Note: Descriptions are shown in the official language in which they were submitted.
7~
-l- RCA 77,583
ELECTRON GVN HAVING AN ASTIGMATIC
BEAM FORMING REGION
This invention relates to electron guns for
cathode-ray tubes and, particularly, to beam forming
electrodes of unitized inline guns used with self-
converging yokes.
Present cathode-ray tube systems, for displaying
color images for television, may include an electron gun
designed to generate three inline beams disposed in a
common hsrizontal plane and a self-converging deflection
yoke designed to maintain the beams converged as they are
scanned over the screen of the tube. In such a system,
the deflection field of the yoke is inhexently astigmatic
by desi~n so as to obtain its self~converging
characteristic. However, this asti~matism, which
desirably produces the self-convergence, at the same time
undesirably produces a distortion on the cross-sectional
shape o the electron beams. Specifically, the yoke field
is over-converging in the vertical plane and under-
converging in the horizontal plane. Thus, if the electron
gun is arranged to produce a circular beam spot at the
center of the screen, the spot will become horizontally
elongated with a vertically extending flare or smear when
it is scanned to the corners of the screen.
In the inllne electron gun disclosed in U.S.
Patent 3,772,554, issued to Hughes on November 13, 1973,
the main focus lens of the gun is formed between two
facing cup-shaped electrodes. Each of these cup-shaped
electrodes has three inline beam apertures formed in a
base portion thereof. Tubular lips surround the acing
a~ertures of the main lens electrode and extend into the
interior of the cup-shaped electrodes. In U.S. Patent
4,350,923, issued to Hughes on September 21, 1982, it is
disclosed that the focus field established between the
cup-shaped electrodes of the main lens is also astigmatic
and, like that of the self~converging yoke field, is
over~conver~ing in the vertical plane and
~,y~
-2- RCA 77,583
under-converging in the horizontal plane, thus further
contributing to undesirable beam spot distor~ion. U.S.
Pakent 4,350,923 suggests adjuc;ting the length of the lips
around the main lens apertures to provide a reversal of
the gun's fringe field astigmalism to compensate for the
yoke's vertical over-convergence stigmatism. This
solution is directly applicable to main lens structures
such as those disclosed in U.S~ Pa-tent 3,772,554; however,
a solution that is applicable 1o a large variety of
electron gun designs is desired. Such a solution requires
a compensating correction to the electron beams before
they enter the main focus lens
One compensating stnlcture is disclosed in U.S.
Patent 4,234,814, issued to Chen et al. on November 18,
1980. That p~tent discloses a screen grid having three
rectangular slot portions with an electron beam aperture
opening into each of -the 510ts~ The slots create an
astigmatic field that produces under-convergence o the
electron beams in one plane to compensate for the flare
distortion of the beam spot at off-center positions on the
image screen. Ho~ever, the slot width is slightly greater
than the apPrture diameter so that the slot is nearly
tangent to the aperture, and the astigmatic field is
strongest at the two oppo~itely disposed points where the
slot is closest to the aperture. Even in grid structures
in which the slot width is equcll to the beam aperture
diameter, such as that disclosed in U.S. Patent 4,251,747
issued to Burdick on February ].7, 1981, the efect of -the
astigmatic field of the slot is strongest only at the
poin-ts of tangency of the slot and the beam aperture.
Thus, there is a need to provicle an astigmatic beam
orming elec~rode which produces an astigmatic field which
has a greater effect than that heretofore achieved in the
prior art.
In accordance with the present invention, a
cathode-ray tube has an electrc)n gun for use with an
.2~7~ii~31
-3- RCA 77,583
astigmatic magnetir deflection yoke, which comprises means
for generating a~ least one elec~ron beam alony a beam
path toward a screen of the tube. The electron gun
includes a beam orming portion proximate to th~
generating means and a main lens portion remote from the
generating means. The beam forming portion comprises a
plurality of electrodes, one of which is an astigmatic
beam forming electrode which introduces a compensating
asymmetry into the electron beam. The astigmatic beam
formin~ electrode has a f~nctional grid portion of a given
thickness with at least one aperture formed there-through.
The aperture is intersected by a slot formed in the
functional grid portion. The slot, which has a depth less
than the thickness of the functional grid portion, ex~ends
across oppositely disposed quadrants of the aperture.
In the drawings:
FIGURE 1 (Sheet 1) is a plan view, partially in
axial section, of a shadow mask cathode-ray tube and
self-converging yoke.
FIGURE 2 (Sheet 1) is a longitudinal section of
an improved electron gun used in the cathode ray tube of
FIGURE 1.
