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Superconductor Science and Technology

Results on a fast digital DROS

M. Podt, A.J. Mieog, B. G.A. Rolink, Jakob Flokstra, Horst Rogalla

Published Nov 21, 2001• 7 citations
Publisher

Abstract

A digital double relaxation oscillation (DROS) superconducting quantum interference device (SQUID) has the potential for a very large dynamic range and a slew rate that is orders of magnitude larger than that of conventional SQUID systems. This is, for example, advantageous when using the SQUID in an unshielded environment. An important characteristic of the digital DROS is that the feedback flux is quantized since this flux is supplied by a superconducting up-down counter. Together with the clock frequency, the quantization unit of the feedback flux determines the maximum slew rate. In an optimized design, the quantization unit is adapted to the broadband flux noise of the DROS, so that the slew rate is maximized without compromising the sensitivity. Simulations on our optimized digital DROS showed proper operation for slew rates up to 5×10 6 Φ 0 s -1 . The relaxation oscillations generated at a frequency of 100 MHz deliver an on-chip clock signal, so that no external clock is required. Slew rates up to 10 8 Φ 0 s -1 can be achieved by increasing the relaxation oscillation frequency to a few GHz. This means that sub-µm 2 Josephson junctions with high critical current densities are required. The first experiments using low- T c ramp-type Josephson junctions are presented in this paper.

