US2336895A - Cathode ray tube - Google Patents

Description

Dec. 14, 1943. v

g. sHELToN ET Al.

CATHODE RAY TUBE Filed July 22, 1941 2 Sheets-Sheet l H/LAEY Moss Dec; 14, 1943 I E. E. sHEL'roN ET A1. V2,336,895

GATHODE RAY TUBE Filed July 22, 1941 2 ySheets-Sheet 2 Fig. 3;

row/ako 15e/c Jaarw Patented Dec. 14, 1943 UNTED STATES PATENT OFFCE CATHODE RAY TUBE Application July 22, 1941, Serial No. 403,570 In Great Britain July 23, 1940 (Cl. Z50- 159) 9 Claims.

This invention relates to the double beam cathode ray tube of the kind in which a single beam is generated in an electron gun and is then divided into two parts by an electrostatic shield, which will be referred to as a splitter plate, disposed in the path of the beam between the lirst (or the only) pair of deiiector plates. The two parts of the beam are rendered independently deiiectable in one direction by the shielding action of this splitter plate.

In tubes of this character, the beam is necessarily generated and focussed,` at least mainly, by electron lenses, because the beam of a tube employing only gas focussing is not sufficiently wide for a splitter plate to be permissible. A small quantity of gas may, however, be admitted to improve the focus.

The term intermodulation will be applied to the defect found in tubes of the kind described, that each part of the beam is to some extent deflected in accordance with variations in the potential of the deector plate from which it is shielded by the splitter plate. Compensation for one form of intermodulation may be ef modulation, which may be experienced in such a tube when the deiiector plate circuits are of high impedance, can be ascribed to the collection by the deflector plates of secondary electrons emitted from the splitter plate. Secondary electrons may be emitted from the edge of the splitter plate nearest to the gun, or from the sides of this plate.

According to the present invention, the secondary electron current which flows from the splitter plate to either one deflector plate is rendered substantially independent of the potential of the other deflector plate.

According to one feature of the invention, the splitter plate is a metallic plate having a carbonized surface. This feature may be used in conjunction with, or independently of, other features of the invention.

According to another feature of the invention, the splitter plate is extended towards the cathode beyond the entrance edges of those deflector plates between which it is disposed. This reduces the penetration of the iield of either deflector plate into the region at the entrance edge of the splitter plate and into the region between this edge and the other deflector plate, and hence reduces the tendency to collection by either deector plate of secondary electrons emitted in these regions. It is essential, however, to avoid appreciable penetration of the splitter plate into any electrostatic focussing field, although penetration into any electromagnetic focussing field is immaterial provided that the splitter plate is of non-magnetic material.

It is preferred that an apertured disc electrode, maintained at the potential of the splitter plate, and disposed normally to the axis, shall be arranged to touch, or almost to touch, the entrance edge of the splitter plate, to reduce still further the penetration of the deector plate iields. The splitter plate may be provided with an extension which reaches within the aperture of this disc. Such an apertured disc may serve also as a part of the final anode of the electron gun.

According to a preferred feature of the invention, as many as possible of the conductors which are connected to either deflector plate are electrostatically shielded from the splitter plate by further shielding members, which do not lie in the path of the beam.

The accompanying drawings illustrate the disposition of the electrodes in the deflection region of a cathode ray tube embodying the invention. This tube is constructed for independent deection of the two beam parts by the potential variations of the Y plates, but simultaneous deilection'of the two beam parts by the potential variations of the X plates. Figure 1 is a sectional view taken on the plane containing the axis and parallel to the direction of Y deection.

Figure 2 is a sectional View on the plane perpendicular to the axis, indicated at A-A in Figure l. Figure 3 is a sectional View taken on the plane containing the axis and parallel to the direction of X deection.

The beam is generated in an electron gun and is Iinally focussed by the electrostatic electron lens formed between the last two anodes I and 2 of the gun. The nal anode 2 comprises the disc 2a, having a central aperture 2b through which the beam issues to pass on both sides of the splitter plate 3. This splitter palte 3 is disposed between the Y deflector plates il and 5 and shields each part of the beam from the deiiector plate on the opposite side. The plate 3 is maintained at the same potential as, and is made long enough to touch, or almost to touch, disc 2a. The plate 3 may have its surface carbonized, in manner known per se, to reduce the tendency tu secondary electron emission therefrom.

When, for example, Y plate 4 is at higher potential than plate 3, the field due to the plate 4 is not appreciable either at the bottom edge of plate 3 in the aperture 2b, or on the right of plate 3 as seen in Figure 1. Therefore secondary electrons generated by the impact of primary electrons on the bottom edge of, and on the said right hand side of, plate 3 are not drawn in appreciable quantity to plate 4.

