We present design, analysis and test of a superconducting microwave power divider for a new superconducting Reciprocal Quantum Logic (RQL). The RQL logic family, using combination of AC power and SFQ data encoding, allows scalable superconducting digital circuits with zero static power dissipation. The Wilkinson 1:8 power splitter/combiner based on λ/4 resonators has been analyzed for geometric series and maximum flat response. Simulated maximum flat response gives one octave of bandwidth with non-uniformity of the bias current distribution within ±10%. This result is valid with up to 40 ps of electrical length mismatch in the power lines that is well within the requirements for complex circuits. We have experimentally confirmed correct operation of the divider/combiner in a frequency band 0-12 GHz in even mode.
We present design, analysis and test of a superconducting microwave power divider for a new superconducting Reciprocal Quantum Logic (RQL). The RQL logic family, using combination of AC power and SFQ data encoding, allows scalable superconducting digital circuits with zero static power dissipation. The Wilkinson 1:8 power splitter/combiner based on λ/4 resonators has been analyzed for geometric series and maximum flat response. Simulated maximum flat response gives one octave of bandwidth with non-uniformity of the bias current distribution within ±10%. This result is valid with up to 40 ps of electrical length mismatch in the power lines that is well within the requirements for complex circuits. We have experimentally confirmed correct operation of the divider/combiner in a frequency band 0-12 GHz in even mode.
Complimentary Medal-Oxide Semiconductor (CMOS) technology is expected to soon reach its fundamental limits of operation. The fundamental speed limit of about 4 GHz has already effectively been sidestepped by parallelization. This increases raw processing power but does nothing to improve power dissipation or latency. One approach for increasing computing performance involves using superconducting digital logic circuits. In this thesis I describe a new kind of superconducting logic, invented by Quentin Herr at Northrop Grumman, which uses reciprocal pairs of quantized single magnetic flux pulses to encode classical bits. In Reciprocal Quantum Logic (RQL) the data is encoded in integer units of the magnetic flux quantum. RQL gates operate without the bias resistors of previous superconducting logic families and dissipate several orders of magnitude less power. I demonstrate the basic operation of key RQL gates (AndOr, AnotB, Set/Reset) and show their self-resetting properties. Together, these gates form a universal logic set and provide memory capabilities. Experiments measuring the bit error rate of the AndOr gate extrapolated a minimum BER of 10−480 and a BER of 10−44 with 30% margins on flux biasing. I describe an analytic timing model for RQL gates which demonstrates the self-correcting timing features. From this model I derive equations for the timing behavior and operating limits. Using this timing model I ran simulations to determine correction factions for more accurate predictions at higher frequencies. Using these results, I also develop Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL) models to describe the combinational logic of RQL gates. To test the timing predictions of the timing model, I performed three experiments on Nb/AlOx/Nb circuits at 4.2 K. The first measured the time of output. The second measured the operating margins of the circuit. The third measured the maximum frequency of operation for RQL circuits. Together, these three experiments showed quantitative agreement with the model for the timing output, qualitative agreement with the limits of operation, and a projected speed limit of 50 GHz for the Hypres 4.5 kA/cm2 process. To power RQL circuits I describe a new design for power splitters and combiners which minimize standing waves. I describe a new kind of Wilkinson power splitter which required numerical optimization but proved to be adequate. I experimentally tested two new designs of the power splitter. Both showed less than 10% variation in standing waves between power splitter and combiner, making it adequate for RQL circuits. I also compared these results with the S-parameters of the power network, which also indicated that the design was adequate for RQL circuits. Finally, I tested an 8-bit Kogge-Stone architecture carry-look ahead adder designed using VHDL models. The adder contained 815 Josephson junctions and was fully functional at 6.21 GHz with a latency of 1.25 clock cycles. The adder produced the correct logical output, had a measured optimal operating point within 8% of the optimal simulated operating point, and measured power margins of 1 dB. It operated best at the designed clock amplitude of 0.88 Ic and dissipated 0.570 mW of power.
We have developed a new superconducting digital technology, Reciprocal Quantum Logic, that uses ac power carried on a transmission line, which also serves as a clock. Using simple experiments, we have demonstrated zero static power dissipation, thermally limited dynamic power dissipation, high clock stability, high operating margins, and a low bit-error rate. These features indicate that the technology is scalable to far more complex circuits at a significant level of integration. On the system level, Reciprocal Quantum Logic combines the high speed and low-power signal levels of single-flux-quantum signals with the design methodology of semiconductor digital logic, including low static power dissipation, low latency combinational logic, and efficient device count.
