US2829303A

US2829303A - Electron beam controlling apparatus

Info

Publication number: US2829303A
Application number: US465865A
Authority: US
Inventors: Ronald C Knechtli
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1954-11-01
Filing date: 1954-11-01
Publication date: 1958-04-01
Anticipated expiration: 1975-04-01

Description

April 1, 195s R. C. KNECHTLI ELECTRON BEAM CONTROLLING APPARATUS Filed Nov. 1. 1954 INVENTOR. RDNHLD E. KNEEHTLI ma/YU stood from the showing of Fig. 42, the phosphor screen is made up of a plurality of horizontal strips of phosphor of different color light emitting characteristics. As the beam is deflected from left to right alongthe horizontal center line of the screen, no rotation thereof is caused by the deflection lields. Similarly, no rotation of the beam is present when the beam is deflected upwardly or downwardly along the vertical center line of the screen, this fact being indicated in the drawing. As the beam is deflected outwardly from the center toward one of the corners of the screen, however, the beam and, therefore, its spot, are rotated through an angle which is proportional to the angle of deflection (both horizontally and vertically). Thus, the closer that a beam spot is toward one of the corners of the screen, the greater is the amountof rotation imparted to it. Moreover, it will be seen from Fig. 2 that, if the screen be divided into its four quadrants, adjacent quadrants are mirror imagesl of each other. Stated otherwise, as the beam is deflected toward the upper right corner of the raster as shown in Fig. 2, the beam spot experiences a gradually increasing counter-clockwise rotation. As the beam isdeected toward the upper left hand corner of the screen, however, the "beam spot experiences an increasing clockwise rotation. in a similar manner, the beam spot is rotated in a clockwise manner and a counter-clockwise manner as it approaches the lower righthand and lower lefthand corners, respectively.

Such rotation of an electron beam as had been described is particularly objectionable in a case of a color kinescope of the type shown, since, as will be understood from the drawing, rotation of the beam spot causes the spot to illuminate two or more of the phosphor strips simultaneously, thereby producing color dilution.

Since, as has been stated, beam spot rotation of the type in question results from the action of the electromagnetic deflection fields, the present invention provides means for compensating for the beam-rotating of the detlection fields. Apparatus in accordance with the principles of the invention is illustrated in Fig. l and comprises the cylindrical electromagnetic coil 4% surrounding the neck of the kinescope between the source of electrons (i. e. the cathode 18) and the deflection yoke 32. The longitudinal or central axis of the compensating -coil is substantially coincident with the axis of the electron beam within the coil. When current is passed through the coil 40 there is produced a magnetic field whose lines of tlux are generally axial of the tube 22. As will be appreciated, axial magnetic fields of special design are employed in many cases for beam focusing, a function not desired of the coil 40. In order to prevent the coil 40 from having any appreciable beam focusing or defocusng effect upon the electron beam while it performs its rotation-compensating function, the coil 40 should be designed as a weak magnetic lens. That is to say, the reciprocal of the focal lenfth f of the magnetic lens afforded by the coils should be a maximum.

The `following equation is useful in expressing the relationship of the focal length f of a coil asa function of the magnetic leld which it produces:

where e is the charge `of an electron, m is the mass of an electron, V is the potential of the electron beam at the coil with respect to the cathode of the tube, Bz

is the axial component of the magnetic ield on the axis is the coordinate parallel to the axis passing through a coil of the type shown in Fig. l may be expressed as follows:

Since it is desired, in accordance with the invention, to impart various amounts of rotation to an electron beam while not appreciably affecting its focus, the quantity in Equation l must be minimized for a given value of 0 of Equation 2 (i. e., minimum lens action for a given angle of rotation). From the foregoing equations, it may be .noted that, when the angle 9 is prescribed, the quantity 1 Similarly, Equation 2 is reduced to the following form:

By dividing Equation 3 by Equation 4 after squaring both sides of the latter, the result of the operations is as fol- (4) [ai: .BL

lows The foregoing relations illustrate the fact that, for an optimum design, the rotation-compensating coils should beaslong as possible for a given angle 0. It has been foundfor example, that where the length of the coil L is greater than one inch, the lens action (i. e., focusing action) of the compensating coil is substantially negligible for angles of rotation up to 10, an amount of correctional rotation which has been found to be sutcient for practical applications.

Once the proper length of the coil 40 has been determined in accordance with the foregoing relationships, the strength of the magnetic field required for given angles of rotation may be found through the use of Equation 2 supra.

Insofar as the energization of the rotation compensating coil 40 is concerned, the angle of rotation of a rectangular spot as a function of deflection has been found -to be, as a first approximation, proportional to the algebraic product of the horizontal and vertical angles of deflection; Since the angle of corrective rotation pro- ,duced by the compensating coil 40 is proportional to its magnetic field (in the absence of a saturated ferro-magnetic material), the corrective rotation is thus proportional to the current applied to the compensating coil.

