US 3798502 A
Planar d.c. gas discharge shift registers are disclosed which comprise a single cathode and two sets of anodes to which voltages are applied in alternate clock phases. The shaping of the electrodes allows a glow discharge to be propagated unidirectionally down the shift register as the clock voltages are applied. The cathodes are of planar geometry and are disposed upon a planar substrate. In one embodiment both sets of anodes are on one side of the cathode, while in another configuration the two sets of anodes enclose the cathode. Another alternate embodiment features two sets of cathodes enclosing respective anodes. A "write" anode is also provided to initiate the flow discharge at one end of the shift register.
Description (OCR text may contain errors)
[451 Mar. 19, 1974 PLANAR GAS DISCHARGE SHIFT REGISTER Primary Examiner-Nathan Kaufman Attorney, Agent, or Firm-William Ryan  Inventor: Peter Dinh-Tuan Ngo, Colts Neck,
 ABSTRACT  Ass gnee el T leph n Laboratories, Planar d.c. gas discharge shift registers are disclosed Inc rp ray Hill, which comprise a single cathode and two sets of andes to which volta es are a lied in alternate clock 7 o 2 PP [221 Filed Dec 26 19 2 phases. The shaping of the electrodes allows a glow  Appl. No.: 318,557 discharge to be propagated unidirectionally down the shift register as the clock voltages are applied. The  Us Cl 315/169 TV 313/220 cathodes are of planar geometry and are disposed  6 37/00 upon a planar substrate. In one embodiment both sets  Fieid 315/169 TV of anodes are on one side of the cathode, while in another configuration the two sets of anodes enclose the  References Cited cathode. Another alternate embodiment features two sets of cathodes enclosing respective anodes. A UNITED STATES PATENTS write anode is also provided to initiate the flow dis- I De Koster et al charge at one end of the register 3.648.093 3/1972 Kupsky 313/217 16 Claims, 9 Drawing Figures l g in n l2 lO-l IO-2 IO-3 lO-6 lO- 7 10-8 PAIENIEUHARIS I974 3,798,502
' WRITE A GAS FILLED 1 PLANAR GAS DISCHARGE SHIFT REGISTER BACKGROUND OF THE INVENTION In the search for compact display devices which are economically attractive, devices based on utilization of gas discharges are becoming increasingly prominent. The physics of gas discharges is for the most part well understood. Moreover, display devices based on gas discharge technology can be fabricated by techniques which promise both compactness and economy.
Most gas discharge display devices in the prior art are con-figured in a matrixform. Half-select techniques are used for applying electrical signals to select particular matrix cells to be ignited. Conventional half-select matrix gas discharge displays are disclosed, for example, in US. Pat. No. 3,559,190, issued to D. L. Bitzer et al. on Jan. 26, 1971. Further examples of matrix displays are disclosed in US. Pat. No. 3,499,167, issued to T. C. Baker et al. on Mar. 3, 1970 and in US. Pat. No. 3,671,936, issued to D. Ngo on June 20, 1972.
The present invention relates to display devices based on principles other than those of the half-select matrix. In particular, principles relating to shift registers are exploited and adapted in an advantageous manner.
As background to the discussion of how shift register principles are applied to gas discharge display technology, it is well to note that there exist in the prior art, outside of gas discharge technology, display devices based on the concept of discrete display elements connected to form a linear shift register. For example, display devices of the rolling, or Times Square variety use a plurality of shift registers to propagate a display message linearly across a rectangular screen of incandescent lamps or other discrete display elements. An example of a shift register display using discrete display elements of modern technology is found in US. Pat. No. 3,651,493 issued to D. Ngo on Mar. 21, 1972. This last-mentioned patent discloses a display wherein solid state cells are connected in tandem via active transfer circuitry to provide a traveling display image. An ac bias is applied in common to all cells in a row to move the image through successive cells.
Gas discharge devices have been found to lend themselves readily to incorporation in devices using the general shift register concept. This fact underlies the application of gas discharge techniques in the counter tube art. For example, US. Pat. No. 2,575,370, issued to M. A. Townsend on Nov. 20, 1951, discloses a gas discharge counter tube wherein a plurality of cathodes are mounted adjacent a common anode. Different voltages are applied to alternate cathodes in such a manner as to propagate a glow discharge from cathode to cathode. Asymmetric cathode geometry is utilized to provide unidirectionality of discharge propagation.
These counter tube devices. are distinguished from the aforementioned display devices using discrete components in that their shifting properties are the result, not of special circuitry designed to provide shifting, but of the physical properties of the counter tube devices themselves. The shifting takes place between electrodes in a single gas-filled envelope, rather than from one discrete display element to another.