Y plates 4 and 5 are supported by the gripping of the ends of their flanges 4a, 5a between micas 29 and Sil. They are positioned relative to mica 3l! by their lugs 4b, 4c, 5b, 5c fitting into slots in the circular inner edge of mica 30.

The connection to Y plate 4 is' provided through the short lead I8, by which its lug ibis joined to the support rod 8. The bucking .wire or' deector element 'I is supported by rods 8 and II. Rod 8 is carried right down to the cathode end of the structure and is connected through a leadin wire to ar: external Contact. Similarly. Y

plate 5 is connected through short lead I9 fromv lug 5c to support rod I2, and bucking wire or deiiector element 6 is supported by rods I2 and I5, one of which is carried right down to the cathode' end' of the structure and is connected through a lead-in wire to an external contact. X plate 20 is supported'by the rods Il and I4,

and rod I 4 is carried down'and connected to an' external contact. The other X plate 4I! is similarly supported from rods I Il and I3, one of which is carried down and connected to an external contact.

Between the Y plates and the X plates there are two interplate shields 2l and 24 disposed substantially perpendicular to the axis. mounted on micas 29V and 28 respectively and both maintained at the potential of the nal anode 2. vThe sides 2Ia and 2lb of shield 2| are bent up as plates parallel to the axis which reach up to the mica 28. Shield 2| has a large rectangular central aperture and the edges 2Ic and 2Id are bent up as plates parallel to the axis at the sides of this aperture. At the ends of the aperture, plates 22a and 22h are provided par'- allel to the axis and extending from the top surface of shield 2lv up tothe mica 28. They are secured by welding to the shield 2l their flanges 22o and' 22d.' Lugs 3a. 3b of plates 3 are bent to be perpendicular to the axis andare welded to interplate shield 2I. Plate 3 is located relative to micas 23, 3B by its ends fitting in slots formed in the inner circular edges or' those micas. l I

Parts of the interplate shield 24 near to the beam-are arched, as indicated at 24a. into a curvature struck from a centre substantially at the apparent centre of Ydeflection. The interplate shield 24 is secured to the mica 28 by lugs 24h which are inserted through holes in the mica and bent back along its under-surface.v VA short lead 23, previously welded to such a lug 24U, is connected to plate 21a to connect the interplate shields 2| and 24 together. Shield 2I is connected by ashort lead 23a to the wire I5, which is carried down to the cathode end of the structure andV connected through a lead-in wire to an external contact. T he straight portions of the bucking wiresf E and 1 are'shielded from splitter plate 3v by plates` 2Ic, ZId. The ends of the bucking wires 6 and l, and the parts of support rods 8, I`I, I2 I5' between the micas 28 and 29, are shielded fronrsplitter plate 3 by plates 22a,` 22h. Theu parts of support rods 9, IIJ, I3,

splitter plate.

I4 between the micas 28 and 29 are shielded from splitter plate 3 by plates 2Ia, 2Ib.

The parts of support rods 8 to I 5 between micas 30 and 3| are shielded from splitter plate 3 by the cylindrical shield 25, which is mounted around anode 2 in electrical contact therewith aid is connected through short lead 26 to wire A tube of which the accompanying drawings are substantially a scale representation, and in which the internal diameter of cylinder 25 was 14 millimetres, was tested in comparison with a tube generally similar but having no provision for shielding the support rods or the bucking wires from" the splitter plate, and having the entrance edges oi the splitter plate and the Y deector plates all spaced from the disc part of the iinalV anode a distance of 3 millimetres. In each case the potential of one of the Y plates was maintained 12 volts positive with respect to the splitter plate, and the potential of the other Y plate was varied over the range from 50 volts negative to 50 volts positive with respect to the The secondary electron current whichl flowed'from the splitter plate to the first Y plate varied between 2.30 and 4.10 microamperes in the tube having no shielding, but in the tube according to the invention the variation was reduced to the limits 2.25 and 2.65 microamperes.

In cathode ray tubes having a partial gas lling, it has been found that the usual coating on the inside of 'the Wall of the envelope, between the gun and the fluorescent screen, is unsatisfactory as it tends to absorb the gas. Instead, a metallic cylindrical hood is provided surroundving the deflector plates and extending for some distance towards the uorescent screen. Such a hood is indicated at 2l; in the structure illustrated it is made a push-fit over the micas 28 to 3l.

