Electron gun for a cathode ray tube

An electron gun for a cathode ray tube is formed such that the spacing between the electron beam passing holes in a static focus electrode is longer than that between the electron beam passing holes in a dynamic focus electrode which forms a dynamic quadrupole lens with the static focus electrode, thereby enhancing the focusing and convergence characteristics of electron beams.

BACKGROUND OF THE INVENTION 
The present invention relates to an electron gun for a cathode ray tube, 
and particularly to an electron gun for a color cathode ray tube, wherein 
the astigmatism is reduced and the convergence characteristic is enhanced. 
Generally, as illustrated in FIG. 1, a cathode ray tube is formed such that 
a panel 21 having a shadow mask frame assembly 24 mounted in the inside 
thereof meets with a funnel 22 which holds an electron gun 1 in a neck 23 
at the end of the funnel, and a deflection yoke 30 is installed on the 
external surface of the neck. 
In the cathode ray tube 20, R (Red), G (Green), and B (Blue) electron beams 
emitted from electron gun 1 are optimally focused on the center of a 
phosphor layer formed on the inner surface of the panel 21. Also, even 
though the R, G, and B electron beams converge at one spot, when the three 
electron beams deflect toward the periphery of the phosphor layer, the 
trajectory of the beam is formed as illustrated in FIG. 1, so that the R, 
G, and B electron beams do not converge at one spot and, moreover, the 
beam spot becomes distorted due to astigmatism. These factors degrade 
color purity and resolution at the periphery of an image. 
FIG. 2 is a schematic view illustrating a conventional electron gun for a 
cathode ray tube designed to solve above-described problem. 
The electron gun illustrated in FIG. 2 is composed of a preceding triode 
consisting of cathodes 2, a control electrode 3, and a screen electrode 4, 
a main lens system having a static focus electrode 5 for focusing and 
accelerating electron beams, a dynamic focus electrode 6, and an anode 7. 
Vertically-elongated electron beam passing holes 5H are formed in the 
outgoing side 5a of static focus electrode 5 to correspond to 
horizontally-elongated electron beam passing holes 6H in the incoming side 
6a of dynamic focus electrode 6. In this electron gun, static focus 
voltage Vsf and anode voltage Ve are respectively supplied to static focus 
electrode 5 and anode 7, and a parabolic dynamic focus voltage Vdf is 
supplied to dynamic focus electrode 6, which is synchronized with the 
vertical/horizontal synchronizing signals of the deflection yoke and its 
lowest voltage is the same as the static focus voltage. 
In the conventional dynamic focus electron gun 1 formed as described above, 
when the electron beams emitted from cathodes 2 deflect toward the 
periphery of the phosphor layer due to the deflecting magnetic field of 
the deflection yoke, dynamic focus voltage Vdf (synchronized with 
vertical/horizontal deflection signals supplied to the deflection yoke) is 
supplied to dynamic focus electrode 6. Thus, a quadrupole lens can be 
formed between static focus electrode 5 and dynamic focus electrode 6, 
which compensates for the astigmatism of the electron beams deflecting 
toward the periphery of the image. 
The specific description of this operation is presented as below. While the 
electron beams deflect toward the periphery of the phosphor layer, a 
dynamic focus voltage higher than the static focus voltage is applied to 
the dynamic focus electrode 6, so that a lens of weaker focusing force and 
stronger diverging force is formed in the vertical direction relative to 
the horizontal direction. Stated conversely, the lens has stronger 
focusing force and weaker diverging force in the horizontal direction 
relative to the vertical direction. Here, this lens is formed by the 
vertically-elongated electron beam passing holes 5H formed in the outgoing 
side 5a of static focus electrode 5, and the horizontally-elongated 
electron beam passing holes 6H formed in the incoming side 6a of dynamic 
focus electrode 6. Therefore, the electron beams passing through the lens 
are under the influence of a force which focuses in the horizontal 
direction and diverges in the vertical direction, so that the 
cross-sectional shape of the beams becomes vertically-elongated. When the 
deformed electron beam having a distorted cross-sectional shape deflects 
toward the periphery of the phosphor layer, the deflecting magnetic field 
of the deflection yoke 30 compensates the distortion of the electron beam 
caused by a non-uniform deflecting magnetic field. As a result, the same 
circular beam spot can be obtained at the periphery as at the center of 
the screen. 
Also, since dynamic focus voltage Vdf whose lowest voltage is the same as 
the static focus voltage Vsf is supplied to dynamic focus electrode 6, the 
potential difference from anode 7 is relatively decreased, which weakens 
the intensity of the major lens formed between these two points and, in 
turn, the focusing distance of the electron beam is lengthened. Therefore, 
the electron beam is optimally focused at the periphery of the phosphor 
layer. 
