JPH0364980B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a deflection yoke of a projection tube suitable for a projection television apparatus.

As the demand for increasing the size of the image reproduction surface of television receivers to obtain more powerful images that cannot be obtained with smaller screens has increased, the images reproduced on the fluorescent screen of cathode ray tubes are being transferred to lenses, reflectors, etc. The so-called projection television apparatus (hereinafter referred to as PTV), which uses a projection optical system to project a large-sized television image onto a screen, has become widely used.

As shown in Fig. 1, such PTVs include a cathode ray tube (hereinafter referred to as a projection tube) 1R that projects a red light image, a projection tube 1G that projects a green light image,
Projection tube 1B and lenses 2R, 2 that project a blue light image
Consisting of G, 2B and screen 4, projection tube 1
There is a system that magnifies each color light image projected by R, 1G, and 1B using lenses 2R, 2G, and 2B, respectively, and projects them onto a screen 4 in an overlapping manner to display the magnified color image.

In such a PTV, one projection tube, for example, projection tube 1G, is installed on a center line 3G perpendicular to the screen 4 (hereinafter referred to as the vertical center line), and the other two projection tubes 1R and 1B are installed vertically. They are provided on the center lines 3R and 3B at an equiangular angle θ in the horizontal direction with respect to the center line 3G.

By the way, assuming that the raster formed on each isometric tube 1R, 1G, 1B is a rectangle with an exact specified ratio, the raster projected on the screen 4 by the isometric tube 1G located on the vertical center line 3G is Although it will be a rectangle with exactly the specified ratio, the raster on the screen 4 by the projection tubes 1R and 1B will not be a rectangle because the raster on each projection tube will be projected diagonally onto the screen 4. It gets distorted.

That is, as shown in FIG.
Raster 5G by projection tube 1G projected above
is a rectangle that is symmetrical about the X and Y axes, but the raster 5R produced by the projection tube 1R is a symmetrical trapezoid that is compressed on the right side of the Y axis and expanded on the left side, and
Conversely, the raster 5B produced by the projection tube 1B becomes a trapezoid symmetrical to the X-axis, elongated on the right side of the Y-axis and compressed on the left side.

Such changes in the horizontal expansion and contraction of the projected rasters 5R and 5G (hereinafter referred to as changes in horizontal linearity)
and trapezoidal distortion (hereinafter referred to as horizontal trapezoidal distortion) means that each point of the raster on the projection tube is This is caused by the difference in distance between the image and each point on the screen where it is projected.

These changes in horizontal linearity and raster distortion on the screen 4 due to horizontal keystone distortion are
When a color image is projected by superimposing R, 1G, and 1B color light images, color shift, ie, misconvergence, naturally occurs. Therefore, a means for correcting this misconvergence is required.

In order to correct the misconvergence, taking the projection tube 1R as an example, the raster 5R on the screen 4 should be
Therefore, it is sufficient to generate a raster 6 on the projection tube 1R, which has a distortion opposite to that of the raster 5R as shown in FIG. 3.

That is, in FIG. 3, straight lines 6a, 6c,
The raster 6 surrounded by 6b and 6d is distorted into a trapezoidal shape to offset the horizontal trapezoidal distortion of the raster 5R shown in FIG. By compressing the raster 5R in the horizontal direction and making the respective horizontal lengths l ₁ and l ₂ different, changes in the horizontal linearity of the raster 5R shown in FIG. 2 are offset.

Conventionally, as shown in FIG. 4a, a projection tube 1 having a deflection yoke 8 equipped with horizontal and vertical deflection coils 12 and 13 shown in FIG. A deflection yoke 9 is provided,
The electron beam 11 from the electron gun 10, which is normally horizontally and vertically deflected by the deflection yoke 8, is further deflected in a predetermined direction by the auxiliary deflection yoke 9 to obtain a raster with distortion as shown in FIG. There is something to do.

However, this conventional method requires an auxiliary deflection yoke and an electric circuit for it, which not only complicates the configuration and increases costs, but also increases power consumption due to the flow of correction current and causes electrical fluctuations. This leads to deterioration of misconvergence due to unstable operation, and thus, until now, it has been impossible to obtain a projection television device with good image quality and low power consumption at a low cost.

