US3427493A - Direct viewing storage cathode ray tube circuit arrangement - Google Patents

Description

ERECT VIEWING STORAGE CATHODE RAY TUBE CIRCUIT ARRANGEMENT A. M, ADAM Filed July 18. 1966 ADJU TABLE 05m) PULSE :auece 8 ouus 71151.5 PULSE sauAcE FIGZZ sa /Rae 0F VARY/N6 SCREEN POTENTIAL.

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: lNvem-rota flaw/m, 2 1;, Adams; ATTORNEY United States Patent 33,235/ 65 US. Cl. 31512 Int. Cl. H013 29/ 41 6 Claims ABSTRACT OF THE DISCLOSURE A direct viewing storage tube arrangement including a viewing screen, a storage target electrode, a reading electron gun for scanning the target electrode for writing information thereon in the form of stored charges and means for providing a repetitively pulsed flood beam for the production of visible images on the viewing screen. Decay pulses are repetitively applied to the target electrode during alternate pulses of the flood beam and the E. H.T. potential applied to the fluorescent screen in varied between a minimum potential corresponding in time to the applied decay pulses and a maximum potential corresponding in time to the remaining flood beam pulses, corresponding to the production of a visible image.

This invention relates to circuit arrangements incorporating direct viewing storage cathode ray tubes and to methods of operating such tubes. More specifically the invention relates to methods of continuously operating direct viewing storage cathode ray tubes (hereinafter termed, for brevity, direct viewing storage tubes) and to circuit arrangements for carrying those methods into effect. The object of the present invention is to provide improved methods of continuously operating direct viewing storage tubes and improved circuit arrangements for carrying out those methods which shall be better suited for the requirements of such equipments as surveillance radars, where good displays, with persistence times of 2 minutes or more, are usually wanted, than are known methods and arrangements.

By continuous operation of a direct viewing storage tube is meant that form of operation in which information to be displayed-for example information provided by signals derived by a surveillance radaris progressively written in the form of an electrical charge image on the storage target of the tube and this image is continuously decayed or attenuated. The at present usual method of achieving such attention or decay is by simply applying periodically repeated short pulses, hereinafter termed decay pulses to the normally provided backing electrode of said target. These may be a single train of applied decay pulses or two (or more) trains of pulses of different durations and amplitudes. Thus, a train of short duration high amplitude pulses and a train of longer duration lower amplitude pulses may both be applied as decay pulses. For clarity in explanation it may be pointed out that continuous operation is in contra-distinction with what is usually called one shot operation in which information is written on the storage target during a scan thereof by the signal modulated writing gun of the tube, the charge image thus obtained being stored indefinitely on the traget and viewable at any time on the fluorescent screen of the tube by switching on the flood gun. In one shot operation, when a stored charge image is no longer required and is to be replaced by another, a long erasing pulse, customarily of about 1 second duration, is applied to the backing electrode of the target. In continuous operation, on the other hand the charge image is attenuated as distinct from being erased. The attenuation being Patented Feb. 11, 1969 effected, in known present day practice, by periodically applying very short pulses to the target backing electrode.

Serious difficulties and limitations are encountered in securing satisfactory continuous operation of a direct viewing storage tube by known methods. The storage elements are charged negatively by the decay pulses and receive positive charges by the action of the writing beam and from positive ion bombardment from residual gas molecules. Equilibrium is reached when the long term averages of negative and positive charging effects are equal. Variations in positive ion charging cause excessive shifts in this equilibrium state when operation at low decay rates is attempted and in practice, with most tubes, the practical lower limit for the decay rate is that which gives a persistence of only about 30-60 seconds. If a lower decay rate is used the defect termed charge spreading (blurring of the stored image) is apt to occur as a result of the locally increased rate of positive-ion charging, which may reach a value great enough to over-ride the decay action. The persistence time of 3060 seconds is, however, insufficient in many cases e.g. in the case of a surveillance radar, where, as already stated, a radar display with a persistence time of 2 minutes or more is commonly required.