A- further type of distortion or" the traces of the electron beams is due to secondary electrons returning from the iuorescent screen being collected by the'Y plates l and 5 during the periods when the potentials of the X plates are nearly equal. If either of the Y plates is connected to a high impedance circuit, such a current of secondary electrons will produce an appreciable variation of potential. This effect may be reduced to negligible proportions by maintaining the hood 21 at a potential of say 35 volts positive to the nal anode 2, so that it collects practically all the secondary electrons returning from the screen. For this purpose the wire I'I is connected to the hood 2l and carried down to an external contact.

It will be appreciated that the features of the invention, being concerned with the electrodes in the deection region, are not limited in their application to cathode ray tubes of the kind having uorescent screens, but that they may also be applied, if occasion should arise, to cathode ray tubes of other kinds.

We claim:

1. In a cathode ray tube the combination of an electron gun for emitting an electron beam comprising an anode having an aperture therein through whichsaid electron beam passes, a pair of deiiector plates between which said electron beam passes, a splitter plate positioned intermediate said deflector plates in the path of said electron beam whereby the said beam is divided into two parts which pass respectively on each side of the splitter plate, said splitter plate comprising a metallic electrode having a carbonized surface.

2. A cathode ray tube comprising an electron gun for emitting an electron beam, an anode having an aperture therein through which said electron beam passes, a pair of deilector plates between which said electron beam passes, a splitter plate positioned intermediate said deiiector plates in the path of said electron beam whereby the said electron beam is divided into two parts which pass respectively on each side of the splitter plate, said splitter plate extending towards the said anode beyond the entrance edges of the said deflector plates between which the said splitter plate is disposed.

3. In a cathode ray tube the combination of an electron gun for emitting an electron beam comprising an anode having an aperture therein through which said electron beam passes, a pair of delector plates between which said electron beam passes, a splitter plate positioned intermediate said deflector plates in the path of said electron beam whereby the said electron beam is divided into two parts which pass respectively on each side of the splitter plate, said splitter plate extending towards the said anode beyond the entrance edges of the said deiiector plates between which the said splitter plate is disposed.

4. A cathode ray tube comprising an electron gun for emitting an electron beam, an anode having an aperture therein through which said electron beam passes, a pair of deector plates between which said electron beam passes, a splitter plate positioned intermediate said deector plates in the path of said electron beam whereby said electron beam is divided into two parts each of which passes respectively on a side of the splitter plate, the entrance edge of said splitter plate being positioned in contact with said anode.

5. In a cathode ray tube the combination of an electron gun for emitting an electron beam comprising an anode having an aperture therein through which said electron beam passes, a pair of deector plates between which said electron beam passes, a splitter plate positioned inter.- mediate said deiiector plates in the path of said electron beam whereby said electron beam is divided nto two parts each of which passes respectively on a side of the splitter plate, the entrance edge of said splitter plate being positioned in contact with said anode.

6. A cathode ray tube comprising an electron gun for emitting an electron beam, an anode having an aperture therein through which said electron beam passes, a pair of deector plates between which said electron beam passes, a splitter plate positioned intermediate said deflector plates in the path of said electron beam whereby said electron beam is divided into two parts each of which passes respectively on a side of the splitter plate, said splitter plate comprising an extension which extends within the aperture of said anode.

7. A cathode ray tube comprising an electron gun for emitting an electron beam, an anode having an aperture therein through which said electron beam passes, a pair of deflector plates between which said electron beam passes, means for supporting said deector plates electrically connected thereto, a splitter plate positioned intermediate said defiector plates in the path of said electron beam whereby the said beam is divided into two parts which pass respectively on each side of the splitter plate, and means for electrostatically shielding said splitter plate from said aforementioned means.

8. A cathode ray tube comprising an electron gun for emitting an electron beam, means comprising a splitter plate for dividing said electron beam into a plurality of parts, means comprising a pair of deector plates cooperating with said splitter plate for independently deflecting each of said parts, deiiector elements each connected to opposite deflector plates and means for electrostatically shielding said deflector elements from said splitter plate.

9. A cathode ray tube comprising an electron gun for emitting an electron beam, an anode having an aperture therein through which said electron beam passes, means to divide the beam into a plurality of parts and to control independently the deflection of said parts in one plane comprising a pair of dei-lector plates between which said electron beam passes and a splitter plate positioned intermediate said deiiector plates in the path of said electron beam whereby the said electron beam is divided into two parts which pass respectively on each side of the splitter plate, a pair of deilector plates between which said electron beam parts pass which serves to control the deiiection of said beam parts in another plane and shielding means comprising a metallic shield surrounding said pairs of deflector plates and said splitter plate.

EDWARD ERIC SHELTON. HILARY MOSS.

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