GB2097994A - Method of operating storage tubes to compensate for positive ion charging - Google Patents

Abstract

In a method of operating a storage tube in bistable mode, after a positive charge pattern has been written on the freshly erased tube storage surface by bombarding the latter with electrons at a potential above the first cross-over potential, a continuous train of "maintain" voltage pulse signals is applied to the tube storage mesh structure backing electrode, the pulse voltage being of such amplitude that, during the pulse, written areas of the storage surface are above, and unwritten areas are below, the first cross-over potential; wherein in operation there is eventually achieved a dynamically stable condition of both written and unwritten areas in which the effect of positive ion charging is substantially or totally cancelled by the effect of flood beam charging.

Description

SPECIFICATION Storage tubes This invention concerns storage tubes. More specifically, the invention relates to the operation in bistable mode of cathode ray tubes of the variety known as direct view mesh type storage tubes.

A direct view mesh type storage tube is a cathode ray tube of the sort which comprises: an electric charge storage structure having a backing grid or mesh with an insulator on one side thereof; a "writing" gun adapted to scan the storage structure with a signal-modulated electron beam so as to produce on the insulator stored electric charges controlled by the signals; a "flood" gun adapted to cover the working area of the storage structure with a collimated, substantially uniform density, "flood" beam of electrons; and a fluorescent viewing screen positioned to receive those electrons of the flood beam which pass through the storage structure.

In operation the stored charge pattern on the storage structure determines whether or not flood beam electrons will pass through it at any particular point to reach the viewing screen on which, accordingly, there appears a visible picture determined by the signals. Tubes of this kind are hereinafter referred to simply as "storage tubes".

Storage tubes are described in some detail in an E.E.V. pamphlet entitled "Preamble: storage tubes" and dated August 1972, and so far as it relevant part of that description suitably modified, is reproduced below with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic transverse section through a typical storage tube; Figure 2 is a "composite" diagrammatic transverse section through part of the tube's screen end; Figure 3 is a graphical representation of the secondary emission characteristic of the storage section of the tube; Figure 4 is a graphical representation of the single pulse erasure method used with the tube; and Figure 5 is a graphical representation of the pulse train erasure method used with the tube.

The essential components of a typical storage tube are shown in Figure 1. A writing gun (generally 10) emits a writing beam (1 1 ) which beam is the same as the electron beam of a conventional cathode ray tube, and is similarly deflected (by deflection system 12), focussed (by focus system 13) and collimated (by collimation system 14). In addition to exciting the screen phosphor (15) directly, the writing beam deposits a charge pattern on the storage surface of a storage mesh structure (16). Two flood guns (as 17) emit low velocity flood beam electrons (18) which continuously approach the entire surface area of the storage mesh structure structure 1 6 but are transmitted only where a charge has been deposited by the writing beam 12.Flood electrons 1 8 which are transmitted are then accelerated by the screen voltage, and produce a visible, continuous image corresponding to the trace of the writing beam 13. The image can be erased in less than a second, leaving the screen clear, or a continuous train of short erase pulses may be used to give variable persistence with repetitive writing.

More comprehensive explanations of the behaviour of the storage mesh structure and of the operation of the tube, are as follows.

Storage tubes rely for their operation on the characteristics of the storage mesh structure 1 6, particularly the high leakage resistance and secondary emission characteristics of the storage surface. Figure 2 shows the arrangement of the storage section of the tube in schematic form. The backing electrode (21) is a fine metal mesh, typically having 250 or 500 lines per inch and an optical transmission of 30 to 60%. On one side of this mesh 21 , facing the electron guns (not shown in this Figure), is a thin layer (22) of a high quality dielectric material. The resistivity of this dielectric is very high so that adjacent positive and negative charges on the storage surface-the gun-side surface of the layer 22-are effectively isolated.

The storage mesh structure 1 6 behaves as a control grid to the flood beam 18. The maximum brightness of any area on the screen 1 5 depends on the screen and flood gun voltages, but intermediate brightness values result from modulation of the flood beam 1 8 by local storage surface potentials.

