Patent Publication Number: US-2023145418-A1

Title: Interference Reducing Passive Transmission Line Receiver

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS - CLAIM OF PRIORITY 
     The present application claims priority to U.S. Provisional Application No. 62/983,392, filed Feb. 28, 2020, entitled “Interference-Reducing Passive Transmission Line Receiver”, which is herein incorporated by reference in its entirety. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     This invention was made with government support under W911NF-17-9-0001 awarded by the Intelligence Advanced Research Projects Activity (IARPA) agency. The government has certain rights in the invention. 
    
    
     TEHCNICAL FIELD 
     This disclosure relates to electronic circuits in general, and more particularly to Josephson junction circuits and transmission lines. 
     BACKGROUND 
     Superconducting circuitry based on Josephson junctions is used to implement digital logic systems. Josephson junctions are typically a circuit element comprising two superconducting electrodes separated, for example, by a thin insulating tunnel barrier, which can support a current that flows indefinitely without any voltage applied. Although the technology has its challenges, one of the major benefits is greatly reduced power consumption relative semiconductor technologies and digital logic circuits. This benefit is realized, even though systems employing Josephson junction circuit elements must be operated at temperatures close to absolute zero. The possibility of realizing this benefit has resulted in a significant amount of interest in Josephson technology. One example of a program that is attempting to take advantage of this benefit is the federal SuperTools program, in which Synopsys, Inc. of Mountain View, California plays a leading role. 
     SUMMARY 
     Embodiments of a method and apparatus are disclosed that operate in a manner that reduces data errors caused by spurious pulses that are generated as a consequence of a combination of particular clock frequencies and line lengths used in circuits with passive transmission lines (PTLs) and PTL driver circuits that emits single flux quantum (SFQ) pulses into the PTL. Also disclosed is an electronic structure for propagating logic states between superconducting digital logic gates. The structure comprises a PTL and transmission line matching circuitry that minimizes the generation and propagation of spurious pulses emitted by Josephson junctions used in the digital logic gates. In some of the embodiments disclosed herein, a new receiver circuit is used that isolates the SFQ-generating junction from the PTL. The PTL terminates into an inductor instead of a Josephson junction. A three-junction interferometer configuration is used for amplification and isolation. With some disclosed embodiments of the receiver circuit, a significant portion of a counter-propagating interference signal that otherwise results is eliminated. What remains is a low-amplitude damped ringing, which dissipates rapidly. This remainder has a far smaller effect than the larger pulse signal generated by conventional receivers resulting in an improved receiver having improved reception and overall performance. 
     In other embodiments disclosed herein, a conventional receiver is used, but a PTL is terminated with a matching resistance at the driver end. In some such embodiments, the driver is designed to be insensitive to the counter-propagating pulses. The counter propagating pulses will disappear without ill effect as a result of a matched termination. Such embodiments eliminate the negative effect of clock frequency and line length dependent data errors from spurious pulses in the transmission lines. 
     This summary and the abstract do not limit the breath, scope or applicability of the claimed invention, but rather the invention as claimed herein is limited only by the language of the claims themselves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed method and apparatus, in accordance with one or more various embodiments, is described with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict examples of some embodiments of the disclosed method and apparatus. These drawings are provided to facilitate the reader’s understanding of the disclosed method and apparatus. They should not be considered to limit the breadth, scope, or applicability of the claimed invention. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale. 
         FIG.  1    shows a simplified schematic of a conventional PTL driver or receiver. 
         FIG.  2    shows a simplified schematic in accordance with some embodiments of the circuit depicted in  FIG.  1   . 
         FIG.  3    shows an example of an input signal and a resulting output signal when the input signal is applied to the circuit of  FIG.  2    with a PTL having a transmission delay of 10 pS. 
         FIG.  4    shows an example of an input signal and a resulting output signal when applied to the circuit shown in  FIG.  2    with a PTL having a transmission delay of 9 pS. 
         FIG.  5    shows a simplified schematic in accordance with some embodiments of the circuit depicted in  FIG.  7   . 
         FIG.  6    shows an example of an input and a resulting output signal when applied to the circuit depicted in  FIG.  5   . 
         FIG.  7    shows a simplified schematic of a receiver circuit in accordance with some embodiments of the method and apparatus disclosed herein. 
         FIG.  8 A  is a simplified schematic of an example of a circuit 800 that comprises the receiver circuit shown in  FIG.  6   . 
