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IEEE Transactions on Applied Superconductivity

High-speed bit-error-rate measurement system for high-temperature superconducting digital circuits

Masahiro Horibe, Y. Tarutani, K. Tanabe

Published Dec 1, 2003• 2 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2003.820509

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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.
We developed a measuring system that has a magnetically shielded wide-band test fixture mounted on a closed-cycle cryo-cooler, and used this system to investigate the feasibility of cryo-packaging technology for high-Tc superconducting digital electronics. In this work, we evaluated the bit-error-rate (BER) performance of a system in which was installed a 50-/spl Omega/ standard microstrip line (MSTL) and a YBa/sub 2/Cu/sub 3/Ox microstrip line to a coplanar waveguide (CPW) transmission line converter (MCC). In the experiment, we used 3-Gbps, 2/sup 15/-1 pseudo-random binary sequence (PRBS) signals. The temperature of the test fixture was 20 K. Results showed that the BER was 3.74/spl times/10/sup -9/ for the 50-/spl Omega/ MSTL when the signal amplitude at the sample was 13.3 mV, and 1.52/spl times/10/sup -9/ for the MCC when the amplitude was 30.8 mV. The effect of the signal loss and impedance mismatch on the BER is also discussed.
We have evaluated and tried to improve the measurement precision of a bit error rate (BER) measuring system for superconducting circuits equipped with a magnetic shield and a cryocooler. A noise model has suggested that the most effective way of improving BER is to reduce the noise figure of the wideband pulse amplifier (WPA). BER measurement using a 3-Gbps, 2/sup 15/-1 pseudo random binary signal showed that by using low-noise WPA (noisefigure=4 dB) a 12.0-mV output voltage is required to obtain a minimum BER of 1.19/spl times/10/sup -15/ (a 10/sup -7/ improvement over the previous system) for a 50 /spl Omega/ standard microstrip. A BER of a La/sub 0.2/Yb/sub 0.9/Ba/sub 1.9/Cu/sub 3/O/sub x//MgO/Au microstrip-line to coplanar-waveguide transition-line converter (MCC) was also measured. It was found that the minimum BER was 4.87/spl times/10/sup -13/ (a 10/sup -5/ improvement over the previous system) when the output voltage was 23.8-mV. However these measured BER values were larger than those expected by the noise model. The cause of the larger measured BER is discussed. A minimum BER of 3.64/spl times/10/sup -55/ in 14.8-mV output voltage for a 50 /spl Omega/ standard microstrip was obtained when the cryo-cooler was not operated (without the noise from a cryo-cooler).
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 report the gigahertz experimental results of time and frequency performance on YBCO high temperature superconductor (HTS) and gold coplanar transmission lines. An on-wafer direct probing measurement system was used to collect data at cryogenic temperatures on both YBCO and gold coplanar lines. The insertion loss of the 6-cm lines at a frequency of 3.0 GHz was measured to be -0.12 dB for the superconducting line compared to -10.7 dB for the gold line at 80 K. Error rate and eye-diagram measurements were performed on the packaged lines and show the correlation between the insertion loss of the line and the attenuation of a Pseudo Random Bit Sequence (PBRS). The superconducting line measured at 77 K had a BER rate of less than 10/sup -11/ at a data rate of 3 Gb/s. The measured eye height was 350 mV for the YBCO compared to 100 mV for a gold line at 3 Gb/s and a temperature of 77 K for the packaged 6-cm lines.
The application of high temperature superconductor (HTS) Josephson junctions in digital rapid single flux quantum circuits requires a careful study of the influence of thermal noise on the bit error rate (BER). We have determined experimentally, for the first time, the BER of a HTS rapid single flux quantum circuit. A comparator, formed by two Josephson junctions, was integrated in a Josephson transmission line ring oscillator, allowing us to perform high speed testing of the comparator at GHz frequencies. For fabrication, focused-electron-beam-irradiated junctions have been used because of their small parameter spread and excellent alignment possibilities. A BER of less than 10−11 was obtained at 39 K.
A new method of analyzing bit error rates (BERs) for SFQ circuits and their outputs at GHz speeds will be presented. This method was used to test four- and six-bit pseudorandom code generators. The code generators were operated in free-run mode, i.e., without a synchronization (Reset) pulse. This enabled us to conduct BER testing to much lower levels than when the code generator is operated in Reset mode. The SFQ circuit output was amplified with a superconducting output latch to give voltage levels suitable for display on a sampling scope. The GHz output data was analyzed by external computer software; this permitted extended tests without user supervision. With the code generators operating in free-run mode, they routinely gave bit error rates (BERs) in the 10/sup -11/ range; the best BER recorded was 1.1*10/sup -13/ (2 errors in /spl sim/2.5 hours) at 2 GHz. We have successfully tested these code generators at frequencies in excess of 3.5 GHz with BERs in the 10/sup -10/ range as well.