Figure 3 shows the secondary electron emission characteristic of the layer 22 forming the storage surface. With further reference to Figure 2, writing beam electrons (as 23) which land with energies between the first and second cross-over potentials result in the emission of secondary electrons (as 24) in greater numbers than those arriving (the secondary electrons 24 are attracted to the collector mesh 26 in both Figures 1 and 2), leaving a net positive charge on the layer 22 storage surface. Flood beam electrons (as 25) landing with energies below the first cross-over potential generate less secondary electrons and so produce a net negative charge.

During operation, the potential on any part of the storage surface is determined by four factors:a) Backing electrode voltage The storage surface, dielectric layer 22 and backing electrode 21 form a capacitance, so that changes in the backing electrode voltage are capacitatively coupled to the whole of the storage surface. The steady-state operating voltage of the backing electrode 21 is typically +2V.

b) Writing beam charging The cathode of the writing gun 10 is typically 1 50or negative, so that an area scanned by the writing beam 11 is bombarded by electrons 23 with an energy above the first cross-over potential, and so driven positive.

c) Flood beam charging An area which becomes positive with respect to the flood gun 1 7 cathode will attract flood beam electrons 25; provided the initial positive voltage is below the first cross-over potential the flood beam 18 will charge it negatively, back to zero (cathode potential).

d) Positive ion charging Since the storage mesh structure 16 is negative with respect to its surrounding, it will attract positive ions produced by collision of electrons with residual gas molecules within the tube and between the collector mesh and the screen. This positive ion current is present whenever the electron beams are operating, and tends to drive the whole storage surface less negative and eventually to zero-so that in the end flood beam electrons 25 can pass through the entire area of the storage mesh structure 1 6.

The writing gun 10 generates a high energy, narrow electron beam 11 which is focussed by the focus electrodes 1 3 to a spot on the storage mesh structure 16 and scanned thereover by the deflection system 12. Although the writing spot is small enough for good resolution, it is considerably larger than the apertures in the storage mesh structure 1 6. Where the writing beam 11 strikes the surface of the dielectric storage layer 22 the surface potential is driven in a positive direction by secondary electron emission, and that part of the mesh structure becomes more transparent to the flood beam 18.

The extent of this positive charging is normally limited by the flood beam to flood gun cathode potential, and in most applications a single scan of the writing beam is sufficient to drive the mesh to zero (fully bright) or intermediate half tone levels.

Once a trace has been written on the storage mesh structure 1 6 it is continuously displayed on the screen by transmitted flood electrons until erased. However, the time during which it can be clearly seen is limited by the positive ion charging mentioned earlier, which drives the whole storage surface slowly towards zero, resulting in a gradually increasing background illumination until the original trace is swamped and the whole screen is at saturation brightness. An erasure operation is then necessary before another trace can be written.

Erasure of the display may be carried out either in a single operation taking less than one second and known as manual, or single pulse, erasure, or over an extended period by pulse train erasure (variable persistence).

Single pulse erasure To erase the display completely in one operation, a positive low amplitude voltage pulse (of about 4V amplitude and one second duration) is first applied to the backing electrode 21 (in Figure 2). This pulse is capacitively coupled to the storage surface, where it produces an instantaneous corresponding (4V) increase in potential (see Figure 5) which enables flood beam electrons (25) to land, so charging the entire surface less positive, and within a few seconds, to zero. Accordingly, when the backing electrode 21 is then returned to +2V at the end of the erase pulse the storage surface is driven uniformly to -4V, which cuts off the flood beam and erases whatever is on the screen.

It should be noted that the erase pulse amplitude is such that, when applied, the storage surface potential is everywhere (in both written and unwritten areas) below the first cross-over potential. Thus, the attracted flood beam electrons do not result in a greater amount of secondary electron emission, and so do not cause the storage surface to be driven further positive.

The erase time can also be reduced by using a pulse of greater amplitude and controlled duration, and in this case the pulse duration determines the storage surface potential after erasure. Again, however, it should be noted that the pulse amplitude is such that, when applied, the storage surface potential is, in both written and un-written areas, below the first cross-over potential.

Pulse train erasure A train of positive pulses above visible flicker frequency applied to the backing electrode has a cumulative erasure effect on the written areas of the storage surface, causing them to be erased in discrete steps. The small amount of positive ion charging between erase pulses is cancelled out during the pulse (see Figure 5). The unwritten areas can be held below cut off if required by increasing the amplitude of the erase pulse train.

