US3413513A - Method and apparatus for increasing writing rate of storage tube - Google Patents

Description

Nov. 26, 1968 J, J, DONOGHUE ET AL 3,413,513

METHOD AND APPARATUS FOR INCREASING WRITING RATE OF STORAGE TUBE Original Filed Jan. 13, 1964 2 Sheets-Sheet 1 l4 /6 TM INPUT VERT. 64 MONITOF AMP.

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JAMES J DONOGHUE' RICHARD B. McM/LLAM dz 8 INVENTORS.

BUG/(HORN, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNE)? I Nov. 26, 1968 J. J. DONOGHUE ETAL 3,413,513 METHOD AND APPARATUS FOR INCREASING WRITING RATE OF STORAGE TUBE Original Filed Jan. 13, 1964 2 Sheets-Sheet 2 G E 0V TIME o v ar 1 5 6 JAMES .1. DONOGHUE mam/#0 a. McM/LLAN, .1. B INVENTORS.

BUG/(HORN, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS United States Patent 3,413,513 METHOD AND APPARATUS FOR INCREASING WRITING RATE OF STORAGE TUBE James J. Donoghue, Portland, and Richard B. McMillan,

Jr., Tigard, Oreg., assignors to Tektronix, Inc., Beaverton, 0rcg., a corporation of Oregon Continuation of application Ser. No. 337,370, Jan. 13, 1964. This application Apr. 7, 1967, Ser. No. 659,826

14 Claims. (Cl. 315-11) ABSTRACT OF THE DISCLOSURE A bistable storage tube and a method of operating the same to increase its writing rate. An enhancement pulse is applied to the flood gun cathode which is much greater than the first crossover voltage to enable storage of written charge images of initially low voltage. The pulse is terminated at such time as to prevent the potential of the unwritten background areas of the storage dielectric from increasing above the first crossover voltage. This is possible because the more positive voltage portions of the storage dielectric charge at a faster rate than the more negative unwritten background target areas.

The present application is a continuation of copending US. patent application Ser. No. 337,370, filed Jan. 13, 1964, now abandoned, by James J. Donoghue et al.

The subject matter of the present invention relates generally to electron image storage apparatus, and in particular to methods and apparatus for operating a bistable storage tube to increase its Writing rate so that the electron beam within such tube may be moved across its storage target at a faster rate and still produce a charge image which is stored an indefinite controlable time.

Briefly, one embodiment of the method of operation of the present invention involves turning off the flood electron guns within such tube to prevent low velocity flood electrons produced by such flood gun from striking the storage target during the time such target is bombarded by a beam of high velocity writing electrons to produce the charge image on such target. Then the target is bombarded with flood electrons of increased velocity immediately after the charge image is formed by the writing electrons after which the velocity of such flood electrons is decreased below that which will drive the potential of the background areas of the storage target above the first cross over voltage of the secondary emission characteristic of such target at which the secondary emission ratio is one. As a result of the momentary increase in velocity of the flood electrons, the potential of any portion of the charge image which is initially below the first cross over voltage, is driven above the first cross over voltage so that the entire charge image can be stored as a bistable image for an indefinite controlable time. The first cross over voltage is the minimum voltage difference between the bombarded rear surface of the storage dielectric of the target and the flood gun cathode which is necessary to enable bistable storage.

The present method of operating the storage target to increase its writing rate is especially useful when employed for a cathode ray oscilloscope having a direct viewing bistable storage tube as its signal display device. This enables high frequency input signals or those having extremely fast rise times to be stored on the target of the storage tube of such oscilloscope even though such signals are above the normal writing rate of such target achieved by conventional operation. In addition, the method of the present invention may be employed for storing both repetitive signals and transient signals. However high frequency repetitive signals may be stored by charge image inte- 3,413,513 Patented Nov. 26, 1968 gration merely by preventing the flood electrons from striking the storage target during the bombardment of such target by the writing beam for several consecutive cycles of such repetitive input signal so that charge images of their successive waveforms are superimposed to increase the potential of the waveform image formed on the target above the first cross over voltage. This charge image integration is more fully discussed in copending US. patent application Ser. No. 302,880, filed by Robert H. Anderson on Aug. 19, 1963, and entitled, Improved Storage Tube and Method of Operation, now Patent No. 3,325,673. The present method may be used in place of charge image integration when there is jitter in the repetitive signal so that successive waveforms are not superimposed on the storage target, or it may be combined with charge image integration to enable faster storage. Since charge image integration is obviously not possible with a transient signal the method of the present invention is even more useful when applied to enable the storage of transient signals having fast rise times which would not otherwise be stored by conventional operating techniques.

