US3230391A

US3230391A - Cryoelectric switching trees

Info

Publication number: US3230391A
Application number: US243427A
Authority: US
Inventors: Richard W Ahrons
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1962-12-10
Filing date: 1962-12-10
Publication date: 1966-01-18
Anticipated expiration: 1983-01-18

Description

Jan. 18, 1966 R. w. AHRONS 3,230,391

CRYOELECTRIC SWITCHING TREES Filed Dec. 10, 1962 2 Sheets-Sheet 1 l6 '8 o M2 Ac, III/l l4 i 4| PRIOR ART [/0 F| s.I 4o

DRIVE CURRENT A SOURCE 0/! 0/0 LEGEND m TIN [:1 LEAD FIG 2 l o o o 22,FF 2|\FF 2 FF PRlOR ART 34 35 36 7o 1 ooo TIN LEAD g1 00' 80%} J PRIOR ART OlO 4; 7i Fm 4 4 RT A 8l loo 1 as i 82 J I 76 no 4 9o 84 CURRENT JLL |u IZ SOURCE ''.1" 1 I I F l o o l 0 2 l 5 2 FF 2 FF 2 FF INVENTOR ard W Ahrons ATTORNEY Jan. 18, 1966 R. w. AHRONS CRYOELECTRIC SWITCHING TREES 2 Sheets-Sheet 2 Filed Dec. 10, 1962 mm vm momDom .PZMEEDO INVENTOR R hard WAhrons u U n g ozwwmj A TTORNE Y United States Patent 3,230,391 CRYOELECTRIC SWITCHING TREES Richard W. Ahrons, Somerville, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 10, 1962, Ser. No. 243,427 11 Claims. (Cl. 30788.5)

This invention relates to new and improved cryoelectric switching trees.

A desired path may be selected through a cryoelectric switching tree by placing impedances in series with all except the desired path. For example, in the case of cryotron trees, the gate electrodes of cryotrons in the nonselected paths are made normal (made to exhibit a finite value of resistance) and the selected path remains superconducting. A drive current applied to the tree steers into the superconductive path in preference to the other paths because it is the path of lowest (zero) resistance.

In prior art ciyoelectric switching trees, the resistance of each cryotron is the same. The present inventor has discovered that improved performance can be obtained by employing impedances of different values in different paths through a selection tree. The impedances effecting course selection (as, for example, selecting 64 paths out of 128) are made to exhibit a substantially higher value of impedance than the impedances effecting fine selection (as, for example, the impedances which effect the selection of one path out of two). The invention is applicable both to resistive (cryotron) switching trees and to inductive switching trees. In the former case, improved operating speed is obtained by following the teachings of this invention. In the latter type of trees, improved current distribution is obtained with the arrangement of the present invention.

The invention is discussed in greater detail below and is illustrated in the following drawings of which:

FIG. 1 is a schematic showing of a prior art cryotron;

FIG. 2 is a schematic drawing of a prior art cry otron selection tree;

FIG. 3 is a schematic drawing of a cryotron selection tree according to the present invention;

FIG. 4 is a schematic showing of the prior art inductive switch; and

FIG. 5 is a schematic showing of an inductive switching tree in accordance with the present invention.

In the discussion which follows, an environment is assumed in which superconductivity is possible. For example, the temperature is assumed to be only a few degrees Kelvin.

The cryotron of FIG. 1 includes a gate electrode formed of a soft superconductor such as tin and a control electrode 12 formed of a hard superconductor such as lead. Both electrodes may be in the form of thin films which are insulated from one another. The cryotron is located over a ground plane 14 which is insulated from the

electrodes

16 and 12.

In the operation of the cryotron, the gate electrode 10 may initially be in the superconducting state. In this condition, a drive current applied to input terminal 16 of insufficient magnitude to cause gate 10 to assume a normal (resistive) state sees a low impedance path through the gate 10. If, however, a control pulse is applied to terminal 18, the magnetic field produced by the control pulse passing through the control electrode is of sufiicient magnitude to drive the gate from its superconducting to its normal state. When this occurs, the gate presents a finite resistance to the drive current applied to terminal 16.

A prior art selection tree employing a cryotron is shown in FIG. 2. While in practice there may be many more than 8 paths for a drive current, for the sake of drawing simplicity, only 8 such paths are shown. These paths Patented Jan. 18, 1966 may be, for example, the X or Y drive lines for a superconductor memory (not shown). The selection tree also has a ground plane which is not shown.

The selection tree of FIG. 2 includes 14 cryotrons 20-33, respectively. The desired one of the 8 paths for the drive current is selected by selection currents applied to certain ones of these cryotrons. These currents may be applied by the flip-

flops

34, 35, 36.