As so far described, it will be appreciated that a freshly erased storage tube has a blank screen because the potential on the storage layer is sufficiently negative (relative to the flood beam cathode) to "repel" all the flood beam electrodes, so that none penetrate the storage mesh structure, and thus non strike the screen. A "weak" writing beam-a rapidly scanning beam, or a beam of low intensitydnves the relevant areas of the storage surface slightly less negative, so that a few flood beam electrons can pass therethrough to cause a faint trace on the screen.

A "stronger" writing beam (or a succession of weak beams in the same place) drives the storage surface less negative again, possibly to zero potential relative to the flood beam cathode, whereupon more of, and eventually all of, the flood beam electrons pass through the area of the storage mesh structure, to produce a strong trace.

A "very strong" writing beam actually drives the storage surface positive relative to the flood beam cathode; instantly flood beam electrons both pass through the storage mesh structure (to cause a screen trace) and are attracted to the storage surface, so rapidly reducing its potential to zero, whereupon all the flood beam electrons pass through the storage mesh structure to cause a strong trace.

Storage tubes (and their associated circuitry) may be designed to operate either in "half-tone mode" in which the intensity of the formed trace is proportional to the strength of the original signal modulating the writing beam, or in "bistable mode", in which it is arranged that any original signal above a certain strength (usually zero or very close thereto) results in a maximum intensity trace, the remainder of the screen being fully dark. For many uses it would be convenient to have a tube system that could be used in both modes, as required, and much effort has been expended to construct such a system. Few, if any, have met with great success; the present invention seeks to provide a more satisfactory such system.

One problem met with storage tube systems of the half-tone variety is that of the trace life-the "memory" of the system-being seriously reduced by positive ion charging, which is briefly referred to hereinbefore. The matter may be explained in more detail as follows. All storage tubes, of whatever sort, inevitably contain minute quantities of gases (despite being highly evacuated) that, upon bombardment with electrons (as from the flood gun), give rise to appreciable amounts of positive ions. These positive ions are naturally attracted towards any negatively charged surface within the tube, and specifically any such positive ions formed between the screen and the collector grid (15 and 26 in Figure 1) are attracted to the negative "dark"-areas of the storage surface.As a result, these negative storage surface areas-the areas where the writting beam has not written (and including any "weakly" written areas that may still be below flood gun cathode potential)-are gradually driven less and less negative, so that flood beam electrons begin to pass therethrough causing the corresponding "dark" areas of the screen to become brighter and brighter.

Eventually, indeed, the screen areas which do not correspond to written storage surface areas become just as bright as those that do, so that the original trace is lost in the increased background illumination. The present invention seeks to provide a storage tube system capable of being operated in both half-tone and bistable mode, in which, when operated in the bistable mode, this screen background brightening is in theory-and, effectively, in practice-totally avoided.

In one aspect, therefore, this invention provides a method of operating a storage tube in bistable mode, in which method: after a positive charge pattern has been written on the freshly erased tube storage surface by bombarding the latter with electrons at a potential above the first cross-over potential, a continuous train of "maintain" voltage pulse signals is then applied to the tube storage mesh structure backing electrode, the pulse voltage being of such amplitude that during the pulse on written areas the storage surface is above, and on unwritten areas it is below, the first cross-over potential; whereby in operation there is eventually achieved a dynamically stable condition of both written and unwritten areas in which the effect of positive ion charging is substantially or totally cancelled by the effect of flood beam charging.

Operating the tube in accordance with the invention causes it eventually to achieve a dynamically stable condition where written areas are stabilised substantially at flood gun cathode potential and unwritten areas are stabilised at a negative potential of a valve substantially equal to the amplitude of the maintain pulses. Changes in potential caused by positive ion charging are cancelled by each successive maintain pulse.

The inventive bistable mode operation of a storage tube may be described as follows, with further reference to the accompanying drawings, in which: Figure 6 is a graphical representation of the voltages applied in accordance with the invention to the tube storage mesh structure backing electrode; Figure 7 is a graphical representation of the corresponding potentials on the unwritten areas of the storage surface; and Figure 8 is a graphical representation of the corresponding potentials on the written areas of the storage surface.

