Abstract:
A method for operating a cathode-ray tube electron gun, suitable for multimode operation, for example, in order to display television images and multimedia images of the SVGA, XGA type, in which the zone for forming the electron beam generated by the cathode has at least two control electrodes. The two control electrodes are connected to voltage sources in such a way that the potential difference between the two control electrodes increases when the beam current decreases.

Description:
The invention relates to a method for operating an electron gun within a cathode-ray tube, and more particularly, to a method for controlling the electrodes of an electron gun to allow optimized operation of the gun for several operating modes, for example, in television mode and in monitor mode, in order to display high-resolution images of the SVGA or XGA type. 
     BACKGROUND OF THE INVENTION 
     An electron gun generally consists of a zone called the beam-forming zone, forming a triode, comprising an emissive cathode, a first electrode G 1 , generally connected to a zero voltage, and a second control electrode G 2 , generally connected to a voltage of about a few hundred volts. In the case of a three-beam gun, for a trichrome tube, three emissive cathodes K are used to form three beams corresponding to the three primary colors to be generated on the screen, and the three beams pass through the grids G 1  and G 2 , which are pierced, for example, with three holes arranged along the axes of said beams. Other electrode means, constituting a main lens, are arranged beyond the zone for forming the beam or beams, in order to focus the beams and/or to make the beams converge on the screen of the tube. 
     There is a rapidly growing tendency which requires the television tube, and the television set equipped with such a tube, to be able to display both television images and images arising from a multimedia application, in which the television set and its tube act as a microcomputer monitor. 
     However, although television images, for contrast and brightness reasons, are generated from high-current electron beams, high-resolution images for multimedia applications are, on the contrary, generated using low-current electron beams. This gives rise to a compromise problem in the design of the gun, since the known triode structure K, G 1 , G 2  is suitable for one type of operation, but cannot be optimized for another, very different type of operation, for example, in a television mode or in a multimedia monitor mode. 
     The invention provides a simple solution to this problem, making it possible to optimize the operation of the gun for various types of images to be displayed on the screen of the tube, this being achieved by virtue of a structure of the beam-forming zone which allows the beam size to be adapted to various desired operating modes. 
     SUMMARY OF THE INVENTION 
     A method for operating a cathode-ray tube electron gun, wherein the gun comprises: a beam forming zone, for forming at least one electron beam, comprising in succession a cathode, and a plurality of electrodes with at least 2 control electrodes, and a main focusing zone consisting of at least one electrostatic lens. In the method, at least one variable potential is applied to at least one of the control electrodes of the beam-forming zone. This variable potential depends on the beam current so as to increase the size of the electron beam at the exit of the beam-forming zone for the low-current beams. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and its advantages will be more clearly understood with the aid of the description and of the drawings, among which: 
     FIG. 1 represents diagrammatically a longitudinal half-section of a gun according to the prior art. 
     FIGS. 2A,  2 B and  2 C are graphs illustrating the effect of the thickness of the accelerating electrode G 2  on the size of the beam exiting the zone for forming the said beam. 
     FIG. 3 is a graph showing, for a gun according to the prior art, the variation in the size of the impact area of the beam on the screen of the tube as a function of the size of the beam in the main lens, for defined beam currents. 
     FIG. 4 represents diagrammatically a longitudinal half-section of a gun according to the invention. 
     FIGS. 5A,  5 B and  5 C are graphs showing the effect of the voltage applied to the novel electrode structure of the beam-forming zone on the beam size. 
     FIG. 6 is a graph illustrating the change in the beam size in the main lens and the size of the impact area of the beam on the screen as a function of the beam current, both for the gun according to the prior art and for the gun according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A gun according to the prior art, as illustrated in FIG. 1, comprises a zone for forming an electron beam, generally consisting of an emissive cathode  1 , a first electrode  2 , called G 1  and connected to earth, and a second, accelerating electrode  3 , called G 2  and connected to a voltage VG 2  of about a few hundred volts. The gun, moreover, comprises electrode means (G 3 , G 4 , G 5 ) constituting a prefocusing lens and electrode means (G 5 , G 6 ) constituting a main focusing lens; the electrodes G 3  and G 5  being connected to a focusing voltage Vf and the final electrode G 6  being connected to the anode high voltage. 
     FIGS. 2A,  2 B,  2 C show the known effect of the thickness of the electrode G 2  on the size of the beam exiting the zone for forming the beam. FIGS. 2A and 2B show the shape of the beam in longitudinal section, for G 2  thicknesses of 20 mils and 8 mils, respectively. FIG. 2C shows the corresponding positions of the electrodes G 1 , G 2 , and the bottom part of G 3 . The left edge  0  in FIG. 1 corresponds to the plane of the cathode K. The electrode G 2 , in dotted lines, corresponds to an electrode G 2  having a thickness of 8 mils, and the electrode G 2 , in solid lines, corresponds to an electrode having a thickness of 20 mils. 
     In a prior art gun, with an electrode G 1  having a thickness of about 4 thousandths of an inch or 4 mils (0.1 mm) lying approximately 4 mils from the cathode, an electrode G 2  lying 20 mils from the cathode and a lower part of G 3  lying approximately 72 mils from the cathode, it is noted that the beam forms a node, also called a cross-over zone, which moves closer to the cathode as the thickness of G 2  decreases, and that the beam exiting the forming zone, close to G 3 , increases in width as the thickness of G 2  decreases. 