In operation, assume that the 1 output terminals of the three flip-flops are active and the 0 output terminals are inactive (at ground potential). This causes the gate elec trodes of

cryotrons

20, 23, 25, 33, 31, 29 and 27 to become resistive. Accordingly, the only path of the 8 paths which remains superconducting in its entirety is the path lea-ding to 111 since the gate electrodes of

cryotrons

21, 22 and 26 all remain in the superconducting state. If, during the time the flip-flops are in the states indicated, a drive current pulse 37 from current source is applied to the convergent end 38 of the tree, the pulse will steer into the superconducting path, since it has zero resistance, in preference to the other paths. This is ind-icated schematically by the dashed arrow 42.

While the above is, in a rough way, how the cryotron selection tree operates, additional factors must be considered in determining the speed capability of the tree. The speed is influenced by the time required for the selection currents (the currents provided by the flip-flops) to drive the cryotrons in series with the unselected current paths normal, and the time required for the drive current to follow the selected (the superconducting) path after the cryotrons have been driven normal. It may be assumed for the purposes of this discussion that the time required to drive the cryotrons normal is so short that it may be neglected. If, when the cryotron in the undesired paths are normal, the drive current pulse 37 is applied to the convergent end 38 of the tree, it secs 8 different paths in parallel, each including an inductive component and some including also a resistive component. As the paths are substantially identical (from a physical viewpoint), the value of inductance in each path is substantially the same.

When the drive current pulse from source 40 is initially applied, its higher frequency components, that is, those components which together provide the steep leading edge of the pulse, see mainly the inductance of each path. Therefore, there arrives instantaneously at each of the 8 paths (legended 000 through 111) a current having an amplitude one-eighth that of the drive current pulse. Thereafter, due to the resistance present in the non-selected paths, the current applied to these nonselected paths decays (see waveform 41), and the drive current pulse steers into the selected path.

There is generally a considerable time At (see waveform 39) between the time t at which the leading edge of drive current pulse occurs and the time t when substantially the full drive current is delivered to the selected superconducting path. As the selection tree becomes larger, it can be shown that At increases. The equations demonstrating this are complex and are not given here. However, the calculated delay At for a 128 path cryotron switching tree similar to the tree of FIG. 2 is the order of 5 microseconds.

In the present invention, as applied to cryotron switch ing trees, the time At defined above is reduced substantially. In the case of 128 path trees, for example, the time At required by the tree of the invention (FIG. 3) is roughly /2 the time At required for a tree like the one of FIG. 2. The time reduction increases as the tree becomes larger. The reduction is achieved by essentially increasing the resistance in series with the nonselected paths through the tree. The increase in resistance causes the current present in each non-selected path due to the initial inductive current division more quick- 1y to decay and consequently the drive .current more quickly to steer out of each non-selected path and into the selected superconductor path. Again, the equations which demonstrate this are quite complex. However, from a qualitative viewpoint, one can consider that increasing the resistance R in series with a non-selected path causes a substantial decrease in the L/R time constant associated with that path, and this essentially speeds up the removal of current from that path.

While it would be possible to improve the operating speed of a cryotron switching tree by increasing uniformly the resistance of all gate electrodes, this is found not to be practical. Increasing the resistance of all gates can be achieved only by making the tree geometry more complex. For example, the tree geometry may require that the control electrodes not follow straight lines. Further, it may require that the spacing between the paths be increased because of the increased complexity of the switching elements. In either case, intricate masking would be required and, moverover, the increased control lead lengths and, in some cases, the zig-zag path they would be required to follow, would increase the time required for the selection pulses to reach the cryotrons in the tree. As is discussed in more detail later, the tree of the present invention is actually sirnplier to construct than the prior art tree of FIG. 2, as well as being faster than the tree of FIG. 2.

A 16 path cryotron switching tree according to the present invention is shown in FIG. 3. The tree includes a first group 50 of 15 cryotrons at the left side of the tree and a second group 52 of 15 cryotrons at the right side of the tree. The group 52 of cryotrons is essentially an inverted mirror image of the group 50 of cryotrons. A ground plane (not shown) may be placed beneath the tree. The control electrodes for the cryotrons, in practice, are as wide as the gate electrodes and are aligned with the gate electrodes, however, for the sake of drawing simplicity, the control electrodes are shown as single lines. The control electrodes are made of a hard superconductor such as lead and the gate electrodes are made of a soft superconductor such as tin.