Shift register techniques somewhat similar to those in the counter tube art have been incorporated in the art relating to matrix display devices. For example, reference may be had to US. Pats. No. 3,631,530, issued to J. A. Ogle on Dec. 28, 1971, and U.S. Pat. No. 3,648,093, issued to G. A. Kupsky on Mar. 7, 1972, and also in The Primed Gas Discharge Cell A Cost and Capability Improvement for Gas Discharge Matrix Displays, by G. E. Holz, 1970 Idea Symposium Digas! of Technical Papers, Society for Information Display (May, 1970), pages 30-31. These references disclose an array with two levels of discharge cells with priming holes connecting corresponding cells on the two layers. The rear layer functions in accordance with shift register principles. It comprises a series of cathodes connected in groups of three to which three-phase driving voltages are applied. The scanned discharge in this rear layer serves to prime and select cells in the front layer by lowering their breakdown voltage. A set of anodes in the front layer is used to drive the selected cells at a controlled brightness. A primary disadvantage of the two layer configuration is the necessity to maintain strict spacing requirements between the two layers in order to achieve adequate operating margins. Another disadvantage is the complexity introduced by the reguirement for a three-phase driving voltage in the rear layer.
The present invention relates particularly to gas discharge shift registers of planar geometry. The electrodes of the shift registers are planar, and they are mounted on a single planar substrate. The planar geometry of the present invention lends itself to fabrication by planar thin or thick film techniques, made economical by modern technology. Since only one planar level is required, the strict interlevel spacing requirements mentioned in the previous paragraph are not present in this invention.
Another shortcoming of most prior art gas discharge display devices is the complexity of the driving circuitry which must be used to address the various cells. The cells in prior art displays are typically subject to random access by half-select matrix techniques. Such techniques require relatively complex driving circuitry. The present invention, in contrast, requires driving circuitry of minimal complexity. The reason for this is that in the present invention, a group of shift registers may be assembled to construct rolling or Time Square displays wherein only each shift register must be addressed, rather than each individual display cell, as in prior art randomly accessed displays.
SUMMARY OF THE INVENTION In the present invention, a single cathode and two 2 sets of anodes are formed, typically by planar thin film or thick film techniques, on a planar substrate. The anodesare disposed adjacent the cathode, which is typically linear, so that anodes from alternate ones of the two sets are encountered as the length of the cathode is traversed. In one embodiment, the two sets of anodes are disposed on opposite sides of the cathode, while in another embodiment, the two sets of anodes are interleaved along the same side of the cathode. Still another embodiment features cathodes disposed on opposite sides of respective anodes.
lllustratively, each anode has a symmetrical shape, in part defining a pickup" tip and in part defining a stable discharge tip. These two tips are separated along the linear dimension of the cathode. This electrode configuration is utilized to initiate a discharge in an unstable path between the pickup tip of a given anode and the cathode due to glow priming. Due to the instability, equilibrium field forces transfer the discharge, once it has been initiated, to a path between the stable dis charge tip of the given anode and the cathode. This latter discharge path is a stable one wherein the discharge can be maintained for any desired length of time.
This electrode configuration, with unstable and stable discharge paths, forms the basis for the shifting features of the present invention. Two-level clock voltages of alternate phases are applied to the two sets of electrodes. The high level of these clock voltages is advantageously chosen to be below the threshold breakdown potential for the paths between anodes and cathode. Thus the clock voltages, by themselves, have no effect. However, when an ancillary anode, called the write anode, is used to initiate a discharge near the pickup tip of a first anode, the effective breakdown potential between this pickup tip and the cathode is lowered to a level below the magnitude of the high" level of the applied clock voltages. Thus a discharge will be initiated between this pickup tip and the cathode. Due to the aforementioned instability, however, the discharge will immediately move to occupy a path between the stable discharge tip of this first anode, and the cathode.
The stable discharge tip of the first anode is adjacent the pickup tip of the next anode, which, as previously mentioned, has applied to it a clock voltage having a phase opposite to that applied to the first anode. Thus when the clock voltage applied to the first anode assumes its low level, a clock voltage of opposite (high level) phase will be applied to the second anode. Accordingly, the discharge will not be extinguished, but will simply pass on to occupy a path between the stable discharge tip of the second anode and the cathode.
These discharge transitions recur so that a gas discharge is propagated from one anode to the next along the length of the cathode, as long as clock voltages are applied. The discharge gives rise to a glow spot in a region immediately above the cathode, so that the net visible effect when the shift register is viewed from above its plane is a glow spot moving along the cathode as the clock voltages are applied.
It is a feature of the present invention that its various embodiments may be fabricated on a single substrate. Accordingly, operating margins are easier to maintain than in matrix type devices of the prior art, for which rather strict tolerances as to spacing between layers of conductors are required.
It is another feature of this invention that its various embodiments have utility as building blocks for various types of displays, including rolling, or Times Square" displays, and also raster scan displays.