DOI
10.1088/0953-2048/14/12/309

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A digital double relaxation oscillation (DROS) superconducting quantum interference device (SQUID) has the potential for a very large dynamic range and a slew rate that is orders of magnitude larger than that of conventional SQUID systems. This is, for example, advantageous when using the SQUID in an unshielded environment. An important characteristic of the digital DROS is that the feedback flux is quantized since this flux is supplied by a superconducting up-down counter. Together with the clock frequency, the quantization unit of the feedback flux determines the maximum slew rate. In an optimized design, the quantization unit is adapted to the broadband flux noise of the DROS, so that the slew rate is maximized without compromising the sensitivity. Simulations on our optimized digital DROS showed proper operation for slew rates up to 5×10 6 Φ 0 s -1 . The relaxation oscillations generated at a frequency of 100 MHz deliver an on-chip clock signal, so that no external clock is required. Slew rates up to 10 8 Φ 0 s -1 can be achieved by increasing the relaxation oscillation frequency to a few GHz. This means that sub-µm 2 Josephson junctions with high critical current densities are required. The first experiments using low- T c ramp-type Josephson junctions are presented in this paper.
A wide-band double relaxation oscillation superconducting quantum interference device (DROS) with on-chip digital flux locked loop (FLL) circuitry, a Smart DROS, has been realized and experimentally investigated. The key element of the FLL circuitry is a superconducting up–down counter. In order to maximize the flux slew rate, the Smart DROS has been optimized with respect to the coupling between the DROS and the up–down counter, and the quantization unit of the feedback flux. For the Smart DROS with an inductance of 280 pH the flux quantization unit was 5 × 10−2 Φ0. The experiments showed proper operation of the superconducting up–down counter. The full functionality of the complete Smart DROS in a closed loop was demonstrated. For operation at a clock frequency of 100 MHz, a maximum flux slew rate of 5 × 106 Φ0 s−1 was calculated from the measured parameters. Numerical simulations showed that the Smart DROS allows a maximum flux slew rate up to 108 Φ0 s−1 by reducing the area of the Josephson junctions.
We designed a digital double relaxation oscillation SQUID (DROS) with an SFQ up/down counter. The DROS is connected to a feedback SQUID loop and a signal magnetic flux is input to the feedback SQUID loop. The DROS generates two kinds of voltage pulses corresponding to the total magnetic flux in the feedback SQUID loop. Two kind of voltage pulses are converted to SFQ and anti-SFQ pulses and input to the feedback SQUID loop to keep the magnetic flux in the SQUID loop constant. The number of the generated SFQ pulses is counted using the SFQ up/down counter. Thus, the count corresponds to the input magnetic flux. Because the SFQ up/down counter is operated in the pulse driven mode, the digital DROS needs only dc bias current for operation. We designed and fabricated a digital DROS with a relaxation oscillation frequency of 3 GHz and an 8-bit SFQ up/down counter. Numerical simulation suggested the designed digital DROS has a dynamic range exceeding ±100 Φ0and magnetic flux slew rate of 3 × 109Φ0/s.
Double relaxation oscillation SQUID (DROS) combined with an on-chip superconducting digital flux locked-loop (FLL) circuit has been proposed. In this study, we described superconducting digital FLL circuit which consists of an 8-bit up/down counter and an 8-bit R-2R ladder D/A converter. The circuit was designed using 4-Junction Logic (4JL)-gates which are driven by a two phase power supply. In the up/down counter, we employed binary carry lookahead (BCL) circuits for high-speed operation. Logic simulations for the BCL 8-bit up/down counter showed correct operation up to 4 GHz, assuming a 2.5 kA/cm/sup 2/ Nb/Al-AlO/sub x//Nb junction technology. From simulation results high slew rate of 10/sup 7//spl Phi//sub 0//s and dynamic range of 2.5/spl Phi//sub 0/ can be expected.
We have realized a two-stage integrated superconducting quantum interference device (SQUID) system with a closed loop bandwidth of 2.5 MHz, operated in a direct voltage readout mode. The corresponding flux slew rate was 1.3×105 Φ0/s and the measured white flux noise was 1.3 μΦ0/√Hz at 4.2 K. The system is based on a conventional dc SQUID with a double relaxation oscillation SQUID (DROS) as the second stage. Because of the large flux-to-voltage transfer, the sensitivity of the system is completely determined by the sensor SQUID and not by the DROS or the room-temperature preamplifier. Decreasing the Josephson junction area enables a further improvement of the sensitivity of the two-stage SQUID systems.
Double relaxation oscillation superconducting quantum interference devices (SQUIDs) (DROSs) based on Nb/AlOx tunnel junctions have been fabricated and characterized. The estimated relaxation frequencies range from 400 MHz up to 14 GHz. Transfer coefficients from flux to voltage of 2 up to 7 mV/φ0 have been obtained. Both the intrinsic flux noise and the performance in a flux-locked loop with direct-voltage readout have been determined. Special attention is paid to the effect of damping resistors on the sensitivity of DROSs. The intrinsic sensitivity improves with increasing relaxation frequency, leveling off to a value of 13h at relaxation frequencies above 2–3 GHz for SQUID inductances of about 30 pH. This sensitivity is very close to the theoretical maximum sensitivity of 6h of a comparable standard type dc SQUID. In a flux-locked loop based on direct-voltage readout, a noise level of 0.55 μφ0/√Hz corresponding to an energy sensitivity of 34h has been obtained for a DROS with a SQUID inductance of 29 pH.
Digital SQUIDs based on the single flux quantum (SFQ) logic have attractive characteristics, such as wide dynamic range and high slew rate. However, the resolution of digital SQUIDs is limited to SFQ. To realize sub-flux quantum resolution for these SQUIDs, we have adopted two types of feedback combinations: one with the SFQ input/output for a main loop and the other with a mutually coupled loop. The main loop receives direct Φ0feedback and sub-flux quantum feedback Φ0/n, where n is an integer, from the mutually coupled loop. We confirmed the correct operation of our device at a clock frequency of 1.25 GHz for a digital SQUID with a sub-Φ0flux feedback resolution of Φ0/8 from circuit simulation. The slew rate is found to be 1.56×108Φ0/s in this simulation (fc=1.25 GHz, n=8). The flux resolution for the digital SQUID shows improvement with increasing n and reached below 40 mΦ0with n=210; however the slew rate is decreased with increasing n and reached 1.22×106Φ0/swith fc=1.25 GHz and n=210.