Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MAGNETIC SHUNT FOR DEFLECTION YOKES
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to display apparatus, and
more particularly relates to apparatus for reducing unwanted
magnetic radiation external to a cathode ray tube display
device without affecting the intended deflection field
within the bore of the yoke.
~ackground Art
Cathode Ray Tubes ("CRTs") generally have associated
coils, or yokes, to provide a varying magnetic field for
electron beam deflection, for example for raster scan. In
addition to manifesting itself within the CRT, for beam
deflection, this magnetic field also extends around the
outside of the CRT and beyond the display device. This
external magnetic field serves no useful purpose and an
effort is frequently made to reduce this part of the yoke
magnetic field. In particular the unwanted frequency range
is from lK to 350K hertz (VLF).
A. A. Seyno Sluyterman of Phillips describes the
radiated field due to the horizontal deflection system in
his paper entitled "The Radiating Fields of Magnetic
Deflection Systems and Their Compensation" presented in 1987
SID Society of Information Display Proceedings. In that
paper it shows that the radiated field of the horizontal
magnetic circuit of the yoke at mid-range, resembles a
vertically oriented dipole, whose mathematical center lies
on the long axis slightly ahead of the yoke,
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Means to provide reduction of this radiation are
proposed in this paper. In one case the Helmholtz coils are
"on top" and "below" the saddle-shaped deflection yoke. In
another case the Helmholtz coils are behind the yoke. The
coils are coupled to the deflection coils and the EMF is
induced therein, giving rise to a magnetic field which tends
to cancel the unwanted radiated magnetic field. However,
this is a relatively expensive and bulky solution to the
problem. A similar top and bottom coil configuration is in
published Finnish Patent Application 861458, April 4, 1986
of Nokia.
Another proposed solution is the placement of shielding
all around the CRT, which results in magnetic radiation
reduction from the eddy currents induced in the shielding.
However, this is also an expensive solution to the problem,
and results in only minimal reduction in the magnetic field
in front of the screen.
Accordingly, there is a need for means to reduce to
acceptable levels the residual magnetic field in front of
the cathode ray tube display device that provides an
inexpensive and compact solution to the problem.
In accordance with the supplemental disclosure in
applicant's co-pending Canadian application Serial No.
572,711, filed July 21, 1988, an apparatus for reducing thej
net distributed magnetic radiation all about the coil uses a
ring disposed about the tube, wherein the ring is of
magnetically permeable material.
While this is acceptable for many applications other
physical constraints such as the shapes of the coils, the
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tube or the presence of wedges for coil alignment can
prevent s~fficient coupling of the ring to the coil.
SUMMARY OF THE INVENTIO~
The present invention finds application in a cathode
ray apparatus including a cathode ray tube ("CRT") having a
screen for viewing and having a charged particle beam
directed at the screen from the rear thereof and aligned
with the central axis of the tube, but that may be
magnetically deflected from the axis, and having a
deflection coil producing a magnetic component from axially
aligned wire segments and a magnetic component from
circumferentially aligned wire segments relative to the
axis, giving rise to a net distributed magnetic field in
about the coil. An apparatus for reducing the net
distributed magnetic radiation all about the coil uses a
ring disposed between the coil and the screen, wherein the
ring is of magnetically permeable material having its
configuration and position relative to the coil selected to
minimize the net distributed magnetic field in front of the
coil. In accordance with the present invention a pair of
coupling wires are looped about the ring and coupled to the
coil to boost the induction of the magnetic field in the
ring to reduce the magnetic field about CRT with negligible
effect within the tube.
The invention may be embodied in forms which are made
of relatively inexpensive linear ferrite materials
configured in shapes that are inexpensive to provide, such
as a flat ring or the like. As such, it permits a
relatively inexpensive solution to the problem. In
addition, in tested embodiments the present invention has
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demonstrated dramatic reductions in the unwanted radiation
in front of CRTs to which it has been applied.
The foregoing and other objects, features and
advantages of the invention will be apparent from the more
particular description of the preferred embodiments of the
invention, as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF_THE DRAWI~GS
Fig. 1 is a diagram showing pertinent portions of an
integrated air core yoke tube component.
Fig. 2 is a simplified diagram of one winding each from
the upper and lower horizontal deflection coils of the
integrated yoke tube component shown in Fig. 1.
Fig. 3 is a computed plot showing the magnetic field
intensity along the Z axis for a typical deflection yoke
such as is shown in Fig. 1.
Fig. 4 is a figure like that of Fig. 1, having added
thereto a ring 50 in accordance with the preferred
embodiment of the present invention.
