Generally, a cathode ray tube is an electron tube where electron beams emitted from an electron gun are directed toward a screen while being deflected in the horizontal and the vertical directions, and land on phosphors of the screen, thereby displaying the desired picture images. The deflection of the electron beams is made by way of a deflection apparatus, which is externally mounted around a funnel to form horizontal and vertical magnetic fields.
A cathode ray tube is mainly used as a color television or computer monitor. Recently, it is spotlighted for use in producing a wide-screened flat panel display, or a high definition digital television (HDTV), depending upon the interests of the consumers.
In order for a cathode ray tube to make a high quality picture image, the deflection frequency of the deflection apparatus should be heightened. Furthermore, in order for a cathode ray tube to be shortened in the electric field, the wide-angled deflection should be made with respect to the electron beams. For this purpose, it has been proposed that the external surface of the funnel externally mounted with the deflection apparatus should be varied in shape from the neck side to the panel side. For instance, the external shape of the funnel may be gradually varied from a circle to a non-circle.
That is, in the above technique, the funnel bears a rectangular sectional shape so that the deflection apparatus can perform its deflection operation well while being placed closer to the scanning route of the electron beams. In this way, the wide-angled deflection for the electron beams can be realized without heightening the deflection power consumption.
Meanwhile, the deflection apparatus usually has deflection coils for forming horizontal and vertical deflection magnetic fields, an insulating member placed between the horizontal and the vertical deflection coils, and a core formed on the insulating member while being connected to the vertical deflection coil.
The deflection apparatus is mounted around the funnel portion of the cathode ray tube, called the “cone portion.” FIG. 9 schematically illustrates the horizontal and the vertical deflection coils of the deflection apparatus mounted around the cone portion with a rectangular sectional shape. Reference numeral 2 indicates the cone portion.
The cone portion 2 has a rectangular section with a long axis X and a short axis Y. The deflection apparatus 4 externally mounted around the cone portion 2 has a pair of horizontal and vertical deflection coils 4a and 4b. The horizontal and vertical deflection coils 4a and 4b have a rectangular section corresponding to the section of the cone portion 2.
Meanwhile, as the deflection coils 4a and 4b of the deflection apparatus 4 have a rectangular section, the section thereof is outlined with a combination of an arc centering around a point on the long axis X, an arc centering around a point on the short axis Y, and an arc centering around a point on the diagonal axis between the long and the short axes X and Y.
Specifically, as shown in FIG. 10, the horizontal deflection coil 4a is formed with a combination of arcs R1 and R2 centering around points x1 and x2 on the long axis X, arcs R3 and R4 centering around points y1 and y2 on the short axis Y, and arcs R5 and R6 centering around points (not shown) on the diagonal axis. The vertical deflection coil 4b is formed with a combination of arcs R7 and R8 centering around points x2 and x3 on the long axis X, arcs R9 and R10 centering around points y2 and y3 on the short axis Y, and arcs R11 and R12 centering around points (not shown) on the diagonal axis.
That is, the deflection coils 4a and 4b of the deflection apparatus have a section outlined with a combination of the arcs centering around the points on the same axes.
However, in the above-structured deflection apparatus, the distribution of the deflection magnetic fields formed by way of the deflection coils is limited so that it is difficult to form the desired distribution of the magnetic fields depending upon the characteristics of the cathode ray tube.
As shown in FIG. 11, in case the distribution of the deflection coils is determined on the basis of a point Xa on the X axis, the thickness thereof close to the X axis is determined to be T1, and the thickness thereof close to the diagonal axis to be T2. If the reference point Xa is shifted to alter the coil distribution close to the diagonal axis, T1 and T2 are altered at the same time while being deviated from the expected value. If the coil distribution is altered on the basis of another point Xc on the X axis, only the thickness of the coil close to the diagonal axis is altered, and this limits alteration in the coil distribution.
In order to alter the coil distribution from the region close to the X axis to the region close to the diagonal axis in a desired manner, a gradual design alteration should be introduced, but this involves a difficulty in the working condition.
Therefore, with the conventional deflection apparatus, the desired coil distribution is not made in a fluent manner, and the degree of freedom in the formation of deflection magnetic fields is lowered so that the deflection apparatus does not function well in an appropriate manner.