US3465200A - Cathode ray tube system including means for varying beam intensity - Google Patents

Description

Sept. 2, 1969 J. E. HIGBEE ET AL 3,465,200

- CATHODE RAY TUBE SYSTEM INCLUDING MEANS FOR VARYING BEAM INTENSITY Filed Jan. 25, 1967 2 Sheets-Sheet 1 CATH. GRID vERT. HoR. 9 I DRIVE DRIVE DEFLEcT DEFLECT 24 CT. CT. MEANS MEANS 1 I 5 BEAM CONT. SI 6.

SOURCE 22 DEFLEC SIGNAL SouRcE BEAM BEAM INTENSITY INTENSITY DEFOCUSING THRESHOLD l s I IN (c) GA! N l G 2 I INI ENTOR.

- MARTIN c. HENDERSON ,JOHN E. HIGBEE ATTORN YS p 1969 .1. E. HIGBEE ET AL CATHODE RAY TUBE SYSTEM INCLUDING MEANS FOR VARYING BEAM INTENSITY 2 Sheets-Sheet 2 Filed Jan. 25, 1967 W U 6 www RR GDC I I l l l I I I l I J AU O 3 w T. M 6 a 5; D R IE K E C v F E A mB LD NE OE 5 M 4 5 FIGQ3 INVENTOR. MARTIN C. HENDERSON JOHN E. HIGBEE BY FIG. 4

ATTORNEYS United States Patent M 3,465,200 CATHODE RAY TUBE SYSTEM INCLUDING MEANS FOR VARYING BEAM INTENSITY John E. Higbee, Santa Susana, and Martin C. Henderson,

Canoga Park, Calif., assignors to The Bunker-Ramo Corporation, Canoga Park, Calif., a corporation of Delaware Filed Jan. 23, 1967, Ser. No. 610,952 Int. Cl. H013 29/52 US. Cl. 31530 8 Claims ABSTRACT OF THE DISCLOSURE Circuit apparatus useful in a cathode ray tube display system for facilitating accurate control of beam intensity thus enabling intensity variations resulting from variations in beam velocity to be compensated for. The nonlinearity of the beam intensity versus control voltage characteristic typical of most cathode ray tubes is compensated for by incorporating a nonlinear feedback path in the amplifier producing the control voltage in response to an input voltage. Additionally, the amplifier incorporates a limiting feed-back path to prevent the grid voltage from increasing beyond a certain threshold level at which defocussing occurs.

The invention herein described was made in the course of or under a contract or subcontract thereunder, with Department of the Air Force, Rome Air Development Center.

BACKGROUND OF THE INVENTION Field of the invention This invention relates generally to cathode ray tube (CRT) display systems and more particularly to means for use therein for facilitating the accurate control of beam.

intensity.

In certain sophisticated display systems, CRTs are called upon to display a variety of significantly different elements or symbols all drawn in constant time irrespective of size. For example, in such a system the CRT may be called upon to describe both a small circle (e.g., inch diameter), and a large circle (e.g., 6 inch diameter) in a single display pattern. It the two circles are drawn in the same amount of time, it should, of course, be apparent that the beam velocity must be much greater to describe the large circle than to describe the small circle. It should also be apparent that if the beam is moved with a greater velocity, it will appear less intense to an observer. In order to prevent this intensity variation, prior art systems have incorporated beam intensity control means responsive to beam velocity. It is found, however, prior art intensity control means are inadequate to provide accurate intensity control over a wide dynamic range because of the nonlinearities typically found in CRTs.

SUMMARY OF THE INVENTION The present invention is directed to a system enabling CRT beam intensity to be controlled extremely accurately over a wide dynamic range.

Briefly, in accordance with one aspect of the present invention, means are provided for introducing nonlinearities between an input signal and an output control electrode (e.g., grid) signal to compensate for nonlinearities in the relationship of beam intensity to the control signal.

In the preferred embodiment of the present invention, the nonlinearities are introduced by an amplifier to which the input signal is applied. A nonlinear feedback path is employed whose impedance changes as the level of the input signal changes. Variations in the feedback impedance, of course, vary the closed loop gain of the amplifier.

Therefore, by proper selection of circuit components, beam intensity can be made to be a substantially linear function of the input signal.

Briefly, a further aspect of the present invention is based on the recognition that beam defocussing or blooming may occur as the control or grid signal exceeds a certain threshold level, and that by automatically limiting the grid signal variation this defocussing can be prevented.