These Figures each show plots of voltage (Yaxis) against time (X-axis); the time axis is not to scale, but is comparable in each Figure.

In accordance with the method of the invention there is applied to the backing electrode of a written, freshly erased storage surface a "maintain" signal constituted by a train of voltage pulses each of which is of a magnitude such that it raises the potential of the backing electrode somewhat above that potential that would, on the storage surface, be the first cross-over potential.

The writing may be carried out in any convenient manner-for example, using a writing gun as described hereinbefore or by means of flood beam electrons accelerated to above the first crossover potential-and the surface may have been erased in any conventional manner-for example, by a single long erase pulse, as shown in Figure 6, of the type described hereinbefore with reference to Figure 4. Consider, now, the actual storage surface potentials in both unwritten and written areas Unwritten area storage surface potential A freshly erased storage surface potential is usually at, or just below, that negative potential constituting cut-off potential, so that (as shown in Figure 2) no flood beam electrons pass through the storage mesh structure to the screen, and the screen is dark.As time goes on, however, positive ion charging drives this potential less negative but before it goes too far there is applied to the storage mesh structure backing electrode the maintain signal pulses. These are of such an amplitude as to raise the unwritten area storage surface potential through zero to a value just below the first cross-over potential, so that during each pulse flood beam charging reduces the potential (eventually to zero) and between each pulse the potential instantaneously drops to a negative value (which is more negative than that of a normal freshly erased surface) which gradually gets less negative as a result of positive ion charging.As the train of maintain pulses continues, the unwritten area potential eventually "stabilises" such that it oscillates between zero (which would give a fully bright screen, but is so for such a relatively short time that it is effectively unnoticeable) and a value approximately as large negatively as the maintain pulses amplitude is positively (which gives a fully dark screen, and is so for such a relatively long time that the unwritten screen appears effectively constantly dark). It will be appreciated that the stable end state is such that the effects of positive ion charging are exactly balanced by the opposite effects of flood beam charging, so that once stabilized the unwritten storage surface areas remain effectively flood-beam-impenetrable, and the corresponding screen areas remain dark, indefinitely.

This, it will be seen, is very similar to the pulse train erasure method described hereinbefore with reference to Figure 5.

Written area storage surface potential Whereas in the unwritten areas of the storage surface the maintain signal pulses periodically raise the surface potential to just below the first cross-over potential, so that during each pulse flood beam charging gradually reduces the surface potential, in the written areas such is the maintain signal pulse amplitude that the surface potential-a combination of the potential caused by the writing beam with that caused by the main pulses raised just above the first cross-over potential, so that (by virtue of the increased secondary emission) flood beam electrons landing in these areas actually result in an increase in the surface potential.Moreover, whereas between pulses in the unwritten areas the potential instantaneously drops to a low negative value such that positive ion charging makes it less negative, between pulses in the written areas the potential (when dynamically stabilized) instantaneously drops to a low but positive value such that flood beam charging makes it less positive (and will normally reduce it to zero). Thus, throughout the entire period while the maintain signal is being applied the written areas of the storage surface are either at a positive or at zero potential, and are passing most or all of the flood beam electrons, so giving rise to a fully bright trace in the corresponding screen areas.

The general mode of operation of the method of the invention will be clear from the preceding comments, and certain preferred features of the method will now be explained.

By way of example, there is now described the application of the inventive method to the bistable mode operation of an E.E.V. E725 storage tube.

This tube is a high writing speed tube of the charge transfer variety (in which the signal is first written by the writing gun onto a first, very low capacitance, storage structure, and is then transferred to a second, high capacitance, storage structure nearer the screen either by further writing gun electrons or by flood gun electrons) and its operation in a half-tone mode is described in detail in the E725 data sheet of September 1980 (and also in the Complete Specification of E.E.V. British Letters Patent No. 896,544). Figures 6, 7 and 8 relate specifically to this example.

It is found that the first cross-over potential of the dielectric (magnesium oxide) used in the E726 tube is approximately 20V. To use the tube in a bistable mode all that is necessary is to apply a continuous train of maintain pulses to the high capacitance storage structure via its backing electrode after the charge pattern has been written or transferred onto the storage surface.

