The present invention relates to convergence yokes which are fixed to a cathode-ray tube of projection televisions and the like and produce two-pole magnetic field for deflecting an electron beam in a horizontal direction or a vertical direction.
With reference to FIG. 1, a convergence yoke comprises a ring core 1 having a horizontal coil 2 and a vertical coil 3 wound therearound. A horizontal convergence correction circuit 4 is connected to the horizontal coil 2. A vertical convergence correction circuit 5 is connected to the vertical coil 3. With the convergence yoke, producing uniform magnetic field within the electron beam passing area is required for suppressing the electron beam distortion caused by the deflection to improve focus performance.
It is proposed, according to the literature xe2x80x9cSEMINAR M-4 1991 SID xe2x80x98Deflection And Convergence Technologyxe2x80x99 Basab B. Dasgupta, Sony Corporation, San Diego, Calif.xe2x80x9d, that winding density distribution of the coil wherein the winding angle xcex8 from the reference axis of the ring core is a variable is in proportion to cos xcex8, whereby the magnetic field within the ring core is made uniform.
Further, Japanese Patent No. 3039988 proposes a convergence yoke wherein ratio A3/A1 of a coefficient (Ak coefficient) when winding distribution is expanded in Fourier series is 0.13 to 0.50 in order to render the magnetic field strength distribution uniform.
However, with use of computer simulation, the present inventor has calculated the magnetic field strength distribution of the horizontal coil of the convergence yoke wherein the winding density distribution of the coil is in proportion to cos xcex8, revealing that contour lines of the magnetic field strength in the ring core are shaped in oblong ellipses as shown in FIG. 20, and particularly the magnetic field strength has nonuniformity on an upper and lower portions of the electron beam passing area. Thus the electron beam has a gross distortion of a circular shape in cross section as shown in FIG. 24(a).
Furthermore, the present inventor, with use of computer simulation, has calculated the magnetic field strength distribution of the horizontal coil of the convergence yoke wherein ratio A3/A1 of Ak coefficient is 0.13 to 0.50, and winding is provided uniformly in the range of 15xc2x0 to 40xc2x0 in winding angle xcex8 from the reference axis (the construction according to claim 2 of the patent stated above), with the result that the magnetic field strength is not uniform at each of four corners as shown in FIG. 21.
Further, in the case where the winding range of the convergence yoke is shifted 0.5xc2x0, i.e. the winding is provided uniformly in the range of 15.5xc2x0 to 40.5xc2x0 in winding angle xcex8 from the reference axis, the present inventor calculates the magnetic field strength distribution of the horizontal coil with use of computer simulation, to obtain the magnetic field strength distribution shown in FIG. 22. In this case whereas ratio A3/A1 of Ak coefficient is 0.1 departing from the scope of the invention of the patent stated above, the magnetic field strength distribution obtained is slightly more uniform than that shown in FIG. 21. Accordingly, it is not always true that ratio A3/A1 of Ak coefficient is most preferably in the range of 0.13 to 0.50.
Further, with use of computer simulation the present inventor calculates the magnetic field strength distribution of the horizontal coil of the convergence yoke wherein the winding is provided uniformly in the range of 10xc2x0 to 30xc2x0 in winding angle xcex8 from the reference axis (the construction according to claim 3 of the patent stated above), to obtain the magnetic field strength distribution shown in FIG. 23. As illustrated, the magnetic field strength is extremely uneven, causing the electron beam to be distorted grossly as shown in FIG. 24(b).
Furthermore, even if ratio A3/A1 of Ak coefficient is in the range of 0.13 to 0.50, the magnetic field strength distribution varies greatly within this range, so that in designing the convergence yoke the coil performance cannot be determined unless the deformation caused by the deflection of the electron beam is actually measured. Presumably, the reason is that the current density distribution represented by the sum of two terms, A1 and A3 of Fourier series, is greatly shifted from the actual current density distribution, as shown in FIG. 25, so that the coil performance cannot be determined only by ratio A3/A1 of Ak coefficient.
As described above, prior art has the problem that the winding distribution having the uniform magnetic field strength cannot be determined uniquely.
An object of the present invention is to provide a convergence yoke having more uniform magnetic field strength than conventionally.
A convergence yoke embodying the present invention is characterized in that the convergence yoke comprises a ring core having a plurality of coils wound therearound, and winding density distribution of each of the coils wherein winding angle xcex8 from a reference axis of the ring core is a variable is in proportion to cos2 xcex8.
As compared with a conventional convergence yoke wherein winding density distribution of a coil is in proportion to cos xcex8, the convergence yoke of the invention is greatly reduced in density as angle xcex8 is increased as shown in FIG. 5, so that an oblong ellipse of the magnetic field strength distribution shown in FIG. 20 is extended upward and downward, rendering the magnetic field strength distribution shaped in concentric circles. As a result, the magnetic field strength distribution is uniform over electron beam passing area, and a cross section shape of the electron beams will not be grossly distorted owing to the deflection.
Stated specifically, the ring core 1 has wound therearound a horizontal coil 2 and a vertical coil 3 based on a horizontal coil reference axis 20 and a vertical coil reference axis 30 which are orthogonal to each other. Each of the two coils 2, 3 has the winding density distribution described.
Further stated specifically, the plurality of coils each comprises a plurality of coil portions. Each of the coil portions is wound within a predetermined angle range wherein a winding center is in the range of 25xc2x0 to 30xc2x0 in winding angle xcex8 from the reference axis on each of quadrants of orthogonal coordinates, and has winding density distribution in proportion to cos2 xcex8 within the angle range. According to the specific construction, the magnetic field strength distribution comprising diamond-shape contour lines is obtained, whereby the uniformity of the magnetic field strength becomes higher than conventionally.
Another convergence yoke of the present invention is characterized in that the yoke comprises a ring core having a plurality of coils wound therearoud, each of the coils comprises a plurality of coil portions, each coil portion is wound uniformly at a first winding density X1 within a predetermined angle range (xcex8s to xcex8e) wherein a winding center is in the range of 25xc2x0 to 30xc2x0 in winding angle xcex8 from the reference axis on each of quadrants of orthogonal coordinates and is wound uniformly at a second winding density X2 within an angle range (0 to xcex8s) from 0xc2x0 to the predetermined angle range in winding angle xcex8, ratio of the second winding density X2 relative to the first winding density X1 is represented by an expression given below.
X2/X1=((cos2 xcex8sxe2x88x92cos2 xcex8e)/2 cos2 xcex8s)xc2x10.1
The convergence yoke described above has approximately uniform magnetic field strength distribution which is equivalent to that of a convergence yoke having winding density distribution in proportion to cos2 xcex8 within the predetermined angle range (xcex8s to xcex8e) wherein a winding center is in the range of 25xc2x0 to 30xc2x0 in winding angle xcex8 from the reference axis.
As described above, with the convergence yoke of the present invention, winding distribution wherein the magnetic field strength distribution is more uniform than conventionally can be determined uniquely, whereby the distortion of the electron beam caused by the deflection is suppressed, and focus performance is improved.