The purpose of the present invention is to eliminate the drawbacks of the above-mentioned prior art,
It is an object of the present invention to provide a projection tube deflection yoke that can produce a desired distortion in a raster with a simple configuration, reduces power consumption, and has stable operation.

In order to achieve this object, the present invention has a structure in which a deflection coil is wound in a hollow shape, and the deflection magnetic field distribution of the electron beam is made asymmetrical with respect to a vertical plane including the tube axis of the projection tube. It is characterized by

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 is a perspective view showing an embodiment of the deflection yoke of the projection tube according to the present invention, and 8 is a deflection yoke;
12 is a horizontal deflection coil, 13 is a vertical deflection coil,
14 is a core, 15 ₁ and 15 ₂ are fringe portions of vertical deflection coils, and 16 is a central axis of a deflection yoke, that is, a tube axis of a projection tube.

FIG. 6 is a configuration diagram of one embodiment of the vertical deflection coil, viewed from the phosphor screen side of the projection tube along the tube axis 16.

In FIGS. 5 and 6, the vertical deflection coil 13 has two coils 13 ₁ , 1 wound in a hollow shape.
3 ₂ as a pair, and the perpendicular surface 18 including the tube axis 16 is provided as an opposing surface. Each coil 13 ₁ , 1
3 ₂ has the same number of coil windings, but if the radii of the fringe portions 15 ₁ and 15 ₂ on the phosphor screen side are R ₁ and R ₂ respectively, then R ₁ < R ₂ and the raster with the normal aspect ratio ( The radii of the fringe portions 15 ₁ and 15 ₂ are made different by making R ₁ smaller and R ₂ larger than the radius of the fringe portion for producing a regular raster (hereinafter referred to as a regular raster). Note that 13a and 13b are coil portions (hereinafter referred to as conductor groups) of the coils 13 ₁ and 13 ₂ , respectively, along the tube axis 16, and 17 is a surface of the deflection coil 13 on the electron gun side.

When a deflection current is passed through the coils 13 ₁ and 13 ₂ configured in this manner, the electron beam is deflected in a predetermined direction by the magnetic field generated by the respective fringe portions 15 ₁ and 15 ₂ , producing a trapezoidal raster on the phosphor screen. .

Next, the action of the vertical deflection coil 13 will be explained in detail with reference to FIGS. 7 and 8.

FIG. 7 is an explanatory diagram for explaining the action of the vertical deflection coil shown in FIG. 6, and parts corresponding to those in FIG. 6 are given the same reference numerals.

Now, if a deflection current is applied to each of the coils 13 ₁ and 13 ₂ , a deflection current will be applied to the tube axis 16 from the electron gun (not shown).
The electron beam _, which has traveled from the back to the front perpendicular to the plane of the _paper along Scan vertically. At the same time, this electron beam is deflected in the direction of arrow B by the magnetic field generated by the horizontal deflection coil ₁₂ (conductor group in FIG. 5) and horizontally scans the fluorescent screen. , a ₂ , a ₃ , a ₄
This results in a rectangular regular raster with vertices indicated by dotted lines.

By the way, the fringe portion 1 of the coils 13 ₁ and 13 ₂
5 ₁ , 15 ₂ causes a plane including the fringe portions 15 ₁ , 15 ₂ perpendicular to the tube axis 16 (hereinafter referred to as a “fringe portion surface”) to move in a direction perpendicular to the fringe portion surface, and therefore to the tube axis. 16 parallel magnetic fields are generated, but the magnetic fields due to the fringe portions 15 ₁ and 15 ₂ are in opposite directions and have different strengths, and are asymmetrical with respect to the tube axis 16.

Therefore, considering the case where the electron beam is scanning the upper half of the raster, each coil 13 ₁ , 13 ₂
The direction of the deflection current flowing in is the direction of the arrow 20, the direction of the magnetic field due to the fringe portion 15 ₁ is from the back to the front of the page, and the direction of the magnetic field due to the fringe portion 15 ₂ is in the opposite direction.

