Abstract:
In order to suppress the Barkhausen oscillation previously observed in fluorescent display tubes, the electron-stream transmission of the grid electrode is controlled by shielding regions of solid or fine mesh form provided in each of the grid meshes in opposing relationship to the exposed insulating portions of the anode substrate lying within the corresponding set of anode segments, which are arranged in a pattern of substantially figure-of-eight configuration.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to fluorescent display tubes of the type commonly employed to display data in numeric and/or letter form. 
     A typical form of fluorescent display tube includes an anode substrate coated with a fluorescing substance (FIG. 1), a grid spacer (FIG. 2) placed over the substrate and the mesh of a grid electrode (FIG. 3) laid over the grid spacer. Other parts such as a cathode and a support structure as assembled on the stack to complete an electrode assembly, and a glass covering is sealingly secured to the electrode assembly. Such configuration is fully described by Takao Kishino in U.S. Pat. No. 4,047,073 (Patented on Sept. 6, 1977). As is well known, the display tube is evacuated at the last stage of manufacture through a gas exhausting tube attached thereto. The anode arrangement of the display tube includes a number of segmented luminescent anodes or groups of anode segments and, in operation, only those anode segments which are held at positive potential are excited to emit light by bombardment of the electrons emitted from the cathode and accelerated by the control-grid potential, as the anode segments in each group are selectively excited to display a numeral or letter desired. 
     In other words, in operation of the fluorescent display tube, the anode segments in each group never emit light all at the same time and the combination of anode segments selected or excited to emit light by electron bombardment usually varies from time to time to display different numerals or letters in turns. In some case, any of the anode segments in some group or other is not excited so that there is no display in the digit place. Specifically, with the conventional mesh pattern of grid electrode (FIG. 3), among the electrons emitted from the cathode and proceeding through the mesh under the acceleration effect of the grid potential, those electrons approaching the anode segments held at positive potential excite such segments effectively and thereby cause them to emit light, but the remaining electrons passing through the mesh and approaching the other segments, not held at positive potential, and the exposed insulating surface (FIG. 1) of the anode substrate are attracted back to the grid electrode. Such behavior of electrons, occurring in repetition, gives rise to a sort of oscillation, which is known as Barkhausen oscillation. Such oscillation is particularly observed in the VHF range, though limited in intensity. In cases where such display tubes are employed with televisions or other electronic appliances utilizing such frequency range, weak electric waves resulting from the Barkhausen oscillation give rise to practical problems, impeding the performance of the associated appliance to a marked extent. 
     SUMMARY OF THE INVENTION 
     It is therefore, an object of this invention to provide a fluorescent display tube suppressing the Barkhausen oscillation. 
     It is another object of this invention to provide a fluorescent display tube comprising improved grid members having a fine mesh region and thereby suppressing the Barkhausen oscillation without disturbing a field of vision. 
     Briefly, those objects of the invention are achieved by providing shielding regions of fine mesh in each of the grid meshes in opposing relation to the exposed insulating portions of the anode substrate lying within the corresponding set of anode segments. 
     The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an anode structure of a fluorescent display tube; 
     FIG. 2 is a perspective view of a spacer frame having grid spacers and lead-in wires; 
     FIG. 3 is a perspective view of a conventional grid frame having an array of mesh patterns; 
     FIG. 4 is a perspective view of a grid frame according to one embodiment of the present invention; and 
     FIG. 5 is a cross-sectional view of a fluorescent display tube employing the improved grid electrodes shown in FIG. 4. 
     FIG. 6 is a perspective view of another form of a grid frame of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a number of groups of anode segments 11 are arranged on the anode substrate 1 in an array of patterns of substantially figure-of-eight configuration 10 and dots 12. Needless to say, a fluorescent material is provided on each of the indicating portions 10 and 12. Each anode segment 11 is electrically connected to an array of connecting portions 13 formed on the anode substrate 1. In FIG. 1, those connecting means are not shown. 