Thus, in accordance with a further aspect of the preferred embodiment of the invention, an additional amplifier feedback path is provided which reduces the amplifier gain after the amplifier output signal exceeds a certain threshold level.

DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of a CRT display system which incorporates the present invention;

FIG. 2(a) illustrates a typical CRT characteristic;

FIG. 2(b) illustrates an ideal CRT characteristic;

FIG. 2(0) illustrates a gain characteristic provided by an embodiment of the present invention;

FIG. 3 is a block diagram of the grid drive circuit of FIG. 1 in accordance with the present invention; and

FIG. 4 is a schematic circuit diagram of the grid drive circuit of FIG. 3.

Attention is now called to FIG. 1 which illustrates a CRT display system which can incorporate the teachings of the present invention. More particularly, the system of FIG. 1 utilizes a CRT 10 having an electron gun including a beam producing electrode or cathode 12 and a control electrode or grid 14. Although other electrodes such as an anode would also be incorporated within the tube 10, such other electrodes are not illustrated herein inasmuch as they do not bear upon the present invention. The cathode ray tube 10 also includes a face 16 coated with an illuminable material such as phosphor. In the normal operation of the tube 10, an electron beam is provided by the cathode 12 and impinges upon the face 16 to develop a light spot.

The position of the beam with respect to the face 16 is controlled by a vertical deflection means 18 and a horizontal deflection means 20 which are responsive to signals provided by a deflection signal source 22. It will be appreciated that by applying appropriate deflection signals to the means 18 and 20, the beam can be caused to describe any desired pattern on the CRT face 16.

The beam provided by cathode 12 can be selectively blanked and unblanked in response to signals provided by the beam control signal source 24 to the cathode drive circuit 26 whose output terminal is coupled to the cathode 12. Thus, for example, the beam can be unblanked by establishing a large accelerating potential gradient between the cathode 12 and the anode (not shown). As an example, assume that the beam is unblanked by applying a +60 volt potential to the cathode 12. On the other hand, the beam can be blanked by raising the potential applied to the cathode 12 to volts, for example, to thus reduce the accelerating potential gradient.

Typically, in the operation of the tube 10, the beam will be moved with considerably different velocities in generating a display. This is so because in many systems each symbol or line is drawn in constant time regardless of the size of the symbol or line. Thus, if a short vector and a long vector are described in the same amount of time, it is apparent that the beam must be moved with a greater velocity to describe the long vector. When the beam moves with a greater velocity, the light intensity developed thereby decreases. In other words, beam intensity is roughly inversely proportional to beam velocity. In order to compensate for these intensity variations and thus enable all information to be displayed with near uniform intensity, some prior art systems have provided means for varying the intensity as a function of beam velocity. Beam intensity can be controlled by controlling the potential applied to the grid 14, for example. Thus, as the potential E provided by drive circuit 30 to the grid 14 is increased toward the potential of the cathode 12, the beam intensity will increase. However, as is illustrated by FIG. 2(a), the beam intensity versus grid potential characteristic of a CRT is typically nonlinear. That is, at low grid potentials when there is a large negative potential gradient from the cathode 12 to the grid 14 the beam intensity varies by only a small increment in response to a unit change of the grid voltage E However, as the grid potential increases and the negative gradient from the cathode to the grid decreases, small changes in grid voltage E will cause much larger changes in the beam intensity. Furthermore, as the grid voltage approaches the cathode voltage the CRT will typically defocus seriously.

It would, of course, be ideal for the beam intensity to vary substantially linearly as a function of the input signal E provided by the control signal source 24 to the drive circuit 30, as is illustrated by linear portion 36 of FIG. 2(b). Additionally, it is desirable to limit the beam intensity, as shown at 38 in FIG. 2(b), to prevent defocussing after the input voltage exceeds a certain threshold level.

In accordance with the present invention, the grid drive circuit 30 is designated to introduce nonlinearities in the characteristic relating the grid signal E to the input signal E In accordance with a prefered embodiment of the invention, th gain (E /E of the drive circuit 30 is varied as the level of the input signal E increases. Thus, from a consideration of FIG. 2(a), it should be apparent that a greater amount of gain is required for low levels of B with decreasing gain required as signal E increases. Thus, the preferred embodiment of the invention as illustrated in FIGS. 3 and 4 provides for different gain levels which decrease as the signal E increases. More particularly, at a low level of signal E a relatively high gain 40 is exhibited by the drive circuit 30. As the level of signal E increases, the gain is successively decreased to level 42, level 44, and level 46. It will, of course, be appreciated that the gain characteristic illustrated in FIG. 2(c) is exemplary only and that the gain characteristic should be selected to approximate the linear portion 36 of the ideal characteristic of FIG. 2(b). It should further be appreciated that it is not at all necessary to actually achieve the ideal characteristic of FIG. 2(b) inasmuch as small amounts of beam intensity variation will be barely noticed by a system user and in any event will not significantly detract from the clarity or esthetic character of the display.

