Cathode ray tube comprising a deflection unit

A cathode ray tube includes a deflection unit. A coil system of the deflection unit is provided with a conductive layer, the value for f.sub.max /.DELTA.f ranging between 0.5 and 10, .DELTA.f being the half-value width of the impedance curve around a peak frequency f.sub.max, and f.sub.max being greater that 1 MHz. This results in a reduction of ringing phenomena.

BACKGROUND OF THE INVENTION
 The invention relates to a cathode ray tube comprising an electron gun, a
 display screen and a deflection unit for deflecting the electron beam,
 which deflection unit includes a line deflection coil system and a frame
 deflection coil system.
 The invention also relates to a deflection unit for use in a cathode ray
 tube.
 Cathode ray tubes are employed, inter alia, in display devices such as
 television receivers, computer monitors and oscilloscopes.
 In operation, the means for generating an electron beam generates one or
 more electron beams. The deflection unit generates electromagnetic fields
 for deflecting the electron beam (or electron beams) across a display
 screen in two mutually perpendicular directions. These directions are
 commonly referred to as the line direction (generally the horizontal
 direction) in which direction the display screen is scanned at a
 relatively high frequency, and the frame direction (generally the vertical
 direction) in which direction the display screen is scanned at a
 relatively low velocity. During deflection of the electron beam(s), a
 phenomenon occurs which will hereinafter be referred to as "ringing". A
 sudden change of the magnetic deflection field generated by the line
 deflection coil system causes an excitation of the line deflection coil
 system and/or the frame deflection coil system. This phenomenon occurs, in
 particular, during flyback of the line deflection and causes a deviation
 in the frame deflection direction on a line written in the line deflection
 direction and/or in the velocity at which a line is written in the line
 deflection direction. This deviation is visible, in particular, in an area
 at the edge of the display screen, that is the location where line
 scanning of the display screen starts.
 A known measure for reducing this problem is a so-called overscan of the
 display screen. An overscan of the display screen means that line scanning
 starts some distance beyond the display screen. As a result thereof,
 ringing is not reduced but the consequences of this phenomenon are less
 visible or invisible on the display screen. This measure has the drawback
 that the velocity at which information is displayed on the display screen
 is increased and that the electron beam(s) must be deflected through a
 larger angle, so that more energy has to be supplied to the deflection
 system.
 SUMMARY OF THE INVENTION
 It is an object of the invention to reduce "ringing" without the
 above-mentioned drawbacks.
 To achieve this, a cathode ray tube in accordance with the invention is
 characterized in that the line deflection coils are at least partly
 provided with a conductive layer, and the impedance of the line deflection
 coil system exhibits a maximum at a frequency f.sub.max of more than 1
 MHz, and in that f.sub.max /.DELTA.f ranges between 0.5 and 10, .DELTA.f
 being the width of the impedance curve around f.sub.max at a value equal
 to 1/2 of the peak value and/or in that the frame deflection coils are at
 least partly provided with a conductive layer, and the impedance of the
 frame deflection coil system exhibits a maximum at a frequency fmax of
 more than 0.3 MHz, and in that f.sub.max /.DELTA.f ranges between 0.5 and
 10.
