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Superconductor Science and Technology

A ring circuit for the determination of dynamic error rates in high-temperature superconductor RSFQ circuits

B Ruck, B. Oelze, Erik Sodtke

Published Dec 1, 1997• 4 citations
Publisher

Abstract

We have designed and simulated a circuit for the experimental determination of the rate of dynamic switching errors in high-temperature superconductor RSFQ circuits. The proposal is that a series-connected pair of Josephson junctions is read out by SFQ pulses circulating in a ring-shaped Josephson transmission line at high frequency. Suitable bias currents determine the switching thresholds of the junction pair. By measuring the voltage across the transmission line, it is proposed that the occurrence of a switching error can be detected. The bit error rate can be determined from the mean time before false switching together with the SFQ circulation frequency. The circuit design allows measurements over a wide temperature range.

DOI
10.1088/0953-2048/10/12/027

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We have designed and simulated a circuit for the experimental determination of the rate of dynamic switching errors in high-temperature superconductor RSFQ circuits. The proposal is that a series-connected pair of Josephson junctions is read out by SFQ pulses circulating in a ring-shaped Josephson transmission line at high frequency. Suitable bias currents determine the switching thresholds of the junction pair. By measuring the voltage across the transmission line, it is proposed that the occurrence of a switching error can be detected. The bit error rate can be determined from the mean time before false switching together with the SFQ circulation frequency. The circuit design allows measurements over a wide temperature range.
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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.
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The use of high-T/sub c/ Josephson junctions for digital applications requires a careful study of the influence of thermal noise on circuit performance. We investigated a balanced current comparator, which contains a basic component of all RSFQ circuits: two overdamped Josephson junctions connected in series. The dependence of the dc-voltage across the Josephson junctions on signal current was measured to determine the threshold uncertainty of switching of the comparator. The experimental data obtained at different sampling pulse frequencies, in a temperature range from 10 to 50 K, were compared with a theoretical model, taking into account the influence of thermal noise as well as of sampling pulse frequency.