Deflection yoke arrangement with overlapping deflection coils

Abstract: A deflection coil system for a CRT has a primary deflection coil (32) and an auxiliary deflection coil (42). The primary and auxiliary deflection coils are each operable to produce a respective magnetic field having a first polarity within a region defined by the coil and an opposite polarity in a second region. Each of the two coils is arranged such that part of the positive polarity field of each coil and all of its opposite polarity field, are in each case coupled to the other coil. This cancels the effects of cross coupling of the primary and auxiliary deflection coils, which are placed in proximity on the same axis, for example on the neck (26) and envelope of a television display tube (28). At least one of the primary and auxiliary deflection coils is a saddle shaped deflection coil and has a flat end turn section (36,46), substantially defining the opposite polarity area. The primary and auxiliary deflection coils are overlapped on the tube over at least part of this end turn section.

Description: This invention relates to the field of deflection yokes for electromagnetic deflection of scanned electron beams, wherein a main yoke section is provided for primary deflection of the beams and an auxiliary yoke section accomplishes a secondary deflection function. In particular, the invention concerns main and auxiliary yoke sections which are overlapped in a manner effective to null adverse effects of cross coupling between the main and auxiliary coils. Deflection yokes for television picture tubes comprise pairs of conductor coils on opposite sides of the tube, which are energized with a current for producing a magnetic field having field lines intersecting the electron beam path, the field lines being disposed perpendicular to the beam path. It is known (see US-A-4524 340) employ main and auxiliary deflection yokes in a television apparatus. A main yoke section provides a large amplitude electromagnetic deflection of the beam for scanning in the horizontal and vertical directions as needed to define a raster. An auxiliary yoke section can accomplish a number of additional functions, including, for example, improving the convergence of the individual red, blue and green electron beams in a color television project apparatus. An auxiliary deflection yoke can be provided for defining alphanumeric characters at a position in the raster determined by the main deflection yoke, using a vector scanning of the beam at the desired position. Another possibility is an auxiliary yoke for modulating the beam scan as a function of the video so as to control contrast, which is modulated by the beam scan velocity. An auxiliary yoke provides a deflection of a smaller amplitude than the main deflection yoke, and can provide deflection at high speed. The auxiliary deflection yoke is placed to the rear of the primary deflection yoke, between the electron guns and the primary deflection yoke. The magnetic field produced by a coil naturally has a magnetic field intensity that extends spatially from the area of the conductors defining the coil. The field decreases in amplitude with distance from the coil conductors, i.e., with distance along the Z axis. To minimize coupling between an auxiliary deflection coil and a primary deflection coil on the same axis, it is possible to space the auxiliary and primary coils from one another along the Z axis. However, the length of the picture tube is thereby increased. In addition to the physical length of the deflection coils along the Z axis, the operative length of the deflection system as a whole (primary plus auxiliary) determines the focal length of the gundeflection system, which forms an electron lens. Accordingly, a longer deflection system must be spaced farther from the screen and results in poorer resolution at the screen. A compact deflection yoke arrangement is desirable as it enables a shorter overall tube length. In a saddle shaped deflection yoke, as shown in FIGURES 2 and 10 of the drawings, the ends of the yoke at the axial extremes along the tube are formed such that the windings are superimposed to protrude radially of the tube. In this manner, the magnetic field proceeding axially along the tube is more sharply cut off at the axial end than occurs if the windings at the axial end are superimposed axially along the tube. As shown in FIGURE 4, the magnetic field intensity proceeding axially tails off to near zero when passing the axial end of a saddle shaped coil of this type. A saddle shaped coil of the type shown in FIGURES 2 and 10 has heretofore been preferred. It is also possible to provide a high permeability magnetic shunt between the auxiliary coils and the primary coils, e.g., a ferrous ring having a minimum extension along the Z axis as shown in FIGURE 10. The magnetic field lines are confined to the high permeability shunt path, tending to localize the fields produced by the respective coils and to better isolate the effects of the primary deflection coils and the auxiliary deflection coils. Notwithstanding these efforts, some coupling of the primary and auxiliary deflection coils remains, in part through the high permeability shunt. Accordingly, modulation of the auxiliary deflection coils by the primary deflection signals (and vice-versa) causes auxiliary deflection to vary with the position of the beam in the raster, and adversely affect convergence and color purity. It is an object of the present invention to eliminate cross-modulation between the primary and auxiliary deflection coils of a scanning electron beam apparatus, by providing at least one coil with a magnetic field section of negative polarity, and deliberately coupling the two coils using the negative polarity sections to null the effects of coupling at positive