The method of operation of the present invention has several advantages over conventional methods of operating storage tubes including increasing the frequency response of the storage tube by effectively increasing th writing rate of the storage target of such tube in a simple and efficient :manner. The present method may be applied to all conventional storage tubes including direct readout tubes employing a grid control or transmission type storage target and a separate phosphor screen spaced from such target, as Well as nonconventional tubes employing phosphor storage targets such as are described in copending US. patent applications Ser. No. 180,457, filed Mar. 19, 1962, by Robert H. Anderson, and entitled, Electron Discharge Display Device, now Patent No. 3,293,473, and Ser. No. 299,422, filed Aug. 1, 1963, by Charles B. Gibson, entitled, Storage Target for Cathode Ray Tube and Photographic Method of Manufacture, now Patent No. 3,293,474.

Apparatus for carrying out the method of operation of the present invention is simple and inexpensive. In order to provide manual operation, one embodiment of this apparatus is in the form of a manual switch having a movable contact connected to the cathodes of the flood guns within the storage tube with one of its fixed contacts connected to a source of positive D.C. voltage source above cut off and another of its fixed contacts connected to the common terminal of a capacitor and a charging resistance which are connected in series between a negative DC. voltage source and ground. Alternatively, the apparatus may be of a type to provide automatic operation, in which case it may include a blanking signal generator which is triggered by the input signal applied to the vertical deflection plates of the storage tube, and applies a negative voltage blanking pulse to the control grids of the flood guns to cut off such flood guns so that a capacitor connected to the cathode of each flood gun charges to a suitable negative DC voltage to increase the velocity of the flood electrons when such flood gun is turned back on by termination of the blanking pulse. This automatic or triggered operation has the advantage that it enables the writing rate enhancement pulse to be applied immediately after the input signal has written its charge image on the storage target before the potential of such charge image decreases appreciably due to leakage. Therefore, such triggered operation is very desirable when storing high speed transient signals whose charge images are initially of very low potential.

It has also been discovered that target areas of different initial voltage potential charge at different rates when low velocity flood electrons bombard such target areas. A target area of higher potential charges at a faster rate than a target area of lower potential so that the potential difference between such target areas increases with time during charging. This means that if the waveform charge image produced on the storage target is of only a slightly greater potential than the background areas of the storage targets, it is still possible to store such charge image bistably by increasing the potential of the charge image and the background areas by applying a negative enhancement pulse to the flood gun cathode until the potential of the charge image is greater than the first cross over voltage but terminating the enhancement pulse before the potential of the background areas exceed the first cross over voltage. It is found that this increased potential difference between written target areas and unwritten target areas due to their different charging rates, may be further increased by employing a spike enhancement pulse having an amplitude greater than the first cross over voltage and exponential trailing edge which can be produced by discharging a suitable capacitor through resistance of the proper value.

It is therefore one object of the present invention to provide an improved method and apparatus for operating a storage tube to increase the writing rate of such storage tube.

Another object of the present invention is to provide an improved method and apparatus for operating a bistable storage tube which allows such tube to store higher speed transient signals as well as repetitive signals of greater frequency.

A further object of the invention is to provide a simple method of increasing the writing rate of a bistable storage tube by preventing low velocity holding electrons from striking the storage target during the time the charge image is produced on such target by high velocity writing electrons and momentarily increasing the velocity of the holding electrons striking the storage target immediately after the charge image is produced.

An additional object of the present invention is to provide a simple and inexpensive electrical circuit which may be operated by a manual switch for increasing the writing rate of any bistable storage tube.

A still further object of the present invention is to provide an electrical circuit for increasing the writing rate of a bistable storage tube automatically by triggering such circuit in response to the input signal to be stored.

Additional objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a schematic diagram of one embodiment of the apparatus made in accordance with the present invention;

FIG. 2 is a curve of the voltage applied to the cathode of the flood guns employed in the storage tube of FIG. 1;

FIG. 3 is a schematic diagram of another embodiment of a circuit for operating the flood guns in the storage tube of FIG. 1 by automatic triggering;

FIG. 4 shows the voltage Waveforms applied to the cathode and control grid of the flood gun of FIG. 3; and

FIGS. 5A, 5B and 5C are diagrams of the waveforms of different types of writing speed enhancement voltage pulses which may be applied to the flood gun cathodes of FIG. 1 and the potentials of written and background target areas effected by such enhancement pulses.