Then, the electron beam is deflected by the magnetic field generated in the conductor groups 13a and 13b of each coil 13 ₁ and 13 ₂ (however, horizontal deflection is also performed at the same time), and when it reaches point P on the fringe surface. , the Lorentzian force is generated by the magnetic field B ₁ due to the fringe part 15 ₁ .
Receive _F1 . The electron beam that has reached this point P is
If the radius R ₁ of the fringe portion 15 ₁ of the coil 13 ₁ is such that a normal _raster is generated, then , the electron beam passing through the iron P reaches the raster point a ₁ (Fig. 8) while being subjected to the Lorentz force F ₁ due to the magnetic field B ₁ , but since the radius R ₁ of the fringe portion 15 ₁ produces a regular raster, , the strength of the magnetic field B ₁ increases due to the fringe portion 15 ₁ and the Lorentz force F ₁ acting on the electron beam at the point P increases. Therefore, the velocity vector v ₁ of the electron beam at point P rotates counterclockwise (hereinafter the same) with respect to the axis perpendicular to the plane of the paper at a larger rotation angle than in the case of producing a regular raster, and point a ₁
The electron beam that should have reached point b 1 will reach point b ₁ to the upper left of this point.

In this way, when the electron beam is scanning the upper right half of the raster, the electron beam is subjected to Lorentz force due to the magnetic field generated by the fringe section ₁₅₁ , and the velocity vector of the electron beam is rotated counterclockwise. However, since the magnitude of the deflection current flowing through the horizontal and vertical deflection coils is changed in order for the electron beam to scan the fluorescent surface, it is natural that the position of the electron beam as it crosses the fringe surface changes. The strength of the magnetic field due to the fringe portion 15 ₁ and the velocity vector of the electron beam differ depending on the angle, and therefore the counterclockwise rotation angle of the velocity vector of the electron beam differs.

That is, in FIG. 8, since the vertical deflection current is zero on the X-axis, the velocity vector of the electron beam does not rotate due to the magnetic field of the fringe portion ₁₅₁ . Furthermore, since the horizontal deflection current is zero on the Y-axis, the velocity vector of the electron beam does not have a component in the X-axis direction, and due to the rotation of the velocity vector by the magnetic field of the fringe section ₁₅₁ , the electron beam is It does not reach the Y-axis of the optical plane, but reaches the lower left of it.

Next, the action of the fringe portion 15 ₂ of the coil 13 ₂ when the electron beam scans the upper left half of the fluorescent surface will be explained.

The radius R ₂ of the fringe portion 15 ₂ is made larger than the radius of the fringe portion when generating a regular raster, and therefore the strength of the magnetic field due to the fringe portion 15 ₂ is greater than that of the fringe when generating a regular raster. This means that the strength of the magnetic field due to the

When the electron beam is scanning the upper left half of the fluorescent surface, the deflection current of the coil 13 ₂ is in the direction of the arrow 20, and the direction of the magnetic field generated by the fringe portion 15 ₂ is from the front to the back of the page. .

The velocity vector v ₂ of the electron beam passing through the magnetic field of the fringe portion _{15 2} is directed diagonally upward and to the left with respect to the plane of the paper due to the action of the magnetic field of the conductor groups 13 a and 13 b of 13 1 and 13 ₂ . Part 1
It rotates counterclockwise due to the action of the magnetic field B ₂ of 5 ₂ .

Therefore, considering the electron beam that has reached point Q on the fringe part surface, if the radius R ₂ of the fringe part 15 ₂ is set to produce a regular raster, then the velocity of the electron beam that has reached point Q is If the vector v ₂ is rotated counterclockwise by the magnetic field B ₂ due to the fringe section 15 ₂ and reaches point a ₂ (FIG. 8) at the upper left corner of the regular raster, then the fringe section 15 ₂
Because _the radius R ₂ of
The strength of the magnetic field becomes smaller, and the counterclockwise rotation angle of the velocity vector v ₂ of the electron beam that reaches point Q becomes smaller. For this reason, the position where the electron beam reaches the fluorescent surface is relatively to point b ₂ to the lower left of point a ₂ .
becomes.

Then, the magnetic field of the fringe portion 15 ₁ of the coil 13 ₁ acts on the electron beam, contrary to the effect on the coil 13 1.
The velocity vector v ₂ of the electron beam rotates clockwise due to the magnetic field of the fringe section 15 ₂ of ₂ , but the magnitude of the rotation angle is smaller than that in the case of producing a regular raster, and the X-axis As the distance from the Y axis increases, the counterclockwise rotation angle decreases.
In the raster area in the upper left half of the fluorescent surface,
The farther the position is from the axis and the Y axis, the more the position where the electron beam hits the fluorescent surface shifts toward the lower left than the position where the electron beam hits the fluorescent surface during regular raster scanning.