polarity. It is also an object of the invention to reduce the dimensions along the Z axis of a deflection system having primary and auxiliary deflection coils, by enabling the coils to be placed close behind one another and preferably overlapping one another, with cross-modulation resulting from the proximity of the coils cancelled. It is a further object of the invention to provide a particular form of deflection coil which produces a reversed polarity field at one end, whereby positive cross coupling of the coils can be nulled by cross coupling the reversed polarity fields in addition to cross coupling the main coil fields. These and other aspects are found in a deflection coil system for an electron beam apparatus having a primary deflection coil and an auxiliary deflection coil. The primary and auxiliary deflection coils are each operable to produce a respective magnetic field having a first polarity within an area defined by the coil and an opposite polarity in a second area. Each of the two coils is arranged such that part of the positive polarity field of each, and all of its opposite polarity field at one end, are in each case coupled to the other coil, thereby cancelling the effects of cross coupling of the primary and auxiliary deflection coils, which are placed in proximity on the same axis, for example on the neck and envelope of a television display tube. In an advantageous arrangement, at least one of the primary and auxiliary deflection coils is a saddle shaped deflection coil and has a flat end turn section, at substantially defining the opposite polarity area. The primary and auxiliary deflection coils are overlapped on the tube over at least part of this section. Advantageously, both the primary deflection coil and the auxiliary deflection coil have such a flat end turn section, the sections of each coil being disposed on the axial end thereof directed toward the other coil, and the two coils being at least partly overlapped. The auxiliary deflection coil can be a saddle shaped coil dimensioned for mounting on a neck of a cathode ray tube, and the primary deflection field can be a flaring saddle shaped coil that extends along the neck and onto the funnel or flaring section of the tube. FIGURE 1 is an elevation view of a deflection coil system according to the invention, only the horizontal deflection coils being shown; FIGURE 2 is a partial section view through an axial end of a different type of deflection coil; FIGURE 3 is a partial section view through an axial end of a deflection coil according to the invention; FIGURE 4 is a graph showing magnetic field intensity versus displacement on the Z axis, characteristic of the deflection coil of FIGURE 2; FIGURE 5 is a graph showing magnetic field intensity versus displacement on the Z axis, characteristic of the deflection coil according to the invention, namely as shown in FIGURE 3; FIGURE 6 is a partial section view through axial ends of two deflection coils, not according to the invention, on the same axis and in a first position; FIGURE 7 is an H vs. Z graph corresponding to FIGURE 6; FIGURE 8 is a partial section view as in FIGURE 6, wherein the axial ends are arranged to overlap; FIGURE 9 is an H vs. Z graph corresponding to FIGURE 8; FIGURE 10 is a partial section view through a modified version of the deflection coil arrangement of FIGURE 2; FIGURE 11 is a partial section view through a full deflection yoke arrangement according to the invention; and, FIGURE 12 is a section view taken along lines 12-12 in FIGURE 11. FIGURE 1 illustrates a deflection coil system for an electron beam apparatus in the form of a television picture tube 20. Electrons produced by one or more electron guns disposed at the rear 24 of the picture tube 20 are accelerated toward viewing screen 22, which bears phosphors on an inner surface, to be excited by the electrons and thereby produce a visual display. The electrons are accelerated along a Z axis, substantially along a center line of tube 20, and with deflection caused by operation of the deflection coils, through neck 26 and funnel shaped envelope 28. The electron beam is deflected magnetically for scanning on the screen to e.g. obtain a raster, when deflection is performed in a raster scanning mode. Magnetic fields are produced via deflection coils placed on the neck 26 and partly on the funnel shaped envelope 28 of the tube 20. In FIGURE 1, only the horizontal deflection coils are shown, however, vertical deflection coils are also provided, as explained in more detail hereinafter. The coils for each axis of the field are provided in pairs, one on each side of the tube. For obtaining horizontal deflection for line scanning, primary horizontal coils 32 are disposed on opposite sides of tube 20. Primary horizontal coils have windings 34 which define loops oriented generally in a horizontal plane. Accordingly, the magnetic field produced by coils 32 has field lines disposed generally vertically across the path of the electron beam. The coils are energized by a sawtooth current at the horizontal line scanning frequency, and therefore cause the electron beams to trace horizontal lines on screen 22. Similarly, primary vertical deflection coils (not shown in FIGURE 1) define loops disposed generally vertically, producing field lines oriented generally horizontally, and deflect the electron beam vertically at the vertical scanning rate. Typically, the horizontal and vertical primary deflection coils are mounted together in a deflection yoke having an external housing (not shown) and residing on the neck 26 at its junction with the funnel shaped envelope 28, such that the forward portion of the deflection coils extend onto the funnel 28. Preferably, the deflection yoke is as short as possible along the Z axis (the tube center line), such that the focal length of the