         FIG.  8 B  is a simplified schematic of an embodiment in which an inductor of the circuit shown in  FIG.  8 A  is split into a first and a second independent inductor and a third bias current source to the receiver is provided between the first and the second independent inductor. 
         FIG.  8 C  shows a circuit used in other embodiments in which the circuit of  FIG.  7    is used. 
         FIG.  8 D  shows a circuit used in other embodiments, in which a receiver circuit similar to the receiver circuit of  FIG.  7   , but without the inductor between the input terminal of the receiver and the reference potential terminal of the receiver. 
         FIG.  9    shows an example of a signal transmitted to the receiver of the circuit of  FIG.  8   . 
         FIG.  10    shows an example of both a current and voltage a signal transmitted to the receiver of the circuit of  FIG.  8   . 
     
    
    
     In the figures, reference signs can be omitted as is consistent with accepted engineering practice; however, a skilled person will understand that the illustrated components when viewed in the context of the figures as a whole, of the accompanying writings about such figures, and of the embodiments of the claimed inventions. 
     DETAILED DESCRIPTION 
     Josephson junctions are found to be useful in digital logic circuits that rely on the quantum mechanical quantization of magnetic flux. The quantum mechanical quantization of magnetic flux results in magnetic flux being quantized as it threads a hole or a loop in a superconductor. One use of quantum mechanical quantization of magnetic flux called single flux quantum (SFQ) logic, is realized in logic circuits. Though SFQ logic itself has numerous variations, all such variations make use of flux storage and transmission, which is affected by pulses emitted by a Josephson junction that is typically shunted by a resistance in SFQ circuits. When a Josephson junction emits a voltage pulse, it induces a single quantum of magnetic flux into a connected inductor. This type of pulse is known as an “SFQ pulse”. Rapid SFQ (RSFQ) devices can achieve pulse widths as narrow as a few picoseconds. 
     Logic circuits can be generated in which logic states propagate from one gate to another through the propagation of such pulses. Historically, SFQ pulses were transmitted via a Josephson transmission line (JTLs) consisting of small series inductors with a Josephson junction connected to ground at the intersections. The inductors tend to have rather small inductance values, enabling a relatively long Josephson transmission line having many junctions. Each junction is biased, therefore consuming current and dissipating power. Further, each junction presents a delay in signal propagation. However, such lines maintain the essential characteristics of an SFQ pulse with high reliability. 
     In a very large scale integrated circuit (VLSI) environment, the use of JTLs for interconnecting gates is not an option, due to a large and variable delay through the JTLs and further due to the lack of a viable automated routing design tool. One alternative is to use passive transmission lines (PTLs), including stripline structures. Superconductive stripline structures have extremely low power loss. However, PTLs require a driver circuit to drive the line with a non-SFQ voltage pulse and then convert the received pulse back into an SFQ pulse at a receiver. 
     Nonetheless, one benefit of using PTL interconnections is that designers can use existing routing tools that are provided within existing Electronic Design Automation (EDA) systems for the designing PTL circuits. Such tools are essential for managing the many design trade-offs and overcoming the technical challenges inherent in designing circuits using PTLs. 
     Some of the challenges that arise in the design of circuits using PTLs are due to the fact that a PTL driver circuit invariably consists of a junction that emits SFQ pulses into the transmission line. This creates two problems. 
     The first problem involves balancing a trade-off between wiring density and transmission line width. To enable high wiring densities, it is desirable to use narrow transmission lines. However, as the transmission line becomes narrower, the characteristic impedance of that line increases. A higher impedance results in less pulse energy being injected into the line, due in part to the limited amplitude of the SFQ pulse. With less pulse energy, it may be necessary to either provide a receiver with higher-sensitivity or reduce the noise immunity of the receiver, or both. For line widths ranging from one to several microns, an approximate impedance range for a PTL is from a few ohms to 20 ohms. Such line widths are considered quite large for CMOS integrated circuits and can increase the difficulties of designing such a CMOS integrated circuit. 
     The second problem caused by a junction that emits SFQ pulses into a transmission line is the generation and propagation of spurious pulses that can cause subsequent data errors. One reason such spurious pulses are generated and propagated is the fact that a Josephson PTL receiver, in its simplest form, consists of a single junction biased near the threshold of the junction. While the pulse from the transmission line has sufficient energy to raise the junction above its threshold, and thus cause an SFQ pulse to be generated (as is desired to generate an output), the same pulse will also propagate backwards along the transmission line toward the driver. Over time, this backwards propagating pulse will die away after reflecting from the ends of the transmission lines. However, if these reflections coincide with a subsequent data pulse, mis-triggering can result, causing a data error. “Tuning” the clock frequency and transmission line delays can avoid having the reflection coincide with the data pulse, however this is difficult to achieve in an LSI environment and reduces the chip flexibility by constraining the clock frequency. That is, tuning the lengths of the interconnects (altering the lengths to suit the application) to avoid certain timing windows when used with an assumed clock frequency would probably not work in densely wired layouts, since it is very difficult to change the physical layout in order to tune (i.e., modify) the transmission line lengths. 