Attention is now called to FIG. 3 which illustrates a block diagram of the grid drive circuit 30 of FIG. 1 which includes means for both introducing nonlinearities to linearize the relationship between beam intensity and input signal E and means for limiting the beam intensity as the signal E exceeds a threshold level. Briefly, the drive circuit 30 is preferably comprised of two or more amplification stages 50 and 52. The signal E is applied through impedance 54 to the input terminal of amplifier 50. The output terminal of amplifier 50 is coupled to the input terminal of amplifier 52. The grid signal E; is provided on the output terminal of amplifier 52.

In order to introduce the nonlinearities previously discussed, a nonlinear feedback path 56 is connected between the output terminal of amplifier 52 and the input terminal of amplifier 50. The impedance of the nonlinear feedback path 56 varies as the input signal E varies to thus vary the gain exhibited to an input signal as illustrated by FIG. 2(c).

In order to limit beam intensity as shown by the porttion 38 of FIG. 2(b), a limit feedback path 60 is provided between the output and input terminals of amplifier 52. The limit feedback path 60 is normally open. However, when the output signal provided by amplifier 52 tries to exceed an established threshold level, the feed back path 60 is closed to thus reduce the closed loop gain of the amplifier 52 to a very small value. Thus, the output signal E provided by amplifier 52 will be maintained at the threshold level, thereby preventing the beam defocussing previously mentioned.

Attention is now called to FIG. 4 which illustrates the grid drive circuit 30 of FIG. 3 in greater detail. More particularly, as can be seen from FIG. 4, the nonlinear feedback path 56 is comprised of a feedback resistor R connected between the output of amplifier 52 and the input of amplifier 50. A plurality of circuit paths are connected in parallel with the feedback resistor R Each of these parallel paths includes a diode which is normally back biased but which becomes forward biased as the input signal E increases to thus gradually insert resistance in parallel with the resistor R to thus reduce gain. It will be recalled that the closed loop gain of an amplifier having a feedback loop coupled from its output to its input is approximately equal to the ratio between the effective feedback resistance and the input resistance, herein resistor 54. Thus, if the feedback resistance can be reduced by gradually introducing resistance in parallel with resistor R the gain can be correspondingly reduced.

More particularly, a first path in parallel with the feedback resistance R is comprised of resistor R connected in series with resistor R and a switch, preferably a diode D A second path in parallel with feedback resistor R includes resistor R connected in series with resistor R and diode D A third parallel path is comprised of previously mentioned resistor R connected in series with resistors R R and diode D Resistor R connects the junction between resistors R and R to a source of negative potential. A variable resistor R similarly connects the junction between resistors R and R to the source of negative potential.

In the operation of the feedback path 56 for low levels of input signal E each of the diodes D D and D will be back biased so that the feedback resistance will be defined solely by the resistor R Thus, the gain of the drive circuit of FIG. 4 will be defined by the ratio between the value of resistor R and the value of input resistor 54. As the level of the input signal E increases, the output signal E also increases, thus increasing the potential at the junction between resistor R and variable resistor R When the level of E increases sufiiciently, diode D will be forward biased to thus insert resistors R and R in parallel with feedback resistor R to accordingly lower the effective feedback resistance and correspondingly lower the closed loop gain. It will be appreciated that the level of signal E required to forward bias the diode D depends upon the setting of variable resistor R As the input signal level increases further, the diode D and subsequently the diode D will become forward biased to thereby introduce additional resistance in parallel with the feedback resistor R to further lower the effective feedback resistance and the closed loop gain.

Accordingly, it should now be appreciated that the gain level 40 represented in FIG. 2(c) is established when all the diodes D D and D are back biased. When diode D becomes forward biased, the closed loop gain decreases to the level 42 and as diodes D and D become successively forward biased the gain decreases further to level 44 and subsequently to level 46.

The limit feedback path 60 includes a diode'D connected in series with a resistor R to the emitter of PNP transistor Q1. The collector of the transistor Q1 is connected to the input terminal of amplifier 52. The base of transistor Q1 is connected to the tap 61 of a potentiometer 62 connected between ground and a source of positive potential.

