shows a schematic sectional diagram for explaining an example of the structure of the color cathode ray tube, of which shape of the electron gun portion is exaggerated for the purpose of explanation.
In
FIG. 5
, the electron gun accommodated in the neck portion is composed of the electron beam generating unit and the main lens unit which accelerates and focuses the electron beam generated from the electron beam generating unit and the electron beam is made to impinge on a phosphor screen
3
composed of three color phosphor materials which are coated and formed on an inner wall of a faceplate portion
2
composing a glass envelope
1
.
The electron beam generating unit is composed of cathodes
7
,
8
and
9
, a first grid electrode (G
1
)
10
, and a second grid electrode (G
2
)
30
. The electron beams which have been emitted from the cathodes
7
,
8
and
9
, are radiated along center axes
35
,
36
and
37
which are disposed approximately in parallel with each other in a common plane (in the horizontal direction) and are incident on the main lens unit after passing through the first grid electrode
10
and the second grid electrode
30
.
The main lens unit is composed of a third grid electrode (G
3
)
31
that is one main lens electrode, a fourth grid electrode (G
4
)
32
and a shield cup electrode
33
. The center axes of electron beam passing holes
70
,
71
,
72
,
76
,
77
and
78
which are formed in the third grid electrode (G
3
)
31
and the shield cup electrode
33
, are on the center axes
35
,
36
and
37
, respectively.
Further, the center axis of a central electron beam passing hole
74
of the fourth grid electrode
32
which is the other main lens electrode, is on the center axis
36
. However, the center axes
38
and
39
of side electron beam passing holes
73
and
75
are not on the center axes
35
and
37
, and are slightly displaced from the center axes
35
and
37
toward the outside, respectively.
In operation, the potential level of the third grid electrode
31
is set lower than that of the fourth grid electrode
32
. The fourth grid electrode
32
and the shield cup electrode
33
having a high potential level is connected to a conductive film
5
such that the potential level thereof is equal to that of the conductive film
5
that is coated on the inner face of the funnel portion by a conductive spring or the like, not shown.
Since the center electron beam passing holes of the third grid electrode
31
and the fourth grid electrode
32
are coaxial, an axisymmetric main lens is formed at the central portions of the two electrodes, and the central electron beam is focused by the main lens and proceeds straight on a trajectory along the axis.
On the other hand, since the axes of the side electron beam passing holes of the two electrodes are deviated from each other, a non-axisymmetric main lens is formed at the side. Therefore, the outside electron beams pass through locations which are deviated from the center axes of the lens toward the central electron beam in a diverging lens region that is formed on the side of the fourth grid electrode
32
, in the main lens region, and receive a focusing action by the main lens and at the same time a converging force toward the central composed of the focusing electrode, the accelerating electrode and the shield cup electrode.
The focusing electrode composing the main lens unit is composed of a first member and a second member, and the electrostatic quadrupole lens is composed by opposing an aperture electrode provided in the first member and planar correction electrodes provided in the second member.
The acceleration electrode is impressed with a final accelerating voltage of 20 through 35 kV that is the highest voltage. Further, a first focusing voltage is applied on the focusing electrode, which is normally a constant voltage of 5 through 10 kV.
On the other hand, a second focusing voltage is applied on the second member of the focusing electrode. The second focusing voltage comprises a constant voltage superposed by a dynamic correction voltage that changes in synchronism with a deflection amount of the electron beam.
The resolution at the peripheral portion of the screen of a color cathode ray tube is considerably improved by using the above electron gun. That is, a correction is performed wherein an astigmatism which elongates in the horizontal direction the electron beam spot that is deflected to the peripheral portion of the screen owing to a self-convergent magnetic deflection field and another astigmatism that elongates the electron beam formed by the electrostatic quadrupole lens in the vertical direction cancel each other.
The distance from the main lens to the center of the screen and the distance from the main lens to the peripheral portion of the screen are different. Therefore, when the electron beam is focused at the center of the image plane in an optimum condition, the focusing condition is deviated from the optimum condition at the peripheral portion of the screen, and this is a curvature-of-field aberration which brings about the deterioration in the resolution. The curvature-of-field aberration is corrected by the above-mentioned dynamic correction voltage, that is, when a dynamic correction voltage is applied, the intensity of the main lens which is a final stage lens formed between the accelerating electrode and the second member of the above-mentioned focusing electrode, is reduced, the deflected electron beam can be optimally focused at the peripheral portion of the screen, and the curvature-of-field aberration as well as the astigmatism are corrected.
However, when the electron gun having this electrostatic quadrupole lens is employed, an electric circuit for generating the dynamic correction voltage is necessary, which increases the production cost especially when the dynamic correction voltage is high. Accordingly, it is necessary to improve a correction sensitivity in deflection aberration.
When the strength of the electrostatic quadrupole lens is increased, the correction sensitivity of the astigmatism in the deflection aberration can easily be improved. However, with respect to the curvature-of-field aberration, the correction sensitivity can not be easily improved, since the curvature-of-field aberration is corrected by the main lens. When the strength of the main lens is increased to improve the correction sensitivity for curvature-of-field aberration, it is not possible to focus the electron beam on the screen, even wh

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