As will become apparent, an important feature of any type of display embodying the shift register devices of this invention is that the driving circuitry can be made quite simple, in contrast to that required, for example, in prior art randomly accessed matrix displays. F urthermore, display devices based on the present invention may be fabricated with a minimum of conductor crossovers, in contrast to the many required in prior art matrix displays.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a planar gas discharge shift register constructed according to the teachings of this inventron;
FIG. 2 is a timing charge showing a series of clock pulses chosen for application to the shift register of FIG. 1 to effect its operation;
FIG. 3 shows another planar gas discharge shift register constructed according to the teachings of this invention;
FIG. 4 is another view of the shift register of FIG. 3, illustrating the gas-filled envelope which surrounds the basic shift register elements;
FIG. 5 is a diagram illustrating variant clock voltages useful in illustrating the operation of the shift registers of FIGS. 1 and 3;
FIG. 6 is a view of a segment of an alternate shift register electrode configuration; and
FIG. 7 shows a section of a planar gas discharge shift register incorporating a dielectric layer between an anode and a cathode;
FIG. 8 shows a configuration of the substrate;
FIG. 9 shows a useful driving circuit.
DETAILED DESCRIPTION FIG. 1 shows a typical embodiment of a planar dc gas discharge shift register constructed according to the teachings of this invention. Cathode 5, write anode 9, and propagating anodes 10-1 through 10-8 are disposed upon planar substrate 1. This manner of construction may be achieved through familiar thin film techniques. Each propagating anode is seen to have a pickup tip 11 and a stable discharge tip 12. Cathode 5 is seen in the typical illustrative geometry to have a protruding region located opposite the stable discharge tip and pickup tip of each pair of adjacent propagating anodes. For example, the region 6 in FIG. 1 is seen to be opposite the stable tip 12 of propagating anode 10-1 and the pickup tip of anode 10-2. Alternate evenand odd-numbered ones of propagating anodes 10-1 through 10-8 are interconnected, the odd-numbered anodes being connected via phase 1 clock line 15, and the even-numbered anodes being connected via phase 2 clock line 16. The substrate and electrodes are encapsulated in a gas-filled envelope (not shown in FIG. 1) such as that discussed below with reference to FIG. 4.
FIG. 2 is a timing chart showing typical waveforms of voltages for application to the electrodes of the shift register of FIG. 1. The voltages are positive digital pulses occurring in regular intervals, or time slots. Time slots lettered a-k are indicated in FIG. 2. Cathode 5 of FIG. 1 is conveniently maintained at a fixed potential. The phase 1 clock voltage, shown in FIG. 2, waveform B, is applied to phase 1 clock line 15. Phase 2 clock voltage, shown in FIG. 2, waveform C, applied to phase 2 clock line 16. Phase 1 and phase 2 clock voltages are positive with respect to the fixed cathode potential and are typically equal in magnitude. FIG. 2, waveform A, shows a write pulse which is applied to write anode 9 to initiate the discharge.
The magnitude, relative to the fixed cathode voltage, of the high level of each of the phase 1 and phase 2 clock voltages shown in FIG. 2, waveforms B and C, is selected to be below the threshold breakdown potential for the paths between the propagating anodes 10-1 through -8 an cathode 5. Thus, the phase 1 and phase 2 voltages when applied to lines and 16 will not alone be sufficient to cause a discharge between the propagating anodes and the cathode. The magnitude of the write pulse shown in time slot 0 of FIG. 2, waveform A, is, however, sufficient to causea breakdown. A discharge is thus created between cathode 5 and the tip 13 of write anode 9 in time slot c. As a result of this discharge, a cathode glow spot will be present above the plane of cathode 5 in the region opposite the tip 13 of write anode 9. The location of this glow spot is indicated by dotted circle 7. Free electrons and ions in the space charge of this glow spot will circulate in the area adjacent pickup tip 11 of propagating anode 10-1 and will thereby lower the breakdown potential between this pickup tip and cathode 5. Accordingly, in time slot d, when the phase 1 clock voltage is applied to propagating anode 10-1 and the write pulse is extinguished, the glow spot will be sustained as a result of a discharge created between pickup tip 11 of propagating anode 10-1 and cathode 5.
The effect which makes possible a discharge between electrodes to which a voltage below the normal breakdown potential is applied is known as priming. It is said that the free electrons generated in the discharge between tip 13 of write anode 9 and cathode 5 of the circuit shown in FIG. 1 prime the discharge location between pickup tip 11 of propagating anode 10-1 and cathode 5, thereby allowing a discharge to be initiated between these latter two electrodes when an otherwise insufficient voltage is applied.
This particular discharge location is unstable, however, due to the fact that a higher electric field strength, and hence a higher possible current density, exists between stable discharge tip 12 of propagating anode 10-1 and cathode 5 than is possible between pickup tip 11 of that anode and cathode 5. This is the case since the stable discharge tip 12 is nearer cathode 5 than is pickup tip 11. Accordingly, the glow discharge rapidly shifts during time slot d to a path between stable discharge tip 12 of propagating anode 10-1 and cathode 5,-and the glow spot will consequently shift to a region above cathode 5 adjacent stable discharge tip 12 of propagating anode 10-1. The location of this'glow spot is indicated by dotted circle 8. The glow spot will be maintained in this position for the duration of time slot d.