     Referring to FIG. 2, an array of grid spacers 21 is supported by spacer frame 20. Each spacer 21 has a grid terminal lead-in wire 22. In FIG. 3, an array of conventional grids 31 is supported by grid frame 30. Each grid 31 comprises a uniform pattern of mesh region 32 surrounded by mesh frame 33. The mesh region 32 has enough area to cover whole region of indicating portions 10 and 12. 
     Referring to FIG. 2 and FIG. 3, as is described in the aforementioned Kishino Patent, a grid frame 30 is put over a spacer frame 20, and after placing each grid 31 upon each spacer 21 and securing them by welding or caulking, a grid member is formed by removing unnecessary portions, namely, frames 20 and 30 and holding piece 34 combined to each mesh frame 33. 
     One of the preferred embodiments of the present invention can be achieved by replacing the conventional grid 31 by a newly invented grid shown in FIG. 4. A new grid 41 is generally of the same pattern of mesh arrangement as shown in FIG. 3, but its mesh region 42 includes solid shielding regions 45 and 46 which allow no passage of electrons. 
     In operation of a fluorescent display tube employing such a grid electrode, it is to be noted that electrons tending to pass through the meshes 42 in the vicinity of any anode segments 11 having no positive potential are attracted by the shielding regions 45 and 46 while electron passage to the insulating regions of the anode substrate 1 that lie within the set of anode segments 11 is precluded by the solid shielding regions 45 and 46. Because of this, it will be readily appreciated that there is no occurrence of any negative electrical resistance and the Barkhausen oscillation is effectively suppressed. 
     In FIG. 5, which illustrates a fluorescent display tube embodying the principles of the present invention, the anode substrate 1 of FIG. 1 with sets of anode segments 11 arranged thereon, the grid spacer 21 of FIG. 2 and the grid 41 of FIG. 4 are used. Other parts, such as a cathode wire 55, cathode supports 56 and a glass covering 57 are located in a conventional manner. A tube 58 for exhausting gases is attached to the substrate 1 and sealed off after the evacuation process. 
     As will be understood from the above description, when the electron passing region is restricted to the area corresponding to the area of the anode segments 11 and 12, Barkhausen oscillation is most effectively suppressed. In that case, however, the field of vision is very narrowly restricted. In this aspect, the areas of the shielding regions 45 and 46 are preferably formed relatively smaller than the area surrounded by the anode segments 11. A typical example of the dimensions of each grid 41 are as follows: the mesh region 42 of 16.5×10 mm; the shielding regions 45 and 46 of 3.5×4 mm and 4×4 mm, respectively; and the mesh frame of 18×14 mm. These dimensions are given for a case where the indicating portions 10 have the dimensions of 15×8.4 mm. 
     In the foregoing embodiment, a grid for dot portion 12 has no shielding region because the actual width of the grid is as narrow as 3 mm wide. Furthermore, as shown in FIG. 4, the mesh region 42 is slightly inclined toward the right hand about 10 degrees depending on the inclination of the figure-of-eight portion 10 (FIG. 1), and thereby restricting the electron passage to the outside of portion 10 as much as possible. 
     Although the solid regions 45 and 46 give a excellent shielding effects, it should be noted that the problem of reflection of exterior light by the solid regions 45 and 46 or emission of light themselves may possibly occur. Such problems, however, can easily be dealt with by blackening the solid regions 45 and 46 as, for example, with an appropriate chemical substance or by hydrogen treatment at an elevated temperature. Alternatively, the solid regions 45 and 46 can be replaced by fine mesh regions as shown in FIG. 6. When a mesh pattern is formed like a honeycomb and the length of one side of a hexagonal opening in the region 42 is 0.2 mm, a fine mesh having a shielding effect is obtained by shortening the length of one side of the hexagonal opening to less than 0.15 mm. In that case, the frame width of each opening changes, for instance, from 40 μm to 90-100 μm. Such dimensions are effective in the range of driving voltage below 30 V. Since the driving voltages of the ordinary fluorescent display tube is around 18 V, the latter dimension is practical. If fine mesh regions are employed instead of solid regions, the field of vision is improved greatly. It will be apparent to those skilled in the art that the invention is not restricted to the features described above and shown in the drawings but may be varied in many ways within the scope of the annexed claim.