One embodiment of the charge image storage apparatus of the present invention is shown in FIG. 1, and includes a conventional bistable storage tube 10 or a direct viewing bistable storage tube similar to that described in the copending U.S. patent application Ser. No. 180,457, referred to above, so that such tube will not be discussed in detail. The electrical input signals to be displayed are applied across a pair of vertical deflection plates 12 at least one of which is connected to an input terminal 14 through a vertical amplifier 16 and a two position selector switch 18. The movable contact of the selector switch 18 is moved to the WRITE position shown, to apply the input signal to the vertical deflection plates during the writing operation of the storage tube. A pair of horizontal deflection plates 20 are also provided within the storage tube and are connected to a horizontal sweep generator 22 through a second selector switch 24 whose movable contact is ganged to that of switch 18. Thus, the horizontal sweep generator 22 applies a conventional saw tooth or ramp shaped sweep signal across the horizontal deflection plates 20 when the selector switch 24 is in the WRITE position shown. As a result, a storage target 26 at one end of the tube 10 is bombarded by a narrow beam of high velocity writing electrons which are emitted from a cathode 28 at the other end of such tube. The writing beam is deflected by the signals on the horizontal and vertical deflection plates so that it produces a charge image on the storage dielectric of such target which corresponds to the waveform of the vertical input signal applied to input terminal 14.

If the current density of the writing beam emitted by cathode 28 is sufficient, the voltage difference between such cathode and the target 26 is high enough, and the speed of the horizontal sweep signal applied is slow enough, the potential of the charge image produced on the storage target 26 will be sufficient to enable bistable storage of such charge image for an indefinite controlable time. This bistable storage is caused in a conventional manner by bombarding such storage target substantially uniformly with low velocity flood electrons emitted by a pair of flood guns 30. Thus, when the potential of the charge image produced by the writing beam on the storage dielectric of target 26 exceeds the first cross over voltage of the secondary emission characteristic curve of such storage dielectric, the holding or flood electrons drive the potential of the charge image up to a stable voltage near the voltage of the collector electrode of such storage target. At the same time the unwritten background areas of the storage target whose potential is below the first cross over voltage are driven downward to a stable voltage adjacent the voltage of the cathodes 31 of the flood guns. In this manner, all areas of the rear surface of the storage target are held at one of these two stable voltages. It has been discovered that the effective first cross over voltage varies with the field produced across the storage dielectric, at least for the phosphor target referred to below, so that this term will be used to indicate the minimum charge voltage necessary for storage and may vary in value with the voltage applied to the target electrode.

The storage target 26 may be similar to the direct viewing target disclosed in copending U.S. patent application Ser. No. 180,457, referred to above, or the split screen target shown in copending U.S. patent application Ser. No. 214,877, filed on Aug. 6, 1962, by Robert H. Anderson, entitled Storage Tube, now Patent No. 3,214,631. In either case, the storage dielectric is a thin layer of phosphor material which serves the dual functions of storing the charge image bistably and of converting the charge image into a light image for direct viewing. This phosphor storage dielectric is supported over a light transparent electrical conductive film of tin oxide coated on the rear surface of the face plate of the tube envelope. This conductive film serves as the collector electrode for the secondary electrons emitted by the phosphor layer due to the porous structure of such layer and is connected to a target voltage produced across a fixed load resistor 32. The load resistor 32 is connected in series with a variable resistor 33 be- I tween a source of positive D.C. supply voltage of about age above which the storage target is driven to a uniformly positive or completely Written condition by the flood electrons.

The low velocity holding or flood electrons emitted from the cathodes 31 are normally transmitted through control grid 34 and anode 36 of the flood guns onto the surface of the storage target 26 after passing through at least one wall band electrode 38 of silver or other conductive material coated on the inner surface of the funnel portion of the envelope. The wall band electrode 38 is connected to a positive DC bias voltage of about +50 volts when the flood gun cathode 31 is normally grounded, in order to spread the flood electrons substantially uniformly over the surface of the storage target and to collimate such flood electrons so that they strike the target at substantially right angles thereto. It should be understood that while the Wall band electrode 38 is shown as a single electrode it may be provided as a plurality of spaced wall bands of varying potential for more precise control of the flood electrons. Thus, while the writing beam of high velocity electrons is focused into a narrow beam by transmitting it through a control grid 40 and a three element focusing anode structure 42 in the writing gun so that the writing beam strikes the storage target as a small circular spot, the flood electrons are focused to cover the entire surface of the storage target. However, this flood gun type storage tube may be replaced by one in which the holding electrons are provided in the form of a narrow beam similar to that of the writing beam but of lower velocity, which is deflected over the surface of the storage target in a TV raster pattern to enable storage of the charge image in a similar manner to the flood electrons.

