US2306663A - Net control of cathode ray tubes - Google Patents

Description

El 2, 1942. K. SCHLESINGER NET CONTROL OF CATHODE RAY TUBES Filed NOV. 5, 1938 Fig. 3b.

GOOD

712 Yen for STATES QFlCE.

Kurt Schlesinger, Berlin, Germany; vested in the Alien Property Custodian Application November 5, 1938, Serial No. 239,173 In Germany March 2, 1938 4 Claims.

So long as the cathode surfaces of television tubes were spot-like in form, i. e. had a diameter of approximately 0.5 mm., the modulation of the ray currents was possible by means of a diaphragm aperture of approximately 1 mm. in diameter closely surrounding the cathode. In modern television tubes, however, the large-surface cathode has been adopted, which has a diameter of 1-2 mm. The emission of these large surfaces is first collected by a system of electric acceleration lenses into a much smaller point, the so-called cross-over point, and the latter is then reproduced on the screen.

Maintenance of the aperture control in large surface cathodes of this kind meets with increasing difficulties. Since the perforated diaphragm must always have an aperture which is larger than the cathode, its control effect is reduced with respect to the centre of the cathode, and for blocking the emission increasing negative biasses of the perforated diaphragm are required. The control acquires the form of characteristic of a tube with variable reciprocal. The characteristic exhibits an increasing requirement for potential up to the blocking point. The half-tone reproduction of the image in black is thus impaired and the requirement for control output is quite considerably increased.

As a remedy it has already been proposed in the copending application Ser. No. (filed Nov. 5, 1938) to replace the control by means of a perforated diaphragm by a net control. In this connection there is disposed in front of the large-surface cathode a net which divides the surface of the cathode into approximately 10 fields per square millimetre. Said net control by the perforated diaphragms is replaced by a longitudinal control with the assistance of the said net, which is located between the cathode and a first suction anode. Unfortunately the execution of this idea meets in practice with three difficulties:

(1) The penetration of anodic fields through nets of this character having a Width of mesh amounting to approximately 0.3 mm. and a thickness of the wire of approximately 0.1 mm. is much too small. It is of the order of 1% or less. On the other hand the anode potentials are definitely determined by the design of the tube system. For the first anode opposite the cathode there are usually available merely potentials of the order of to of the maximum anode potential, 1. e. approximately 200 volts. There accordingly extends through the net so little shift potential, i. e. in the present case 1-2 volts, that in the negative range of the grid potential no appreciable emissive currents are ob tainable.

(2) The operation of the nets in the positive range is prohibited in the case of the usual construction by the occurrence of very strong grid currents, which then very quickly represent a multiple of the useful electronic currents passing through the meshes, and unnecessarily load the cathode. Similar objections, viz. overloading of the cathode and excessive complication of the system, are also to be made against the provision of a special inner space charge net.

(3) A cathode behind a control net supplies electronic rays of particular distribution of energy, which considerably differs from the thermal distribution according to Maxwell. It is accordingly found that the treatment of net controlled electronic rays is not possible with the same optical systems which have been employed for the free surfaces of hot cathodes. If the concentration of large-surface cathodes with net control in a common focal point is to be successful, it is therefore necessary in conjunction with the net control to develop special electronoptical methods.

In the following methods are set forth for the purpose of avoiding the diificulties referred to under (l)-(3):

According to the invention, for the purpose of avoiding the reciprocal difficulty (1), the net control is carried out substantially within the range of positive grid biasses. The normally strong grid currents occurring in this connection are considerably reduced by production of the net from the finest tungsten wire obtainable, there being a ratio of 1:10 or less between the diameter of the Wire and the Width of mesh. The Width of mesh is so great and the distance from the first anode so small that all of the electrons in the free opening of the meshes are sucked up entirely into the ray and do not pass to the wire net. After these measures have been carried out there passes to the net merely that proportion of the total emission which corresponds to the covering factor, i. e. the surface of the cathode which is shaded by the net. This can be made less than 10% of the total emission, so that the difiiculty mentioned above under (2) is overcome in practice.

The electron-optical problem referred to under (3) and referring to the manner in which the bundles of rays passing out of the net are to be combined in a focal point is solved by a special Figs. 3a and 3b illustrate modifications of the system shown in Fig. 2.

In Fig. 1a there is shown the plan view of a control net for cathode ray tubes, such as has been developed by the applicant.