     In order to overcome some of these problems, it would be desireable to reduce or eliminate the need to make tradeoffs between circuit density, line widths and impedances of circuits that use both PTLs and associated PTL driver circuits that emit SFQ pulses into the PTL. In addition, it would be desireable to reduce the potential for data errors due to spurious pulses without having to tune clock frequencies and transmission line delays in such circuits. 
     All of the circuits disclosed herein can be designed, simulated and optimized with electronic circuit design tools that are either commercially available or that can be attained through open source resources. These tools make use of libraries of device and circuit primitives developed for general circuit design and optimization. Resulting circuit designs can then be used to fabricate devices at an appropriate foundry. The concepts and general implementation of the circuits presented here are independent of the specific library and foundry employed. Accordingly, the circuits disclosed herein are improved circuits with respect to circuits in which single flux quantum (SFQ) pulses were propagated between gates of such a circuit using a series of Josephson junctions, making the use of such electronic circuit design tools impossible. 
       FIG.  1    shows a simplified view of a conventional passive transmission line (PTL) receiver  100 . In the case of a driver, one would flip the diagram left-to-right, so that a signal would be seen originating as an SFQ pulse across a Josephson junction  101 . The generated pulse is applied to a PTL  105  through a resistor  103 . In the case of the receiver  100 , a pulse from the PTL  105  is applied to a biased Josephson junction  101  through the resistor  103 , inducing an SFQ pulse from the junction  101 . In some embodiments, the resistor  103  has a resistance of 0 Ω (i.e., a short-circuit). 
     “Resonance” effects in the system can result in data errors that occur for very specific transmission line delays (or physical line lengths) when operating at a particular clock frequency. 
       FIG.  2    shows an example of a circuit  200  in which two Josephson junctions  202 ,  203  are used within a PTL driver  204 . A third Josephson junction  205  is used in a receiver  207 . Some component values are provided as an example and will vary in other embodiments of the disclosed circuit. A PTL  208  between the PTL driver  204  and the receiver  207  has a delay of 10 picoseconds. A resistor  206  in series with the PTL  208  has a value of 1 ohm in the example shown and is used to break the superconducting loop. In this example, the value of the resistor  206  is much lower than the 16 ohm impedance of the PTL  208 . 
     A pulse from the PTL driver  204  will enter the PTL  208 , propagate left to right, and be applied to the receiver junction  205 . The receiver junction  205  will switch as a consequence, emitting an SFQ pulse. This pulse will enter the PTL  208  on the right and propagate right to left along the PTL  208 . As the pulse reaches the end of the PTL  208 , it will be reflected and become a forward propagating pulse with an amplitude that is reduced by the reflection coefficient (R-Z)/(R+Z), where R is the value of the resistor  206  and Z is the impedance of PTL  208 . For the situation in  FIG.  2   , the reflection coefficient is a negative value close to one. If the resulting reflected pulse is coincident with a data pulse, the two will at least partially cancel, producing a data error. This is the basis of the “resonance” effect. In some embodiments of the disclosed circuit, the impedance of the resistor  206  is chosen to be the same as the impedance of the PTL  208 . This results in the reflection coefficient being zero. Therefore, there is no reflected pulse and the “resonance” effect is eliminated. 
     The resonant affect occurs infrequently, because the pulse widths are narrow, resulting in a low probability that the timing will result in cancellation. However, in VLSI circuits, where there may be thousands to millions of such transmission lines, the joint probability of a resulting cancellation is much greater. Because simple circuits typically work without significant errors, designers are not aware of the effect. 
       FIG.  3    shows an example of an input signal  302  and the resulting output signal  304  when the input signal  302  is applied to the circuit  200 . The signals  302 ,  304  are generated from a simulation in which the PTL  208  has an impedance of 16.0 ohms and there is a 10 picosecond delay through the PTL  208 . The output signal  304  is essentially a faithful representation of the input signal  302 , although delayed by the transmission line delay of the PTL. In particular, note that the series of 3 pulses that include the pulse  306  just before the 0.2 ns mark is nearly identical to the series of 3 pulses that include the pulse  308  in the input signal  302 . Thus, it appears that the circuit behaves in an acceptable manner even though reflections are occurring, since the reflections seem to be occurring at times that result in a minimal distortion to the output signal. 