Fig. 5 is a diagram like that of Fig. 2, having added
thereto a ring 50 in accordance with the preferred
embodiment of the present invention.
Fig. 6 is a set of curves, on the same set of axes as
in Fig. 3, showing the effect on the net field A of ring 50.
Fig. 7 is a set of curves showing the effect of ring 50
on the end turn field shown in Fig. 3.
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Fig. 8 is an expanded view of the portion of the curve
shown in Fig. 6 beyond approximately 2.5 centimeters.
Fig. 9 is a plot like that of Fig. 8, wherein ring 50
is a slightly different distance from the yoke.
Fig. 10 is a diagram like Fig. 8, in which the inner
diameter radius of ring 50 is slightly different from that
of Fig. 8.
Fig. 11 is a curve like that of Fig. 8, but wherein the
distance of the ring 50 from the end of the yoke is
different from that of Fig. 8 and Fig. 9.
Fig. 12 is a diagram showing a CRT with yoke with a
ferrite core and the associated fields.
Fig. 13 illustrates the system of Fig. 12 with the
compensating ring.
Fig. 14 is a sketch of the top view of the core, coil
and ring of Fig. 13 illustrating magnetization currents and
fields.
Fig. 15 is a sketch of the front view illustrating
magnetization currents and fields.
Fig. 16 shows a preferred embodiment of the ring for
color tubes in which the ring is split providing two
portions.
Fig. 17 is a sketch of the split-ring illustrating the
shunt fields across the base of the tube.
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Fig. 1~3 is a cross-sectional diagram through a portion
of a still further embodiment of ring, made with
conventional ~ metal laminates.
Fig. 19 shows a further embodiment, having a hexagonal
shape.
Fig. 20 is a sketch of one embodiment of the present
invention which is a top view of the CRT, choke coil and
ring.
Fig. 21 is an electrical schematic diagram of the
present invention.
Fig. 22 is a diagram of one winding of the upper and
lower horizontal deflection coils with the wire loops
connected thereto.
Fig. 23 is a screen end view of the ring illustrating
the pair of loops.
Fig. 24 is a sketch of the ring with quadrature placed
pairs of holes through the ring through which the loops are
passed before returning to the driver terminal 4.
Fig. 25 is a sketch illustrating how the loops pass
through the holes in the ring in Fig. 24.
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DETAIL~D DESCRIPTIO~ OF EMBODIMENTS
Fig. l shows the pertinent portions of an integrated
yoke tube component ("ITC") 10 which includes CRT 12, having
a front screen 14, and upper and lower horizontal deflection
coils 16, 18. ~he deflection coils 16, 18 generate a vary-
ing magnetic field between them, inside CRT 12, to deflect
the electron beam within the tube 12 for horizontal sweeping
across the fact of the screen 14, as is well known in the
art.
Fig. 2 is a simplified diagram of one winding each from
the upper and lower deflection coils 16, 18 of Fig. 1.
Thus, loop 20 is a single loop from coil 16, while loop 22
is a single loop from coil 18. As illustrated, a current i
flows through each of the coils so as to generate the above
described varying magnetic field for horizontal deflection
of the electron beam.
In Fig. 2, X, Y, and Z axes are depicted, having their
origin in the plane of circumferential coil portions 34, 38
and centrally located between them. The X axis coincides
with the central axis of CRT 12 (Fig. 1). Note that the
upper and lower halves 20, 22 are symmetrical about the x z
and y-z planes.
In actual operation the upper and lower loops 20, 22
are interconnected to produce a dipole field on the Z axis
as is known. From the known coil shape and current, the B
field is given by:
B = 1l ~JXR dl
R2
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where J is the current, R is the direction and R is the
distance to a point of interest T on the Z axis. This
equation is used in computing the field distribution of
Figs. 3 and 7 through 12.
A plot of the computed B field distribution of an air
core horizontal deflection coil, such as is shown in Fig. 1,
without any high permeability material, like ferrite shield-
ing, is shown in Fig. 3. The actual B field is a direction-
al field, and the plot shown in Fig. 3 shows only the magni-
tude, or intensity, of such magnetic field along the Z axis.
The units depicted on the horizontal axis are centimeters,
while the units in the vertical axis are gauss. The curve
reflects a typical coil having current flowing so as to
produce a field which deflects a 20 kilovolt electron beam
to an angle of about 40 degrees.
