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IEEE Transactions on Applied Superconductivity

Interface circuit using JTLs as control lines of SQUID array

Y. Tarutani, K. Saitoh, Kazumasa Takagi

Published Mar 1, 2001• 17 citations
Publisher

Abstract

Interface circuits for the RSFQ circuit system were designed, fabricated and their frequency characteristics were investigated. Key elements of the interface circuit were the series array of SQUIDs and a JTL as a control line of the SQUID. The interface circuit was designed to cover the operating frequencies much greater than 10 GHz. A high-frequency signal generator circuit consisting of a ring oscillator, a frequency divider, and an interface circuit was fabricated. High frequency signals produced in the ring oscillator could be detected by using the interface circuit. The h/2e relationship between the oscillation frequency and the ac Josephson voltage was verified.

DOI
10.1109/77.919352

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Interface circuits for the RSFQ circuit system were designed, fabricated and their frequency characteristics were investigated. Key elements of the interface circuit were the series array of SQUIDs and a JTL as a control line of the SQUID. The interface circuit was designed to cover the operating frequencies much greater than 10 GHz. A high-frequency signal generator circuit consisting of a ring oscillator, a frequency divider, and an interface circuit was fabricated. High frequency signals produced in the ring oscillator could be detected by using the interface circuit. The h/2e relationship between the oscillation frequency and the ac Josephson voltage was verified.
RSFQ circuit elements were fabricated based on the YBa/sub 2/Cu/sub 3/O/sub 7-x/ ramp-edge junctions and their characteristics were investigated. In particular, two types of interface circuits were designed, fabricated and their amplification functions were evaluated. The interface circuits comprised series array of SQUID's that shared a common inductor each other. Each SQUID was switched between zero voltage state and voltage state by current injection. Input signal was AC Josephson current for the type-one interface circuit. The input signal was an SFQ for the type-two interface circuit. Both interface circuits could successfully be operated. In particular, an output voltage more than 1 mV was obtained for the type-one interface circuit that comprised the stack of five SQUID's.
We have designed ring oscillators to test rapid single flux quantum (RSFQ) circuits with passive transmission lines at high frequencies. To reduce the width of the transmission line, design methods for increasing the characteristic impedance of the transmission line have been investigated and then we designed a 5-stage of Josephson transmission line (JTL) receiver which can transform the impedance from 1 to 4 /spl Omega/ by changing the critical current and the inductance. The receiver, driver, and micro-strip line were inserted in a ring oscillator. The ring oscillator fabricated with the Nb/AlOx/Nb junction process was successfully operated at frequencies ranging from 6 to 10 GHz for line lengths of 0.6, 3, 6, and 9 mm. The usefulness of the impedance transformation was then confirmed experimentally. A propagation delay per unit length of 8.4 ps/mm was also obtained.
The authors evaluated the operation of high-temperature superconducting quantum interference device (SQUID)-array interface circuits (IFCs) with normal-metal control lines. Transimpedance amplification was obtained at an operating speed of 1 Gb/s using a cryocooler. The effect of the number of SQUIDs connected in series and the number of arrays connected in parallel on the level of output from the SQUID-array IFCs was examined by Josephson circuit simulation, and then the effect of statistical spreads of junction characteristics was evaluated by Monte Carlo simulation. It was found that the configuration of two parallel SQUID arrays with 64 SQUIDs gives the highest output when the junction characteristics in the arrays have a certain spread. The authors fabricated the IFCs by using the conventional interface-engineered junction process. The process reproducibility was 100 /spl mu/A /spl plusmn/25% for junction I/sub c/, and 3.02 pH /spl plusmn/5% and 2.57 pH /spl plusmn/17% for the sheet inductance of the upper and lower electrodes, respectively. The transimpedance at low frequencies reached 20 and 4 V/A for input levels of 20 and 100 /spl mu/A, respectively. Output voltages as high as 4.4 mV at 4.2 K and 2.3 mV at 40 K were obtained. Furthermore, an output voltage of 600 /spl mu/V was obtained for a 1-Gb/s 2/sup 15/-1 pseudo-random binary signal input at 40 K.
An integrated quasioptical receiver circuit comprising planar complementary log-periodic antenna, SIS mixer with microstrip matching transformers and parallel array of unshunted SIS junctions operating as a local oscillator has been designed, fabricated and dc tested. The array of parallel SIS junctions united in a microstrip transmission line can be described as a Josephson transmission line (JTL) with single flux quanta synchronously moving along the array of quantum interferometers. The JTL with 20 junctions 4 /spl mu/m in diameter has critical current 4.5 mA, normal resistance 0.3 /spl Omega/ and a self-detection step at about 1.1 mV. The step width is over 1.5 mA in current scale and 50 /spl mu/V in voltage scale that corresponds to 2.5 /spl mu/W oscillation power and 25 GHz tuning range with 550 GHz central frequency. A current applied along the top electrode of JTL allows to vary the output power of such oscillator. The advantages of such oscillator are higher impedance of microstrip line about 10 /spl Omega/ that makes easier matching to SIS mixer, lower losses at frequencies about the energy gap for Nb and the possibility to tune output power. The SIS junction IV curve shows self-resonant steps at about 1.2 mV that corresponds to the matching circuit central frequency about 600 GHz and is close to the JTL central frequency.
An integrated quasioptical receiver circuit comprising planar complementary log-periodic antenna, SIS mixer with microstrip matching transformers and parallel array of unshunted SIS junctions operating as a local oscillator has been designed, fabricated and tested. The array of parallel SIS junctions united in a microstrip transmission line can be described as a Josephson transmission line (JTL) with single flux quanta synchronously moving along the array of quantum interferometers. The JTL with 20 junctions each 4 micrometers in diameter has critical current 4.5 mA, normal resistance 0.3 (Omega) and a self-detection step at about 1.1 mV. The step width is over 1.5 mA in current scale and 50 (mu) V in voltage scale that corresponds to about 1 (mu) W oscillation power and 25 GHz tuning range with 550 GHz central frequency. A current applied along the top electrode of the JTL allows to vary the output power of such oscillator. The advantages of such oscillator in comparison with Flux-FLow Oscillator are higher impedance of microstrip line about 10 (Omega) that makes easier matching to SIS mixer, lower losses at frequencies about the energy gap for Nb, narrow linewidth and the possibility to tune output power. THe SIS junction IV curve shows current steps at about 1.2 mV that corresponds to the matching circuit central frequency about 600 GHz that is close to JTL central frequency.
We have experimentally demonstrated high-speed operation of RSFQ circuits using on-chip testing system and confirmed normal operation up to 30 GHz. The system includes high-frequency pulse generators and shift registers (SRs) serving as an interface circuit between high-speed internal signals and low-speed external signals. We adopted two ladder-type pulse generators. One generates clock pulses of 10 GHz and the other generates 25 GHz. By increasing bias currents of JTLs, clock pulses can be generated at higher frequency. By using the system, an SR and a T flip-flop (TFF) gate are examined. The circuits were fabricated by NEC's standard process based on Nb/AlO x /Nb junction technology with J c = 2.5 kA cm -2 . We optimized circuit parameters by using Monte Carlo simulation. In addition, the effect of parasitic inductances can be reduced by decreasing I c . As a result, the systems for SR and TFF worked correctly up to 30 GHz and 27.5 GHz, respectively. The SR and TFF have wide bias margins of ±39% and ±20%, respectively. We confirmed that the bias margins of the SR are independent of operation frequency. The limits of operation frequency are thought to be much higher than 30 GHz.