It is shown in respect of the typical but by no means only possible case in which there is to be controlled a circular cathode having an emissive surface of approximately 1 square millimetre. In Fig. 1 the cathode is designated l and the spot of oxide embedded in the same is designated 2. This useful surface of 1 square millimetre in size is divided by 2 X 4 wires (3) into, say, 10 fields. The wires have a relative spacing a of approximately 0.3 mm. The shading portion of the surface of the wire on the controlled area amounts to approximately 2d/a per unit of area, d being the thickness of the wire. If this shading proportion is to be less than 10%, the thickness of the wire (1 must be made equal to 0.015 mm. or 15 on the market, but are produced by placing the wires 3 of the net in accordance with Fig. 11) over a mounting t, the edge of which is preferably grooved, i. e. furnished with recesses, in the stated divisioning. After they have b en placed in'the correct position the wires are firmly welded to a layer 4a. As material there is preferably employed a heat resisting material, such as platinum or tungsten.

To obtain a certain preliminary concentration of the electrons the cathode can conveniently be furnished with a surface curved in the manner of a hollow reflector.

Tests have shown that the sensitivity of the control, with conditions otherwise unchanged, is proportional to the number of wires or the square root of the number of meshes. On the other hand it is to be considered in accordance with the above that with increasing number of meshes the current loss is also increased.

It is found that in accordance with the invention nets having the very small shading factor of approximately 10% can b controlled into the positive range of up to approximately 30 volts positive bias without consuming in interfering fashion large proportions of the total emission. The reciprocal in conjunction with nets of this kind having a d:a ratio of 0.1 and less amounts to several per cent. The suction of the total mesh emission with anodes having a potential of approximately 200 volts is accordingly still possible at a distance of several millimetres away from a net of this kind, so that 80-90% of the emission of approximately 1-2 ma. per square millimetre obtainabl in the case of positive grid biasses is supplied to the ray.

Difiiculties are caused by the electron-optical treatment of these rays. In connection with the net control in the case of negative grid bias electron-optical reproducing systems have. already Nets of this kind are not obtainable .ill

been developed such as would appear typical quite generally with respect to net controlled cathode ray tubes. According to the present invention a new electron-optical system is provided which produces a real reduced image of the object of reproduction in or near the intersection-point diaphragm. In this way all difiiculties, which resided more particularly in a complete unreliability as regards selection of the electrode spacing of the concentration system and in the impossibility of keeping the angle of divergence of the rays small when leaving the intersection-point diaphragm are solved at one go, as by means of th known laws of electron-optical reproduction this optical system is also now under control.

In Fig. 2 there is shown a typical electronoptical system of this kind such as can be employed in conjunction with net control of all kinds.

The problem involved is: To unite all electrons passing through the net from the surface I in a focal point 5, or to allow them to pass through a diaphragm of prescribed size, in which connection the ray leaving the diaphragm should not exceed a certain angle a of divergence in order that it can be completely embraced later by the main reproducing system.

In order to concentrate at all in a point rays disposed approximately parallel to the axis, there must be provided an electric lens 6, which in the present case merely requires to be in th form of an aperture lens with acceleration of the ray, as it is only with lenses of this kind that with utilisation of the root-from-potential factor in the case of reasonably small object space the requisite strong reductions of approximately 1210' can be obtained. In this connection the spacing Z between the aperture lens 6 and the diaphragm 5 is in a ratio of approximately =tang a to the aperture 0 of the lens. On the other hand the lens aperture 0 is selected to be at most equal to the useful cathode surface of 1. In this way 0 and Z are definitely determined in conjunction with the remaining tube construction, length of the tubular member, position of the main lens etc. Care accordingly requires to be taken that so far as possible all electron rays reach the opening 0. For this purpose, according to the invention, there is employed a second condensing lens 1. This lens must be located at a distance l1 from the lens 5, which distance allows the lens aperture to be reproduced according to the known laws of acceleration lenses on to the dimensions of the intersection-point diaphragm 5.

In the case of a desired reduction of km the desired spacing Z1 results from the theoretically derived formula:

In this connection e0, c1 and 62 are the potentials of the intersection-point diaphragm 5, the collecting lens 6 and the collecting lens I. Tests have provided a good confirmation of the theory.

The aperture 01 of the lens I must be larger than the net or cathode surface. The distance from the latter amounts to approximately 1 mm. By further adjusting the potential 62, which is conveniently tapped at a common potentiometer 9. a otential. Qf. lemen 7 c n. e o d at w h the loss of current in relation to the lens 6 is at a minimum or is even reduced to zero. The larger the aperture 01 is selected to be, the more negative will be the bias ea in th case of fixed distances, and the greater the distance mg is selected to be, the more positive will be the bias e2 with respect to the theoretical value. A suitable position of 01 in relation to g is found empirically, so that the already weak anode fields are not additionally weakened by the bias of i.