When an input signal of extremely high frequency or fast rise time is applied to the vertical deflection plates 12 the potential of the charge image of such input signal produced on the storage target is sometimes below the first cross over voltage so that the charge image is not stored normally because the flood electrons drive its potential down toward the voltage of the flood gun cathode along with the potential of the unwritten background target areas. It has been discovered that if the flood or holding electrons are prevented from bombarding the storage target during the time the writing electrons are producing the charge image thereon, an increase in the potential of such charge image results which is frequently suflicient to enable bistable storage of such charge image when the holding electrons are subsequently allowed to bombard the storage target. Furthermore, if the input signal is a repetitive signal it is possible to increase the potential of the charge image even more by maintaining the flood guns turned off during several successive cycles of the input signal so that charge image of these successive input signals are superimposed on the storage target and their potentials effectively added together to increase the total potential of the resulting charge image over the first cross over voltage. This charge image integration technique is described in greater detail in copending US. patent application Ser. No. 302,880, by Robert H. Anderson, filed Aug. 19, 1963, entitled Improved Storage Tube and Method of Operation.

The increase in the potential of the charge image achieved by preventing the holding or flood electrons from striking the storage target during writing is due to the fact that such holding electrons oppose the writing operation since they tend to drive the potential of the charge image downward to the voltage of the flood gun cathode. This opposing action continues until the potential of such charge image is increased above the first cross over voltage. After this the holding electrons aid writing by driving the potential of such charge image upward to the voltage of the collector electrode.

It has also been discovered that separate areas of the same storage target initially at different potentials are charged at different rates by the holding electrons to higher voltages when the voltage on the flood gun cathodes is lowered below the first cross over voltage, as shown in FIGS. 5A, 5B and 50. Thus, target areas of higher initial voltage which have been struck by the writing beam are charged at a faster rate than unwritten background target areas of lower initial voltage so that the voltage difference between such target areas increases with time. It is therefore possible to diflerentiate between a charge image of very low initial voltage and the background areas of the target because of their different charging rates and to store the charge image by decreasing the flood gun cathode voltage back to zero after the potential of the charge image exceeds the first cross over voltage but while the potentials of the background target areas are still below such first cross over voltage. This allows the charge images of high frequency signals to be stored and is especially useful when storing high speed transient signals but may also be employed along with the charge image integration to store repetitive signals more quickly.

The writing speed enhancement technique described above is accomplished in FIG. 1 by means of a manual switch 44 having three different positions with its movable contact connected to the flood gun cathodes 31. The fixed contact of switch 44 labeled WRITE is connected to a source of positive DC. bias voltage of about volts through an isolating resistor 46 so that the flood guns are cut off when the switch 44 is in the WRITE position due to the fact that the control grids 34 are connected to negative DC bias voltages of about 20 volts and such flood gun cathodes are therefore reverse biased by about volts. This prevents the flood electrons from bombarding the storage target 26 during the time the writing beam emitted by cathode 28 is producing the charge image on the storage target. In the STORE position of the manual switch 44 the flood gun cathodes are connected across a capacitor 48 having one terminal connected to ground and its other terminal connected to a source of negative D.C. supply voltage of about 70 volts through a fixed resistor 50 and a variable resistor 52 connected in series. The capacitor 48 is initially charged to -70 volts bu current flowing through resistors 50 and 52 so that by moving the switch 44 to the STORE position the voltage on the flood gun cathode changes initially to 70 volts. This causes the flood guns to be rendered conducting since their cathodes are then forward biased by 50 volts. Thus, flood electrons are transmitted to the storage target of a higher voltage initially. However, the capacitor 48 immediately begins discharging toward the voltage set at the common connection of such capacitor with resistor 50 to decrease the velocity of the flood electrons exponentially. The voltage of the flood gun cathode 31 after discharge of capacitor 48 may be set to 0 by adjusting the setting of resistor 52 to vary the current so that the voltage drop across resistors 50 and 52 is 70 volts. As the voltage on the flood gun cathodes is maintained at about 0 volts with respect to ground, the storage tube 10 operates in a conventional manner to store the charge image.

