US2439504A

US2439504A - Flat beam cathode-ray tube and circuit

Info

Publication number: US2439504A
Application number: US581281A
Authority: US
Inventors: Broadway Leonard Francis
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1944-03-09
Filing date: 1945-03-06
Publication date: 1948-04-13
Anticipated expiration: 1965-04-13
Also published as: GB580860A

Description

April 13, 1948 L. F. BROADWAY A 2,439,504

FLAT BEAM CATHODE RAY TUBE AND CIRCUIT Filed Marh e, 1945 H VVENTOR. LeazzafaFm/zczf Ervan/way l ATTORNEY Patented Apr. 13, 1948 FLAT BEAM CATH AND CIR ODE-RAY TUBE CUIT Leonard Francis Broadway, Ickenham, England,

assigner to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application March 6, 1945, Serial No. 581,281

' In Great Britain March 9, 1944 (Cl. o-158) 7 Claims.

This invention relates to cathode ray tubes and circuit arrangements embodying such tubes.

For some purposes cathode ray tubes are required in which the sensitivity of deiiection of the beam across the screen of the tube in one direction is required to be much greater than the sensitivity of deflection in another direction perpendicular to the first. For example, in cathode ray tubes employed in radar equipment a high -sensitivity of deflection is required in the signal direction and also in cathode ray tubes employed for television purposes the sensitivity of deection is required to be much greater in the direction of the line scan than in the direction of the frame scan.

Usually the cathode ray beam is scanned across the screen of the tube either by. electrostatic deflecting means cr by electromagnetic deflecting means, each deflecting means comprising either a pair of plates or a pair of coils which may be associated with magnetic polepieces provided within the envelope of the tube. It is known that in order to obtain optimum sensitivity of deection the deflecting plates or the pole pieces of the coils should be placed as close as possible to the electron beam. Since the beam, which is lusually of circular form in cross-section, is brought' to a focus at the screen and since it is necessary to dispose the deecting means at some distance from the screen in order that the focussed beam can scan an area of a desired size, the deflecting means must be disposed at a position where the diameter of the beam is fairly large which results in the sensitivity of deflection being relatively small. If it is desired to reduce the diameter of the beam at the position Where theA deflecting means are disposed, then it is necessary to step down the beam diameter which is an undesirable expedient owing to the consequent, reduction in the beam current.

It is the object therefore of the present invention to provide an improved cathode ray tube and circuit arrangement embodying such tube in which the sensitivity of deiiection can be increased in one direction without necessitating a decrease in the available beam current.

According to one feature of the invention there is provided a circuit arrangement embodying a cathode ray tube in which the electron beam of the tube is of elongated form in cross-section, said tube having means for causing said elongated beam to converge to form a small spot. on the screen of the tube and in which deflecting means are provided for deecting said spot relatively to said screen in one direction and de- Cil fleeting means are provided for deecting said spot relatively to said screen in a co-ordinate direction, deflection inone direction occurring at a more rapid rate than the deection in the other direction, wherein the deilecting means for deiecting the spot at said more rapid rate are disposed at a position Where the beam is of elongated form` and embrace the longer sides of the beam cross-section thus permitting said deecting means t0 be placed closer together than would otherwise be possible, whereby the sensitivity of deflection at said more rapid rate is increased,

According to another feature of the invention there is provided a cathode ray tube having means for generating an electron beam of elongated form in cross-section and having means for causing said electron beam to converge to form a small spot on the screen of the tube and in which deiiecting means are provided for deflecting said spot relatively to said screen in one direction and deiiecting means are provided for deecting said spot relatively to said screen in a co-ordinate direction, one of said deflecting means being capable of deflecting the spot at a more rapid rate compared with the rate of clelection produced by said other deflecting means, wherein the deflecting means capable of deecting the spot at said more rapid rate are disposed at a position where the beam is of elongated form and embrace the longer sides of the beam crosssection thus permitting said deecting means to be placed closer together than Would otherwise be possible, whereby the sensitivity of deection at saidmore rapid rate is increased.

Usually, the deflecting means are disposed at separated points along the path of said Abeam and the deecting means which deect the beam at the more rapid rate are disposed in a position Where the beam is more elongated compared with the elongation of said beam at the point where the other deecting meansI are disposed.

Witha cathode ray tube employed in accordance with the invention it is possible to arrange that the beam is of a substantially rectangular form in crosssection, the depth of the crosssection being, for example, of the order of 0.005, the width of the beam being of any suitable size in order to obtain a desired beam current. Where the cathode ray tube employs electrostatic deiiecting plates the distance apart from said plates at the point Where they approach closest to the beam may be only slightly larger than the depth of the beam thus providing a substantial improvement in the sensitivity of deflection.