FIGURE 3 (Sheet 3) is a plan view of a novel
astigmatic control grid ~lectrode.
FIGURE 4 (Sheet 2) is an enlarged plan view of
an aperture and intersecting slot oxmed in the novel
astigmatic electrode of FIGURE 3.
FIGURE 5 (Sheet 2) is a sectional view of the
aperture and slot taken along line 5-5 of FIGURE 4.
FIGUR~ 6 ~Sheet 2) is an enlarged plan view of
an aperture and alternative intersecting slot formed in
the novel astigmatic ~eam forming electrode.
FIGURE 7 (Sheet 2) is ~ sec tional view of the
aperture and alternative slot taken along line 7-7 o~
FIGURE 6.
FIGURE 8 (Sheet 3) is a longitudinal section of
an electron gun embodying a modified novel asymme tric
control grid electrode.
q
-4~ RCA 77,583
FIGURE 9 (Sheet 4) is a graph of electron beam
size versus ca~hode potential :for the novel con~rol grid
electrode of FI~URE 4 having a slot depth of 0.20 mm.
FIGU~E 10 (Sheet 5) :is a graph of electron beam
size versus cathode potential :Eor a prior art control grid
elec-trode having a slot depth o~ 0.20 mm.
FIGURE 11 (Sheet 3) :is a plan view of a novel
astigmatic screen grid electrode.
FIGURE 1 is a plan v:iew o a rectangular
cathode-ray tube, e.g., a color pic-ture tube, hav1ng a
glass envelope 10 comprising a rectangular faceplate panel
or cap 12 and a tubular neck 14 connected by a rec~angular
funnel 16. The panel 12 compr:ises a viewing faceplate 18
and peripheral flange or sidewclll 20 which is sealed to
the funnel 16. A three-color phosphor screen ~2 is
carried by the inner surface o.E the faceplate 18. The
screen is preferably a line screen with the phosphor lines
extending substantially perpen(~icular to the hi.gh
frequency raster line scan of the tube (i.e., normal to
the plane of FIGUR~ 1). A multiapertured color sel~ction
electrode or shadow mask 24 is removably mounted, by
conventional means, in predetermined spaced relation to
the screen 22. An improved inline electron gun 26, shown
schematically by ~otted lines in FIGURE 1, is centrally
mounted within the neck 14 to generate and direct three
electron beams 28 along coplanar convergent paths through
the mask 24 to the screen 22.
The tube of FIGURE 1 is designed to be used with
an external magnetic deflection yoke, such as the
self-converging yoke 30 schemal;ically shown surrounding
the neck 14 and funnel 16 in the neighborhood of their
junction. When activated, the yoke 30 subjects the three
beams 28 to magnetic fields whi.ch cause the beams to scan
horizontally and vertically in a rectangular raster over
-the screen 22. The initial plane of deflection ~at zero
deflection) is shown by the line P-P in FIGUR~ 1 at about
the middle of the yoke 30. Because of fringe fields, the
i3
-5~ RCA 77,583
zone of deflection of the tube extends axially, from the
yoke 30 int.o the region of the gun 26. For simplici-ty,
khe actual curvature of the deflected beam paths in the
deflection zone is not show~ in FIGURE 1. The fringe
fields of the yoke 30 cause the beams to be deflected
slightly off axis and into more aberrated portions of the
electron le~s of the gun 26. The result, frequently, is a
flare distortion of the electron beam spot which extends
from the spot toward the center of the screen.
The details of the improved electron gun 26 are
shown in FIGURE 2. The gun comprises two glass support
rods 32 on which various electrodes are mounted. These
electrodes include three equally spaced coplanar cathode
assemblies 34 (one for each beam3, a beam forming region
comprising a novel control grid electrode 36 ~G1) and a
screen grid electrode 38 (G2), and a main lens region
comprising a first accelerating and focusing electrode 40
(G3), and a second accelerating and focusing electrode 42
(G4), spaced along the glass ro~ds 32 in the order named.
All of the post-cathode electro~es have at least three
inline apertures in them to pen~it passage o ~hree
coplanar electron beams. The m,ain electrostatic focusing
lens in the gun 26 is formed between ~he G3 electrode 40
and the G4 electrode 42. The G3 electrode 40 is formed
with cup~shaped elements whose open ends are attached to
each other. The G4 electrode 4:2 also is cup-shaped, but
has its open end closed with a r,hield cup 44. The portion
of the G4 electrode 42 facing the G3 electrode ~0 includes
three inline apertures 46, which are aligned with
corresponding apertures 48 in the G3 electrode 40.