When scanning the lower half region of the fluorescent surface, the polarity of the deflection current flowing through the coils 13 ₁ and 13 ₂ is reversed, so the direction of the magnetic field due to the fringe portions 15 ₁ and 15 ₂ is reversed, which has been explained so far. As is clear from this, in the lower right half of the fluorescent surface, the position where the electron beam hits the fluorescent surface is shifted to the lower left compared to during regular raster scanning, and in the lower left half of the fluorescent surface, Displaced to the upper left.

Therefore, the raster obtained by the magnetic field generated by the asymmetrical fringe portions 15 ₁ and 15 ₂ of the coils 13 ₁ and 13 ₂ is a solid line whose vertices are points b ₁ , b ₂ , b ₃ , and b ₄ in FIG. The area surrounded by the left side of the raster,
The right boundary lines 6c and 6d are curved to the right, and the upper,
The lower boundary lines 6a and 6b form trapezoidal rasters that are straight lines approaching each other in the left direction. Note that the radius of the fringe portion on the electron gun side may be made different as described above.

In this way, a raster with a trapezoidal shape and distortion is obtained, but as is clear from the boundaries 6c and 6d, it contains upward and downward curved distortion, and this curved distortion must be corrected. No. This correction is performed by creating an asymmetrical magnetic field by a group of conductors in the vertical deflection coil.

Next, this correction will be explained with reference to FIGS. 9 to 11.

In FIG _. 9, opposing ends _13'a ,
13'b, these ends 13'a, 13'
b and the tube axis 16 (hereinafter referred to as the opposing surface of the conductor groups 13a, 13b) 19,1
9' is a plane that intersects with the tube axis 16. That is,
Conductor groups 13a and 13 of coils 13 ₁ and 13 ₂
In b, the winding angles are set so that the opposing surfaces 19 and 19' are shifted by an angle β toward the coil 13 ₁ whose fringe portion has a smaller radius with respect to the vertical plane 18 that includes the tube axis 16. .

The direction of the magnetic lines of force due to the conductor groups 13a and 13b is perpendicular to the vertical plane 18, that is, parallel to the horizontal plane 22. However, when the winding angle of the conductor groups 13a and 13b is set as described above, the conductor The lines of magnetic force caused by the group 13a become relatively dense, and the lines of magnetic force caused by the conductor group 13b become relatively sparse.

The lines of magnetic force at this time are as shown in FIG. FIG. 10 shows the lines of magnetic force due to the conductor groups 13a and 13b in a plane perpendicular to the tube axis 16 in FIG. When the electron beam scans the upper part of the fluorescent surface, the direction of the magnetic field lines 21 is curved upward to the left, as shown by the solid line above the horizontal plane 22. When the electron beam scans the lower part of the fluorescent surface than the horizontal center line, the vertical deflection current Since the locality of is reversed, as shown by the dotted line below the horizontal plane 22, the lines of magnetic force 21'
The direction is curved upward to the right. The curved upward left and right curves become smaller as they are closer to the horizontal plane 22.

When an electron beam passes through such a magnetic field,
The electron beam is subjected to a force perpendicular to the power line.
In this way, the electron beam is vertically deflected, but if the magnetic lines of force 21 are curved upward to the left as described above, the electron beam is directed slightly to the right with respect to the vertical upward direction. Furthermore, if the magnetic lines of force 21' are curved upward to the right, the electron beam is deflected slightly to the right with respect to the vertical downward direction.

Therefore, as shown in FIG. 11, the raster contour obtained by the magnetic field generated by the conductor groups 13a and 13b is such that the upper and lower boundaries 6a and 6b are straight lines approaching each other in the left direction, but the left and right boundaries are straight lines. line 6
c and 6d are curves curved to the left in the opposite direction to that in FIG.

Therefore, if a vertical deflection coil is formed by a coil pair having a conductor group and a fringe portion formed as described above, the curvature distortions in the left and right directions in FIGS. 8 and 11 cancel each other out, and as shown in FIG. A raster with trapezoidal distortion is thus obtained on the fluorescent light surface, and the horizontal trapezoidal distortion of the projected image on the screen 4 (FIG. 1) by the projection 1R shown in FIG. 2 can be corrected.