However, according to experiments of this applicant, for electron beams 
deflected toward the right and left sides of the phosphor layer, the 
electron beams land optimally when the potential difference between focus 
electrode 5 and dynamic focus electrode 6 is 900 V. When deflecting toward 
the phosphor layer's corners, the potential difference should be 1500 V. 
The aforesaid conventional electron gun enables the focus characteristic to 
be enhanced and astigmatism to be improved. However, this electron gun has 
a limitation in that the convergence characteristic which makes the R, G, 
and B electron beams converge on one spot of the phosphor layer cannot be 
enhanced. As illustrated in FIG. 3, in order to overcome the functional 
restriction of the conventional electron gun, a cylindrical blade 5b' is 
fixed to the edge of the central electron beam passing hole among the 
electron beam passing holes formed in the outgoing side 5a' of a focus 
electrode 5', and semi-circular blades 5c' toward the central electron 
beam passing hole are each fixed to the inner edges of the flanking 
electron beam passing holes. Also, semi-circular blades 6b' are fixed to 
the outer edges of the flanking electron beam passing holes, and are 
formed in the incoming side 6a' of a dynamic focus electrode 6'. 
Therefore, the convergence is enhanced throughout the entire phosphor 
layer. However, in this case, even though dynamic focus voltage Vdf is 
supplied to dynamic focus electrode 6', since a quadrupole lens for 
compensating astigmatism is not formed, the distortion of the electron 
beam due to a non-uniform magnetic field of the deflection yoke 30 at the 
periphery of the phosphor layer cannot be corrected when the electron beam 
deflects. For this reason, the image resolution of the cathode ray tube 
deteriorates. 
SUMMARY OF THE INVENTION 
The present invention is designed to solve the above-described problems. 
It is the object of the present invention to provide an electron gun for a 
color cathode ray tube, capable of improving both astigmatism compensation 
and the convergence characteristic of electron beams which are emitted 
from cathodes and land on a phosphor layer. 
To achieve the object, there is provided an electron gun for a cathode ray 
tube comprising: 
a triode consisting of cathodes for emitting thermoelectrons, a control 
electrode and a screen electrode for transforming the emitted 
thermoelectrons into electron beams; and a main lens system for focusing 
and accelerating of the generated electron beams, consisting of a static 
focus electrode which has three vertically-elongated beam passing holes in 
its beam outgoing plane and is supplied with a static focus voltage, a 
dynamic focus electrode which has three horizontally-elongated electron 
beam passing holes in its beam incoming plane opposing to the beam 
outgoing plane of the static focus electrode and is supplied with a 
dynamic focus voltage, and an anode supplied with an accelerating voltage 
which is the highest among the three, wherein 
the spacing between electron beam passing holes in the incoming side of the 
dynamic focus electrode is shorter than the spacing between the electron 
beam passing holes in the outgoing side of the focus electrode. 
The electron gun of the present invention can have improved characteristics 
by forming the spacing between the electron beam passing holes of the 
static focus electrode to be longer than that between the cathodes.

DETAILED DESCRIPTION OF THE INVENTION 
As illustrated in FIG. 4, an electron gun 10 for a cathode ray tube 
according to the present invention has a triode consisting of cathodes 11, 
a control electrode 12, and a screen electrode 13, and a main lens system 
including a static focus electrode 14, a dynamic focus electrode 15 and an 
anode 16. 
As illustrated in FIGS. 5 and 6, vertically-elongated electron beam passing 
holes 14C and 14S, and horizontally-elongated electron beam passing holes 
15C and 15S are formed in the beam outgoing plane 14a of static focus 
electrode 14, and in incoming plane 15a of dynamic focus electrode 15, 
respectively. These two electrodes together form a dynamic quadrupole 
lens. According to the characteristic of the present invention, a spacing 
S1, i.e., the interval between the centers of vertically-elongated 
electron beam passing holes 14C and 14S in beam outgoing plane 14a of the 
static focus electrode 14, is longer than a spacing S2 which is the 
interval between the centers of horizontally-elongated electron beam 
passing holes 15C and 15S in beam incident plane 15a of dynamic focus 
electrode 15. Thus, the vertically-elongated outer electron gun passing 
holes 14S on either side of static focus electrode 14 are more outwardly 
positioned than the horizontally-elongated electron beam passing holes 15S 
on either side of dynamic focus electrode 15, so that the electron beams 
stray from the center lines C of the electron gun. 
Referring back to FIG. 4, an electron gun 10 of another aspect of the 
present invention is formed such that the spacing S1 between electron beam 
passing holes of static focus electrode 14 is longer than a spacing S3 
between cathodes, while spacing S1 and S2 have the same relationship as 
the above-stated structure. A reference symbol Vsf is a static focus 
voltage supplied to static focus electrode 14, and Vdf is a dynamic focus 
voltage supplied to dynamic focus electrode 15. 