       FIG.  4    shows an illustration of an input signal  402  and an output signal  404  for the same circuit in  FIG.  2   . However, the time delay through the PTL  208  is reduced from 10 ps to 9 ps. As a result of just a 1 picosecond difference, the output signal  404  is significantly distorted, with some of the output data pulses, such as the pulse  306  that was seen in the output signal of  FIG.  3   , missing from the output signal  404 . The missing pulses reflect the distortion resulting from the resonance effect at certain clock frequencies and with certain transmission line delays. The resonance effect causes backward propagating pulses to be reflected from the Josephson junction  205  of the receiver  207  with a polarity inversion with regard to the driver side, resulting in some of the pulses being cancelled at the receiver  207 , as can be seen in the output signal  404 . 
     To analyze the signals in the PTL  208 , one can conceptually replace each of the Josephson junctions  202 ,  203 ,  204  with an ideal voltage source whose voltage is precisely the same as that which would be seen across the respective Josephson junctions. The signal found in the PTL  208  is then a superposition of the signals generated by the voltage sources and the reflections induced at the ends of the PTL  208 . 
     Ideally, by matching the resistance at both ends of the PTL  208 , reflection-free transmission can be achieved. However, in practice this is not done because it is considered impractical due to the amount of signal power that would be lost in a resistor required to create the resistance match. This is because the amplitude of the pulses launched onto the PTL  208  is limited by the amplitude produced by the Josephson junctions, which is characteristic of the junctions for a given manufacturing technology. For example, in the fabrication process for the Lincoln Laboratory SFQ5ee Josephson circuit (as used to model particular characteristics of a Josephson junction in the federal SuperTools program), the amplitude produced is about 0.69 mV. The peak current of the pulse in the line is the value of the amplitude produced by the Josephson junction divided by the sum (R+Z) of the resistor  206  and the characteristic impedance of the PTL  208 . In a practical design, the characteristic impedance Z is made to be as high as possible, in order to have the line width be as narrow as possible to maximize the wiring density. Accordingly, twice as much current is obtained in the pulse by substantially reducing the value R of the resistor  206  (e.g., setting R = 0 Ohms). 
     It should further be noted that even if the impedances at receiver end of the PTL  208  is matched, there is still a spurious pulse generated when the receiver junction  205  switches in response to a received pulse. That is because the receiver  207  becomes a generator, launching a counter-propagating pulse into the PTL  208 . The counter-propagating pulse will be reflected as a negative forward-propagating pulse due to a reflection at the assumed unmatched driver end of the PTL  208 . In the typical case, the driver end of the PTL  208  is driven directly by the driver junction  203  and the reflection coefficient is -1, as determined from the formula for the reflection coefficient (R-Z)/(R+Z) when the resistance R=0 for the driving resistor  206 . If the negative parasitic pulse is precisely coincident with a data pulse, the two will cancel and a data error will result (the receiver will not detect any pulse). 
       FIG.  5    shows the same circuit topology as in  FIG.  2   , however the resistor  506  has a value equal to the impedance of the PTL  508 , which is 16.0 ohms in this example. In one example, the delay of the PTL  508  is kept at 9.0 picoseconds, which is known to be a value susceptible to the resonance effect. 
       FIG.  6    shows a simulated input signal  602  and output signal  604 . Compared to the output signal  404 , the circuit with the values are shown in  FIG.  5    produces an output signal  604  with minimal detectable distortion. That is, the output pulse train of the output signal  604  is a faithful delayed representation of the pulse train at the input signal  602 . Accordingly, matching the impedance of the PTL  508  to the impedance of the resistor  506  successful mitigates the “resonance” effect. 
     In other embodiments disclosed herein, receiver circuits are disclosed which reduced the effects of the counter-propagating signals by providing some degree of input/output isolation. 
