US2455321A - Cathode-ray beam deflection system - Google Patents

Description

-Nv.3o,194s. Y f W, L STOLZE EAL 2,455,321

. CATHODE-RAY BEAM DEFLECTION SYSTEM Filed Oct. 28, 1947 I i 2 Sheets-Sheet 1 /Me Peanut/N6 cnr/loos MY VERTICAL 72\= Pwff? Tues Patented Nov. 30, 1948 Price.

2,455,321 d cA'rHonE-RAY BEAM DEFLECTION SYSTEM William .lL Stolze, Astoria, Long Island, and Earl I. Anderson, Manhasset, N. Y., assignors to Radio Corporation of America, a corporation o Delaware Application October 28, 1947, Serial No. '782,490

i claims. (c1. S-2er The present invention relates to cathode ray beam deflection systems, and more particularly relates to systems of this nature in Vwhich at least one direction of deflection of the cathode ray beam iscarried out through the action of an electrostatic field.

It is known in the art that negative feedback may be employed in connection with -amplier circuits in order to reduce distortion ofthe output waveform. This is usually accomplished by taking a portion of the voltage output ofthe ampliiier and applying this portion of the output voltage to the amplifier input in such a manner that its phase is negative. Thus any distortion introduced between the points bridged by the feedback connection is decreased in magnitude.

A system such as the above is` satisfactory in many cases, but has the disadvantage that the gain of the amplier is reduced to an appreciable extent. It has been found, however, that this lossin amplification may be cut down, and at the same time an improvement may be obtained in waveform fidelity, bythe provision of ya separate network connected to the output of the amplier but preceding the network causing the distortion." This separate network is `specifically designed. to produce the same type of distortion present in the output of the system, but to "an even greater degree. If the voltage output of this separate wave-shaping network is then applied to the input of the amplifier proper, the resulting output of the system more closely resembles the input waveform than in cases where the usual feedback arrangement is used.

The above concept has been utilized, `in one embodiment of the present invention, in connec-` tion with a cathode ray beam` deflection circuit of the electrostatic type, One of the requirements for the vertical, oriield, deflection circuit of `a television receiver employing an electrostatically deflected cathode ray tube is that the condensers for coupling the verticaldeflection amplifier to the cathode ray tube deflection plates? must be normallyof fairly large capacitance due to the relatively low frequency of vertical deflection.A Furthermore, these condensers must have a high coupling condensers large in a physical sense, but in addition they are relatively expensive and tend to increase materially the total cost of the television receiver.

By using the principle of the present invention, it has been. found that both the size and cost of the above-mentioned coupling condensers may be considerably reduced. It will be appreciated that the normal effect of reducing the capacitance of each coupling condenser is to introduce distortion into the waveform of the Voltage applied to the deflection plates of the cathode ray tube, since the time constant of the coupling circuit is not high enough to pass the sawtooth voltage generat ed by the power output tubes in its undistorted form. The resulting non-linearity of vertical deflection results in a poor television image and hence is highly undesirable.

By the use of the present invention, however, a portion of the sawtooth voltage output from the power tube (or from one to two push-pull operated power tubes) is applied to a separate waveshaping network. This separate network is designedto produce in the sawtooth wave a type of non-linearity similar to that produced by the relatively small capacitance of the coupling condenser, but to an even greater degree. The highly distorted Voltage thus obtained from this sep arate network is fed back in proper phase to the input of the power tube, so that the output of the'latter is no longer of sawtooth wave-form, but instead contains distortion in opposite phase to the distortion produced by the coupling con* denser. Hence, after passing through the con pling condenser, the two types of distortion in effect cancel one another, and the resulting voltage wave as `applied to the vertical deflection plates of the cathode ray tube is substantially a pure sawtooth which is effective to bring about a linear deflection of the cathode ray scanning beam. i

O'ne object of the present invention, therefore, is to provide an improved type of vertical, or field, deflection system for use with electrostatically deflected image-reproducing cathode ray tubes.

Another object of the invention is to provide an improved cathode ray beam deflection system of the electrostatic type in which improved linearity of deflection is obtained through the use of a separate wave-shaping network, the output of which is applied to the deflection system to reduce distortion resulting from the employment of less Vbulky and more economical circuit components.

A still further object of the present invention is to provide, in a cathode ray beam deection circuit of the electrostatic type wherein the normal sawtooth voltage output of a push-pull amplifier is applied to the field deflection plates of -a cathode ray tube and which voltage is then distorted 4lcy-theuse` of coupling Vcondensers of small capacity resulting in a time constantwhich is inadequate for the frequency of field deflection of the cathode ray scanning beam of the cathodeV ray tube, a separ-ate wave-shaping network designed to distort a portion of the output voltage of the amplifier to an even greater degree than that produced by the couplingA condensers', and further means for applying the output ci this wave-shaping network to the input of the pushpull -amplier with such magnitude andphase as to substantially cancel on the deflecting plates the distortion produced by the presence of the coupling condensers.

