COLOUR CATHODE RAY TUBE

Abstract: An improved picture sensitivity to the magnetic field produced by a field deflection coil (28) for an in-line gun shadow mask display tube is obtained by positioning magnetic field conducting elements (32, 34) within the envelope of the display tube in the central deflection area of the field produced by the field coil (28). By positioning the magnetic field conducting elements (32, 34) inside the envelope, the distances between the elements (32, 34) and the yoke ring (30) and between the elements (32, 34) and the field coil (28) are such that the unnecessary flux is significantly less compared to a known situation in which these elements are located between the line and field coils, thus improving the field deflection sensitivity.

Description: The present invention relates to a colour cathode ray tube comprising in an envelope including a faceplate, a cone and a neck, an in-line electron beam producing means disposed within the neck for producing three electron beams which lie , when undeflected, substantially in a single plane on which plane the longitudinal axis lies, the axis of the central of said beams, when undeflected, substantially coinciding with said longitudinal axis, a cathodoluminescent screen on the faceplate, a shadowmask adjacent to but spaced from, the faceplate, a deflection unit comprising a field deflection coil and a line deflection coil, and magnetic field conducting means arranged such that, the deflection coil being energized, the magnetic field deflection field produced at the central deflection area is barrel shaped. British Patent Specification 2013973 discloses a deflection unit comprising a line deflection coil which is constituted by two diametrically oppositely arranged coil portions for deflecting an electron beam in a first, horizontal direction, a field deflection coil which is constituted by two diametrically oppositely arranged field coil portions for deflecting an electron beam in a second, vertical direction transverse to the first direction, and magnetic field conducting means comprising two soft-magnetic plates located diametrically opposite to each other between the field and line deflection coils adjacent the centre of the field deflection coil. The purpose of these plates is to provide field astigmatism correction by adjusting the magnetic field produced when the deflection unit is energized in such a manner that it is barrel shaped at the central deflection area. In the known deflection coil unit the soft-magnetic plates are located radially outwards of the line deflection coil so that the plates have only a slight influence, if any, on the line deflection magnetic field. The soft magnetic plates are normally held captive within the thickness of a resin shell inside which the line deflection coil, which may comprise two saddle-type coil portions, is arranged. If the field deflection coil comprises two saddle-type portions then they are mounted externally of the resin shell, rotated 90° relative to the line coil portions, and an annular core member of soft magnetic material, otherwise termed the yoke ring, encloses the field deflection coil. Alternatively if the field deflection coil comprises two toroidally wound coil portions then these are wound on the annular core member of soft magnetic material, which member encloses the resin shell. Whilst such a deflection unit functions satisfactorily, there is always a requirement by set makers to improve the sensitivity of the deflection unit whilst minimising coma, raster and astigmatic effects. According to the present invention there is provided a colour cathode ray tube of the type described in the first paragraph of the specification characterized in that said magnetic field conducting means are disposed within the envelope. Providing the magnetic field conducting means, which may comprise curved plates, within the envelope, by for example affixing them to the internal surface of the envelope or attaching each plate to a non-magnetic field conducting member secured to the electron beam producing means, provides a number of advantages over the known arrangement disclosed in British Patent Specification 2013973. The field deflection sensitivity is improved because the distances between the magnetic field conducting elements and the copper of the field deflection coil and between the magnetic field conducting elements and the yoke ring are in each case greater which results in less unnecessary flux between the magnetic field conducting elements and the yoke ring. It is no longer necessary for the resin shell to be of a relatively large thickness because it does not have to accomodate the magnetic field conducting elements. Consequently the shell can be made thinner and the field deflection coil can be slightly smaller and closer to the beams. This means that a further improvement in field deflection sensitivity can be gained. The disposition of the magnetic field conducting means within the envelope, particularly in the neck, makes use of unused space above and below the in-line electron beams and therefore does not require any special modifications of the envelope. These advantages generally outweigh the possible disadvantages of the influence of the field conducting means on the line deflection field. The axial position of the magnetic field conducting means is determined by the deflection field produced by the field coil. If the magnetic field conducting elements are too close to, or at, the rear (gun) end or the front (screen) end of the frame deflection field then they will affect adversely the coma and raster effects, respectively, without improving the sensitivity and reducing field astigmatism. However in order to be effective the magnetic field conducting elements should be in an axial position where the electron beams have already experienced some deflection, i.e. adjacent to the central deflection area. The present invention will now be explained and described, by way of example, with reference to the drawings, wherein: Figure 1 is a vertical cross-sectional view through a colour cathode ray tube made in accordance with the present invention, Figure 2 is a cross-sectional view on the line II-II′ of Figure 1, Figure 3 is a cross-sectional view of the colour cathode ray tube and deflection unit as disclosed in British Patent Specification 2013973, and Figure 4 is a diagram illustrating the central deflection area in the magnetic field produced by the field coil. In the drawings corresponding reference numerals have been used to indicate the same parts. Figure 1 illustrates a shadow mask colour cathode ray tube comprising an envelope formed by an optically transparent faceplate 10, a cone 12 and a neck 14. An in-line, triple electron beam gun structure 16 is disposed in the neck 14. A cathodoluminescent screen 18 comprising triplets of phosphor elements is provided on the inside of the faceplate 10. The triplets of phosphor elements can have a striped structure which is generally used in television tubes or a hexagonal structure as used in some datagraphic display tubes. A shadow mask 20 is mounted inside the faceplate 10 adjacent to, but spaced from, the screen 18. A deflection unit 22 is mounted externally of the envelope at the neck-cone transition. In the embodiment illustrated the unit 22 comprises a line deflection coil 24 comprising two saddle type coil portions 24A, 24B (Figure 2) arranged on opposite sides of the plane containing the electron beams. The coil portions 24A, 24B are fitted inside a resin shell 26 on the external surface of which is mounted the field deflection coil 28. The coil 28 comprises two saddle type coil portions 28A, 28B arranged orthogonally to the coil portions 24A, 24B. A yoke ring 30 is arranged about the coil portions 28A, 28B. In an alternative non-illustrated embodiment the field deflection coil 28 comprises toroidal windings on the yoke ring 30, which windings lie traverse heightwise of the plane containing the electron beams. A pair of oppositely disposed magnetic field conducting elements consisting of plates 32, 34 are mounted within the envelope, more particularly the neck 14, so as to be within the deflection field produced by the field coil 28. The plates 32, 34 are of a soft magnetic material having a permeability greater than 100 and of generally rectangular shape. These plates 32, 34 are separated heightwise on opposite sides of the plane of the electron beams. They can be mounted by means of arms 36 of a non-magnetic material which are secured to the centering cup of the electron gun structure 16 or alternatively they may be held captive against the internal surface of envelope. The axial position of the plates 32, 34 relative to the deflection field produced by the field coil 28 has to be determined taking into account a number of factors. These factors include that a deflection field should be present. The plates 32, 34, which serve to improve the sensitivity and field astigmatism by rendering the deflection field barrelshaped at the central deflection area, should not influence coma and raster more than necessary. The plates 32, 34 should be located in the unused space within the neck 14 which unused space terminates at the forward end when the plates 32, 34 interfere physically with the paths of the electron beams. Figure 4 illustrates diagrammatically the field coil portions 28A, 28B and the yoke ring 30. The central deflection area 38 is indicated by the broken lines and it is the field in this area which it is desired to be rendered barrel shaped. In the case of a toroidally wound field coil, the magnetic field it produces extends rearward beyond the end of the coil and in consequence the central deflection area, is located further back, relative to the screen, than with a saddle-type field coil. By way of comparison Figure 3 is a cross-sectional view of the deflection unit disclosed in British Patent Specification 2013973. For ease of comparison the same reference numerals have been used to indicate corresponding parts in Figures 2 and 3. Disposing the plates 32, 34 inside the envelope (Figure 2) provides an improved sensitivity compared to disposing them between the line and field coils 24, 28, respectively (Figure 3). In Figure 3 the plates 32, 34 are very close to the copper wire of the field coil 28 and only a short distance, a₁, from the yoke ring 30. This arrangement gives rise to unnecessary flux which affects adversely the sensitivity of the field coil. By way of comparison, in Figure 2 the plates are further from the copper wire of the field coil 28 and at a distance, a₂, from the yoke ring 30. By the distance a₂ being greater than the distance a₁ there is less unnecessary flux and hence a better picture sensitivity.

