The authors argue that superconducting digital electronics is the future trend for high-speed and low-power applications. Low-T/sub c/ technology based on stable Nb-AlO/sub x/-Nb tunneling junctions has been successful in constructing most of the digital circuit logic elements. A four-stage shift register constructed with latching logic operated up to 4 GHz with a power consumption of about 60 mu W per gate. A 12-b analog-digital coverter constructed with a single flux quantum (SFQ) logic showed proper operation with the total power dissipated being only about 100 mu W. Fabrication of Josephson tunneling junctions with high-T/sub c/ materials has been tougher than with low-T/sub c/ materials. However, recently developed very high speed rapid single flux quantum (RSFQ) logic circuits can be applied to weak link type junctions fabricated with YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. Simulations of the various circuits with weak link type junctions, e.g., shift registers, analog-digital converters, etc., show fast operation and reasonable margins. Applications of similar concepts can be applied to NbN circuits operating at above 10 K.>
The authors argue that superconducting digital electronics is the future trend for high-speed and low-power applications. Low-T/sub c/ technology based on stable Nb-AlO/sub x/-Nb tunneling junctions has been successful in constructing most of the digital circuit logic elements. A four-stage shift register constructed with latching logic operated up to 4 GHz with a power consumption of about 60 mu W per gate. A 12-b analog-digital coverter constructed with a single flux quantum (SFQ) logic showed proper operation with the total power dissipated being only about 100 mu W. Fabrication of Josephson tunneling junctions with high-T/sub c/ materials has been tougher than with low-T/sub c/ materials. However, recently developed very high speed rapid single flux quantum (RSFQ) logic circuits can be applied to weak link type junctions fabricated with YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. Simulations of the various circuits with weak link type junctions, e.g., shift registers, analog-digital converters, etc., show fast operation and reasonable margins. Applications of similar concepts can be applied to NbN circuits operating at above 10 K.>
This paper presents a study on the merits of single flux quantum (SFQ) logic in superconducting digital electronics. With no requirements for an external RF clock source, the speed of SFQ logic circuits can exceed 100 GHz, orders of magnitude higher than latching logic circuits. Use of overdamped junctions in SFQ circuits makes this logic available for already developed high-Tc junctions. Simulations show the operation of SFQ circuits at above 10 GHz with the junctions having characteristics similar to those of YBa2Cu3O7-x/Au/YBa2Cu3O7-x planar junctions. A similar concept is applied to NbN circuits operable above 10 K. A flip-flop counter using NbN junctions with Vm (1.5 mV)=3 mV and Delta =1.9 mV to model 12 K operation has been successfully simulated up to 5 GHz. Operating margins of only 26% on MVTL shift register were obtained by simulations, which is comparable to the margins for SFQ logic circuits. A counting SFQ analogue-to-digital converter can be improved by using an output buffered quantizer.
Superconductive electronics offers a unique combination of high bandwidth and low power for mission critical hardware such as digital processors at 20 to 100 GHz, microwatt per bit analog to digital converters, large digital switches at multi-Gb/s data rates, and low noise parametric amplifiers. This cryogenic technology, typically operating at a few degrees or a few tens of degrees Kelvin, depends upon the unique physics of superconducting materials such as Nb, NbN, and YBaCuO, and the fundamental circuit element, the Josephson junction (JJ) to create unique architectures of unprecedented design efficiency and performance. These circuits are based on the single flux quantum (SFQ) logic family, a technology that has no parallel in semiconductor electronics. Digital data is transmitted by voltage pulses with amplitude less than 1 mV, but at extremely high speeds. Individual gates have been operated as high as 370 GHz. In this paper, we present a brief overview of superconductive digital technology, describe some recent circuits, and discuss how the challenges to superconductive technology are being met, including the use of multi-chip module (MCM) technology, and the potential use of III-V HBT and HEMT circuits to provide interface drivers and amplifiers, possibly operated cryogenically.
Key digital circuits for signal processing with tens of gigahertz speed and a few microwatts power dissipation have been recently demonstrated using a simple, 2 /spl mu/m linewidth process. These are the fastest digital circuits reported to date. These circuits use novel rapid single flux quantum (RSFQ) technology that takes advantage of the fundamental properties of superconductivity. In this technology, data and clock are picosecond-long quantized voltage pulses corresponding to the transfer of single magnetic flux quanta, /spl Phisub 0/=h/2e=2 mVps. These SFQ pulses are regenerated at each gate and are processed by circuits comprised of overdamped Josephson junctions interconnected via microstrip inductances. The inherent internal memory of all RSFQ gates (primarily flip-flops), combined with high switching speed, makes serial, bit-pipelined circuit design advantageous. This paper presents experiments in RSFQ digital circuits, including an N/spl times/8b serial multiplier, and on-chip test systems based on RSFQ shift registers and on SFQ samplers. They enable test and study of RSFQ circuits at full GHz speed while communicating with semiconductor electronics at low speed.>
A new generation of the Josephson digital circuits-rapid single flux quantum (RSFQ) devices-based on overdamped externally shunted high quality Nb-AlO x -Nb tunnel junctions have been designed, fabricated and experimentally investigated. All-refractory material 11-layer 5 mu m integrated technology has been successfully employed for the preparation of the RSFQ microcircuits. The T flip-flop has been demonstrated to operate properly up to clock frequencies of 100 GHz at 25% margins. The proper operation of the buffer stage for a one-directional SFQ pulse transmission at clock frequencies up to 125 GHz has been achieved. The possibilities to convert the information from the usual DC form to SFQ and vice versa have been demonstrated. An output signal in an SFQ/DC converter as high as 220 mu V has been measured. A new counter type A/D converter which includes a comparator and reversible binary counter with DC outputs has been fabricated and tested. The proposed ADC seems to be very promising for high-accuracy measurement of medium-bandwidth signals. The possibilities of creating a number of SFQ devices with outstanding parameters are discussed.
Superconductive Rapid Single Flux Quantum (RSFQ) logic and memory, in which ones and zeros are represented by the presence or absence within a timing window of quantized picosecond voltage pulse (/spl int/v(t)dt=h/2e=2.07 mV/spl middot/ps), corresponding to one SFQ, can be integrated into a digital computing system with an operating rate of several tens of GHz, based on the present Nb Josephson junction integrated circuit technology. It is the most promising technology beyond semiconductor transistors for low-power high-end computation. However, as the operating speed of circuits and systems increase, timing uncertainty from fabrication process variations makes global synchronization very hard. In this paper, we present a globally asynchronous, locally synchronous timing methodology for RSFQ digital design, which can solve the global synchronization problem. We also demonstrate the recent experimental results of some asynchronous circuits and systems implemented in RSFQ technology. Several key components such as a self-timed shift register, a self-timed demultiplexor, a Muller-C element, a completion detector, and a clock generator have been designed and tested. High speed operation has been confirmed up to 20 Gb/s for a prototype data buffer system, which consists two self-timed shift registers and an on-chip 5-38 GHz clock generator.
Single flux quantum (SFQ) logic circuits, in which a single flux quantum is used for an information carrier, have a possibility to open a new digital system operated with over 100 GHz clock frequencies at extremely low power dissipations. Recently, much progress in basic technologies for designing SFQ circuits and operating circuits with high speeds has been done. By taking advancements in these designing tools, circuits including more than several thousands junctions easily operated with the clock frequency of more than several tens GHz. Applications of SFQ circuits such as high performance computers, high end router and high end servers are being developed in the world taking advantages of the high through-put nature and the low power dissipation of the SFQ logic.