Curves A, B, and C of Fig. 3 represent the total field,
the partial field from the axial wires and the partial field
from the end turns, respectively. Curve A is the magnitude
of the vector sum of the fields represented by curves B and
C. In typical uncompensated yokes, at 55 centimeters in
front of the yoke the field can be in range of approximately
1,000 - 2,000 nano-Tesla. Clearly, this is not very large
magnetic field. However, in accordance with the present
invention this field can be reduced to an even smaller
quantity. In actual experiments using the preferred embodi-
ment described below, reductions to below 200 nano-tesla at
55 centimeters was measured.
Fig. 4 shows the ITC 10 of Fig. 1 having added thereto
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a ring 50 of linear ferrite operating as a magnetic shunt,
in accordance with the supplementary disclosure in Appli-
cant's co-pending Canadian application no. 572,711.
Fig. 5 shows the loops 20, 22 of Fig. 2, with the
ferrite ring 50 disposed in front of it, to illustrate the
relative shape and position of ring 50.
Ring 50, as mentioned above, is a linear ferrite.
Linear ferrite is a well known material commonly used in
transformer and yoke production. According to the preferred
embodiment the ring 50 has a relatively high magnetic perme-
ability, (~ above 2,500). It also has a high volume resis-
tivity, for example 1 Meg Ohm or more per cubic centimeter.
The high resistivity value keeps eddy currents at a minimum.
Otherwise the loading effects on the yoke would result in a
need for more energy to drive the yoke. While embodiments
could be constructed, for example out of conventional
laminates, having this loading effect, and be in accordance
with the present invention, it was deemed desirable to keep
the eddy currents low, and avoid this loading effect in the
preferred embodiment. The cross section of the ring 50 is
large enough to avoid saturation.
Fig. 6 is a set of curves, on the same set of axes as
those of Fig. 3, showing the effect on the net field A shown
in Fig. 3 of a flat ring, such as ring 50 in Fig. 4, in
accordance with the preferred embodiment of the present
invention. Curve A in Fig. 6 is the same as curve A in Fig.
3. Curve D in Fig. 6 represents the field contribution from
the magnetization effect of the ring 50, while curve E
represents the resultant curve from the combination of
curves A and D.
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To better understand the effect of field D on the
overall magnetic field A, a set of curves is shown in Fig. 7
including curve D, and the end turn magnetic field component
C. Curve C is the same curve C as shown in Fig. 3. Curve F
is a curve representing the resultant field from the combi-
nation of curves D and C. Note that in Fig. 7 the horizon-
tal axis is the same in Figs. 3 and 7 while the vertical
scale has been expanded, to aid in clarity.
As mentioned above, curve D is the theoretical field of
the ring alone. This is an intrinsic field which is created
by the magnetization force of the end turn field. It should
be noted that the presence of the ring attenuates the end
turn field. The degree of attenuation is controlled by the
variables such as ring dimensions and ring yoke separation,
as is discussed in more detail below. It should be further
noted that the end turn field combines with the main deflec-
tion field, and the area in front of the CRT screen, to form
the net measurable residual field whose reduction is an
object of this invention. At optimum attenuation, the
modified end turn field F is equal it magnitude but opposite
in direction to the main deflection field, resulting in a
zero vector sum. As a practical matter, the net measurable
residual field in front of the CRT screen can never be
reduced to zero. However, by application of the principles
of my previous invention as disclosed herein, this field can
be reduced to very small levels.
The portion of Fig. 6 beyond approximately 2.5 centime-
ters to the right thereof is shown in Fig. 8. In order to
see clearly the curve behavior in that region, the scale is
expanded in the vertical direction as compared with Fig. 6.
Curves A and E are described in Fig. 6. Curve D is not
shown in this figure in the interest of providing more
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clarity for curves A and E. Note that Curve E is very
nearly at a zero field magnitude at approximately 9.5 centi-
meters.
The compensated curve E is for a typical CRT-yoke
configuration, having a ring 50 of ferrite with a permeabil-
ity of 1,000 - 3,000, and high volume resistivity, and
having an inner dimension of 4 centimeters, a thickness of
.2 centimeters, a width of 1 centimeter, placed at a dis-
tance of .4 centimeters from the end of the yoke. As used
herein, the width of the ring refers to its radial extent
from inner diameter to outer diameter.
Figs. 9-11 are plots like the plot shown in Fig. 8, for
slightly different ring configurations from the configura-
tion producing the curves of Fig. 8. Thus, in Fig. 9 all of
the parameters for the ring are the same as those corre-
sponding to Fig. 8, except the distance of the ring from the
end of the yoke. In Fig. 9 the curves correspond to a
configuration in which this dimension is .3 centimeters. It
will be appreciated that this reveals over-compensation, as
the curve E' is slightly above the horizontal axis, for
example of 9.5 centimeters and slightly above curve E in
Fig. 8.