Where Ih is the horizontal or line deflection current whichvis passed through the deflection yoke 32 for producingA horizontal deection of the electron beam and "vpvherefl,l is the vertical deflection current passed through the yoke 32 for producing vertical beam deflection, the correction current 1 required for passage through the 'compensating coil may be expressed as follows for the case in 'which the direction'of vthe longitudinal of the elongated beam spot is required to remain constant'over compensate,'in addition to the'rotation ofthe spot caused by deliection, for a misalignm'ent between the longitudinal axis ofA the` undeected spot and the lines of the screen.

In the foregoing Equation 6, Ih and Iv areV algebraic v quantities representing sawto'oth` currents of zero average value '(i. e., no direct currentcomponent), 'the period of Ih being equal to the television horizontal sweep period ($45,750 second) `and thelperiod of Iv lbeing equal to the vertical sweep period (1,60 second).

It is Aalso to be noted thatthe sawtooth component of the correction currentlc should be equal in amplitude but'opposite in sign (i. e., polari-ty) for any pair of horizontal lines of sweep'which are symmetrical with'respect to the horizontal centerline `of the screen.

In accordance with the'foegoing requirements, Fig. 1 illustrates apparatus provided for energizing the coil 40 with suitable current forcompensating forthe beam rotation resulting from magnetic deection elds. Current corresponding to the 'horizontal deection current is'catised` to flow from the `source 33 through the primary i Winding 44'of a transformer T, the secondary winding of which is indicated bythe reference numeral 46. The upper` endIof the secondary-vvinding'46 which is center tapped Vat A48 isconnectedto the control electrode 50`of a multigrid electron tube 52. The lower end of the winding 46 is similarly lconnected to the control electrode 54` of ak second multigrid electron tube 56. The

cathodes of the tubes 52 and l56 are connected to each other and to the positive terminalof a biasing battery 58 whose negative" terminal islconnected to thecenter tap'of the secondary winding 46 and whichserves to bias the controlgrids'S and'54`ls`oAthat `they operate in accordanoewith Class AA amplijieation principles.

A Vertical frequency deiiection currents from the source 34are applied via the transformer T2A to the screen electrodes 60and 62, respectively, of the'tubesSZ and 56. These latter electrodes are connected through the secondary winding of the transformer T2 to ya biasing battery 64 which biases the electrodes 60 and 62 for Class B operation. The anodes 66 and 68 of the two electron tubes are connected together andto the upper end of a common load resistor 70 whose other terminusV is connected to a source of positive operating potential at the terminal 72 (+B). The common load terminal 74 of the amplifier tubes 52 and 56 is coupled via a capacitor 76 to one end of the winding 40, the other end of the winding 40 being connected to ground, as shown.

In the operation of the apparatus of Fig. 1 as thus far described, voltages such as those shown by waveforms (a) and (b) are applied, respectively, to the control electrodes 50 and 54 of the two electron tubes. At the same time, vertical frequency waveforms corresponding to those shown by curves (c) and (d) are applied to the screen electrodes 62 and 60, respectively.

By virtue of the Class B polarization of the screen electrodes of the tubes, the tube 52 will be rendered conductive during the iirst half of each television field (i. e.,

` during the period t1), while the tube 56 will be rendered conductive during the second half (i. e. period t2) of each eld. By virtue of the varying degree of ampliiication provided by the tubes to the horizontal frequency voltages of waveforms (a) and (b), the resultant waveform available 'at the output terminal 74, which resultant waveform constitutes the combined output of tEetwo form (e)` is, therefore, illustrative of the correction current applied to the compensating coil 40 and will also tubes, will beas shown in curve (e) of Fig.Y 3. Wave- Y be understood as being representative of the amount and f I direction of rotational correction imparted to the eleczontal line scan, the amount of correction is its maximum at the extremities of the line and gradually delV creases to zero at the center of the line. Moreover, the direction of correctional rotation changes as the scanning beam passes through the horizontal center of the raster. v Y Y From the foregoing, it will be understood that with 'a current of the waveform (e')'of. Fig. 3 passing through the coil 40, theaxial magnetic field produced by that current will dynamically compensate for the rotational'effects upon the beam produced by the deflection elds from the yoke -32. In other words, Vthe auxiliary iield produced by the coil 40 may be considered as a de-rotating eld in that it imparts to the beam a rotation of the same magnitude but of the opposite direction from that which results from the deflection iields.

