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Physica C Superconductivity

High-speed operation of SQUID–array-type interface circuits using a cryocooler

Masahiro Horibe, Yoshinobu Tarutani, K. Tanabe

Published Jun 25, 2004• 3 citations
Publisher
DOI
10.1016/j.physc.2004.01.175

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IEEE Transactions on Applied Superconductivity•Mar 1, 2001•17 citations
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.
We have investigated and tried to improve the packaging technologies for a bit-error-rate (BER) measurement system for high-T/sub c/ superconducting circuits. Signal cables, test fixture, and magnetic shields were specially designed and assembled for the BER measurement system, taking into account electrical losses, signal reflection, heat inflow, and power consumption of a cryocooler. BER was further improved by changing the way of connection between the semiconductor amplifiers outside the sample vacuum chamber. These improvements led to a BER less than 10/sup -12/ at an output voltage (V/sub out/) of 1.7 and 2.3 mV for a 50-/spl Omega/ standard microstrip transmission line and a superconducting microstrip-line to coplanar-waveguide transmission-line converter, respectively. The temperature rise of the test fixture was as low as about 2 K. These values are approximately one order of magnitude smaller than those in the previous measurement system and small enough to be achieved by using superconducting interface circuits. Furthermore, 1-Gb/s operation of a superconducting quantum interference device array-type interface circuit was demonstrated in the BER measurement system at 40 K.
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.