As show~ in FIGURES 2 and 3, the novel G1
electrode 36 has three functional grid porkions 50 and a
support portion 52. Three circular recesses 54 having a
diameter of about 3.05 mm are formed in the cathode side
of the Gl electrode 36. The cathode assemblies 34 are
disposed within the recesses 54 and are located about
0.152 mm (6 mils) from the G1 e:Lectrode 36. An aperture
56 havlng a diam~ter of 0.635 ~n [25 mils~ and comprising
2~ 3
-6- RCA 77,583
four quadrants 56a, 56b, 56c and 56d, shown in FIGURES 4
and 5, is formed through the center o~ each of the
recesses 54. Three slots 58 a:re ormed respectively
within the three func~ional po:rtions 50 on the G2 side of
S the G1 electrode 36. The thickness of each of the
functional grid portions 50 within the recesses 54 is in
the range of 0.20 to 0.30 mm 58 to 12 mils). Each of the
slots 58 intersects a differenl~ one of the apertures 56.
An enlarged view of the functional grid portion 50 of the
G1 electrode 36, showing one o:E the apertures 56 and a
slot 58, is provided in FIGU~ES 4 and 5. The slot 58 has
a maximum longitudinal dimension, L, of about 2.03 mm ~80
mils) and a maximum transverse dimension, W, of a~out
1.02 mm (40 mils). As shown in FIGURE 4, the slot 58
intexsects the aperture 56 at two oppositely disposed
quadrants 56a and 56c. The width, w, of the slot 58 where
it intersects the aperture 56 is given by the equationo
w = r~
In this case r, the radius of the aperture, is 0.3175 mm0 and therefore
w ~- 0.449 n~. (2)
Thus, the width, w, of the slo~: 58 (about 0.449 mm) is
less than the diam~ter (0.635 n~) of the aperture 56. The
depth, d, of the slot is direct:ly proportional to the
strength of the astigmatic correction. Typically, ~he
510t depth is within the range of about 0.10 to 0.20 mm ~4
to 8 mils); ho~ever, shallower or deeper slots are within
the scope o~ this invention. For a slot depth of 0.20 mm,
a functional grid thickness of about 0.30 mm is required,
but where the slot is only 0.10 mm deep a grid thickness
of 0.20 mm is satisfactory. The slot 58 may be formed by
either electrical discharge mac~hining (EDM), coining, or
conventional drawiny methods. As shown, the longitudinal
dimension of the slot 58 extencLs perpendicular to the
inline direction of the three inline beam forming
apertures 56.
An alternative slot 158 is shown in FIGUR~S 6
and 7. The slot 158 extends across oppositely disposed
-7- RCA 77,583
guadrants of an aperture 156 and is similar to the slot
58, except that the slot is substantially rectangular wi-th
a transverse dimension that is constant along the slot
length. In this embodiment, the aperture 156 has a
diam~ter of 0.635 mm. The S101_ width is about 0.45 mm,
the length is about 2.03 mm, and ~he depth of the slot is
preferably within the range of 0.10 to 0.20 mm. The
rectangular slot 158 i5 moxe easily formed by conventional
drawing methods and is ~herefore less expensive to form
than a slot which is EDM produced.
Tn order to determine the effectiveness of the
slot 58 to provide an astigmatic electron beam, an
experimental electron gun 226, an embodiment of which is
shown in FIGURE 8, was constructed. The gun 226 produces
only a single electron beam from a cathode 234; however, a
single be~m is sufficient to evaluate the effect of the
slot.
The gun 226 comprises the cathode ~34, a Gl
electrode 236, a G2 electrode 238 and a G3 electrode 240
spaced along a pair of glass support rods (not shown) in
the order indicated. The ~1 electrode 236 is similar to
the novel control grid electrocle 36, differing there~rom
in that it has a single Gl aperture 242 therethrough. The
G2 electrode 238 of the experimental gun 226 includes a
support cup 244 having a first plate member 246 afixed to
the end proximate to the G1 electrode 236. The first
plate member 246 has a first plate aperture 248
therethrough which is coaxially aligned with the G1
aperture 242. Both the Gl aperture 242 and the first
plate aperture 248 have a diameter of 0.635 mm. A second
plate member 250 is attached to the opposite end of the
support cup 244, and a thi.xd plate member 2S2 is affixed
to the second plate member 250. A second plate aperture
254, having a diameter of about 3.05 mm, is formed ln the
second plate m~mber 250. A third plate aperture 256,
having a diameter greater than that of the second plate
aperture 254, is formed in the third plate mernber 2S2.