The curves of Fig. 10 are for a configuration in which
the dimensions are the same as those corresponding to Fig.
8, but wherein the inner diameter radius is 5 centimeters,
instead of 4 centimeters. It can be seen that significantly
less compensation is provided, as curve E" is here below the
horizontal axis.
Fig. 11 shows a curve for a configuration wherein the
dimensions are as in Fig. 8, but wherein the distance of the
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ring from the end of the yoke is .6 centimeters, instead of
.4 centimeters. It can be seen that slightly less compensa-
tion is provided, causing curve E''' to cross the horizontal
axis of 9.5 centimeters. This was deemed to represent
optimum compensation.
While curves are not provided showing the effect of
change of width of the ring on the compensation effect, in
general, decreasing the width will tend to reduce the com-
pensating effect, while increasing the width will tend to
increase the effect.
Thus, from the above Figs. 8-11 it will be appreciated
how changing the various dimensional parameters of the ring
affects the performance of the ring in compensating by
cancelling the magnetic field components on the X axis in
front of the screen due to yoke winding and components.
Through an understanding of these effects, one practicing
the present invention can provide the adjustments deemed
desirable to optimize the cancellation affect.
In the above described arrangement the CRT tube has an
air core horizontal deflection coil without any high permea-
bility shielding about the neck of the tube. The direction
of the horizontal deflection field to move the beam toward
the right edge of the screen as viewed from the front is
represented by arrow 70 in Fig. 12. In common commercial
type yokes the horizontal deflection coils have ferrite
shielding (ferrite core) 68 about the horizontal deflection
coils as shown in Fig. 12. There is also vertical deflec~
tion coils tnot shown) about the horizontal deflection coils
and under the ferrite core. The radiated field produced by
the horizontal coils with the end loops 32, 34, 36 and 38
extending beyond the ferrite core is a
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dipole centered forward of the loop nearest the screen as
shown by arrows 70a in Fig. 12. Note the ferrite core
reverses the polarity of the radiated field. A ferrite ring
50 as shown and illustrated in Fig. 13 is mounted forward of
the horizontal deflection coil near the radiation center of
the horizontal coil. The manner in which this ring compen-
sates for the field radiation without measurably affecting
the deflection is illustrated in connection with Figs. 14
and 15. Fig. 14 is a sketch of the top view of the coil 16,
core 68 and ring 50 illustrating the deflection current in
the deflection coil, the magnetization currents and the
resulting fields. Fig. 15 is a front view of Fig. 14. The
counterclockwise current of the horizontal deflection coil
seen in the top view is represented by 71. The magnetic
field produced is represented by OH at the center that
points toward the viewer. This corresponds to 70 in Fig.
12. The ferrite core 68 is coupled to the deflection coil
and produces an even stronger equivalent magnetization
current Ml represented by the lines 72. The coupled current
72 circulates in the opposite direction (clockwise in Fig.
14) with current along adjacent surfaces of coil and core
flowing in the same direction. The result is a magnetic
field X1 (with a direction into the paper) in the center of
the core and l (with a direction out of the paper) in front
of the ring. The field X1 combines with field OH and pro-
duces a net radiated field l or 70a of Fig. 12 which is the
vector sum of OH and X1. The radiated field l is a dipole
field and is the major component of the magnetic radiation.
The exposed end-turns are radiating a minor quadrapole field
which is designated with Xe. Symbols "X's" and "O's" are
consistent with the sign convention established earlier
where X means the field is pointing down
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into the paper, O means the field is pointing up toward the
viewer. The sum X1 and Xe is the total radiated field
without the presence of the ring.
When a ferrite ring is placed in front of the yoke as
illustrated in Figs. 13 and 14 the ring will be magnetized
as described below. Magnetization currents Ml in the yoke
shield induce equivalent magnetization current M2 in the
ring in the counter clockwise directions. The resulting
field is pointing up within the ring (2) and pointing down
outside of the ring (X2). The polarization of this field is
also indicated in Fig. 15 with "N" (north) on top and "S"
(south) on the bottom of the ring. The front end-turns of
the horizontal coils (top, bottom) induce equivalent magnet-
ization currents M3 in the ring in a clockwise direction.