The drive current pulse source is shown at 54 and is connected to the input end 56 of the tree. The control electrodes for the various cryotrons are connected to the 1 and output terminals of the four selection flip-flops 5861. The latter supply the selection currents.

It may be observed that the cryotrons employed are in-line cryotrons rather than crossed-field cryotrons. An in-line cryotron is one in which the control electrode is arranged parallel to the gate electrode, whereas a crossed-field cryotron is one in which the control electrode extends at right angles to the gate electrode. It may also be observed that the gate electrodes have a length which is related to the coarseness of the selection step. The coarser the selection, the greater the gate electrode length, and therefore, the larger the resistance introduced. In the tree illustrated, the 2 flip-- flop 61 produces the coarsest selection ste-p since the cryotrons it controls eliminate 8 of the 16 possible paths. Of the remaining 8 paths, the 2 flip-flop 60' eliminates 4, and so on.

Another feature of the cryotron selection tree of FIG. 3 is that the gate electrodes corresponding to a given selection bit are aligned. This makes the masking required for vacuum depositing the gate electrodes relatively simple, and also makes the masking required for the control eleqtrodes relatively simple as they have a straight geometry.

The configuration of the paths through the tree of FIG. 3 is also advantageous for a reason which is not so obvious. In practice, when one wishes to vacuumdeposit two lines which extend at right angles to one another as, for example, the lines 57 and 59 of the tree of FIG. 2, it is necessary to do this in two steps,

using a separate mask for each step, even though the two lines actually lie in the same plane. The difiicutly with using a single mask is that the inner corner of the opening in the mask is unsupported, and, especially when the lines such as 57 and 59 are long, may vibrate or move during the vacuum deposition process. This, of course, results in faulty patterns. With the arrangement of FIG. 3, all of the paths through the tree, that is, all of the portions of the tree made of lead, are straight lines which extend horizontally. Thus, all of these paths can be laid down through a single mask. In a similar manner, all of the gate electrodes (tin) lie on vertical lines and can be laid down through a single mask.

The operation of the system for FIG. 3 may perhaps be better understood by specific example. Assume that it is desired to select the line 1101. This corresponds to active 1, l, 0, 1 output terminals of the 2 2 2 and 2 flip-flops, respectively. The active 1 terminal of the 2 flip-flop causes the gate electrode of the cryotron 62 to go normal. This eliminates paths 0111 through 0000. The active 1 terminal of the 2 flip-flop causes the gate electrodes of cryotrons 64 and 66 to go normal. The normal gate 66 eliminates the four paths 1011 through 1000. The active 0 terminal of 2 flip-flop causes the gate electrodes of

cryotrons

68, 70, 72 and 74 to go normal. The normal gate 74 eliminates paths 1111 and 1110. The active 1 terminal of the 2 flip-flop causes the gate electrode of cryotron 76 and of all cryotrons aligned with cryotron 76 to go normal. The normal gate 76 eliminates path 1100. Therefore, the only path which remains superconducting is the path through line 1101, as indicated by dashed line 78.

In the operation of the selection tree of FIG. 3, the drive current pulse applied to input lead 56 initially divides equally among the 16 paths. It then decays from all of the non-selected paths and steers substantially entirely into the path 1101. The decay is greatly accelerated in the tree of the invention because of the increased resistance in the non-selected paths. For example, the last 8 paths include the very long gate electrode of the cryotron 62 (some of these 8 paths also include other normal gate electrodes) and this electrode has a relatively large resistance. In a similar manner, the paths 1011, 1010, 1001 and 1000 have in series the relatively long gate electrode of the cryotron 66 and so on.

A prior art inductive switching element is shown in FIG. 4. It includes a control element formed of a soft superconductor such as tin and a controlled element 72 which is preferably formed of a hard superconductor such as lead. When the control element 70 is in its superconducting state, the inductance exhibited by the controlled element 72 is relatively low due to the shielding elfect of the element 70. However, when the penetration of depth A of the element 70 is greatly increased as, for example, by driving element 70 into its intermediate or normal state, the inductance of the controlled element 72 very greatly increases. While not shown, the controlled element 70 may have a resistance of very low value in shunt therewith to permit element 70 to be driven into the intermediate rather than the normal state. (A more detailed discussion of the inductive switching element of FIG. 4 may be found in Appl. Ser. No. 195,- 462, filed May 17, 1962, by R. A. Gange and assigned to the same assignee as the present invention.)