@therI objects and advantages will-be'apparen-t from the following description :o-fa preferred forml of the inventionzan-d iromfthe-drawingsin which:

Fig. l is a circuit diagram of a preferred embodiment of the present'nventiomand Fig. 2 is a set of voltage waveforms illustrating the operati-on ofthe circuit fof-Fig. l.

Referring now to the-drawings, there is shown an image-producing cathode ray tube generally .indicated by thefreference: numeral l; Tube l maybean electro-statically deflected cathode ray tube of any suitable: type which includes a pair of vertical, or eld, deection plates 2f'and I4.

Although tube l 0 is also assumed to include a pair of horizontal, or line, deflection plates, these have mit been showriin-ordertosirnplify the drawing, inasmuch as the explanation is based on application of the principle to the vertical plates., It is furtherA assumed that a beamof electrons isL de.- veloped and directed toward the cathode ray tube screen it. by an electron gun I8 Iin the customary manner. part a conductive coating on a portion of the inner surface o-f tube lili) accelerates the electrons emitted from the electron, gun I3 toward the luminescent screen l G.

A pair of vertical, or field, power output tubes, illustrated asl two triodesl 22 and-2d, may actuallyl comprise separate. tubes or they may constituteV the two sections of ay twin triode (such as a GSNJGT) having a common envelope.

a control electrode or grid 3l), Tube 24 is similarly provided with an anode 32, a cathode 34, and a control electrode or` grid 3d, The-joined cathodes 28 yand Sof tubes-22 and.24 respectively are connected through a cornmorry cathode resistor 3l to the grounded negative terminal 38'of the sourceoi operating potential for the power output tubes, designated as B+ in the drawing;

Anode .E of tube 22 'is connected to the positive terminal Il@ of the source ofA tube operating'A potential through the load resistor a2. Anode 32 ot tube 2li is likewise connected tothe same positive terminal t0 through another load resistor-A4 which preferably is of the same value asresistor 32. A charging condenser G'Sihas one 4of its plates connected and its other plate connected to the positive supply voltage source terminal 58. through two series-connected resistors E and '52,. the former `being adjustable in ordery to vary the .positive charge developed on condenser 46. A: vertical discharge'tube` Sd'ise'ectively connected in shunt with the condenser 45, andzis Av second anode 20.' (which maybe in;

Tube 22' is provided with an anode 26; a cathode 28', and:

`-voltage appearing on grid 30 of tube 2.2.

arranged to periodically discharge the condenser 136 so that the voltage developed thereon (and as applied to the grid 30 of tube 22) will be of substantially sawtooth waveform, in a manner well known in the art.

The vertical power tubes 22 and 2d are arranged'to ofperatein pushepull relation by means of a sawtooth voltage applied to grid 3.6 of tube 24 which is 180 out-of-phase with the sawtooth This out-of-phase voltage is obtained by the use of two resistors 55 and 58 connected in series between groundand the anode 2li of tube 22. These resistors 5e and t8 act in eifect as a voltage divider to produce a sawtooth wave of desired amplitude on grid 3%, and, due to the phase-inversion properties of tube 22, this voltageon grid 35' will be such as to cause the tubes 22 and 24 to'operate in the required push-pull manner.

The sawtooth voltagenormally appearing on anode Vof tube 22. is applied to. the vertical sawtooth voltage normally appearing on.. anode SZof tube 2li. is applied to the vertical deflectingA plate l-i through a coupling condenserv 52. In order that the operating potential of thedeflecting plates I2v and i4 be at or near the potential4 appliedfto the'second anode. 2t .of the cathode ray tube and thus preclude a defocusing of the cathode ray beam developed by the electron gun 8-, the derlecting plates I2y and lll are connetced to the high-voltage source (H. V.) of the system thro-ughtwo resistorsteand 66, respectively.`

It has been found in practice that a deflection of the cathodev ray scanning beam of tube IE! at the usual vertical, or field, frequency oiOv cycles persecondrequires thatthe coupling condensers Gilandfz be of relatively large capacitance in order accurately to pass the sawtooth voltage wave normally produced by the push-pull amplier 122-2@ In order that the time constant of the RC networks Sli-t and SZ-G be suiciently high, therefore, the condensersr 6@ andZ should.

preferably be in the order of .05 microfarad. The use of condensers having such a rating is. undesirable, however, since they are quite large ina physical sense and also of fairly high cost. It has not. been practicable, heretofore, to reduce their size in previously used deflection systems, since to do so in previously used systems would result in considerable distortion of the sawtooth output of the push-pull amplifier as it appears on the cathode ray tube deflecting plates.

In accordance with one embodiment of the present invention, the capacitance. of the condensers 60 and 62 may be reduced to a small fraction of the size normally required, andat.

the same time any distortion which would normally resultfrom such an Jact is overcome.

separate wave-shaping network connected to receive a vportion of the normal sawtooth voltage; output of tube 22 as it appears on anode This wave-shaping-network includes a condenser 68,

one plate of which is connected to the anode 2liV of tube 22 `and the other plate of which isconnected to one end of a resistor 10. The remaining end of resistor 'I0 is connected to the grounded- The means for accomplishing this result comprises a quency of eld deflection of the cathode ray I2 and I4, as shown in Fig. 2. In fact, the distortion may be made even more pronounced, according to the values chosen for condenser 68 and resistor 10.