     Moreover, FIG. 3 shows that, for each beam current, the size (in Mils) of the point of impact of the beam on the screen depends on the size of the beam in the main lens, which, in turn, depends on the size of the beam exiting the forming zone described above, and therefore, ultimately, depends on the chosen thickness of the electrode G 2 . 
     The prior art gun structure usually is optimized for high currents, greater than 1 milliamp, because the minimum size of the point of impact of the beam on the screen corresponds to the minimum size of the beam in the main lens. However, for smaller currents, less than 1 mA, the minimum size of the point of impact of the beam on the screen no longer depends on a smallest size of the beam in the main lens, but rather result from a larger size of the beam in the main lens. For the gun considered above, with a G 2  thickness of about 20 mils, the sizes of the beam in the main lens and those corresponding to the impact area of the beam on the screen are represented by solid rectangular points. 
     A principle of the present invention consists in varying the G 2  thickness virtually, so as to benefit from the advantages of an electrode G 2  having a low thickness at high currents and from the advantages of a thin electrode G 2  at low currents. To do this, at least one control electrode G 2 ′ is added to the electrode means of the beam-forming zone and voltages corresponding to the defined operating mode are applied to the control electrodes (G 2 , G 2 ′, etc.). For example, in an embodiment illustrated in FIG. 4, in which the gun is optimized for two types of operating mode corresponding to television mode and to XGA multimedia mode, two control electrodes G 2  and G 2 ′ are placed in succession between the electrode G 1  and the bottom part of the electrode G 3 . 
     FIGS. 5A,  5 B and  5 C show that by applying a voltage to G 2 ′, which is equal to or greater than that applied to G 2 , the cross-over node of the electron beam moves closer to the cathode as the voltage VG 2 ′ increases, and likewise the size of the beam exiting the forming zone increases with VG 2 ′. 
     In the television-type operating mode, the two electrodes G 2  and G 2 ′ will be connected to the same constant potential, for example, between 200 volts and 300 volts. In order to display multimedia-type images, a potential difference will be applied between G 2  and G 2 ′ so that the potential of G 2 ′ is above that of G 2 , for example, by raising the potential of G 2 ′ to a constant potential of about 100 V. In this way, the potential Pv applied to G 2 ′ changes from one constant value to another constant value, the switch from one value to another taking place, for example, automatically on the basis of a defined current threshold of about 1 mA. The switch may be made not by considering the value of the beam current but the chosen or detected operating mode, i.e., television mode or multimedia monitor mode. 
     FIG. 6 illustrates the improvement obtained by the device according to the invention. In the Figure, the device according to the prior art, having a conventional structure K, G 1 , G 2  with an electrode G 2  having a thickness of 20 mils, is compared with a structure, according to the invention, with two control electrodes G 2  and G 2 ′ having a thickness of 8 mils and separated by a space of 4 mils. The predetermined beam-current threshold was fixed at 1 mA, VG 2  and VG 2 ′ are equal to 260 V, and above 1 mA, VG 2 ′ is increased to 1000 V, VG 2  remaining at its previous value. It may be seen that the size of the beam in the main lens is effectively enlarged in the region of low currents with respect to the prior art, which means, in this current range, that there is a significant improvement in the size of the impact area of the beam on the screen, allowing access to high resolution necessary for displaying multimedia images, for example, of the SVGA or XGA type. 
     More than two operating modes of the gun may be envisaged and, in this case, the voltage Pv will assume as many values as there are operating modes. For more precise control of the size of the beam exiting the zone for forming the said beam, it is possible to vary the voltage Pv not in a jump but gradually, depending on the detected value of the beam current. In this way, the size of the beam may be controlled over all the values of the beam-current range used. 
     Modifying the value of the voltage applied to one of the control electrodes may require modifying the voltage applied to another control electrode. This is because, in the usual triode structure of the beam-forming zone, K, G 1 , G 2 , the beam is modulated from the cathode by a negative modulation voltage, the G 1  being raised to earth and the G 2  to a preset threshold voltage, called the cut-off voltage, in order for there to be no beam current in the absence of modulation on the cathode. The switch from a mode in which the G 2  and G 2 ′ are at the same cut-off voltage to a mode in which the voltage of the G 2 ′ becomes greater than the voltage of G 2  requires modification of the voltage applied to the G 2 , so that the new voltage applied to the G 2  corresponds to the threshold for emission of a beam current. 
     This modified structure may be developed for any type of gun by replacing the control electrode G 2  by at least two electrodes G 2  and G 2 ′, the longitudinal dimension of which is substantially the same as that of the initial electrode G 2 . In this way, the design of the gun in the region of the focusing stages will not have to be changed. Preferably, the space between the two electrodes is as small as possible so as to maintain a sufficient thickness at the two metal components which form the electrodes G 2  and G 2 ′ in order to ensure good mechanical integrity of the said components. G 2  and G 2 ′ will, for example, have a thickness equal to 8 Mils, and the space between the two electrodes will be 4 mils. 
     The principle of the invention may be discriminantly and advantageously applied to a single-beam gun or a multibeam gun for cathode-ray tubes.