An electrical readout signal may be produced on the target coating electrode by scanning the phosphor layer of the storage target with a reading beam of electrons as described in copending US. patent application Ser. No. 245,716, filed Dec. 19, 1962, by Robert H. Anderson and entitled Electrical Readout for Storage Tube, now Patent No. 3,219,316. This reading beam may be produced by the same electron gun which provided the writing beam merely by changing the position of the switches 18 and 24 to the READ position indicated so that a raster signal generator 54 is connected to the horizontal and vertical deflection plates of tube 10 to move the beam in a TV raster pattern over the surface of the target. In order to prevent this reading beam from storing the raster pattern, it is necessary either to connect the cathode 28 to a more positive voltage to decrease the velocity of the reading electrons below that of the writing electrons, or to increase the negative voltage applied to the control grid 40 in order to decrease the current density of the reading beam. This latter technique is more desirable and may be 7 accomplished by means of a switch 56 whose movable contact is connected to the control grid 40 so that in the WRITE position of switch 56 the control grid is connected to a negative DC. voltage source of about 3,025 volts, while in the READ position of such switch the control grid is connected to a voltage of about -3,050 volts. The electrical readout signal produced on the conductive film of the storage target is transmitted through a coupling capacitor 58 to a low input impedance preamplifier 60. The output of preamplifier 60 is connected through a conventional voltage amplifier 62 before being transmitted to the Z-axis input at the control grid or cathode of a remote TV monitor tube 64. The horizontal and vertical deflection plates of the monitor tube 64 are also connected to the raster signal generator 54 so that the same or related saw tooth raster signals may be applied to these deflection plates as are applied to the horizontal and vertical deflection plates of the storage tube during the readout operation. As a result, the wave form image stored on the storage target 26 is reproduced on the fluorescent screen of the monitor tube 64. Of course, electrical readout is not necessary when a direct viewing storage target is employed but the present method may also be employed with storage tubes having nondirect readout targets.

The wave form image stored on the target 26 may be removed by a conventional erase operation merely by varying the resistor 33 so that the voltage across load resistor 32 is first increased above the fade positive voltage to enable the flood electrons to cause the storage target to fade uniformly positive. Then the target voltage is decreased below the first cross over or retention threshold voltage to cause the potential of the rear surface of the storage dielectric to be driven negative back to the voltage of the flood gun cathode. Next, the voltage across resistor 32 is slowly increased above the first cross over voltage so that the conductive film target electrode is provided with a voltage within the stage range without causing the rear surface of the storage dielectric to follow such target electrode voltage. This erase operation may also be accomplished by pulsing the target electrode. Also, while it is not essential, it has been found desirable to connect the flood gun cathode to a source of positive DC. bias voltage of, for example, about +50 volts during the erase operation. This may be accomplished by rotating the movable contact of switch 44 to the ERASE position.

The potential of the flood gun cathode during the operation of the tube of FIG. 1 is shown by the curve 66 in FIG. 2. It should be noted that the writing rate enhancement pluse portion 68 is applied immediately after the WRITE operation during which the flood gun cathode is cut off. This enhancement pulse 68 decreases to a negative voltage which may be substantially below the first cross over voltage and then rises in an exponential manner towards volts at a rate determined by the RC. time constant of the circuit including capacitor 48 and resistors 50 and 52. Thus, the width of such enhancement pulse is approximately equal to 3 RC and should be inversely proportional to the voltage amplitude of such enhancement pulse. A large voltage writing rate enhancement pulse will cause the background areas of the target to charge more rapidly in a positive direction due to the increased secondary emission caused by the greater velocity of the flood electrons so that the enhancement pulse must be terminated sooner to prevent the voltage of such background areas from exceeding the first cross over voltage. In order to provide an enhancemeent pulse of the correct width, the proper values of resistance and capacitors must be selected for resistors 50 and 52 and capacitor 48.

Another embodiment of the method and apparatus for increasing the writing rate of the storage tube of FIG. 1, is shown in FIG. 3. The flood guns 30 are automatically turned off at the start of the vertical input signal applied through the vertical amplifier 16 to the vertical deflection plates of the storage tubes. This may be accomplished by transmitting a portion of the vertical input signal through a sweep trigger generator 69 to produce trigger pulses at the start of such vertical signal which are transmitted to the horizontal sweep generator 22 to start the operation of such sweep generator in a conventional manner. The sweep trigger generator 69 may also be connected to the input of a flood gun blanking multivibrator 70 to trigger such multivibrator so that it produces a negative voltage output pulse 72. This negative output pulse is then transmit-ted to the control grid 34 of the flood gun and functions as a blanking pulse to reverse bias the cathode of such flood gun to cut 01?. This allows the capacitor 48 to charge by current flowing through resistors 50 and 52 from the 70 volt DC. voltage source until voltage across such capacitor reaches that of the voltage source. After the charge image of the vertical input signal is written on the storage target, the blanking pulse 72 is terminated to return the control grid 34 to a more positive voltage so that the flood gun is again rendered conducting. At this time the voltage on the flood gun cathode 31 is equal to the 70 volts of the fully charged capacitor 48. However, thi cathode voltage decreases as the capacitor discharges to a more positive voltage which may be set to zero by varying resistor 52 until the beam current produces a voltage drop of 70 volts across resistor 50 and 52.