The resulting field X3 is pointing down within the ring and
pointing up outside of the ring O3. The polarization of
this field is also shown in Fig. 15 with letters "N"'
(north) and "S"' (south). From the distribution and polar-
ization of the induced magnetization current end fields we
conclude that the radiated field of the yoke shield X1 sets
up a dipole magnetization 2 in the ring which opposes the
radiating dipole. Similarly, the quadrapole component of
the radiated field due to the exposed end-turns of the
horizontal deflection coils induce a quadrapole magnetiza-
tion in the ring which cancels the radiating quadrapole.
Variables such as ring thickness, inside diameter, outside
diameter, permeability and yoke-ring separation can be used
to tune for optimum performance. Naturally, the lower limit
of the ring dimensions are dictated by the given CRT and
yoke combination. In practice, the tendency is to bring the
ring as close to the front of the yoke as possible without
adversely effecting the deflection field in the bore of the
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tube. This reduces the ring dimensions and assures minimum
cost. The ring has the lower limit of permeability of about
1,000 with the ring placed closest to the yoke. The higher
the permeability the greater the distance the ring can be
from the yoke.
Despite the effort to eliminate interference between
the ring and main deflection field, it was found that the
presence of the solid ring moves the center of deflection of
the vertical deflection field slightly back toward the
electron gun. This ls not noticeable in the monochrome
system, however, it causes about 10 6 meter mis-registration
in a color system and that is detectable. This problem is
fixed with a split ring, configuration Fig. 16. Here, part
of the induced dipole field as shown in Fig. 17 which is
normally conducted by the ring is forced to enter the bore,
and to join and strengthen the vertical deflection field,
thereby causing the center of deflection to move forward.
In practice, it was found that 2mm air-gap can compensate
10 6 meter mis-registration.
In actual prototype experiment, in conjunction with an
ITC manufactured by Matsushita Company having a serial
number of M34JDJOOX1, a ferrite ring of ordinary linear
ferrite was provided, having a u of approximately 1,000 -
3,000 and a volume of resistivity of greater than 1 meg ohm
per cc, ring dimensions of: an inner dimension of 4-3/8", a
width of 3/8", and a thickness of 1/8". This ring was found
to produce excellent cancellation effects when it was placed
against the circumferential wire portions (end closest to
the screen) of the yoke provided with this ITC with spacing
resulting only from the insulation of the yoke wire~.
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Embodiments may be made with conventional ~ metal
laminates, yielding rings having a cross-section as shown in
Fig. 18.
Fig. 19 shows a hexagonally shaped ring, representing a
still further embodiment for use with, for example, a hexag-
onally configured yoke.
As mentioned previously the compensation effect of the
ring is dependent on its width and other size and material
dimensions. According to the teaching herein these size and
material dimensions and the spacing from the end turns of
the coil can be overcome by a pair of coupling wire loops
about the ring 50 as shown in Figs. 20 - 22. Fig. 20 is a
top view of the yoke, coil and tube and ring. Fig. 21 is an
electrical diagram showing the terminals. Terminal 1 is
coupled to one end of the driver and terminal 4 is the
return to the driver. The upper coil refers to the upper
saddle yoke and the lower coil refers to the lower saddle
yoke. Fig. 22 illustrates the upper saddle yoke 220 and the
lower saddle yoke 221. The first loop 210 begins at termi-
nal 1 on the rear bundle terminal, passes clockwise about
the ring 50a and terminates at terminal 2. The upper and
lower saddle yokes are coupled at one end to terminal 2 and
terminate at terminal 3. The second loop 211 extends from
terminal 3 and passes clockwise about the ring 50a on the
opposite side of the ring (diametrically opposite surfaces
in the horizontal plane) and terminates at terminal 4. In
this manner the loops are in series with each other and in
series with the parallel yoke coils. An end view sketch of
the ring as seer. from the screen with the loops is illus-
trated in Fig. 23. Note the direction of arrow 215 and 216
match in Figs. 20 and 22.
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In accordance with another embodiment of the present
invention correction for the quadrapole effect can be by the
placement of the loops passing about the ring and through
quadrature placed holes in the ring as shown in Figs. 24 and
25.
It is recognized that the ring may be any of the
shapes, sizes, dimensions and material discussed herein and
that the number of loop turns can be selected according to
the required coupling to achieve the desired reduced radia-
tion.
While the invention has been described herein with
respect to the preferred and various other embodiments, it
will be understood by those skilled in this art that still
other modifications and variations may readily be conceived
by one of ordinary skill in the art to which it pertains,
without departing from the spirit and scope of the invention
as set forth herein. It is contemplated that all such
variations, modifications and embodiments are encompassed
within the scope of the appended claims.
KI9-89-002 -17-