At the beginning of time slot e, the voltage applied to propagating anode 10-1 is removed, and the phase 2 clock voltage applied to propagating anode 10-2 assumes its high state. As this occurs, the glow spot moves to a region above cathode 5 adjacent the pickup tip of propagating anode 10-2 and then instantaneously to a region above cathode 5 adjacent the stable discharge tip of propagating anode 10-2. The glow spot will be maintained in this position for the duration of time slot e.
At the beginning of time slot f, the high" voltage applied to propagating anode 10-2 is removed, and the high" level of the phase 1 clock voltage is applied to propagating anode 10-3. As this occurs, the glow spot moves to a region above cathode 5 adjacent the pickup tip of propagating anode 10-3. The glow spot will then instantaneously move to the more stable region above cathode 5 adjacent the stable discharge tip of propagating anode 10-3. The glow spot will be maintained in this region for the remainder of time slot f.
It may be noted that at the beginning of time slot f, when the high level of the phase 1 clock voltage was applied to propagating anode 10-3 as described above, that the same high level of the phase 1 clock voltage was also applied to propagating anode 10-1. (All oddnumbered propagating anodes are connected to phase 1 clock line 15.) It may be asked, therefore, why the glow spot, previously in a location opposite propagating anode 10-2, moved to a location opposite propagating anode 10-3 instead of a location opposite propagating anode 10-1. The directional preference is due to the asymmetry in the propagating anodes. The pickup tip of propagating anode 10-3 is much nearer the location of the stable discharge and glow spot which occurred in time slot e than is the stable discharge tip of propagating .anode 10-1. It is much easier for the glow spot to traverse the shorter distance to the region adjacent the pickup tip of propagating anode 10-3 at the beginning of time slot f. This is particularly the case since the priming effect decreases very sharply as the distance from the glow spot increases.
This technique for achieving unidirectionality of discharge propagating through the use of two phase driving voltages and asymmetric electrodes is similar to techniques employed for the same purpose in the counter tube art. As is known from that art, if it is desired to use anodes which are symmetrical, driving voltages of three, or a greater number, of phases must be employed.
In time slot g of FIG. 2 the glow spot will move to the cathode region opposite the stable discharge tip of propagating anode 10-4. In time slots h through k the glow spot will move successively to cathode regions opposite propagating anodes 10-5 through 10-8. It is clear that as many propagating anodes as are desired may be added to the illustrative embodiment shown in FIG. 1. As long as alternate clock phase voltages are applied to alternate anodes, the discharge will continue to be propagated unidirectionally down the length of the shift register.
Although the discharge paths in the shift register of FIG. 1 are paths between the cathode 5 and the several propagating anodes 111-1 through 10-8, it should be remembered that the visible glow is not uniform throughout the length of these discharge paths. Rather the glow is concentrated near the cathode. (Hence the wellknown terminology, cathode glow.) The visible result of the operation of the shift register of FIG. 1 is therefore a glow spot which moves from position to position along the linear cathode 5. The exact shape of the glow spot will vary, depending upon the exact shape of the electrodes and the current. At higher current, the glow spot will depart from circularity and tend toward elongation in the direction of the adjacent propagating anode. This phenomenon is referred to as glow spread.
FIG. 3 illustrates a shift register with two sets of propagating anodes on opposite sides of a cathode. It is seen that with this shift register geometry it is possible to in terconnect alternate propagating anodes by metallization directly on the substrate without conductor crossovers. (This is not possible with the structure of FIG. 1.) For this reason the shift register of FIG. 3 will be more desirable than that of FIG. 1 whenever it is desired to fabricate a display device with several like shift registers closely spaced and parallel to each other. In that case, connections can be made at the ends of each shift register without any required conductor crossovers.
The shift register of FIG. 3 is designed to be enclosed in a gas-filled envelope such as that discussed below in conjunction with FIG. 4.
The shift register of FIG. 3 is seen to comprise a lower set of propagating anodes 21, 23, 25, and 27, each with a pickup tip and a stable discharge tip (e.g., pickup tip 41 and stable discharge tip 31 of propagating anode 21), connected by conductor 51. An upper set of similar propagating anodes 22, 24, 26 and 28 is connected by conductor 52. A write anode 55 is provided for initiation of a discharge at the left end of the register.
The time charts of FIG. 2, referred to previously with reference to the shift register of FIG. 1, may also be used to explain the operation of the shift register of FIG. 3, since the actual operation of the two shift registers is similar. Phase 1 clock voltage, shown in FIG. 2, waveform B, applied to the lower set of propagating anodes via conductor 51. Phase 2 clock voltage, shown in FIG. 2, waveform C, applied to the upper set of propagating anodes via conductor 52. Cathode 20 is maintained at a fixed potential. A write pulse, as shown in time slot of FIG. 2, waveform A, is applied to write anode 55 when it is desired to initiate a discharge at the left end of the register.