       FIG.  7    shows a receiver circuit  700  using a three junction interferometer  712 . The three Josephson junctions  702 ,  704 ,  706  together with two associated inductors  708 ,  710 , form the three-junction interferometer  712 . The interferometer  712  is sensitive to current injected into an input terminal  714 . The critical current of a first Josephson junction  702  is twice that of a second and a third Josephson junction  704 ,  706 . Two bias current sources  720 ,  722  are provided. The current injected through the input terminal  714  will cause the second Josephson junction  704  to switch, which in turn causes the first and third Josephson junctions  702 ,  706  to switch. The switching of the first Josephson junction  702  isolates the input terminal  714  from the switching of the third Josephson junction  706 . The inductor  708  coupled between an input terminal  714  and a reference potential terminal  716  coupled to a reference potential, such as ground, serves as a sink for residual current through the first Josephson junction  702 , preventing the formation of an interference signal that might otherwise be emitted through the input terminal  714  back into a PTL (see PTL  208  in  FIG.  8 C ) coupled to the input terminal  714 . The present circuit  700  reduces the amplitude of interference signals that would otherwise be produced to the point of being negligible. 
       FIG.  8 A  is a simplified schematic of a circuit  800  that comprises the receiver circuit  801  similar to the circuit  700 . It should be noted that in the particular embodiment of the receiver circuit  801 , no source of bias current  722  is needed. In the particular circuit  801 , the Josephson junction  706  has a critical current that is much lower than the Josephson junction  704  and so can operate as desired without the bias current source  722 . However, in other embodiments (such as the embodiment shown in  FIG.  8 C ) it might be necessary or desirable to provide the bias current source  722 . 
       FIG.  8 B  is a simplified schematic for a circuit  802  used in other embodiments in which the inductor  710  may be split into two independent inductors  808 ,  810  and a bias current source  806  provided between the inductor  808  and the inductor  810 . Specific values for the components are provided as an example of an embodiment of the circuit  800 . However, other values are to be used in other embodiments designed for particular implementations. In this example, a 16 Ohm resistance  506  is provided at the receiver side of a PTL  208  ahead of the receiver  803  to provide an impedance match to the 16 Ohm characteristic impedance of the PTL  208 . With the circuit element values as specified in  FIG.  8 A , the PTL  208  is matched on the receiver side. The inductance  708  to ground (in this example, 18 picoHenries) reduces the transient signal that is otherwise launched back into the PTL  208  when the three receiver junctions  804 ,  806 ,  808  switch. 
       FIG.  8 C  shows a circuit  804  used in other embodiments in which the circuit  700  is used. As noted above with respect to  FIG.  7   , the receiver  700  has three Josephson junctions  702 ,  704 ,  706 , two inductors  708 ,  710 , two bias current sources  720 ,  722 , an input terminal  714 , and a reference potential terminal  716 . 
       FIG.  8 D  shows a circuit  805  used in other embodiments, in which a circuit  807  is used. The circuit  807  is similar to the circuit  700 , however the inductance between the input terminal  714  and the reference potential terminal  716  is absent. 
       FIG.  9    shows an input signal  902  and an output signal  904  for the circuit  800 . The output signal  904  has minimal distortion (i.e., is a faithful representation of the input signal  902 , delayed by the PTL  208 ). For other values of transmission line delay (different lengths), there is essentially no “resonance” effect. The new receiver essentially eliminates the resonance by not emitting a large pulse signal back into the PTL  208 . 
       FIG.  10    shows details of signals obtained from the simulation performed with the circuit  800  and element values shown in  FIG.  8 A . In this case, in addition to input and output pulses  902 ,  904  illustrated in  FIG.  9   , the current  1002  that flows into the driver side of the passive PTL  208  is shown. This shows a current pulse  1004  coincident with the voltage pulse  1006  generated by the driver, and at a later time on the same trace, the return from the receiver appears as a small transient signal  1008  when the incident pulse is received. Note that this signal is much smaller than the incident pulse  1006  and is essentially balanced with respect to positive and negative current flow. This is unlikely to cause errors. 
     Features, structures, functions, or characteristics described herein may be used in any combination of two or more such features, structures, functions or characteristics, irrespective of whether such features, structures, functions or characteristics, or combinations thereof, solve any problems disclosed herein. Furthermore, particular embodiments disclosed are not intended to limit the scope of the Claims of the patent, but rather support such claims. 
     In view of the Detailed Description, a skilled person will understand that many variations of the disclosed embodiments may be possible and would support the claimed inventions to such. 
     The disclosed embodiments are neither exhaustive nor limiting of the precise structures claimed. It is intended that the scope of the claimed inventions and embodiments be defined by the following Claims and their equivalents. 
     A number of embodiments of the claimed invention have been described. It is to be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, some of the steps described above may be order independent, and thus can be performed in an order different from that described. Further, some of the steps described above may be optional. Various activities described with respect to the methods identified above can be executed in repetitive, serial, or parallel fashion. It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the claimed invention, which is defined by the scope of the following claims, and that other embodiments are within the scope of the claims.