It is clear that in order to correct the horizontal trapezoidal distortion to the projection tube 1B during the first reception, the shape of each vertical deflection coil may be reversed from that described above.

Next, correction of changes in horizontal linearity will be explained.

This correction is performed by the horizontal deflection coil 1 shown in FIG.
Perform according to 2.

In FIG. 13, the horizontal deflection coil 12 consists of a pair of vertically opposed coils 12 ₁ and 12 ₂ . These coils 12 ₁ and 12 ₂ are symmetrical with respect to a horizontal plane 22 that includes the tube axis 16. ₁ ,
By specifying the angle of the winding width of the coil portion (hereinafter referred to as side conductor) that forms the horizontal deflection magnetic field of 12 ₂ , the coil portion is formed asymmetrically with respect to the vertical plane 18 .

In other words, the side conductor 12 _a of the coil 12 _1
−1 and 12 _a-2 , the winding width angles α ₁ and α ₂ are different,
Side conductors 12 _{b-1 ,} 12 _b-2 of coil 12 2
The angles α ₃ and α ₄ of the winding width are made different. However, angles α ₁ and α ₃ are equal, and α ₂ and α ₄ are equal.

FIG. 14 shows lines of magnetic force due to the side conductors of the horizontal deflection coil formed in this manner.

In the same figure, as mentioned above, the coil pair 12 ₁ ,
12 ₂ , the angle γ 1 looking into the side conductors 12 _a-1 and 12 _b-1 from the tube axis 16 is different from the angle γ ₂ looking into the side conductors 12 _a-2 and 12 _{b-2 .} .

Therefore, side conductors 12 _a-1 , 12 _a-2 ,
The magnetic field lines 21' generated by 12 _b-1 and 12 _b-2 are
It has a vertical curve shape that curves to the left in the horizontal direction.

Now, assuming that the electron beam scans to the right of the vertical line 18, the lines of magnetic force 21' become as shown by solid lines, and the electron beam receives a force F ₃₁ perpendicular to the lines of magnetic force 21' and is deflected horizontally. However, because the direction of the magnetic field lines 21' is curved, when the electron beam is above the fluorescent surface, it is deflected to the lower right, and when it is below, it is deflected to the upper right, and the tube axis 16 is When it is on the horizontal line 22 containing the beam, it is deflected to the right.

On the other hand, if the electron beam scans to the left of the vertical line 18, the lines of magnetic force 21' will be as shown by dotted lines, and the electron beam will be horizontally deflected by receiving a force F ₃₂ perpendicular to the lines of magnetic force 21'. , Contrary to the above, when the electron beam is above the fluorescent surface, it is deflected to the upper left, when it is below, it is deflected to the lower left, and when it is on the horizontal line 22, it is deflected to the left.

Note that when the electron beam is on the vertical line 18, the horizontal deflection current is zero, so it is not horizontally deflected.

Therefore, the raster obtained on the fluorescent surface is
As shown in Figure 5, it has a trapezoidal shape in the horizontal direction. Then, in the horizontal direction, the right side of the raster is expanded to have a width of _l1 , and the left side of the raster is compressed to have a width of _l2 . These widths l ₁ and l ₂ are l ₁ and l ₂ in FIG. 3, and are amounts for correcting changes in the horizontal linearity of the raster 5R in FIG. 2.

Due to the magnetic fields of the vertical deflection coil 13 and the horizontal deflection coil 12 formed as described above, the raster appearing on the fluorescent surface is created by distorting the trapezoidal raster shown in FIG. 13 in the horizontal direction as shown in FIG. The resulting raster is the raster shown in FIG. Here, trapezoidal distortion occurs in the raster shown in FIG. 15 in the opposite direction to the raster shown in FIG. 12, but these trapezoidal distortions are combined to form the desired trapezoidal distortion shown in FIG. 3. do.

It is clear that in the projection tube 1B of FIG. 1, the horizontal deflection coil 12 must be formed in the opposite shape to that of the projection tube 1R.