The phase 1 and phase 2 clock voltages shown in FIG. 2 each are selected, as before, to be below the threshold breakdown potential for discharge between each of propagating anodes 21 through 28 and cathode 20. The write pulse in time slot c in FIG. 2 is, on the other hand, sufficient to initiate a discharge between write anode 55 and cathode 20. The discharge thus initiated remains for the duration of time slot 6, giving rise to a glow spot above cathode in the region opposite write anode 55.
Free electrons from the discharge between write anode 55 and cathode 20 prime the region between pickup tip 41 of propagating anode 21 and cathode 20, so that at the beginning of time slot d, when the high level of the phase 1 clock voltage is applied to propagating anode 21, a discharge will take place between pickup tip 41 and cathode 20. It may be noted that this requires that the discharge jump" across cathode 20. However, there is no great obstacle to prevent this from taking place. The gaseous medium through which the discharge must jump completely surrounds the upper surface of cathode 20, as will become clear from the discussion of FIG.-4.
The discharge between pickup tip 41 of propagating anode 21 and cathode 20 is not stable, however. Therefore, the current, field interaction and the absence of a glow from the previous cell will force the discharge to instantaneously jump to an equilibrium path between stable discharge tip 31 of propagating anode 21 and cathode 20. The discharge will remain there for the duration of time slot d, giving rise to a glow spot above cathode 20 opposite stable discharge tip 31.
At the beginning of time slot e, the phase 1 clock voltage on propagating anode 21 goes to zero, and the high" level of the phase 2 clock voltage is applied to propagating anode 22. As a result of the priming effect, a discharge commences which occupies a new path between pickup tip 42 and cathode 20. This discharge path is unstable, however, for the same reasons cited in the previous paragraph with reference to the instability of the discharge path between pickup tip 41 and cathode 20. The discharge will then instantaneously move to occupy a path between stable discharge tip 32 of propagating anode 22 and cathode 20 as the glow from the discharge at the previous cell extinguishes. The discharge will remain in this location for the duration of time slot 6, giving rise to a glow spot above cathode 20 opposite stable discharge tip 32.
In succeeding time slots f through k, the discharge path will shift from the stable discharge tip of one propagating anode to the next, via instantaneous intermediate stops at the pickup tips. Unidirectionality of discharge propagation is achieved, as before, by means of the asymmetric electrode geometry. That is, it is much easier, when the clock voltages shift, for a discharge to shift the relatively short distance from a stable discharge tip of one propagating anode across the cathode, to the pickup tip of the next-numbered propagating anode (which is subjected to a strong priming effect), than it is for that discharge to traverse the much longer jump across the cathode to the stable discharge tip of the previous-numbered anode (which is subjected to a very weak priming effect).
The net visible effect of the unidirectional discharge propagation is quite similar to that indicated previously for the shift register geometry of FIG. 1. What will be observed is a glow spot which moves linearly from position to position along cathode 20, each position being opposite a stable discharge tip of a particular propagating anode.
The shift registers of FIGS. 1 and 3 are, of course, designed to be operated in a gaseous environment. FIG. 4 shows a gas discharge shift register of the type shown in FIG. 3 enclosed in a gas envelope. The view is a cross section, looking at the edge of the shift register. Substrate 60 is typically made of glass or ceramic material and forms the bottom portion of the envelope. Cathode and anode metallization 62 is formed upon the upper surface of substrate 60. Upper envelope 61, advantageously made of glass, is joined to the substrate to form an integral envelope. Leads 63 and 64 pass through the glass envelope to provide electrical contact between the shift register and external power sources. The envelope comprising substrate 60 and upper envelope 61 is filled with a gas mixture of composition and pressure suitable for maintaining a glow discharge between electrodes of the shift register when voltages are applied to leads 63 and 64. The gas pressure is determined by the spacing between the electrodes and the desired range of operating voltages. For a listing of various typical gas mixtures and their pressure-determined breakdown potentials, reference may be had to G. F. Weston, Cold Cathode Glow Discharge Tubes, London, FLIF F E Books, Ltd. (1968), pp. 291ff. The composition of the gas mixture chosen may be determined by the color and intensity of the display desired as well as by the desired breakdown voltage and speed. A typical gas mixture of general utility is that of 99 percent Ne and 1 percent Ar.
Of course, it is not necessary that the substrate be made of glass. Instead, it may be made of any other dielectric material suitable for bonding to a glass or other transparent upper envelope. Altemately the substrate itself may not be part of an integral envelope, but may be suspended by means of spacers in a separate glass envelope in accordance with techniques well known in vacuum tube technology.