The backing electrode is normally operated at a potential of +0.5V with respect to flood gun cathode potential, and a 5V erase pulse is required to cut off the flood beam. The writing or charge transfer process increases the potential of the storage surface from the -5V erased potential to -2V with respect to flood gun cathode potential. As can be seen from Figure 6, the maintain pulses have an amplitude of 23V at a pulse repetition frequency of 250 Hz and a duty cycle of approximately 1% (that is, each pulse lasts for 1/99 of the time between pulses).

It will be seen from Figure 7 that on the unwritten areas the first maintain pulse will cause the storage surface to rise to + 1 8V (which is below the 1 st cross-over potential, 20V) and the surface will, therefore, charge negatively for the duration of the pulse. Each successive maintain pulse charges the surface negatively until the storage surface is at cathode potential during the pulse period. When this condition is attained, then immediately after a maintain pulse the storage surface is at -23V, and any positive ion charging during the pulse off period is cancelled by the next maintain pulse. A dynamically stable condition is therefore achieved where the background is held "dark" except during the maintain pulses.With 1% duty cycle pulses the mean background luminance is very low, and good contrast is achieved.

It will also be seen (from Figure 8) that on the written areas (which have been charged to -2V) the first maintain pulse raises the storage surface to +21 V (which is above the first cross over potential, 20V) and the surface charges positively, due to secondary electron emission, during the pulse period. Subsequent maintain pulses continue the positive charging processes until the storage surface potential is greater than +23V at the end of a maintain pulse (the 3rd one in the Figure). When this occurs the storage surface potential, immediately after the maintain pulse, is above cathode potential, and flood electrons land and charge the surface back to flood gun cathode potential before the next maintain pulse arrives.A stable condition is therefore reached where positive charging during a maintain pulse is cancelled by negative charging before the arrival of the next maintain pulse. The written areas are, therefore, maintained substantially at cathode potential, which gives maximum screen luminance and consequently very high contrast when compared with the background. For 1% duty cycle maintain pulses the contrast ratio is 100:1.

To achieve rapid stabilisation of the written and background areas it is sometimes advantageous to increase the duty cycle of the maintain pulses initially to approximately 10% to 20% and then reduce to 1% for viewing.

Storage times of several hours without image deterioration have been regularly achieved by this technique.

It should also be noted that this method of bistable operation does not limit the writing speed-as happens in other methods of bistable operation where the writing process has to produce large changes in the storage surface potential before bistable operation can be achieved. Also, by careful adjustment of the potentials it has proved possible to achieve bistable operation and contrast improvement by applying maintain pulses to a stored half-tone image where the background is not initially black.

On the E725 it has been possible to achieve bistable stored images at approximately twice the writting speed of half-tone stored images from a "dark" background.

Claims (7)

Claims

1. A method of operating a storage tube in bistable mode, in which method: after a positive charge pattern has been written on the freshly erased tube storage surface by bombarding the latter with electrons at a potential above the first cross-over potential, a continuous train of "maintain" voltage pulse signals is then applied to the tube storage mesh structure backing electrode, the pulse voltage being of such amplitude that during the pulse on written areas the storage surface is above, and on unwritten areas it is below, the first cross-over potential; whereby in operation there is eventually achieved a dynamically stable condition of both written and unwritten areas in which the effect of positive ion charging is substantially or totally cancelled by the effect of flood beam charging.

2. A method as claimed in Claim 1, in which the storage surface is erased by a single long erase pulse.

3. A method as claimed in either of the preceding claims, in which the writing of the positive charge pattern is carried out using a writing gun.

4. A method as claimed in any of the preceding claims, in which the maintain pulses generally have a pulse repetition frequency of 250 Hz and a duty cycle of 1%.

5. A method as claimed in any of the preceding claims, in which the duty cycle of the maintain pulses is initially 10% to 20%, and thereafter it is 1%.

6. A method as claimed in any of the preceding claims, in which the tube is a high writting speed tube of the charge transfer variety.

7. A method as claimed in any of the preceding claims and substantially as described hereinbefore.