The flood gun blanking pulse generator 70 may be a monostable multivibrator whose frequency is separately controlled so that it is considerably lower than the frequency of the input signal applied to input terminal 14 in order to enable charge image integration by maintaining the flood gun cut off during several successive cycles of such vertical input signal. However, it may also be desirable to connect the flood gun blanking pulse generator 70 as a bistable multivibrator which is triggered by sweep trigger pulses and is reverted to its initial stable state by a signal from the horizontal sweep generator corresponding to the retrace portion of the horizontal sweep signal so that the blanking pulse 72 is discontinued immediately after the horizontal sweep signal. This bistable flood gun blanking multivibrator may be desirable when operating the storage tube to store transient vertical input signals. It should be noted that a different type of signal generator may be employed in place of the blanking multivibrator 70 and the output of this signal generator may be connected to the flood gun cathode 31 to apply a positive voltage blanking pulse as well as a writ ing speed enhancement pulse to such cathode without employing the separate pulse forming circuit of resistors 50 and 52 and capacitor 48.

It is extremely desirable to provide the triggered writing rate enhancement operation of FIG. 3 when attempting to store a high speed transient input signal because the charge image produced by such transient signal on the storage target could decrease below the minimum voltage level necessary for storage due to leakage if an enhancement pulse is not applied to turn on the flood gun im mediately after the charge image is produced. For this reason a bistable multivibrator is more desirable as the flood gun blanking multivibrator.

The flood gun blanking pulse 72 produced by the apparatus of FIG. 3, is shown in time relation to the flood gun cathode voltage 74 in FIG. 4. Thus, when the blanking pulse 72 is applied to the control grid 34, the flood gun cathode voltage gradually decreases from "0 to about -70 volts as the capacitor 48 charges. When the blanking pulse 72 is terminated, the flood gun cathode voltage then increases from 70 volts to 0 volts as the capacitor 48 discharges. This positive going portion of the flood gun cathode voltage 74 provides the writing speed enhancement pulse. It should be noted that the flood gun blanking multivibrator 70 of FIG. 3 is provided with a lock out control which prevents such multivibrator from being retriggered during storage and until after the charge image has been erased from the storage target.

The effect of operating a storage target in accordance with the methods of the present invention is shown by the curves of FIGS. A, 5B and 5C. When the writing speed enhancement pulse applied to the flood gun cathode is in the form of a negative spike voltage 76 as shown in FIG. 5A, the best results are obtained. This is due to the separation of the charge image voltage 78 from the background voltage 80 of unwritten target areas by a greater amount Y when the charge image voltage exceeds the first cross over voltage V Before the enhancement pulse 76 is applied the charge image voltage 78 differs from the background voltage 80 by a small amount X after such charge image is written on the storage target, but before the flood electrons strike such target. When the enhancement pulse 76 is applied to the flood gun cathode, the potential of the charge image and the potential of the background target areas begin to rise due to the charging action of the flood electrons. As stated previously, the charge image voltage increases at a faster rate than the background voltage due to the fact that it Was initially at a higher potential. Therefore, at some time after the enhancement pulse is applied, the charge image voltage 78 and the background voltage 80 are separated by a greater voltage difference than X. This enables the enhancement pulse 76 to be terminated within a wider time range without causing background to fade positive since the charge image voltage 78 exceeds the first cross over voltage V much sooner than the background voltage. Of course, the writing speed enhancement pulse 76 must be terminated or decreased in voltage to such a value that it can no longer increase the background voltage above the first cross over voltage, before such background voltage exceeds V in order to cause bistable storage.

When the charge image voltage 78 is driven above the first cross over voltage, the flood electrons cause such charge image voltage to be driven upward to a high voltage stable state V which is slightly greater than the voltage applied to the collector electrode At the same time such flood electrons cause the background voltage 80 of the unwritten target areas charged to a potential below V to decrease in voltage to a potential approximately equal to thatof the flood gun cathode.