As mentioned previously, gas discharge shift registers such as those shown in FIGS. 1 and 3 can be economically fabricated using thin film techniques. In mass production of such devices, it is important that adequate operating marginsbe maintained. A brief analysis will reveal that the distance between a propagating anode (as 9 F 9.1 9913f G. ns ihesa hqds (e.g., 5 of FIG. 1 or of FIG. 3) ofa planar shiftregister is not a highly critical dimension. The priming effect in such a shift register is proportional to the coupling of the field lines of the primed" anode (i.e., the one to which the high level of a clock voltage is just ap plied at the beginning of a particular time slot) with the space charge region of the primer anode (i.e., the adjacent anode which has its clock voltage reduced to zero at the beginning of the same time slot). Since the space charge region is most densely concentrated near the cathode, an increase in the distance between the primed anode and the cathode will result in a spreading of the field lines from the primed anode outward so that more of the space charge region opposite the adjacent anode is intercepted by these field lines. This effect tends to increase the coupling, and thus the priming effect. At the same time, however, the increased distance results in a weakening of the field strength between the primed anode and the cathode. This effect tends to decrease the net coupling and priming effect. These two phenomena thus tend to have opposite influences on the overall priming effect. Although they may not exactly cancel, they do tend to counteract one another, and thus significantly reduce the effect of such a change in electrode spacing. For this reason, dimensional tolerances do not significantly influence operating margins.
It is worthwhile at this point to make some observations regarding the clock pulses to be applied to the gas discharge shift registers. The clock pulses discussed with reference to FIG} have a5 pe r ce nt duty cycle. That is to say each phase is on for a length of time equal to that in which it is off. It is clear that, although a 50 percent duty cycle is a useful value, tflgqg pulses having other duty cycles may be used as well. FIG. 5 shows segments of clock pulses having three different duty cycles. Waveforms A in FIG. 5 show a clock pulse segment with a 50 percent duty cyclesimi; lar to that shown in FIG. 2. Waveforms B in FIG. 5 show a clock pulse semgent having a duty cycle greater than 50 percent. Here the phase 1 and phase 2 clock pulses overlap. The overlap of clock pulses make it less likely that a glow spot may accidentally be extinguished before it can be picked up and transferred to the next propagating anode in its path. An excessive overlap, however, may cause the glow spot to become stuck at a particular propagating anode.
FIG. 5, waveform C, shows a clock pulse segment having a duty cycle less than 50 percent. Here there is a nonzero time lag after the extinction of a clock pulse of one phase before the next clock pulse of alternate phase is applied. This time lag insures that the glow spot will not become stuck. Too large a time lag (too small a duty cycle) on the other hand, will allow the glow spot to become completely extinguished during the time lag. If this occurs, the discharge will not then be reignited at the occurrence of the next clock pulse, since the magnitudes of the clock pulses are below the threshold breakdown potential in the absence of a priming discharge. These considerations must be kept in mind when selecting an appropriate duty cycle for the clock pulse.
The clock pulse rates are dependent upon the speed at which it is desired to propagate the glow spot along the shift register. Maximum rates will depend upon several factors, including the gas pressure and composition, the magnitude of the applied clock voltages and the spacing between adjacent propagating anodes.
It will be clear to those skilled in the art that modifications may be made to the basic shift register geometries discussed with reference to FIGS. 1 and 3. For example, FIG. 6 shows a segment of a shift register with a modified cathode geometry. Apart from the modified cathode with its angular sections 76 and 77, this shift register is quite similar to that discussed with ref erence to FIG. 3. The angular sections 76 and 77 allow a glow spot to protrude further on the sides of the cathode closest to the pickup tips 72 and 73 than does the cathode geometry of the shift register of FIG. 3. Thus the design of FIG. 6 tends to optimize the glow discharge coupling from stable anode tip to pickup tip. The angle 6 is chosen small enough to achieve a gradual transition in the spacing between the anodes and the cathodes in the vicinity of sections 76 and 77. This prevents instability in discharge propagation by minimizing shifts in the device load line which may occur when the discharge jumps from one propagating anode to the next.
Typical values for linear dimenions are A 0.01 inches, B 0.012 through 0.015 inches, and D 0.016 inches. Other geometry may be used, of course, depending on gas pressure and composition as well as desired operating characteristics.
Another class of modifications to the presently disclosed shift register designs involves the interchange of cathode and anode functions. In the designs previously mentioned the cathode has taken the form of a single linear electrode except as shown in FIG. 6. Thus in the operation of these shift registers, the glow spot has moved along this single essentially linear electrode. In some cases however it may be desirable to have the position of the glow spot move between different electrodes. One way in which this may be accomplished is to interchange the roles of the cathodes and anodes in the shift registers discussed with reference to FIGS. 1 and 3. If this is done, the single cathode in each of these shift registers becomes a single anode and the propagating and write anodes become propagating and write cathodes, respectively.
In fact, such an interchange of roles may be accomplished by simply reversing the polarity of the clock pulses which are applied to the various electrodes. It will readily be seen that, if this is done, the operation of the modified shift register is actually analogous to that discussed previously, with the exception that the visible glow spot now occurs in regions above the several cathodes, instead of above a single cathode as before. If such an interchange were effected the shift register of FIG. 3, for example, could be used to provide, on one side of the central anode, direct access to the glow spot in alternate time slots.