The operation of the above-described electron gun for the cathode ray tube 
according to the present invention is as follows. 
In the electron gun 10 for the cathode ray tube of the present invention, 
when predetermined voltages are applied to each electrode, a prefocusing 
lens is formed between screen electrode 13 and static focus electrode 14. 
Also, a quadrupole lens whose horizontal and vertical intensities are 
different, is formed between static focus electrode 14 and dynamic focus 
electrode 15 in accordance with the fluctuations of dynamic focus voltage 
Vdf supplied to dynamic focus electrode 15. Additionally, a major lens is 
formed between dynamic focus electrode 15 and anode 16 to finally 
accelerate the electron beams. 
When the R, G, and B electron beams emitted from respective cathodes 11 of 
electron gun 10 land on the center of the phosphor layer without being 
deflected by deflection yoke 30, dynamic focus electrode 15 is supplied 
with dynamic focus voltage Vdf which is the same as the static focus 
voltage of focus electrode 14. Therefore, a quadrupole lens is not formed 
between static focus electrode 14 and dynamic focus electrode 15, so that 
the R, G, and B electron beams RB, GB, and BB pass through this portion 
unaffected, and then pass through the center of the major lens, thereby 
optimally focusing and converging on the center of the phosphor layer. 
As the electron beams RB, GB, and BB respectively emitted from cathodes 11 
of electron gun 10 deflect toward the periphery of the phosphor layer by 
deflection yoke 30, the quadrupole lens is formed between static focus 
electrode 14 and dynamic focus electrode 15 due to the potential 
difference between the static focus voltage and the dynamic focus voltage. 
Here, spacing S1 between vertically-elongated electron beam passing holes 
14C and 14S in outgoing plane 14a of static focus electrode 14 is greater 
than spacing S2 between horizontally-elongated electron beam passing holes 
15C and 15S in incoming plane 15a of dynamic focus electrode 15. That is 
to say, the centers of vertically-elongated electron beam passing holes 
14S on either side of static focus electrode 14 are positioned further 
away from the center line C of electron gun 10 than the 
horizontal-elongated electron beam passing holes 15S of dynamic focus 
electrode 15 from the center lines C of electron gun 10, so that both 
sides of the quadrupole lens become asymmetric as shown in FIG. 7A. 
Accordingly, R and B electron beams RB and BB on either side among the 
electron beams emitted from respective cathodes 11 first deflect outwardly 
while passing through the quadrupole lens at a predetermined angle, and 
then pass through the periphery of the major lens, meanwhile the central 
electron beam GB passes through the center of major lens unaffected by the 
central symmetric quadrupole lens. Thus, the electron beams can focus and 
converge at the periphery of the phosphor layer in the optimum condition. 
In other words, when R and B electron beams RB and BB respectively emitted 
from cathodes 11 pass through asymmetrically formed quadrupole lens at 
both outer sides, since the force F affecting the electron beams is 
exerted at right angles to the equipotential line, and the electron beams 
are moving in the direction of the electron gun's axis by a predetermined 
speed V, the actual advancing path of the electron beams outwardly deflect 
by a predetermined angle. As a result, the electron beams having passed 
through the quadrupole lens optimally focus and converge on the periphery 
of the phosphor layer in the optimum condition after passing through the 
center of the major lens, as the trajectory of the electron beams 
illustrated in FIG. 4. 
In another embodiment of the electron gun according to the present 
invention, if a spacing S3 (FIG. 4) of cathodes 11 is shorter than spacing 
S2 of static focus electrode 14, both sides of prefocusing lens formed 
between screen electrode 13 and static focus electrode 14 are 
asymmetrically formed, which is not shown in the drawings. Therefore, as 
shown in FIG. 7B, the trajectory of the electron beams inwardly moves as 
compared with the case that spacing S3 of cathodes 11 is the same as the 
spacing S2 of static focus electrode 14. Also, since the electron beams 
advance by being biased toward the inner part of both sides of the 
quadrupole lens formed between static focus electrode 14 and dynamic focus 
electrode 15, the electron beams outwardly advance as described above, 
thereby optimally focusing and converging on the periphery of the phosphor 
layer. 
As described above, in the electron gun for the cathode ray tube according 
to the present invention, the spacings of the electron beam passing holes 
of static focus electrode and dynamic focus electrode which form the 
dynamic quadrupole lens are different from one another, so that the 
present invention is advantageous in that the astigmatism compensation, 
focus and convergence characteristics of the electron beams are improved 
to realize an image of good quality.