There is also a second way of obtaining the focussing of the electrons. Th condensing lens 1 can be omitted and it is possible to manage with the lens field of 5 alone. In this case, in accordance with Fig. 3a, the spacing Z2 between the control grid g and the lens 5 requires to be determined empirically. The smaller Z2 is selected to be, the better is the suction of the mesh electrons into the ray owing to increasing field intensity on the part of the diaphragm 6, which in The arrangement according to Fig. 3a appears to be more simple than the arrangement with intermediate diaphragm i according to Fig. 2, but has the disadvantage that the object spacing Z2 between the object of reproduction and the lens 6 is of a fixed kind by reason of an auxiliary requirement, viz, the preliminary concentration, so that it is not possible to obtain any desired scale of reproduction between cathode l and diaphragm 5. After the determination of 12 it is accordingly necessary so to determine the cathode surface I that its image exactly fills out the diaphragm aperture 5, i. e. it is not possible to employ cathode surfaces of a desired size. The method according to Fig. 3a is capable of being employed more particularly for simple tubes of medium output.

Since the rays are very slow and accordingly readily capable of deflection in the space between grid and condenser lens 6, it is desirable to screen off this portion of the path of the electrons against external electro-static fields by means of a cylinder Pia disposed in front of the cylinder 8 supporting the control electrode and the cathode. The presence of a cylinder of this kind does not influence the paths of the rays so long as the diameter of the cylinder is selected to be greater than approximately three times the diameter of the grid.

The cylinder 8a can either be mechanically connected with the diaphragm supporting the net, as shown in Fig. 3a, or it can be designed in the form of a separate electrode, as illustrated by Fig. 3b, in which case it is preferably connected with the potential of the cathode.

The distances Z2 can be determined in each case experimentally. For cathodes having a surface of 1 square millimetre and nets as shown in Fig. 10. there are to be found in practice approximately the following values:

Distancel (5/6) mm 7 Aperture 0 mm 1 Distance 11 mm 5 Aperture 01 Iclaimt v 1. In a television arrangement a cathode ray tube comprising a cathode having a circular emissive surface, one apertured control diaphragm mounted in front of said cathode, an apertured object diaphragm the size of the aperture of which amounts to approximately 1/ 10 of the size of said emissive surface, and an electron optical preconcentrating system adapted to focus the cathode ray in the aperture of said object diaphragm, said preconcentrating system consisting of said cathode, said control diaphragm, said object diaphragm, a cylindrical electrode mounted adjacent to said control diaphragm at its side facing said object diaphragm, and a further apertured diaphragm mounted between said cylindrical electrode and said object diaphragm, the diameter of said cylindrical electrode being larger than the diameter of the aperture of said control diaphragm, the aperture of said further diaphragm being smaller than said emissive surface but larger than the aperture of said object dia phragm, the aperture of said control diaphragm being at least as large as said emissive surface and being covered by a plurality of wires crossed at right angles, said wires being so dimensioned that at most 10% of said emissive surface is shaded by said wires, means for supplying said control diaphragm with a positive bias with respect to said cathode, and means for supplying said further diaphragm and said object diaphragm with positive potentials increasing in the direction away from said cathode.

2. The invention set forth in claim 1 wherein said cylindrical electrode is connected with the potential of said cathode.

3. The invention set forth in claim 1 wherein said cylindrical electrode is mechanically as well as electrically connected with said control diaphragm.

4. In a television arrangement a cathode ray tube comprising a cathode having a circular emissive surface, one apertured control diaphragm mounted in front of said cathode, an apertured object diaphragm, and an electron opticalpreconcentrating system adapted to focus the cathode ray in the aperture of said object diaphragm, said preconcentrating system consisting of said cathode, said control diaphragm, said object diaphragm, and two further apertured diaphragms mounted between said control diaphragm and said object diaphragm, the apertures of all of said diaphragms being circular and so dimensioned that their diameters decrease in the order 1 in which said diaphragms follow each other in the direction towards said object diaphragm, the size of the aperture of said object diaphragm being approximately equal to 1% of the size of said emissive surface, the aperture of said control diaphragm being at least as large a said emissive surface and being covered by a plurality of wires crossed at right angles, said wires being so dimensioned that at most 10% of said emissive surface are shaded by said Wires, means for supplying said control diaphragm with a positive bias with respect to said cathode, means for supplying said further diaphragms with positive potentials increasing in the direction towards said object diaphragm, and means for supplying said object diaphragm with a positive potential at least as high as the potential of the nearest of said further diaphragms.

KURT SCHLESINGER.

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