The inductive switching element of FIG. 4 may be substituted for the cryotron switching elements of FIG. 2 to provide an inductive switching tree. In such a tree, all controlled elements are of the same length. When so employed, the controlled element '70 of the inductive switching elements in all except a desired path through the tree are placed in the intermediate or normal state. Thus, all except the desired path'through the tree exhibit a relatively large value of inductance. Accordingly, when a drive current pulse is applied to the input end of the switching tree, that pulse steers instantaneously into the desired path since its inductance is substantially smaller than that of the remaining paths. Actually, the division of current is in accordance with the inductances seen by the drive pulse. While this current division is instantaneous, there is no later redistribution of current as all paths are always superconductive. Therefore, if the inductance of a desired path is say one-tenth that of the inductance of all other paths combined, then ninetenths of the drive current will flow into the desired path and remain in the desired path.

An improved inductive switching tree in accordance with the present invention is shown in FIG. 5. In this tree, the inductive switching elements which control the coarse selection are made to have much longer controlled and control electrodes than the inductive switching elements which control the fine selection. The inductance L exhibited by a controlled element is a function of its length. The important advantage of this tree over the prior art inductive switching tree just described is that the drive current distribution will be such that a greater proportion of the drive current will flow into the desired path. This is because the effective value of inductance in shunt with the desired path will be much greater in the switching tree of FIG. 5 than in the prior art inductive switching tree already discussed and therefore less current will steer into the undesired shunt paths.

The operation of the switching tree of FIG. 5 may perhaps be better understood by specific example. Assume that it is desired to select a path 010. This corresponds to the 0, 1, 0 output terminals active, of the 2 2 and 2 flip-flops, respectively. The active 0 terminal of the 2 flip-flop causes the inductive switch 74 to be actuated. This places a relatively large value of inductance in series with paths 1%, 101, 110 and 111. The active 1 terminal of the 2 flip-flop actuates

inductive switches

76 and 78. Active switch 78 places a relatively large value of inductance in series with

paths

000 and 001. The active 0 terminal of the 2 flip-flop actuates inductive switches 8084. Active inductive switch 81 places a relatively large value of inductance in series with path 011. The only path remaining which has only a relatively small value of inductance associated with it is path 010. This is because the control elements of the

inductive switches

86, 87 and 88 remain in the superconductive state. If now a drive current pulse 90 is applied by current source 92 to the input end 94 of the inductive switching tree, it will steer substantially instantaneously into the path 010. The amount of current which divides into the remaining paths is greatly minimized in view of the relatively large values of inducmnce in series with the remaining paths. For example, the large inductive switching element 74 exhibits a large value of inductance in series with paths 1%, 101, 110 and 111.

What is claimed is:

1. In a cryoelectric switching tree in which there are a plurality of paths through the tree and which includes controllable individual impedances which in one condition exhibits substantially higher value of impedance than in the other condition, essentially in series with different groups of said paths, some said groups including more paths than others, the improvement comprising the impedances for the groups containing larger numbers of paths having larger values when in their higher impedance condition than the impedances for the groups containing smaller numbers of paths when the latter impedances are in their higher impedance condition.

2. In a cryoelectric switching tree in which there are a plurality of paths through the tree and which includes the respective gate electrodes of cryotrons essentially in series with different groups of said paths, some said groups including more paths than others, the improvement comprising the gate electrodes of the cryotrons for the groups containing larger numbers of paths exhibiting a larger resistance, when driven normal, than the gate electrodes of the cryotrons for the groups containing smaller numbers of paths, when the latter gate electrodes are driven normal.

3. In a cryoelectric switching tree in which there are a plurality of paths through the tree and which includes the respective controlled elements of inductive switches which in on condition exhibits a substantially larger value of inductance than when in the other condition, essentially in series with different groups of said paths, some said groups including more paths than others, the improvement comprising the controlled elements of the switches for the groups containing larger numbers of paths exhibiting a larger value of inductance, when their respective control elements are driven out of the superconducting state, than the controlled elements of the inductive switches for the groups containing smaller numbers of paths, when the control elements of the latter inductive switches are driven out of their superconducting state.

4. In a switching tree,

11 paths through the tree;

a switching element connected to one end of the paths essentially in series with a first group containing 11/111 of the paths;

a second switching element connected to the other end of the paths essentially in series with a second group containing n/ m of the paths, where none of the first group of paths is common to the second group of paths;

a third switching element connected to one end of the paths essentially in series with a sub group containing 11/ mp of the first group of paths;

and a fourth switching element connected to the other end of the paths essentially in series with a sub group containing n/ mp of the second group of paths, where n, n/m, and n/mp are all integers.