Where electromagnetic deflecting means are employed the pole pieces -of the deflecting coils need only be spaced apart by a distance slightly larger than the depth of the beam although in this case it will b e appreciated that the direction in which increased sensitivity of deection occurs will be at right ang-les to that obtained with electrostatic 4 As shown in the drawing-the referencemlmeral l indicates the envelope of a cathode Iray tube, having a screen S. VThe reference numeral 2 indicates a cathode of elongated form`mounted within a cathodeshield 3 of rectangular form, the shield' 'having a rectangular slot'll, `as shown, for defining vthe size of the beam emitted from the cathode 2. The shield L3 in, for example, a television reproducing `tube may serve as a modulating electrode to which modulated potentials can be applied for varying the intensity of the beam. The beam emanating4 through the slot 4 is thus of elongated rectangularform and the depth of the slot may be Say 0.005" whilst its width willbe of a suitable size to permit the passage of a beam of a desired current. `Adjacent to the shield 3 (is a tubular electrode 5 of rectangular form in cross-section, the end of the electrode adjacent the shield `3 being closed by a plate provided with a rectangular slot 6 therein. The electredev may be maintained at apositive potential when operating the tube and lits slot Yt3, in conjunction with Vthe -slot vl and the potential applied in operation tothe shield 3, lwhich will be negative with respectto ythe electrode '5, serves to produce lan electrostatic lens which maintains the beam emanating through the slot sensibly parallel or slightly convergent, with Vrespect to the plane containing the major axis of the slot il; thus the operation of the electrostatic lens should be such that the velectron beam does Anot increase in depth Y-beyond the depth of the' slot 4. It may in some casesbe desirable to insert a further electrode or electrodes suitably slotted yand maintained at suitable potentials Vbetween Vthe electrodes 3 and or'thesurface ofthe cathode be suitably curved in its shorter dimension order to ensure this result. The electron Ybeam indicated bythe dotted lines l' passing through the electrode 5 is'then acted upon'by a further electrostatic lens in order to causeV the beam to converge so as --to form a small spot of a desired size on thescreenrS. This converging action may be accomplished vby employing an electrostatic lens of one of the .types described .in the specification of British patent application No. 4628/42. Such a lens may comprise afurther tubular electrode 8 of rectangular form Vin cross-section which cfs-operates with the end of the electrode 5 reinotefrom the cathode, the adjacent ends of the electrodes 5 .and being suitably shaped and provided with slots in the ,manner described in the aforesaid specification so as lto afford a convergeht beam, the action of the lens formed by the electrodes5 .and 8 being such thatthe depth of the beam is substantially unchanged whilst its width'converges to form a spot of the desired size. v In operation the electrode 8 will usually be maintained at, a hgherpotential than the electrode 5 and the electrode 8 may be provided with a slot 9 in order to define the size of the beam emanating from the electrode 8.

Deecting means are provided for deflecting the beam relatively to the screen of the tube in Vonedirection and deflecting means are provided Vfor deflecting said beam relatively to said screen in a co-ordinate direction. As shown in the drawing, the electron beam is arranged to be dellected ein -a directionfperpendicular to the plane containing the major axis of the beam cross-section Y, by a pair of plates I0 to which suitable poten- -tials are applied -for setting up an electrostatic deeetinvgjeld. 4The pair of plates I0, it will be observed, embrace the longer sides of the beam cross-section and are disposed in a position where .fthe beam'is stillof elongated form. The deflecting means ll for deflecting the beam at right.. angles -to the deflection imparted by the plates l0, are spaced from the plates l0 along the path of the beam and are closer to the screen .S of the tube than said plates l0 so that the beam at the point where the other deflecting means ll are disposed will be less elongated compared with the elongation of the beam at theApOillt where the plates `lll are disposed. AIn oneof the directions of deection the beam is required ,to'be deflected at a more rapid rate compared ,withthe rate of deflection in the other direction. Yinthe case of a cathode ray tube employed in Ytelevision apparatus deflection of the beam at -the linescanning frequency is far more rapdthan'the deflection beamat the frame-scanning frequency. With the tube shown in the drawing the plates l0 will serve to deflect the beam at the more rapid rate whilst the other deecting means `l l, (not shown), will serve to deflect the beam at the slower rate. By arranging the plates lin the position shown the sensitivity of deflection at the more rapid rate is vincreased since the distance apart of the edges of the plates 1-0 where they are closest to the beam need only b eslightly larger than the depth of the Vbeam, i. e.; slightly larger than 0.005". In-this manner the sensitivity of dellection can thus be made very'high whilst at the same time the plates l0 can'be disposed at a position jto permit an area of a desired Asize ,to be scanned. The deflection of the'beam Vin the other direction maybe accomplished `by the'further pair of defleoting plates `II orbyelectromagnetic deflecting means. If desired, of course, the plates I'U ,may be replaced by the polepieces of electromagnetic deflecting means the pole pieces extending within the envelope l andbe.- .ing disposed close to the beam. vWhere .the plates l04 are replaced 'by pole pieces it will of course be appreciated. that the deflection will [be in a direction mutually perpendicular to .thedirection of deection produced by `the ,plates lll.