In this way, by making the shape of the deflection coil asymmetrical, the generated magnetic field is made asymmetrical with respect to the tube axis, and a raster of a desired shape can be formed on the fluorescent surface of the projection tube with a simple configuration. In this case, the asymmetric magnetic field due to the conduct group of the vertical deflection coil and the shape of the fringe section combine to cause a horizontal trapezoidal distortion in the raster, and the asymmetric magnetic field due to the shape of the side conductor of the horizontal deflection coil distorts the raster in the horizontal direction. I'm letting you do it.

As explained above, according to the present invention, horizontal,
By forming the vertical deflection coil asymmetrically,
Since it generates a horizontal and deflecting magnetic field asymmetrical about the tube axis to produce a raster with the desired distortion, it does not require any additional deflection means to distort the raster, and is simple to configure and consumes little power. The operation is stable without increasing the
It is possible to sufficiently eliminate horizontal keystone distortion and changes in horizontal linearity of the projected image on the projection screen, and it is possible to provide a projection deflection yoke with excellent functions at a low cost while eliminating the drawbacks of the conventional technology. .

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing a projection television apparatus, Fig. 2 is an explanatory drawing showing the projection raster of each projection tube on the screen, and Fig. 3 is a raster on the projection tube for correcting the projection raster. An explanatory diagram showing an example, FIG. 4 is a configuration diagram showing a conventional example for correcting the distortion of the image on the projection screen shown in FIG. 1, and FIG. FIG. 6 is a front view showing one embodiment of the vertical deflection coil, and FIG. 7 is a perspective view showing the embodiment.
FIG. 8 is an explanatory diagram showing the raster of the projection tube due to its action. FIG. 9 is a front view showing another embodiment of the vertical deflection coil of the deflection yoke in FIG. 5. Figure 10 is an explanatory diagram showing the action of the vertical deflection coil in Figure 9;
Fig. 1 is an explanatory diagram showing the raster of the projection tube due to its effect, Fig. 12 is an explanatory diagram showing the composite raster of the rasters shown in Figs. 8 and 11, and Fig. 13 is an explanatory diagram showing the horizontal deflection of the deflection yoke in Fig. 5. FIG. 14 is an explanatory diagram showing the action of the horizontal deflection coil shown in FIG. 13, and FIG. 15 is an explanatory diagram showing the raster of the projection tube due to the action. 12...Horizontal deflection coil, 12 _a-1 , 12 _a-2 ,
12 _b-1 , 12 _b-2 ...Horizontal deflection coil side conductor, 13...Vertical deflection coil, 13a, 1
3b...conduct group of vertical deflection coil, 15 ₁ ,
15 ₂ ...Fringe part of vertical deflection coil, 19,
19'... Opposite surface.

Claims (1)

[Scope of Claims] 1. A vertical projection system consisting of a pair of coils wound in a saddle shape, used in a projection television device that projects an image from a direction oblique to the vertical center line of a projection screen. In the deflection yoke of a projection tube equipped with a deflection coil and a horizontal deflection coil consisting of a pair of coils wound in a hollow shape, the radii of the fringe portions of the first and second coils forming the pair of vertical deflection coils are set to each other. The distortion of the raster on the projection screen can be offset by making the magnetic field distribution by the vertical deflection coil asymmetric with respect to the tube axis of the projection tube and causing distortion in the raster on the projection tube. A deflection yoke for a projection tube, characterized in that it is configured as follows. 2. In the projection tube deflection yoke according to claim 1, the ends of the first conductor group along the tube axis of each of the first and second coils are parallel to each other, and , the ends of the second conductor group along the tube axis of each of the second coils are also parallel to each other, and the end portions of the first conductor group of each of the first and second coils are parallel to each other. a first tube including the tube axis;
and the second plane of each of the first and second coils.
A deflection yoke for a projection tube, characterized in that a second plane passing between the ends of the conductor group and including the tube axis is non-parallel to each other. 3. In the projection tube deflection yoke according to claim 1 or 2, the first conductor group forming a horizontal deflection magnetic field in each of the first and second coils forming a pair of the horizontal deflection coils. The second conductor group is defined by a horizontal plane including the tube axis, in which the angles of the winding widths seen from the tube axis are different from each other, and the first and second coils are arranged symmetrically with respect to the horizontal plane, and A deflection yoke for a projection tube, wherein the first and second coils are arranged asymmetrically with respect to a vertical plane including the tube axis.