753
-8- RCA 77,583
The G3 electrode 240 has an enlarged G3 aperture 258
formed in the end proximate to the G2 electrode 238~ The
aperture 258 has a diameter of 3.05 mm. The remote end of
the G3 electrode has been removed, and a longitudinally
adjustable viewing screen 260 is locate~ therein. The
viewing screen 260 comprises a transparent support plate
262 having an aluminized phosphor coating 264 on one side
thereof. For the tests reported herein, the longitudinal
spacing between the ~l electrodle 236 and the G2 electrode
238 was about 0.28 mm (11 mils), and -the spacing between
the G2 electrode 238 and the G3 electrode 240 was 1.52 mm
(60 mils). The Gl electrode 236 was main~ained at ground
potential, the G2 electrode 238 was at 1047 volts, and the
G3 electrode 240 was operated at 7000 volts. The cathode
potential was varied from about 10 volts to about 150
volts, and the height and width of the resulting electron
beam on the screen 260 were measured and plotted. Three
G1 slot configurations for the electrode 236 were
evaluated. A prior art rectangular slot, with the slot
tangent to the aperture and having a slot depth of about
O.20 mm, was compared to two configurations of the novel
slot 58, shown in FIGURES 4 and 5. One configuration of
the novel slot had a slot depth of 0.10 mm, and the other
had a slot depth of 0.20 mm. The maximum length of each
of the Gl slots was 2.03 mm. The maximum width for the
novel slots 58 was 1.02 mm, and the minimum width was 0~45
mm. The maximum width for the rectangular prior art slot
wa~ 0.635 mm. The electron beam current in each test was
adjusted for 3 milliamperes of beam current, and the
screen 260 was located a distance of 22.86 mm (900 mils)
from the inside surface of the G3 electrode 240 nearest to
the G2 electrode 238. The beam height and width
dimensions, in millimeters, and a beam axial ratio,
defined as beam width divided by beam height, are listed
in the TABhE below for the three Gl configurations
described above.
i3
"
-9 RCA 77,583
TABLE
Slot Slot Dep~h Beam Height Beam Width Axial
Sample Type mm mm mm Ratio
A - Novel 0.20 2.54 7.16 2.82
5B Prior Art 0.20 2.84 5.33 1.87
C Novel 0.10 2.44 4.0~ 1.66
From the TABLE, it can be seen that the ~reatest
asymmetry, i.e., the strongest lens effect~ as measured by
the largest axial ratio, is produced by a novel slot 58
(Sample A) havin~ a depth of 0.20 mm; and that there is
little difference in lens effec-t between a deep
rectangular prior art slot ~sample B) and a relatively
shallow novel slot 58 (sample C). The change in electron
beam spot configuration as a function of Gl slot shape and
cathode potential is shown in FIGURES 9 and 10. In FIGURE
9, the electron beam height and width for a control grid
electrode having the novel slot 58 of sample ~ is shown.
In this test, the screen 260 is positioned a distance of
about 10.03 m~ from the bottom of th~ G3 electrode 240.
In FIGURE 10, the electron beam height and width for a
control grid electrode having the prior art rectangular
slot of sample ~ is shown. In the test of the prior art
rectangular slot, the screen 260 îs positioned at a
distance of 14.22 mm. A comparison of FIGURES g and 10
shows that at cathode potPntials below about 100 volts,
the beam axial ratio, defined as beam width divided by
beam height, is greater for the novel slot 58 than for the
prior art rectangular slot having the same depth. This
means that the novel slot 58 provides a stronger lens
e~fect than a prior art rectangular slot.
Rather than provide an astigmatic corxection to
the Gl electrode 36 as described above, it is possible -to
make the astigmatic correction to a G2 electrode 38'. As
shown in FIGURE 11, the G2 electrode 38' has three inline
apertures 70 therethrough. Each o the apertures 70
comp~ises four quadrants and has a diameter of 0.635 mm.
Three slots 58' identical in size to the slots 58 are
formed in the G1 side of the G2 electrode 33'. The slots
58l intersect the apertures 70 and ex-tend across
~2~3~7~3
~:LO- RCA 77,583
oppositely disposed quadrants of the apertures so that the
longikudinal dimensions of the slots extend in the inline
direction of the apertures. This slot orientation on the
Gl side of the G2 electrode 38'' produces the same
astigmatic effect as that p.rev:'Lously described for the
slots 58 ormed in the G2 side of ~he G1 electrode 36.
While a novel slot configura~ion is shown in FIGURE 11, a
rectangular slot configurationp such as tha~ shown in
FIGURE 6, is also within the scope of the invention.