The tap 61 of potentiometer 62 is utilized to establish a reference potential on the base of transistor Q1. As long as the output signal E provided by the amplifier 52 is not greater than the potential applied to the base of transistor Q1, the diode D will be back biased and the transistor Q1 will be held off. On the other hand, when the signal E increases to a level two diode drops greater than the potential established on the potentiometer tap 61, the diode D will become forward biased and turn the transistor Q1 on. The introduction of the low impedance feedback loop around amplifier 52 through transistor Q1 reduces the closed loop gain of the amplifier 52 to a very small value such that the level of signal E cannot rise much above the potential established by the potentiometer 62.

It is pointed out that the feedback path 60 is connected around amplifier stage 52 rather than around both stages 50 and 52 primarily to assure stability. That is, inasmuch as the feedback through path 60 is around fewer stages than the principal signal loop through path 56, it will introduce less phase shift and therefore will not tend to oscillate.

From the foregoing, it should be appreciated that a circuit apparatus has been disclosed herein for enabling beam intensity in a cathode ray tube to be controlled extremely accurately over a wide dynamic range. Accurate control is achieved by introducing nonlinearities in the grid electrode drive circuit which nonlinearities tend to linearize the relationship between beam intensity and input signal E Additionally, the disclosed circuit apparatus introduces limiting means for preventing the grid signal E from increasing beyond a certain level at which severe beam defocussing can occur.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination with a cathode ray tube having beam producing means and a control electrode, drive circuit means responsive to an input signal for providing a control signal to said control electrode for controlling the intensity of said beam, said cathode ray tube having a nonlinear characteristic relating beam intensity to control signal level, said drive circuit means including:

an amplifier having input and output terminals;

means for applying said input signal to said amplifier input terminal;

means coupled to said amplifier for nonlinearly varying the gain thereof as a function of input signal level to substantially linearize the characteristic relating beam intensity to input signal level;

means coupling the output terminal of said amplifier to said control electrode;

means including a normally open feedback path connected between said output terminal and said input terminal for limiting said control signal to a predetermined level; and

means for closing said normally open feedback path in response to said control signal reaching said predetermined level.

2. The combination of claim 1 wherein said normally open feedback path includes a transistor having an emitter, a collector, and a base;

means connecting said emitter and collector between said amplifier input and output terminals; and

means applying a variable potential to said base.

3. The combination of claim 1 wherein said means for varying gain includes a second feedback path including a feedback impedance coupling said amplifier output terminal to said amplifier input terminal; and

means for varying said feedback impedance as a function of input signal level.

4. The combination of claim 3 wherein said feedback impedance includes a fixed resistor; and wherein said means for varying said feedback impedance includes a plurality of impedance paths connected in parallel With said feedback resistor;

a plurality of switch means, each connected in a different one of said paths; and

means for closing each of said switch means in response to a different level of control signal.

5. The combination of claim 4 wherein each of said switch means comprises a diode.

6. In combination with a cathode ray tube having beam producing means and a control electrode, drive circuit means responsive to an input signal for producing a control signal for application to said control electrode for controlling the intensity of said beam, said cathode ray tube having a nonlinear characteristic relating beam intensity to control signal level, said drive circuit means includng:

an amplifier having at least first and second stages, each stage having input and output terminals;

means for applying said input signal to said input terminal of said first stage;

means coupling said first stage output terminal to said second stage input terminal;

means coupling said second stage output terminal to said control electrode;

first feedback means coupled between said second stage output terminal and said first stage input terminal for nonlinearly varying the gain of said amplifier as a function of input signal level to substantially linearize the characteristic relating beam intensity to input signal level; and

second feedback means connected between said second stage output terminal and said second stage input terminal for limiting said control signal to a predetermined level.

7. The combination of claim 6 wherein said first feedback means includes a feedback impedance; and

means for varying said feedback means as a function of input signal level.

8. The combination of claim 6 wherein said second feedback means comprises a normally open feedback path 1ncluding a transistor having an emitter, a collector, and a base;

means connecting said emitter and collector between said second stage output terminal and said second stage input terminal; and

means coupling a controllable potential source to said transistor base.

References Cited UNITED STATES PATENTS 2,218,720 10/1940 Rinia 315-30 2,240,289 4/1941 Dillenburger et a1. 315-30 2,567,377 9/1951 Holbrook 315-30 2,671,871 3/1954 Haynes 315-30 2,860,284 11/1958 McKim 315-30X 3,277,335 10/ 1966 Moser et al 315-30 RODNEY D. BENNETT, 111., Primary Examiner M. F. HUBLER, Assistant Examiner

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