The intentionally distorted output of the waveshaping network 68`70 is applied through an additional condenser 12 to the grid 30 of the vertical power tube 22, and, since the voltage applied to `the wave-shaping network 68-10 is derived from the output of tube 22, it follows that the voltage thus fed back through condenser 'l2 (see Fig. 2) will be of such phase as to cancel in the Voltage waveform appearing on deflecting plate I2, the distortion produced by what normally would be an inadequate capacitance rating for the coupling condenser 60. Y

From the above, it will be seen that the only additional components required in the circuit of Fig. 1 in order to overcome the non-linearity produced by the use of a reduced capacitance for the coupling condensers 60 and 62 are the two condensers 68 and 12 and the resistor 10. If the condenser 68 is chosen to have a value of .0015 microfarad, for example, and the resistor 'l0 is chosen to have a value of 2.8 megohms, then it is possible to reduce the value of each of the coupling condensers 60 and 62 from their standard rating of .05 microfarad to a lower rating of .005 microfarad, or, in other words, the capacitance of each may be reduced to of its lower value. Condensers of the latter rating are of course, much less bulky and expensive. A corresponding ten-fold increase in the values of resistances 64 and 66. to retain the original time constant, would obviously be impractical. The present invention makes it possi-ble to materially .reduce the size of the coupling condensers 60 and 62 and still obtain the desired linearity of deflection even though the time constant of the elements in the deflection circuit is materially and correspondingly decreased.

The two condensers 12 and 46 are connected effectively in series between ground and the output of the wave-shaping network 68-10. Therefore, the ratio between the values of condensers 12 and 46 detemines the percentage of the voltage output of network 68-10 which is fed back to the grid 30 of tube 22.

While the above description has assumed a cathode ray beam deflection system using a pair of power output tubes arranged in push-pull relation, nevertheless it will be apparent that applicants invention is equally applicable to a dellection system of the so-called single-ended type in which but a single power output tube is employed.

While normally the ratings of the components 68, l0 and 12 are chosen so that the amount and degree of distortion fed back to the power tube 22 is just sucient to cancel the distortion introduced by the low-value coupling condensers 6I] and 62, it will be appreciated that an even greater degree of distortion may be produced by the wave-shaping network so as to over-compensate for the effect of condensers 60 and B2 if and when such a result is desirable.

Having thus described our invention, we claim:

1. In a cathode ray beam deflection circuit of the electrostatic type, wherein the normal sawtooth voltage output of a push-.pull amplier as applied to the field deflection plates of a cathode ray tube is distorted by the use of coupling condensers of small capacity relative to the frescanning beam of said cathode ray tube, the combination of a wave-shaping network, a circuit for applying a portion of the normal sawtooth voltage output of said amplifier to said wave-shaping network to thereby produce in the sawtooth voltage output from said network a degree of distortion at least as great as that which normally appears on the said deflecting plates due to the relatively small capacity of said coupling condensers, and a further circuit for applying the distorted sawtooth voltage output from said wave-shaping network to the input of said push-pull amplifier, the distorted sawtooth voltage thus fed back being of such magnitude and phase in the output of said amplifier as to substantially cancel on said deflection plates the distortion produced by the presence of said coupling condensers.

2. The combination of claim 1 in which said wave-shaping network comprises a resistancecondenser combination.

3. The combination of claim 2 in which the capacitance of the condenser of said resistancecondenser combination is equal to or less than the capacitance of each of the said coupling condensers.

4. In a cathode ray beam deflection circuit of the electrostatic type, wherein the normal sawtooth vol-tage output of an amplifier tube as applied across a pair o cathode ray bea-m deflecting plates of a cathode ray tube is distorted by the use of a coupling condenser of relatively small capacity, the combination of a wave-shaping network connected to receive a portion of the normal sawtooth voltage output of said amplier and to distort such voltage variation in a manner similar to the distortion produced by said coupling condenser, and a feed-back connection from the output of said wave-shaping network to the input of said amplifier tube.

5. In a device of the class described, the combination of an electrostatically deilected cathode ray tube, a pair of grid-controlled electron discharge tubes connected in push-pull relation and adapted to deliver substantially balanced voltages to -two dei'lecting plates of said cathode ray tube, to deflect the cathode ray beam developed therein at a predetermined frequency, the anodes of said electron discharge tubes being respectively connected to the said deflecting plates through coupling condensers of small capacity resulting in a time constant which is inadequate for linear deection of the beam at the predetermined frequency, a further condenser, a resistor, a circuit connecting the anode of one of said electron discharge tubes to a point of constant potential through the series combination of said further condenser and said resistor, and a connection from a point between said further condenser and said resistor to the grid of said one electron discharge tube.

6. The combination of `claim 5 in` which said further condenser and said resistor act in effect as a differentiating network.

7. `The combination of claim 5 in which the capacitance of said further condenser is less than capacitance of any one of said coupling condensers.

WILLIAM J. STOLZE. EARL I. ANDERSON.

No references cited.

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