Again, with known methods of continuous operation the decay pulses tend to degrade the black level of the viewed image, for the screen flashes to full white during each pulse. In practice this defect may not be serious because of the low duty ratio and high repetition rate of the decay pulses usually employed. However, even if not unacceptably serious in itself, it does restrict the decay pulses which can be used. Although, for other reasons, the use of a low amplitude high duty-ratio decay pulse train might be preferred, the greater degradation of black produced by such a train as compared with a high amplitude, low duty-ratio decay pulse train may prevent the adoption of the former. The present invention seeks to overcome the foregoing defects and limitations.

According to the invention in its broadest aspect, the flood beam of a direct viewing storage tube having decay pulses applied to its target backing electrode to obtain continuous operation of the tube is pulsed.

In one way of carrying out the invention successively occurring time periods during which the flood beam is effectively pulsed on are employed for the decay phase and for the viewing phase of operation, said phases alternating with one another.

According to a feature of this invention apparatus for continuously operating a direct viewing storage tube comprises means for writing information in the form of a charge image on the storage target of the tube, means for applying periodic decay pulses to said target to attenuate the charge images stored thereon and means for translating the said charge images into visible images on the viewing screen of the tube in pulsed fashion by means of a pulsed flood beam which intermittently floods the target and passes therethrough to said screen.

The duty-ratio of the pluse flood beam (i.e. the pulse duration for a given pulse repetition frequency) may be adjustable to provide brightness control.

By means of the invention it is possible to reduce the level of decayerase to one corresponding to a persistence time several times as long as that which would be permissible when using the customary known method of continuous operation without causing serious charge spreading or blurring of the charge image. Thus if the flood beam is pulsed with a duty ratio of (say) 1 to 10, charge spreading, as compared to that which occurs when the flood beam is unmodulated, as in normal present day practice, is reduced by a factor of 10. Although the invention involves a comparative reduction of brightness of the viewed image (approximately in proportion to the duty-rates of the flood beam pulsing), in practice an image bright enough to be viewed in daylight is readily obtainable.

The flood gun conditions during on conditions of flood beam pulsing are, of course, those normally specified by the manufacturers for the tube considered. The effective extinction of the Hood beam between pulses can be effected in any of a variety of diflerent ways: for ex ample by reducing, in pulsed fashion, the potential of one or more of the normally provided collimating electrodes of the tube; or, in the case of tubes provided with a space-charge grid in front of the flood gun, by taking this grid to a negative potential; or by taking the normally provided backing electrode of the target to a potential of, say, to volts more negative than the normal value; or (equivalently) by taking the cathode of the flood gun to a potential of, say 5 to 10 volts more positive than the normal value. It will be appreciated that, in the last mentioned two cases, the flood beam will not be physically cut off but will be diverted to the collector mesh. From the viewpoint of the present invention, however, this may be regarded as the equivalent of cutting off the flood beam.

A further improvement is obtained if the E.H.T. applied to the screen of the tube is reduced to a low value or to zero, during a proportion of the flood beam pulses, all decay pulses being confined to periods of low E.H.T. Thus, for example, the E.H.T. may be varied so that it is a minimum at the times of alternate flood pulses and a maximum at the times of the remaining flood pulses. If this is done, degradation of the black level of the viewed image by decay pulses can be avoided altogether.

The decay pulses may be variable in amplitude and/or in duration.

The invention is illustrated in and further explained in connection 'with the accompanying drawings in which FIGURE 1 is a much simplified schematic representation of one embodiment of the invention and FIGURE 2 is an explantory graphical figure.

Referring to FIGURE 1 the tube therein represented is as known per se and comprises with an evacuated envelope 1 a fluorescent screen 2 on the large end wall of the envelope, a storage target or mesh 3, and a collector mesh 4. 5 represents a flood gun system for flooding the storage target with electrons to translate a stored electrical charge image thereon into a corresponding visible image on the screen 2. In the neck of the tube is a writing gun 6 for scanning the target 3 through the mesh 4 and producing stored charge images thereon in well known manner. The tube is provided, on the walls of its enlarged bulb portion, with the customary electrodes 7 for collimating the flood gun electrons.