 The invention is based on the fact that the application of a conductive
 layer to (a part of) the line deflection coil system and/or the frame
 deflection coil system may have a positive effect on the "ringing"
 phenomenon. The line deflection coil system as well as the frame
 deflection coil system can be regarded as a resonance circuit with a
 natural frequency. The impedance is frequency-dependent and exhibits a
 maximum at or around the natural frequency of the resonance circuit. The
 steeper the slope of the resonance characteristic, the more "ringing"
 occurs. A measure of the slope of the resonance characteristic is the
 width of the resonance peak, that is f.sub.max /.DELTA.f, where f.sub.max
 is the frequency at which the impedance exhibits a maximum and .DELTA.f is
 the width of the impedance curve around the maximum. These quantities
 result from a measurement of the impedance of the line deflection coil
 system as a function of the frequency. f.sub.max /.DELTA.f is large for
 (almost) undamped resonance circuits and low for heavily damped resonance
 circuits. For customary line deflection coil systems, f.sub.max /.DELTA.f
 is greater than 10, typically approximately 20, at a natural frequency in
 the range between 1.5 and 6 MHz. The values for an f.sub.max /.DELTA.f
 frame deflection coil system are comparable; the value of the natural
 frequency ranges between 0.4 and 1 MHz. The application of a conductive
 layer reduces the value of f.sub.max /.DELTA.f, so that resonances in the
 line deflection coil system are damped more rapidly, which reduces
 ringing. However, the application of a conductive layer also has a further
 effect, namely that the natural frequency of the resonance circuit formed
 by the line deflection coil system and/or frame deflection coil system is
 reduced. The invention is also based on the realization that this second
 effect may have an opposite result, namely an increase of "ringing"
 phenomena. As the natural frequency of the line deflection coil system
 and/or frame deflection coil system is reduced, the time necessary to damp
 vibrations in this system is increased. In addition, the distance (in
 frequency) between the natural frequency of the line deflection coil
 system and frequencies of other resonance circuits (such as the frame
 deflection coil system) and of stresses generated during operation in the
 cathode ray tube (such as the line frequency and harmonics thereof) are
 reduced, which generally increases the risk of crosstalk between
 resonances (and hence of ringing). By ensuring that the natural frequency
 of the line deflection coil system is above 1 MHz and/or the natural
 frequency of the frame deflection coil system is more than 0.3 MHz, the
 above-mentioned negative effects caused by the application of a conductive
 layer remain much smaller than the positive effects. If the natural
 frequency is smaller than 1 MHz, ringing generally increases. Hereinabove,
 the invention has been explained by means of the effects on a line
 deflection coil system. The same applies for a frame deflection coil
 system.
 Preferably, f.sub.max /.DELTA.f ranges between 1 and 5. In this case,
 damping is very effective without the natural frequency being influenced
 to a substantial degree. A reduction of the value of f.sub.max /.DELTA.f
 also has the effect that the dissipation in the coil is increased, which
 increase in dissipation is acceptable for values ranging between 1 and 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The Figures are diagrammatic and not drawn to scale, and in the different
 embodiments like reference numerals generally refer to like parts.
 FIG. 1 is a partly cut-away perspective view of a cathode ray tube, for
 example a 110.degree. monochrome monitor. The invention can also be used
 in color monitors and (color) television receivers. The cathode ray tube
 comprises a glass envelope 1 which includes a display window 2, a cone 3
 and a neck 4. Said neck accommodates an electron gun 5. The term "electron
 gun" is to be taken to mean within the scope of the invention a means for
 generating one or more electron beams. The electron beam 6 is focused to a
 spot 8 on a display screen 7. The electron beam 6 is deflected across the
 display screen 7 in two mutually perpendicular directions x, y by means of
 deflection unit 9. The tube is provided with a base 10 having connections
 11. In the Figure, the x and y-directions are indicated, as well as the
 z-direction which extends at right angles to the x and the y-direction.
 A sectional view along the y-direction of an example of a deflection unit 9
 is shown in FIG. 2A. The deflection unit comprises a line deflection coil
 system 12 for deflecting the electron beam in the line deflection
 direction (the x-direction) and a frame deflection coil system for
 deflecting the electron beam in the frame deflection direction (the
 y-direction). In this example, the line deflection coil system 12 includes
 two saddle-shaped coils and the frame deflection coil system 13 includes a
 toroidal coil. A support 14 is situated between the systems 12 and 13.