Biasing remains one of the key scalability bottle necks in superconducting digital circuits, as traditional DC bias networks impose excessive power dissipation, large direct current requirements resulting in significant power overhead. Fast Phase Logic (FPL), an all-Josephson Junction (JJ) logic family, achieves ultra-high density by eliminating resistors and large inductors; however, existing FPL designs still rely on DC bias. This paper introduces a novel architecture: AC-biased FPL (ACFPL), which integrates single-phase AC/SFQ converters with FPL cells to deliver bias currents as quantized pulses, thereby eliminating the need for a global DC distribution network. Circuit-level simulations at 10 GHz demonstrate that AC biasing achieves up to 25× lower power consumption compared to DC bias for RSFQ circuits, and 14.3× reduction in power consumption for FPL circuits in our design-under-test (DUT), which combines FPL JTL, FPL DFF, and FPL AND cells. These results estab lish ACFPL as a scalable and energy-efficient alternative for large-scale superconducting digital systems, effectively combining compact FPL cell design with practical AC/SFQ bias delivery, allowing superconductor VLSI.
Superconductor single flux quantum (SFQ) technology is one of the promising candidates for energy-efficient high-performance computing. A new generation of SFQ logics, Reciprocal Quantum Logic (RQL) with no static power dissipation in bias resistors, offers an opportunity to dramatically decrease energy consumption in superconductor processors. Although several low complexity RQL processing units have already been demonstrated, the use of RQL for local storage design has not been explored yet. The objective of this dissertation is to design on-chip local storage structures such as memory, register files, and caches with RQL technology and analyze their energy efficiency, complexity, and performance characteristics. The physical chip design of these RQL storage units is not feasible at this point because both CAD tools for physical VLSI chip design and as well as a target fabrication process are under development at Northrop-Grumman Systems Corp. (Baltimore, MD) and MIT Lincoln Laboratory, respectively. In order to achieve our goal, the layout-aware cell-level design process using VHDL RQL cell library developed at the Ultra High Speed Computing Lab in Stony Brook University has been used. The SBU VHDL RQL cell library specifies the dynamic and stand-by energy consumption, latency, JJ complexity, and approximate sizes of individual cells based on the input received from the JJ-level RQL designers. Clock propagation skew and wire delays are accounted for during circuit simulation. The circuit simulation is done with Mentor Graphics design and verification tools. As a result of the work, key characteristics of the 8.5 GHz multiported RQL storage structures with their capacity in the range of 1-4 kbit have been determined. The average energy consumption of the RQL storage designs is ~3.0-9.5 fJ/bit/operation at room temperature and the cryo-cooling efficiency is 0.1%. The data from this dissertation also reveal the critical issues that need to be considered in the RQL storage design. These will be helpful for further development on superconductor VLSI design. | 124 pages
High current requirements in single flux quantum (SFQ) circuits pose scalability challenges, contributing to issues such as high thermal loads, static power dissipation, and increased sensitivity to flux trapping under non-ideal shielding or cool down conditions. AC biasing, successfully used in other superconductive logic families, has been integrated into rapid single flux quantum (RSFQ) circuits and typically includes AC-to-DC converters, introducing area and complexity overhead. This work proposes an alternative method for applying AC biasing to existing RSFQ circuits using inductively coupled bias junctions, eliminating the need for AC-to-DC converters. This approach also combines resistive DC biasing with inductive AC biasing, reducing static power dissipation. The proposed approach targets clock splitters, which are significant contributors to on-chip current demand, achieving an average 94% reduction in splitter current and a 62% reduction in overall system current with a Josephson junction overhead of 6.5%. The technique is compatible with conventional RSFQ system architectures and logic families while significantly enhancing energy efficiency and scalability.
A superconductor frequency divider based on rapid single flux quantum (RSFQ) logic has been demonstrated to operate from dc to 750 GHz with an error rate less than the measurement limit (25 MHz). This high operating frequency is made possible by the use of small (0.25 μm2), high critical current density Josephson junctions along with an optimized design having parameter margins of ±30%. Simulations based on the Werthamer model are in reasonable agreement with the data and give a SFQ pulse width of 1 ps and a maximum power dissipation of 1.5 μW.