In the illustrated embodiment the flood beam from the gun is pulsed by pulsing one of the collimating electrodes though any other method of pulsing the beam, such as those hereinbefore described, may be used. Block 8 represents a source of pulses each of which, when applied, permits the flood beam to flood the target 3 through the collector mesh. Between pulses the flood beam is efiectively cut off. The arrow on block 8 indicates that the pulses are of adjustable duration.

Block 9 represents a source of decay pulses which, as indicated by the arrow, may be adjustable in amplitude and/or duration, and which are applied to the target 3. There may be more than one train of applied decay pulses, e.g. a train of short duration high amplitude decay pulses and a train of longer duration lower amplitude decay pulses. Such trains could be obtained from a plurality of pulse generators such as generator 9, operating at different subharmonics of the frequency from generator 8.

Block 10 represents a source of periodically varying E.H.T. applied to the viewing screen 2.

FIGURE 2 illustrates the operation graphically. The

pulses in line (a) are two successive pulses from block 8 and represent periods when the flood beam is on. The dotted lines indicate that the pulse widths are adjustable. As will be seen the first of the two pulses of line (a) occurs in a decay phase of operation and the second in a viewing phase.

Line (b) of FIGURE 2 shows in full line a decay pulse applied from block 9. The decay pulses can be varied in amplitude, duration or timing (so long as they occur in decay phases) as is indicated by the dotted line pulse shown alongside the full line pulse in line (b) and, as already stated, a plurality of trains of decay pulses of different durations and amplitudes may be applied.

Line (0) shows the varying E.H.T. wave applied from block 10 to the screen. As will be seen the E.H.T. is zero, or approximately so, during the decay phase, and is of approximately normal working value during the viewing phase.

The periodicity of flood beam pulsing may be adjustable if desired but if it is the periodicity of the decay pulses should be adjusted therewith to ensure that the decay pulses and flood beam pulses occur in the correct relationship illustrated by FIGURE 2, i.e. the decay pulses must occur within the on period of the flood beam pulses.

I claim:

1. A direct viewing storage tube arrangement including a tube having a display producing viewing screen, a storage target, means for writing information on the target by the production of a charge image thereon, means for applying periodically recurring decay pulses to said target for attenuating the charge images stored thereon, means for applying a repetitively pulsed flood beam of electrons to said target to produce visible images on said viewing screen, and means for applying to said viewing screen a potential repetitively varying between a maximum potential corresponding in time to the production of images on said viewing screen by the flooding thereof and a minimum potential corresponding in time to the application of decay pulses to said target by said means for applying periodically recurring decay pulses.

2.The arrangement according to claim 1 wherein said means for applying periodically recurring decay pulses to said target comprises means for producing said decay pulses duringalternative applications of said repetitively pulsed flood beam to said target to provide visible image production during the remaining alternative applications of said repetitively pulsed flood beam to said target.

3. The arrangement according to claim 1 wherein said means for applying a repetitively pulsed flood beam is adjustable for altering the duty-ratio of said pulsed flood beam for controlling the brightness of the visible image.

4. The arrangemeent according to claim 1 wherein said means for applying a repetitively pulsed flood beam comprises means for periodically stopping the production of said flood beam.

5. The arrangement according to claim 1 further comprising a collector mesh within said tube, said means for applying a repetitively pulsed flood beam periodically providing diversion of said flood beam to said collector mesh.

6. The arrangement according to claim 1 wherein said means for applying a repetitively varying potential to said viewing screen comprises means for applying said mini mum potential and maximum potential during alternative applications of said pulsed flood beam to said target.

References Cited UNITED STATES PATENTS 2,931,938 4/1960 Patrick et al. 315-12 3,277,333 10/1966 Williams et a1. 3l512 RODNEY D. BENNETT, 111., Primary Examiner.

C. E. WANDS, Assistant Examiner.

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