As shown in FIG. SE, a similar charging operation takes place when a negative rectangular writing rate enhancement pulse 82 is applied to the flood gun cathode, However, the increase in the voltage difference between the charge image voltage 78 and the background voltage 80' is not as great. Thus, the voltage difference Y between curves 78' and 80 when curve 78 exceeds V is less than the voltage difference Y in FIG. 5A even though the initial voltage difference X is the same in both cases. This means that the width W of the rectangular enhancement pulse 82 is more critical than the width of the spike enhancement pulse 76, so that such rectangular enhancement pulse must terminate immediately after the charge image voltage 78' exceeds V Of course, the maximum amplitude V of both of the enhancement pulses 76 and 82 may also exceed the amplitude of the first cross over voltage V because they are maintained above that voltage for only a short time.

In FIG. 5C a negative stair step enhancement pulse 84 is applied to the flood gun cathode. Since this stair step enhancement pulse is maintained at its maximum voltage V and does not return to 0 its amplitude must be less than that of the first cross over voltage V Also, the sum of the voltage V of the enhancement pulse and the initial voltage A of the charge image voltage 7 8" must be greater than the first cross over voltage, while the sum of V and the initial voltage A of the background voltage 80" must be less than the first cross over voltage. In other words, A must be greater and A less than the difference in the voltage AV between the stair step enhancement voltage V and the first cross over voltage -V in order to enable bistable storage. Of course, this technique is less desirable than the other two discussed because the differential charging action of the methods of FIGS. 5A and 5B is not employed. Thus, the initial voltage difference X between the charge image voltage and the background voltage must be sufficient to enable the enhancement voltage 84 to cause the charge image voltage to exceed the first cross over voltage immediately without causing the background voltage to exceed the first cross over voltage. This means that any appreciable fiuctation in the voltage V of the enhancement pulse 84 would prevent the bistable storage of charge images of very low initial potential due to the fact that there is always some noise in the background voltage. It should be noted that this noise voltage is less for smooth surface storage targets such as the phosphor storage target shown in copending US. patent application Ser. No. 180,457, described above, than it is for conventional direct viewing storage targets of the current transmission type employing a mesh structure, due to the fact that the thickness of the phosphor target is substantially uniform while the thickness of the mesh target varies considerably.

Of course it is possible to use a rectangular or spike enhancement pulse in place of pulses 82 or 76 whose maximum negative voltage V is less than V if the potential difference AV between V and V is maintained with the same relationship to A and A as in FIG. 5C. However the width of the enhancement pulse must be increased considerably with written areas of low initial voltage in order to raise the potential of such areas above V As a result of this increased charging time the potential of the unwritten target areas may decrease along curve 80" of FIG. 5 C to a negative voltage which is greater than the potential difference between the target electrode voltage V, and the fade positive voltage, before the enhancment pulse terminates. If this happens the electrical field across the target dielectric collapses and the potential of the bombarded surface of the target increases to a uniformly positive voltage equal to the voltage of the target electrode so that it has the effect of erasing the charge image. When an enhancement pulse having a maximum negative voltage V greater than V is employed, this problem is avoided because the unwritten target areas charge positively along curves 80 and 81 of FIGS. 5A and 5B and decrease, rather than increase, the field across the target dielectric.

It will be obvious to those having ordinary skill in the art that various changes may be made in the details of the above preferred embodiment of the present invention without departing from the spirit of the invention. For example, when an ion repeller electrode or a secondary electron collector electrode is employed between the storage target and the flood gun, it would be possible to apply the blanking pulse to such electrode in order to prevent the flood electrons from bombarding the storage target during writing. Also, the writing speed enhancement pulse may be applied to the storage target electrode instead of the flood gun cathode merely by reversing the polarity of such pulse. Furthermore it is not essential that the flood guns be completely cut off during writing to prevent all target areas from being bombarded by flood electrons at this time. On the contrary, it is only necessary to prevent the flood electrons from striking those areas of the storage dielectric at a potential less than the first cross over Voltage, during writing. Thus flood electrons may be allowed to strike target areas charged above such first cross over voltage at this time since they do not oppose writing in these areas. This may be accomplished by increasing the potential of the flood gun cathodes with respect to the rear surface of the target dielectric in the positive direction until it is slightly above the first cross over voltage. The potential of the control grid of the flood gun is also increased so that flood electrons are still emitted therefrom. However most of these flood electrons are collected by the Wall band electrodes 38 and no flood electrons strike the storage target except in those written areas which the writing beam has driven above the first cross over voltage. As in the other methods described above, the negative writing rate enhancement pulse is applied to the flood gun cathodes after the writing operation to enable flood electrons to strike all target areas. Therefore, the scope of the present invention should only be determined by the following claims.