Claims: A colour cathode ray tube comprising in an envelope including a faceplate, a cone and a neck, an in-line electron beam producing means disposed within the neck for producing three electron beams which lie , when undeflected, substantially in a single plane on which plane the longitudinal axis lies, the axis of the central of said beams, when undeflected, substantially coinciding with said longitudinal axis, a cathodoluminescent screen on the faceplate, a shadowmask adjacent to but spaced from, the faceplate, a deflection unit comprising a field deflection coil and a line deflection coil, and magnetic field conducting means arranged such that, the deflection coil being energized, the magnetic field deflection field produced at the central deflection area is barrel shaped, characterized in that said magnetic field conducting means are disposed within the envelope. Colour cathode ray tube as claimed in claim 1, characterized in that said magnetic field conducting means is disposed adjacent to the central deflection area. Colour cathode ray tube as claimed in claim 1 or 2, characterised in that said magnetic field conducting means comprise a pair of oppositely disposed magnetic field conducting elements, separated heightwise on opposite sides of the plane of the electron beams. Colour cathode ray tube as claimed in claim 1, 2 or 3, characterized in that said magnetic field conducting elements comprise plates of a soft magnetic material. Colour cathode ray tube as claimed in any of the preceding claims, characterized in that said magnetic field conducting means are affixed to the internal surface of the envelope. Colour cathode ray tube as claimed in any of the preceding claims, characterized in that said magnetic field conducting means are carried by the in-line electron beam producing means. Colour cathode ray tube as claimed in any of the preceding claims, characterized in that said field conducting elements are secured to a non-magnetic field conducting member secured to the in-line electron beam producing means.

Inventors: SLUYTERMAN, ALBERTUS AEMILIUS SEYNO
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J29
IPC codes: H01J29