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Superconductor Science and Technology

New SFQ/DC converter for RSFQ logic/memory family

Stanislav V. Polonsky

Published Sep 1, 1991• 7 citations
Publisher

Abstract

Single flux quantum (SFQ) pulses used by the elements of RSFQ logic/memory family should be transformed into the usual DC-voltage form before leaving an RSFQ chip. To provide a high output voltage level (about the gap voltage V g ) the author suggests an SFQ/DC converter using unshunted hysteresis Josephson junctions. The converter employs a DC flip-flop of a HUFFLE type. It does not require AC power supply and can easily be integrated with the RSFQ elements.

DOI
10.1088/0953-2048/4/9/017

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Single flux quantum (SFQ) pulses used by the elements of RSFQ logic/memory family should be transformed into the usual DC-voltage form before leaving an RSFQ chip. To provide a high output voltage level (about the gap voltage V g ) the author suggests an SFQ/DC converter using unshunted hysteresis Josephson junctions. The converter employs a DC flip-flop of a HUFFLE type. It does not require AC power supply and can easily be integrated with the RSFQ elements.
We report on a voltage-state logic family based on shunted Josephson junctions and on the Rapid Single Flux Quantum (RSFQ) concept. Storing information in a voltage-state form makes this Multi-Flux-Quantum Logic (MFQL) easier to match to complementary metal-oxide semiconductor (CMOS) circuits. The switching of MFQL element between the voltage and zero-voltage states takes a minimum of two SFQ pulses, and the switching time of this logic is comparable to that of RSFQ logic. The key logic element is an inverter in the form of an asymmetric interferometer, and the underlying idea is to use a separate superconducting wire to supply a standard dc output voltage for the elements. The asymmetric interferometer, directly coupled to the other elements, is actually a Josephson junction based three-terminal device or transistor. Elements such as NOT, OR, NOR, XOR, XNOR, a toggle flip-flop, and an RS trigger have been simulated, and show margins better than /spl plusmn/24%. The performance of this logic with current circuit fabrication technologies is also discussed.
We are introducing a new subfamily of rapid single-flux-quantum (RSFQ) digital cells, based on a single B flip-flop template. The template can be considered as an SFQ flip-flop with up to 4 inputs and 6 outputs. Each input SFQ pulse can change the flip-flop's internal state. Each output presents (in the form of SFQ pulses a specific logic function of the initial state of the cell and input signals. Simple connection of various inputs and/or outputs, combined with shortening or opening of certain branches of the template allows one to implement a variety of RSFQ cells with wide margins. Of these various RSFQ cells, we have designed and successfully tested the T1 cell (asynchronous toggle flip-flop with synchronous destructive readout), single-bit full adder, and single-bit stage of an up-down counter. Experimentally measured margins for dc power supply voltage for these circuits were /spl plusmn/24%, /spl plusmn/24%, and /spl plusmn/17%, respectively.>
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.
Recent progress on reciprocal quantum logic (RQL) has renewed interest in ac powering of superconductor digital circuits, which had been abandoned since the famous IBM project of 1970s. In this paper, we propose and demonstrate new ac-biased single flux quantum (SFQ) circuits, and search for synergy of ac and currently dominating dc biasing schemes. As the first step, we suggest an on-chip ac/dc converter capable of feeding a few dc-biased gates surrounded by their ac-biased counterparts. As the second step, we introduce and present the first successful demonstration of a new ac-powered circuit-an 8192-bit shift register with over 32800 Josephson junctions (JJs) and JJ density of about 6 · 105 JJ per cm2. We suggest a few niche applications for this type of ac-biased circuits, which require high clock rates. For example, these circuits, scalable to millions of JJs per chip, can serve as a convenient benchmark for new SFQ fabrication technology nodes, allowing the operating margins of individual cells to be extracted and thus “visualize” individual fabrication defects and flux trapping events. The circuit can also be developed into a megapixel imaging array for a magnetic field microscope.
A new family of counter-type Josephson-junction A/D converters using ultrafast rapid single-flux quantum (RSFQ) logic/memory elements is proposed. The converters are built in two main blocks: a low-bit differential-code A/D converter running with subterahertz clock speed and a RSFQ-logic-based digital processing unit providing digital low-pass filtering and sample rate reduction. It is shown that such converters can be treated as digital SQUIDs possessing an extremely high slew rate (up to 10/sup 11/ phi /sub 0//s) combined with flux resolution close to that of analog DC SQUIDs (up to 10/sup -6/ phi /sub 0/Hz/sup -1/2/). Several ways of implementing both main blocks to make it possible to tradeoff these parameters against converter complexity are presented and discussed.
We demonstrated interface circuits between a half-flux-quantum (HFQ) circuit and a rapid single-flux-quantum (RSFQ) circuit to obtain SFQ/HFQ and HFQ/SFQ converters. These converters were created by simply inserting a resistor between an RSFQ circuit and the adjacent HFQ circuit, where both circuits are based on pulse logic. These converters enable us to create combinatory circuits that make the best use of the features of the two circuits. Using a DC/SFQ, SFQ/HFQ, HFQ/SFQ, and SFQ/DC converter, we measure an HFQ Josephson transmission line (JTL) composed of eight 0-0-π superconducting quantum interference devices (SQUIDs), which are the essential elements of an HFQ circuit. The 0-0-π SQUIDs are composed of two conventional Josephson junctions and a π-shifted ferromagnetic junction. The HFQ JTL demonstrated in this study is driven by a bias current 11-fold lower than that of a similar RSFQ JTL. These experimental results indicate that the HFQ circuit requires power that is two or more orders of magnitude lower than that of an RSFQ circuit.