Electric lamp and discharge devices – Cathode ray tube – Beam deflecting means
Reexamination Certificate
1999-06-14
2001-06-05
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Beam deflecting means
C313S432000
Reexamination Certificate
active
06242856
ABSTRACT:
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
max
of more than 1 MHz, and in that f
max
/&Dgr;f ranges between 0.5 and 10, &Dgr;f being the width of the impedance curve around f
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
max
/&Dgr;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
max
/&Dgr;f, where f
max
is the frequency at which the impedance exhibits a maximum and &Dgr;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
max
/&Dgr;f is large for (almost) undamped resonance circuits and low for heavily damped resonance circuits. For customary line deflection coil systems, f
max
/&Dgr;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
max
/&Dgr;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
max
/&Dgr;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
max
/&Dgr;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
max
/&Dgr;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.
REFERENCES:
patent: 5039922 (1991-08-01), Ogasa et al.
Doedee Robert T. M.
Harberts Dirk W.
Van Den Berg Hendrik D.
Patel Vip
U.S. Philips Corporation
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