 Said toroidal coil is wound on a core 15. This example is not to be
 construed as limiting the scope of the invention. The deflection coil
 systems may be of the saddle-shaped type, the toroidal type or of any
 other type. FIG. 2B shows a front view of the deflection unit. The line
 deflection coil system includes two line deflection coils 12. These coils
 have flange portions extending more or less transversely to the electron
 beams. Said flange portions comprise, inter alia, a central portion 21 and
 outermost portions 22. The line deflection coils further include portions
 23 which extend approximately parallel to the cone.
 The screen is scanned in a large number of lines. The line deflection coil
 system 12 deflects the electron beam in the x-direction. Each time, one
 line, for example line 31 shown in FIG. 3, is scanned. After line 31 has
 been scanned, the electron beam is rapidly brought to the start of the
 next line. This return of the electron beam is referred to as line
 flyback. The frame deflection coil system 13 deflects the electron beam in
 the y-direction. The line flyback causes an excitation in the frame
 deflection coil system 13 and/or in the line deflection coil system 12.
 FIGS. 4A and 4B schematically show the effect of an excitation of the frame
 deflection coil system. During line flyback, the electromagnetic field
 generated by the line deflection coil system 12 changes in a very short
 period of time. As a result, a voltage is induced in the frame deflection
 coil system causing a current to flow through the frame deflection coil
 system, so that an electromagnetic interference field is generated which
 deflects the electron beam in the y-direction. Line 41 in FIG. 4A
 represents the voltage across the frame deflection coil immediately after
 line flyback. In FIG. 4A, the voltage V is plotted on the vertical axis
 and the time t in .mu.sec is plotted on the horizontal axis, where t=0
 represents the line flyback. Line 41 exhibits an approximately sinusoidal
 deviation at the start of the flyback, that is immediately after the line
 flyback; the amplitude of the deviation decreases with time. FIG. 4B shows
 the effects of the induced voltage V in the frame deflection coil system
 on a line in the image represented. A deviation occurs at the beginning of
 a line 42 written in the line direction. Said line 42 is not straight but
 undulated. The disturbance is invisible to an observer if the deflection
 of the electron beam immediately after line flyback is such that the
 electron beam falls beside the visible part of the display screen. In this
 case, for example, the visible part of the display screen starts at line
 43. Although this solves the problem, the solution is far from ideal. The
 speed at which information can be represented on the display screen is
 reduced because, during a part of the time, the electron beam does not
 scan the visible part of the display screen. The electron beam must be
 deflected further, which requires additional energy. The higher the line
 frequency the longer, in general, the part of the line 42 is for which the
 deviation is visible. For HDTV (High Definition television) and for
 monitors having a high resolution, the aim is to increase the line
 frequency. It is an object of the invention to provide a cathode ray tube
 in which ringing of the frame deflection coil system is reduced. FIG. 4C
 schematically shows the disturbing ringing effect on the voltage across
 the line deflection coil system. Immediately after the line flyback 44,
 the voltage across the line deflection coil system exhibits deviations 45.
 As a result of these deviations, the speed at which the lines are written
 on the screen varies, which becomes visible as a pattern of stripes 46, as
 schematically shown in FIG. 4D.
 In a combination of a line and a frame deflection coil system, "ringing"
 may develop in various ways. If oscillating currents develop in the line
 deflection coils after the line flyback, this is referred to as line coil
 ringing, if the oscillating currents develop in the frame deflection
 coils, this is referred to as frame coil ringing. Also combinations
 thereof may occur. The invention is based on the realization that these
 oscillations are largely determined by the resonance behavior of the line
 and the frame deflection coil system. This resonance behavior can be
 measured by measuring the impedance of the relevant coil systems as a
 function of the frequency. These curves exhibit a peak value at a number
 of natural frequencies, the lowest natural frequency being the most
 important. The ratio f.sub.max /.DELTA.f is a measure of the amplitude and
 the damping time of the oscillations. The higher this ratio, the more
 disturbing the ringing phenomena are. This ratio can be reduced by
 applying a conductive layer, however, in accordance with a further
 realization, the natural frequency should not be reduced to a value below
 1 MHz because the ringing phenomena increase again below said value.
 FIG. 5 shows the impedance of a known line deflection coil system as a
 function of the frequency. The impedance Z (in Ohm) is plotted on the
 vertical axis, the frequency f (in MHz) is plotted on the horizontal axis.