5. In a cryoelectric switching tree,

:1 paths through the tree;

a first cryotron the gate electrode of which is connected to one end of the paths essentially in series with a first group containing n/m of the paths;

a second cryotron the gate electrode of which has a resistance, when normal, which is substantially equal to that of the gate electrode of the first cryotron, said gate electrode of the second cryotron being connected to the other end of the paths essentially in series with a second group containing 11/111 of the paths, where none of the first group of paths is common to the second group of paths;

a third cryotron having a gate electrode which exhibits a lower value of resistance, when normal, than the first cryotron, said gate electrode of the third cryotron being connected to said one end of the paths essentially in series with a sub gnoup containing n/mp of the first group of paths;

and a fourth cryotron having a gate electrode which has a resistance, when normal, which is substantially equal to that of the gate electrode of the third cryotron, said gate electrode of the fourth cryotron being connected to the other end of the paths essentially in series with a sub group containing n/mp of the second group of paths, where n, n/m, and 11/ mp are all integers.

6. In a cryoelectric switching tree,

2 paths through the tree;

a first cryoelectric switching element connected to one end of the paths essentially in series with a first group containing 2 /2 of the paths;

a second cryoelectric switching element connected to the other end of the paths essentially in series with a second group containing the remaining 2 2 of the paths, where none of the first group of paths is common to the second group of paths;

a third cryoelectric switching element connected to one end of the paths essentially in series with a sub group containing 2 /4 of the first group of paths and with the first cryoelectric switching element; and a fourth cry oelectric switching element connected to the other end of the paths essentially in series with a sub group containing 2 /4 of the second group of paths and with the second cryoelectric switching element, Where n in an integer.

7. In the cryoelectric switching tree of claim 6 said switching element's comprising the gate electrodes of inline cry otrons, and the first and second of said electrodes being longer than the third and fourth of said electrodes.

8. In a cryoelectric switching tree, the improvement comprisingthe paths in one plane through the tree formed of a hard superconductor being arranged parallel toone another and each having a straight line geometry, and cryotron gate electrodes in series with the paths lying in the same plane as and at an angle to the paths and arranged parallel to one another, said electrodes being formed of a soft superconductor and each also having a straight line geometry.

9. In a cryoelectric switching tree, the improvement comprising the paths in one plane through the tree formed of a hard superconductor being arranged parallel to one another and each being continuous and having 13. straight line geometry, and cryotron gate electrodes connected to the paths lying in the same plane as and at an angle to the paths and arranged parallel to one another, saidelectrodes each also having a straight line geometry.

10. In a vacuum deposited cryoelectric switching tree, the improvement comprising the paths in one plane through the tree formed of a hard superconductor being arranged parallel to one another and each being continuous and having a straight line geometry, whereby all said paths may be deposited through a single mask, and cryotron gate electrodes connected to the paths lying in the same plane as and at an angle to the paths and arranged parallel to one another, said electrodes each also having a straight line geometry, whereby all of said electrodes may be deposited through a single mask.

11. In combination,

first and second superconductor lines;

a first superconductor gate element connected to one end of the first line and a first superconductor bypass element which is joined to the first gate element, connected to the same end of the second line;

a second superconductor gate element connected to the other end of the second line and a second superconductor by-pass element which is joined to the second gate element, connnected to the other end of the first line;

means for applying a current to the first gate [and bypass elements; and

means for selectively driving one of the first and second gate elements normal.

References Cited by the Examiner UNITED STATES PATENTS 7/1962 Buckingham et al. 30788.5 X

OTHER REFERENCES DAVID J. GALVIN, Primary Examiner.

ARTHUR GAUSS, JOHN W. HUCKERT, Examiners.

Claims

1. IN A CRYOELECTRIC SWITCHING TREE IN WHICH THERE ARE A PLURALITY OF PATHS THROUGH THE TREE AND WHICH INCLUDES CONTROLLABLE INDIVIDUAL IMPEDANCES WHICH IN ONE CONDITION EXHIBITS SUBSTANTIALLY HIGHER VALUE OF IMPEDANCE THAN IN THE OTHER CONDITION, ESSENTIALLY IN SERIES WITH DIFFERENT GROUPS OF SAID PATHS, SOME SAID GROUPS INCLUDING MORE PATHS THAN OTHERS, THE IMPROVEMENT COMPRISING THE IMPEDANCES FOR THE GROUPS CONTAINING LARGER NUMBERS OF PATHS HAVING LARGER VALUES WHEN IN THEIR HIGHER IMPEDANCE CONDITION THAN THE IMPEDANCCES FOR THE GROUPS CONTAINING SMALLER NUMBERS OF PATHS WHEN THE LATTER IMPEDANCES ARE IN THEIR HIGHER IMPEDANCE CONDITION.