deflection system, which defines a magnetic lens, is short and the overall length of the apparatus in minimal. The deflection yoke itself is formed of a plurality of individual conductors and may include a high permeability body or core, not shown in FIGURE 1, for example of ferrite, for confining flux leakage. The auxiliary deflection coil system is provided according to the invention to produce any of a number of additional deflections that may be desirable, as stated previously. The auxiliary deflection coils 42 are provided to the rear of the primary deflection coils 32. FIGURE 1 illustrates horizontal auxiliary deflection coils, however, vertical auxiliary deflection coils may be included to obtain mutually perpendicular magnetic deflection fields. Typically, the amplitude of the auxiliary deflection field is relatively smaller than the amplitude of the primary coils, which provide horizontal and vertical scanning. A coil having a current passing through it produces magnetic field lines having a first polarity within the loop defined by the coil, and an opposite polarity outside of the loop. Of course in connection with controlled deflection, it is normally desirable to minimize the extent to which the opposite polarity field deflects the electron beam. As shown in FIGURE 2, a saddle shaped coil 54 having a laterally protruding section 52 at its axial end along the Z axis can be used to minimize the concentration of the negative polarity field within the tube 20. This configuration provides a magnetic field intensity H0 versus Z axis displacement characteristic of curve 58, as shown in FIGURE 4, where H0 is the main or Gaussian component (i.e. uniform field) of the deflection field. A more complete depiction of all the respective coils of a primary/auxiliary deflection system that does not have the benefit of the invention is shown in FIGURE 10, wherein a high permeability annular disc 94 is disposed between the primary coils and the auxiliary coils. Each of the primary horizontal coil 84, primary vertical coil 86, auxiliary horizontal coil 88 and auxiliary vertical coil 92 has radially directed or turned up end turns. Inasmuch as the respective coils of a deflection system are disposed along a common axis (the Z axis), some cross coupling occurs notwithstanding efforts to control the positions of the magnetic field lines. Cross coupling is normally undesirable because the extent of auxiliary deflection is thereby modulated as a function of the position of the beam in the raster, as set by the primary deflection coils. According to an invention arrangement, however, means are provided to deliberately overlap the fields of the primary and auxiliary deflection systems in a manner that cancels the effects of the cross coupling. A primary deflection coil 32 is configured to produce a deflection field having a first polarity within a first region defined by the coil, and an opposite polarity in a second region. The auxiliary deflection coil 42 is also configured to produce a deflection field having a first polarity in a first region and an opposite polarity in a second region. The primary deflection coil 32 and the auxiliary deflection coil 42 are then cross coupled such that the opposite polarity areas of the primary and auxiliary deflection coils cancel cross coupling at the first polarity. The primary and auxiliary deflection coils 32 and 42 may be deflection coils of a video display tube 20, wherein cross coupling results at least partly from proximity of the primary and auxiliary coils on the tube, the respective coils being disposed on a common Z axis defined by the tube 20. At least one of the primary and auxiliary deflection coils 32 and 42 is a saddle shaped deflection coil, however, a section 62 of the coil as shown in FIGURE 3 is a flat end turn at one end of the coil. This configuration produces an H0 versus Z curve 68 as shown in FIGURE 5. A region 70 occurs along the Z axis, wherein the polarity of the field produced by the coil is opposite from the polarity within the loop of the respective coil. The opposite polarity region corresponds to the elongated section 62 at the axial end of the coil 64. With reference to FIGURES 6 and 7, two coils 632 and 642, having end turn sections 636 and 646, respectively, are spaced apart and are disposed end to end along a tube 26. The negative polarity section 70 of the deflection field obtained at the axial end of each one of the coils still couples with the positive polarity field of the other coil. The cross coupling is each at the negative polarity, i.e., 180 degrees out of phase. This cross coupling is undesirable because it produces a modulation of the auxiliary deflection as a function of beam position in the raster. According to the invention, as shown in FIGURES 8 and 9, the primary and auxiliary deflection coils 32 and 42 are juxtaposed so as to be, e.g., overlapped on the tube over at least part of their respective end turn sections 36 and 46. As a result of the juxtaposition, the coils are cross coupled in part along their positive polarity area, shown as 72 in FIGURE 9, as well as along their negative polarity areas 70. By correctly positioning the two coils 32 and 42, and in particular by overlapping the end turn sections 36 and 46 thereof by the required extent, it is possible to substantially eliminate the cross coupling by providing equal amounts of coupling in-phase and 180 degrees out-of-phase. An embodiment of the invention is shown in cross section in FIGURES 11 and 12. The auxiliary deflection yoke includes a horizontal deflection coil 106 and a vertical deflection coil 108, both disposed to the rear of the primary deflection yoke along the tube. FIGURE 11 is a cross section through the center line of the deflection apparatus showing the upper half of the deflection apparatus, a mirror