 The impedance curve exhibits a sharp peak at approximately 2 MHz. The
 squares indicate measured values, the curve indicates a calculated value
 for an equivalent circuit diagram as shown in FIG. 6. In said Figure, the
 reference numerals 41 and 42 represent connection terminals of the line
 deflection coil system. The drawn line shown in FIG. 5 indicates the
 impedance, as a function of the frequency, for a diagram as shown in FIG.
 6, where R=34.6 kOhm, L=170 .mu.H and C=48 pF.
 FIG. 7 illustrates the effect of the application of a conductive layer on
 the line deflection coil system, in this example a line deflection coil
 system in which C=26 pF and L=200 .mu.H. In all cases, the central part 21
 (see FIG. 2B) of both line deflection coils is covered with a conductive
 layer. Of these conductive layers, the surface resistance is measured when
 they are provided on a glass plate. Curve 71 shows the impedance in the
 absence of a conductive layer. For curves 72, 73, 74, 75 and 76, the
 surface resistance of the conductive layer on a glass plate is,
 respectively, 500 MOhm/square, 0.5 MOhm/square, 0.1 MOhm/square, 0.01
 MOhm/square and less than 1 Ohm/square. The values of f.sub.max /.DELTA.f
 are 16 (curve 71), 6.5 (curve 72), 3 (curve 73), 1 (curve 74), 1 (curve
 75) and 1.5 (curve 76). The natural frequencies are 2.2 MHz (curve 71),
 2.2 MHz (curve 72), 2.2 MHz (curve 73), 2 MHz (curve 74), 0.38 MHz (curve
 75) and 0.31 MHz (curve 76). The natural frequencies for curves 75 and 76
 are below 1 MHz, and an increased degree of ringing occurs. Curves 72, 73
 and 74 illustrate embodiments in accordance with the invention, curves 73
 and 74 showing preferred embodiments. Within the scope of the invention,
 the conductive layer may be applied in various ways. In a first way, a
 conductive material is provided in the adhesive layer of the wire used for
 winding the coils. During the formation of the line deflection coils, the
 adhesive layers melt together and a conductive layer is formed in and on
 the line deflection coils. For the conductive materials use can for
 example be made of carbon, organic conductive materials such as PEDOT or
 inorganic conductive materials such as ITO (indium tin oxide) or ATO. An
 alternative method of application consists in impregnating the deflection
 coils with a solution of a conductive material and, subsequently, allowing
 the solution to dry. The use of PEDOT, ITO or ATO solutions is preferred.
 The impedance of the line or frame deflection coil system can be measured,
 for example, by means of a commercially available impedance analyzer such
 as the BP4192A. The measurement is carried out by connecting the
 connection wires of the relevant deflection coil system to the measuring
 apparatus (care should be taken, however, that apart from the deflection
 coil system no other elements (for example auxiliary coils or resistors
 are connected in series or in parallel) across the connection wires)). By
 means of such an apparatus, the impedance can be determined, for example,
 by applying a sinusoidal voltage and measuring the resultant current. The
 impedance is equal to the ratio between the voltage amplitude and the
 current amplitude.
 The invention can be briefly summarized as follows:
 A cathode ray tube includes a deflection unit. A coil system of the
 deflection unit is provided with a conductive layer, the value for
 f.sub.max /.DELTA.f ranging between 0.5 and 10, .DELTA.f being the
 half-value width of the impedance curve around a peak frequency f.sub.max,
 and f.sub.max being greater than 1 MHZ for a line deflection coil system
 and/or greater than 0.3 MHz for a frame deflection coil system. This
 results in a reduction of ringing phenomena.