It is possible that with the construction described employing a beam o f elongated cross-section, a higher modulationslopeand a lesscurved modulation characteristic may .be obtained compared with a tube employinga vcircular,cathode. Furthermore, less .de-focussing may occur with the modulation potentials ,and with the .potentials applied to the delecting plates.

I claim:

1. A circuit arrangement embodying a cathn ode ray tube having ,means .for produclnga thin at electron beam rectangular in cross-section. means ,for causingsaid flatbeam to converge in Width to forma smallspot ,Qn thescreen of the tube deilecting means fordeflecting said spot relatively to s aid screen in one Idirection ,and deflecting means for deflecting said spot relatively to said screen in a co-ordinate direction, de dcction in said one direction occurring at a more rapid rate than the deection in the other direction, wherein the defiecting means for deiiecting the spot at said more rapid rate are disposed a,J a position where the beam is rectangular in cross-section to embrace the longer sides of the beam cross-section and placed closed togetlier than would otherwise be possible, whereby the sensitivity of deflection at said more rapid rate is increased.

2. A circuit arrangement according to claim 1, wherein said deflecting means are disposed at separated points along the path of said beam and the deecting means which deect the beam at the more rapid rate are disposed in a position where the at beam is wider than at the point where the other deflecting means are disposed.

3. Television apparatus incorporating a circuit arrangement according to claim 1, in which said deecting means serve respectively to deect the beam of the tube at the line-scanning frequency and at the frame-scanning frequency, said deflecting means which embrace the longer sides of the beam cross-section serving to deflect the beam at the line-scanning frequency.

4. A cathode ray tube having means for generating an electron beam rectangular in crosssection and having means for causing said electron beam to converge to form a small spot on the screen of the tube deflecting means for deiiecting said spot relatively to said screen in one direction and deflecting means for deecting said spot relatively to said screen in a co-ordinate direction, one of said deecting means being disposed at a position where the beam is rectangular in cross-section to embrace the longer sides of the beam cross-section, said deecting means comprising parallel members spaced a distance only slightly greater than the depth of the cross-section of said beam between said memlbers, whereby the sensitivity of deection at said more rapid rate is increased.

5. A cathode ray tube according to claim 4, wherein said delecting means are disposed at separated points along the path of said beam, one of the deecting means being disposed at a point where the width of the beam is greater than at the succeeding point along the path of the beam where the other deecting means are disposed.

6. A cathode ray tube comprising an evacuated envelope enclosing means for producing a thin at beam of electrons, a fluorescent screen in the path of said beam, beam focusing means eifective only on the width of said beam to focus said beam to a spot on said screen, deflecting means for producing a deiiecting eld perpendicular to the sides of said dat beam at a region near the said focusing means Where said beam is wide, a deecting means for producing a deecting field parallel'to the sides of said beam at a region more remote from said focusing means and where the Width of said beam is less than at the first deiiecting eld.

'7. A cathode ray tube having a thermionic cathode, a beam forming electrode for forming the electron discharge from said cathode into a thin fiat beam rectangular in cross-section, a tubular accelerating electrode for said beam, a focusing electrode adjacent the beam exit end of said accelerating electrode for forming with said accelerating electrode an electrostatic lens effective to converge said beam only in the direction of its width to focus said beam, a pair of deecting plates positioned near said focusing electrode and disposed on opposite sides of the normal path of said beam to be parallel to the sides of said flat beam and spaced a, distance only slightly greater than the depth of the crosssection of the beam between said plates to deflect said beam in a direction perpendicular to the sides of said beam, and a second pair of deflecting plates more remote from said focusing electrode and positioned to deflect said beam in a direction parallel to the sides of said flat beam.

LEONARD FRANCIS BROADWAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,036,532 Knoll et al. Apr. 7, 1936 2,053,537 Schlesinger Sept. 8, 1936 2,081,942 Lubcke June 1, 1937 2,152,363 Ruska Mar. 28, 1939 2.183.309 Flechsig et al. Dec. 12, 1939