image of the configuration shown being provided on the lower side. The auxiliary horizontal deflection coil 106 has a flat, front end turn section 46 at a forward end of the auxiliary deflection coil 106. The primary horizontal deflection coil 102 has a flat, rear end turn section 36 at a rear end. The sections of the primary and auxiliary deflection coil are at least partly overlapped, namely by the amount needed to cancel the effects of cross coupling by coupling the coils at both positive and negative polarity, in a balanced manner. The particular extent of overlap depends in part on the geometry of the elongated sections 36 and 46. Adjustment means (not shown) can be provided, for example axially oriented screws attaching the respective primary and auxiliary coils to one another or to a yoke housing such that the exact overlap needed for cancelling cross coupling can be obtained adjustably. A similar overlapping arrangement is provided for the vertical coils 104 and 108 of the primary and auxiliary deflection yokes, respectively. The overlap can be seen in FIGURE 12, which shows a cross section in the area of the overlap. The axial ends of the coils which face away from the area of overlap can be provided with turned up end turns. Except for the particular ones of the end turns which are overlapping ends of the coils, the coils can be arranged in a known manner. The overall deflection system provided by the invention is substantially shorter along the Z axis than a comparable configuration that does not use inventive concepts, as will be apparent from a comparison of FIGURES 10 and 11. While the arrangement of FIGURE 10 is characterized by spacing of the primary coils 84, 86 from the auxiliary coils 88, 92, as well as the use of a flux confining element 94, the overlapped arrangement of the invention illustrated in FIGURE 11 is shorter than the individual lengths of the primary and auxiliary coils. The focal length of the deflection system as a whole is short, as is the overall length along the Z axis, whereby a shorter picture tube is made possible. In addition, the precision of primary and auxiliary deflection is improved.

Claims: A deflection coil system, comprising: a primary deflection coil (32) operable to produce an electron beam deflection field having a first polarity within a first region (70) defined by the coil and an opposite polarity in a second region (72); an auxiliary deflection coil (42), also operable to produce an electron beam deflection field having said first polarity in a first region (70) and said opposite polarity in a second region (72); characterized in that at least one (42) of the primary deflection coil and the auxiliary deflection coil has a flat end turn section (46), substantially defining the opposite polarity area, and wherein the primary and auxiliary deflection coils are overlapped on the tube over at least part of the end turn section to thereby cross couple part of the positive polarity areas and all of the opposite polarity areas. The deflection coil system according to Claim 1, characterized in that the primary and auxiliary deflection coils are deflection coils of a television display tube, and wherein the cross coupling of the primary and auxiliary deflection coils results at least partly from proximity of the primary and auxiliary coils on the tube. The deflection coil system according to Claim 2, characterized in that at least one of the primary and auxiliary deflection coils is a saddle shaped deflection coil. The deflection coil system according to Claim 1, characterized in that the auxiliary deflection coil is disposed to the rear of the primary deflection coil along the tube, and the auxiliary deflection coil has said flat end turn section adjacent a forward end of the auxiliary deflection coil. The deflection coil system according to Claim 4, characterized in that the primary deflection coil also has a flat end turn section (36), the end turn sections of the primary and auxiliary deflection coils being at least partly overlapped. The deflection coil system according to Claim 1, characterized in that the auxiliary deflection coil is a saddle shaped coil dimensioned for mounting on a neck of a cathode ray tube, and wherein the auxiliary deflection coil has an end turn section (46) which is flat against the neck of the tube, said end turn section providing the opposite polarity field. The deflection coil system according to Claim 6, characterized in that the primary deflection coil is a saddle shaped coil provided with a flat end turn section (36), the primary and auxiliary deflection coils being mountable successively on the tube so as to overlap at least partially in the area of the respective flat end turn sections. The deflection coil system according to Claim 7, characterized in that the flat end turn sections of the primary and auxiliary deflection coils are disposed on facing ends thereof, whereby the primary deflection coil is mountable against a funnel-shaped envelope (28) of a television tube and the auxiliary deflection coil is mountable on a neck (26) of the tube, partially overlapping the primary deflection coil at the flat end turn sections . The deflection coil system according to Claim 8, further characterized by a primary deflection yoke and an auxiliary deflection yoke, each of said yokes having horizontal deflection coils (102,106) and vertical deflection coils (104,108) oriented to produce a magnetic field in axes perpendicular to an axis of the electron beam, and wherein each of said coils has at least one negative polarity section coupled to an additional one of said coils.

Inventors: ROUSSEL BRUNO, THIBAUDIN DIDIER, ROUSSEL, BRUNO, THIBAUDIN, DIDIER
Applicants: THOMSON TUBES & DISPLAYS, THOMSON TUBES & DISPLAYS SA
Application year: 1990
Main classification: H01J29
IPC codes: H01J29