Still another modification of the basic shift register geometries is illustrated in FIG. 7. There, a small section of substrate 79 is shown together with an anode stable discharge tip 80 and a cathode 81. The modification takes the form of a raised dielectric layer 82 formed on the substrate between anode stable discharge tip 80 and cathode 81. The function of this dielectric layer is to prevent electrode edge burning. The dielectric layer accomplishes this by trapping the very high field lines which are closest to the plane of the substrate and preventing electrons and ions from moving along these very high field lines and thus bombarding the edges of the electrodes. Since the electrons and ions cannot flow along the lines of highest field strength, they are constrained to move along field lines which curve above the dielectric layer. These lines are of smaller electric field strength. Moreover, field lines curving above the dielectric layer tend to terminate on the surface of electrodes 80 and 81 instead of on the electrode edges. Therefore, the electron and ion bombardment is distributed over a larger area, thus minimizing electrode edge burning. Typical thicknesses for dielectric layer 82 are in the range of 1 mil (perpendicular to the substrate plane).
Although the techniques mentioned above as useful in fabricating shift registers in accordance with the present invention were said to include well-known thin film techniques, such thin film techniques are by no means exclusive. Thus in particular, so-called thickfilm techniques described, for example, in Thick Film Materials for Electro-Optical Applications, by S. J. Stein, Proc. 1972 Electronic Components Conference, Washington, D. C., May -17, 1972, and other references cited therein.
A useful extension of the earlier fabrication techniques that is particularly applicable to the thick film approach is the construction of islands" for isolating the conductors (cathodes, anodes, write electrodes and interconnecting circuitry) from the substrate. These islands typically assume the form of dielectric thick film layers underlying the actual electrodes. This structure, shown in FIG. 8 has the advantage that cathode 86 and anode 87 are elevated from substrate 85 by dielectric layers 88 and 89, thereby decreasing the likelihood of shorting between cathode and anode due to the accumulation of sputtered metal 90 accumulating between the electrodes. Additionally, a more uniform field across the glow zone between electrodes results from this arrangement. This increased uniformity results from the tapered electrodes (as shown in FIG. 8) which may be realized using thick films. This geometry and resultant field shaping also improves the efficiency of operation; fewer field lines intercept the substrate and less capacitive loading results.
A useful driving (clock) circuit technique will now be described in connection with FIG. 9 which improves operating margins. FIG. 9 shows a battery 90 acting in concert with controlled voltage sources 91 and 92 (supplying voltages e p 1 and e a to supply phase 1 and phase 2 currents, i 1 and i a to drive terminals 95 and 96 by way of respective resistors R v 1 and R identified as 93 and 94, respectively. Also shown in FIG. 9 is a common cathode resistor R The resulting phase 1 and phase 2 voltages may be taken as those shown in FIG. 2 as waveforms B and C, respectively.
The resulting operation of the circuits of FIGS. 3 and 9 will now be discussed for the case where the output terminals 95, 96 and 97 in FIG. 9 are applied to respective electrodes 51, 52 and in FIG. 3. During time slot d, a stable discharge exists between cathode 20 and stable anode tip 31. The sustain voltage may be readily identified as When the phase 2 clock assumes a positive level during time slot e as shown in FIG. 2, the voltage at the stable tip of anode 22 is since the discharge at anode 21 has not yet ceased, and that at anode 22 has not yet begun. This is higher than s y i v i w 2- When, however the breakdown commences at anode 22, the voltage at anode 21 is reduced by i a R thereby falling below the required sustaining voltage. This causes the discharge at anode 21 to terminate. The existence of these signal conditions further represses any tendency of the shifting to occur in the reverse direction.
Several shift register configurations and modifications have been discussed. There are many others which will no doubt occur to those skilled in the art. Clock pulses having more than two levels may be useful. For example, it might be desirable to have each high level of clock voltage followed by a negative voltage to aid in stabilizing the shifting of the discharge. Also clock voltages of other than rectangular shape may be useful.
Another useful modification might include the use of a nonplanar (e.g., cylindrical) substrate with electrodes lying flat on the surface of the substrate. Also it may be desirable to construct a shift register which is not linear, but curved. All of these modifications will be seen to be encompassed by the present invention. Accordingly, the present invention is intended to be limited only by the scope and spirit of the appended claims.
Although the detailed discussion proceeded above in terms of the propagation of a single flow discharge, it should be understood that any number of write pulses may be applied in respective time slots, thereby causing any desired pattern of discharges to propagate along the electrodes.