As has been mentioned in connection with Equation 6 supra, it is often desirable to provide a direct current component for'the correction coil 40, as for the purpose of correcting for static misalignment between the longitudinal axis of the undeected spot andV a reference line drawn horizontally of the screen, for example. Such a direct current component isprovided in the apparatus of Fig. l from a D. C. source'i) which will be understood as being variable in magnitude and reversible in direction and which is connected to that end of the coil winding 40 remote from the grounded end through a choke coi-l 32'which sei-vesto prevent passage of the dynamic correcting current fromthe terminal 74 through junction with a'cathode ray tube,"said' apparatus com prising: a'substantia'lly'cylindrical electromagnetic coil adapted to surround the path of an electron beam; a source of line deflection signals; a source of eld deiiection signals; and means coupled to each of said signal sources and to said coil for passing through said coil a magnetic field-producing current generally proportional to the product of said line and field deflecting signals.

2. Electron beam controlling apparatus for use in conjunction with a cathode ray tube, said apparatus comprising: a substantially cylindrical electromagnetic coil adapted to surround the path of an electron beam; a source of line deflection signals; a source of eld deflection signals; and means coupled to each of said signal sources and to said coil for passing through said coil a magnetic ield-producingcurrent generally proportional to the product of said line and eld deflecting signals and continuously variable in intensity and direction as a function of such signals. Y

3. Electron beam controlling apparatus for use inconjunction with a cathode ray tube of the type having electromagnetic yoke means associated therewith for causing a beam produced therein to scan a raster in accordance with line and eld deflection iields, such detlection fields being such as to produce rotation of such beam as a function of deection, said apparatus comprising a generally cylindrical electromagnetic coil adapted to surround the path of such beam; a source of line deflection signals; a source of iield deiiection signals; and

Ymeans coupled to each of said sources and to said elec- '7' capable of causing rotation of such beam in the same amount, as but of the opposite direction from the r0- tation produced by such deliection fields.

4. Apparatus for compensating for rotation of an electron beam in a cathode ray tube resulting from raster-scanning electromagnetic fields applied to such beam in its passage toward a target screen, said apparatus comprising: a source of line scanning deflection signals; a .source of field scanning deflection signals; electromagnetic fieldv producing means for producing a substantially axial eld in the path of such beam; and means for energizing said last-named means with current proportional to the algebraic product of such line'and field scanning deilection signal in such manner as to produce rotation of such beam of such magnitude and sense as substantially to cancel such rotation resulting from raster scanning.

5. The invention as defined by claim 4 wherein said axial electromagnetic field producing means comprises a cylindrical coil having great enough axial length as to render its magnetic lens focusing eiect negligible.

6. Electron beam controlling apparatus for use in conjunction with a cathode `ray tube of the type having means associated therewith for causing a beam therein to scan a eld raster made up of a plurality of lines, said apparatus comprising: a source of line rate deflection sawtooth signals; a source of field rate deflection sawtooth signals; an electron beam rotating eld producing means, first and second amplifier having a common output terminal; means coupling said output terminal to said field producing means; and means for applying line and field deflection signals from said sources to each of said amplifier means in such manner that said amplifiers provide at said output terminal a waveform comprising a line rate sawtooth wave of gradually decreasing amplitude and of a certain polarity during the first half of each raster field and of gradually increasing amplitude and of the opposite polarity during -the second half of each raster field.

7. Electron apparatus for use in conjunction with a cathode ray tube of the type having means associated therewith for causing a beam therein to scan a field raster made up of a plurality of lines, said apparatus comprising: a source of line rate deflection sawtooth signals; a

source, of fieldl rate sawtooth signals; voltage multiplying means having an output terminal; means for applying line and field deflection signals from said sources to said voltage multiplying means in such manner that said multiplying means provides at said output terminal a waveform comprising a line rate sawtooth Wave of gradually decreasing amplitude and of a certain polarity during the first halfof each raster field and of gradually increasing amplitude and of the opposite polarity during the second half of each raster eld; means for producing an electron beam rotating field; and means coupling said output terminal'of said multiplying means to said last-named means.

- 8. Electron beam controlling apparatus for use in conjunction with a cathode ray tube of the type having means associated therewith for causing a beam therein to scan afield raster made up of a plurality of lines, said apparatus comprising: a source of line rate deflection sawtooth signals; a source of field rate deflection sawtooth signals; an electron beam rotating field producing means comprising electromagnetic coil in the form of a cylindrical winding having substantial axial length; voltage multiplying means having an output terminal; means coupling said output terminal of said voltage multiplying means to said beam rotating eld producing means; and means for applying line and field deflection signals from said sources to said voltage multiplying means in such manner that said multiplying means provides at said output and for passage through said winding a waveform comprising a line rate sawtooth wave of gradually decreasing amplitude and of a certain polarity during the first half of each raster field and of gradually increasing amplitude and of the opposite polarity during the second halt` of each raster eld.

Schlesinger Dec. 29, 1953