We claim: 1. A method of increasing the writing rate of a bistable storage tube comprising the steps of:

forming a charge image on the storage dielectric of a storage target in said tube with at least a portion of said charge image having a potential less than the first crossover voltage of the secondary emission characteristic of said storage dielectric which is the minimum voltage necessary for bistable storage;

bombarding said storage dielectric with low velocity holding electrons to raise the potential of said portion of said charge image above said first crossover voltage without increasing the potential of the unwritten background areas of the storage dielectric above said first crossover voltage in order to cause bistable storage of the entire charge image;

applying an enhancement voltage to said tube during said bombardment by said holding electrons to produce a positive voltage difference between the cathode emitting the holding electrons and the storage dielectric which is greater than the first crossover voltage, thereby increasing the velocity of said holding electrons for a brief time at the beginning of bombardment of said storage dielectric by said holding electrons to increase the potential of said portion of said charge image above said first crossover voltage; and

terminating the enhancement voltage to reduce said voltage difference below said first crossover voltage thereby decreasing the velocity of said holding electrons to a lower velocity in time to prevent the potential of said background areas from exceeding said first crossover voltage while continuing to bombard said storage dielectric with said holding electrons to store said charge image.

2. A method in accordance with claim 1 in which the charge image is formed by bombarding the storage dielectric with high velocity writing electrons.

3. A method in accordance with claim 2 in which the low velocity holding electrons are prevented from striking the storage dielectric during bombardment of said storage dielectric by the Writing electrons to form the charge image.

4. A method in accordance with claim 3 in which a substantially constant voltage is applied to a target electrode over which the storage dielectric is supported, during the time the holding electrons are prevented from striking the storage dielectric.

5. A method in accordance with claim 3 in which a repetitive input signal is applied to the tube to modulate the writing electrons and form a charge image of the input signal, and several successive cycles of said input signal are applied during the time the holding electrons are prevented from striking the storage dielectric to enable integration of the superimposed charge images of said succcessive cycles.

6. A method in accordance with claim 1 in which the enhancement voltage is a spike shaped pulse whose trailing edge decreases at a slower rate than its leading edge increases.

7. A method in accordance with claim 1 in which the enhancement voltage is a rectangular pulse.

8. Electron image storage apparatus, comprising:

a storage target including a secondary emissive storage dielectric capable of bistable storage;

means for bombarding said storage dielectric with high velocity writing electrons to produce a charge image on said storage dielectric, at least a portion of said charge image having a potential less than the first crossover voltage of the secondary emission characteristic of said storage dielectric;

means including a holding cathode for bombarding said storage dielectric with low velocity holding electrons so that said holding electrons cause bistable storage of said charge image when its potential is above said first crossover voltage;

means for applying an enhancement voltage to said tube to produce a positive voltage difference between the holding cathode and the storage dielectric which is greater than the first crossover voltage, thereby increasing the energy of said holding electrons for a brief time at the beginning of their bombardment of said storage dielectric to increase the potential of said portion of said charge image above said first crossover voltage; and

means for terminating said enhancement voltage to reduce said voltage difference below the first crossover voltage, thereby decreasing the energy of said holding electrons to a lower energy while continuing to bombard said storage dielectric with said holding electrons, in time to prevent the potential of the unwritten background areas of said storage dielectric from exceeding said first crossover voltage.

9. An apparatus in accordance with claim 8 in which the storage target is contained within the evacuated envelope of a storage tube forming part of said apparatus.

10. An apparatus in accordance with claim 9 which also includes means for preventing the holding electrons from striking the storage dielectric during the bombardment of said storage dielectric by said writing electrons to form the charge image.

11. An apparatus in accordance with claim 10 which also includes means for applying a repetitive input signal to the storage tube to modulate the writing electrons and form a charge image of the input signal, and for applying several successive cycles of said input signal to said tube during the time the holding electrons are prevented from bombarding the storage dielectric to enable integration of the superimposed charge images of said successive cycles.

12. An apparatus in accordance with claim 10 which also includes means for applying a substantially constant voltage to a target electrode over which the storage dielectric is supported.

13. An apparatus in accordance with claim 10 which includes a switch means for applying a negative bias voltage between the holding cathode and its control grid to cut off the flow of holding electrons to the storage dielectric, and for applying the enhancement voltage to the holding cathode.

14. An apparatus in accordance with claim 9 in which the storage dielectric is a layer of phosphor material provided over a light transparent conductive film coated on the inner surface of the face plate of the storage tube.

References Cited UNITED STATES PATENTS 3,259,791 7/1966 Jensen et al 315-12 FOREIGN PATENTS 11,519 7/1961 Japan.

RICHARD A. FARLY, Primary Examiner.

C. L. WHITHAM, Assistant Examiner.

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