What is claimed is:
1. Apparatus for propagating a gas discharge glow spot in accordance with first and second clock signals of alternate first and second phases, comprising:
a planar substrate,
a substantially planar electrode of a first polarity disposed in the plane of said substrate,
first and second interconnected groups of substantially planar electrodes of a second polarity disposed adjacent said electrode of a first polarity, one of the electrodes of a second polarity of said first group being located between each pair of successive electrodes of a second polarity of said second group, and one of the electrodes of a second polarity of said second group being located between each pair of successive electrodes of a second polarity of said first group, each of said electrodes of a second polarity of said first and second groups having a pickup tip and a stable discharge p.
means for initiating a glow discharge,
means for applying said first clock signal to said first group of electrodes of a second polarity in said first phase, and means for applying said second clock signal to said second group of electrodes of a second polarity in said second phase, so as to transfer said glow discharge along said electrode of a first polarity between regions of said electrode of a first polarity adjacent the stable discharge tips of successive ones of said electrodes of a second polarity.
2. The apparatus of claim 1 wherein said electrode of a first polarity comprises a cathode, wherein said electrodes of a second polarity comprises anodes, wherein said cathode is maintained at a fixed potential, and wherein said first and second clock signals are positive with respect to said fixed potential.
3. The apparatus of claim 1 wherein said pickup tips of said electrodes of a second polarity are spaced apart from said electrode of a first polarity a distance greater than the distance said stable discharge tips of said electrodes of a second polarity are spaced apart from said electrode of a first polarity so that a higher field strength is created between said stable discharge tips and said electrode of a first polarity than is created between said pickup tips and said electrode of a first polarity.
4. The apparatus of claim 1 wherein said planar electrodes of a second polarity are disposed along a single side of said electrode of a first polarity, each of said electrodes of a second polarity being congruent upon translation with each of the remaining electrodes of a second polarity.
5. The apparatus of claim 1 wherein said first and second groups of planar electrodes of a second polarity are disposed along opposite sides of said electrode of a first polarity.
6. The apparatus of claim 1 wherein said means for initiating a glow discharge comprises a write electrode positioned adjacent the pickup tip of a first one of the electrodes of said first interconnected group.
7. The apparatus of claim 1 further comprising a substantially flat envelope enclosing said substrate and all of said electrodes, for confining therein a gas atmosphere susceptible of electrical breakdown.
8. The apparatus of claim 1 further comprising at least one raised dielectric layer positioned on said substrate between said electrode of a first polarity and each of said first and second groups of electrodes of a second polarity.
9. Apparatus for propagating a gaseous discharge comprising a substantially planar substrate;
interleaved first and second pluralities of metallization paths positioned on said substrate;
a unitary metallization path positioned on said substrate in spaced-apart relation to each of said paths in said first and second pluralities of metallization paths,
each of said paths in said first and second pluralities having both a region defining a stable discharge location between that path and said unitary metallization path and a region defining an unstable discharge location adjacent said stable discharge location;
means for alternately creating a potential difference between said first plurality of paths and said unitary path, and between said second plurality of paths and said unitary path,
so that a gaseous discharge present in a stable discharge location between one path in said first or second plurality and said unitary path is caused to propagate to successive unstable and stable discharge locations along a path defined by reference to said unitary path.
10. The apparatus of claim 9 wherein said first and second pluralities of metallization paths are disposed along opposite sides of said unitary metallization path, each of said paths in said first plurality defining said stable and unstable discharge locations being directly opposite regions of said paths in said second plurality defining said unstable and stable discharge locations, respectively.
11. Apparatus according to claim 9 further comprising an additional metallization path located adjacent one of said unstable discharge locations.
12. Apparatus according to claim 9 wherein said unitary path is formed so as to be closer to said first and second pluralities of paths at said stable locations than at said unstable locations.
13. Apparatus according to claim 9 further comprising a dielectric path on said substrate between said unitary path and said first and second pluralities of paths.
14. Apparatus according to claim 9 further comprising dielectric means intermediate each of said metallization paths and said substrate.
15. Apparatus according to claim 9 wherein said means for alternately creating a potential difference comprises first and second pulse generators and means for alternating applying said first and second pulse generators between said unitary metallization path and said first and second pluralities of metallization paths, respectively.
16. Apparatus according to claim 15 further comprising a common resistor interconnecting said unitary path and said first and second pulse sources.
UNITED STATES RATEN' I OFFICE CERTIFICATE OE CORRECTION Patent No. 3,79 ,5 I Dated March 1 9, 197M Inventor(s) Peter Dinh'luan Ngo It is certified that error appears in the above-identified patent and that said Letters Patent are herebycorrected as shqwn below:
Col. 2, line 63, change "a symmetrical" to --an asymnetriqel e.
Col. 3, line 67, after "many insert --crossover's. Col. 4, line 6, change-"charge" to -chart-; and
line 57, before apfalied" insert -is*.
Col. 9, line &7, change "semgerm" to ,segment-.
Col. 10, line '2, change "pulse to pulses.
Col. 13, line 7, change "anodes" to --anode--.
Signed and sealed this 24th day of September 1974.
MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents F (10459) USCOMM-DC 60376-P69 i U.5I GOVERHIENT PRINTING OFFICE 199 0-356-334.