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Applied Physics Letters

Superconductor digital frequency divider operating up to 750 GHz

W. Chen, Alexander V. Rylyakov, Vijay Patel, James E. Lukens, K. K. Likharev

Published Nov 9, 1998• 106 citations
Publisher

Abstract

A superconductor frequency divider based on rapid single flux quantum (RSFQ) logic has been demonstrated to operate from dc to 750 GHz with an error rate less than the measurement limit (25 MHz). This high operating frequency is made possible by the use of small (0.25 μm2), high critical current density Josephson junctions along with an optimized design having parameter margins of ±30%. Simulations based on the Werthamer model are in reasonable agreement with the data and give a SFQ pulse width of 1 ps and a maximum power dissipation of 1.5 μW.

DOI
10.1063/1.122600

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IEEE Transactions on Applied Superconductivity•Mar 1, 1993•133 citations
A superconductor frequency divider based on rapid single flux quantum (RSFQ) logic has been demonstrated to operate from dc to 750 GHz with an error rate less than the measurement limit (25 MHz). This high operating frequency is made possible by the use of small (0.25 μm2), high critical current density Josephson junctions along with an optimized design having parameter margins of ±30%. Simulations based on the Werthamer model are in reasonable agreement with the data and give a SFQ pulse width of 1 ps and a maximum power dissipation of 1.5 μW.
The paper presents the determination of the high-frequency performance of Rapid Single Flux Quantum (RSFQ) circuits fabricated in superconductor-insulator-normal metal-insulator-superconductor (SINIS) technology. Circuits of different designs have been realized and were experimentally investigated, including specially designed T flip-flop (TFF) structures and circuits consisting of a switch terminated in Josephson transmission lines. For TFF circuits, the operational functionality was investigated in the frequency range from dc up to nearly 200 GHz. Two frequency domains were found. In the first domain, ranging up to frequencies of 80 GHz (f/sub C//spl cong/Y/sub C///spl Phi//sub 0/), correct digital operation of the device was ascertained. Beyond this range a second domain was found, ranging to nearly 200 GHz, for which quasi-analog frequency dividing is assumed. The bias current margins are /spl plusmn/30% at 40 GHz and /spl plusmn/15% between 130 GHz and 200 GHz. For circuits consisting of a switch, the operational functionality was proved in the frequency range from dc up to 50 GHz, with bias current margins of more than /spl plusmn/20%. The critical current density of the circuits is j/sub C//spl cong/750 A/cm/sup 2/. The smallest junction area is about A=12 /spl mu/m/sup 2/, and the characteristic voltage is V/sub C/=170 /spl mu/V.
A low-skew frequency divider and clock controller have been designed for high-frequency timing of superconductor rapid single-flux-quantum (RSFQ) digital systems. The circuits have only about 10-ps skew between input and output signals and are applicable for multirate digital systems (e,g., oversampling analog-to-digital converter and bit-serial digital systems). Several circuits have been fabricated in conventional Nb-trilayer technology with a critical current density of 1 kA/cm/sup 2/. The most complex clock controller generates trains of 2/sup 24/ single-flux-quantum pulses with a period of less than 70 ps. The long-term relative stability of these intervals has been measured to be better than 6/spl times/10/sup -5/. The basic component of the controller, a frequency divider, operates at input frequencies above 85 GHz.>
A simple test circuit of the rapid single-flux-quantum (RSFQ) logic family has been implemented in a planarized all-refractory technology using only two superconducting layers and small Nb/AlOx/Nb Josephson junctions (nominally 1.5×1.5 μm2) with critical current density jc≊6 kA/cm2. A special layout design of the integrated circuit (including a SFQ pulse generator, Josephson transmission line, and T flip–flop) was used to reduce 3D inductances resulting from absence of the ground plane. For samples with various junction parameters, the maximum frequency of the SFQ pulse train, which could be divided by 2 by the flip–flop, ranged from 200 to 370 GHz.
A new generation of the Josephson digital circuits-rapid single flux quantum (RSFQ) devices-based on overdamped externally shunted high quality Nb-AlO x -Nb tunnel junctions have been designed, fabricated and experimentally investigated. All-refractory material 11-layer 5 mu m integrated technology has been successfully employed for the preparation of the RSFQ microcircuits. The T flip-flop has been demonstrated to operate properly up to clock frequencies of 100 GHz at 25% margins. The proper operation of the buffer stage for a one-directional SFQ pulse transmission at clock frequencies up to 125 GHz has been achieved. The possibilities to convert the information from the usual DC form to SFQ and vice versa have been demonstrated. An output signal in an SFQ/DC converter as high as 220 mu V has been measured. A new counter type A/D converter which includes a comparator and reversible binary counter with DC outputs has been fabricated and tested. The proposed ADC seems to be very promising for high-accuracy measurement of medium-bandwidth signals. The possibilities of creating a number of SFQ devices with outstanding parameters are discussed.
The operational functionality of rapid-single-flux-quantum (RSFQ) T-flipflop circuits has been experimentally investigated in the frequency range from dc up to nearly three times the characteristic frequency . The operational range found is divided into two domains different in nature. In the first domain, ranging up to frequencies of 120 GHz , correct digital operation of the device has been found. Beyond this range a second domain has been found, ranging from about 200 GHz to nearly 350 GHz, in which quasi-analogous operation of frequency dividing is suggested. Typical bias current margins are in the first domain and in the second. The circuits have been fabricated using - Al/Nb trilayer technology with externally shunted Josephson tunnel junctions of critical current densities of and smallest areas of about . The characteristic voltage is and the McCumber parameter .
Several novel circuits of the rapid single-flux-quantum (RSFQ) family of Josephson-junction digital devices have been designed, fabricated using a 2.5- mu m 1000-A/cm/sup 2/ Nb trilayer technology, and tested at low frequencies. Numerical simulation and measurements have shown that these circuits have considerably wider parameter margins, due to application of several novel design methods. The authors have also carried out an experiment to measure the rate of errors in a simple RSFQ circuit including an inverter, confluence buffer, and Josephson transmission line. Near the middle of the parameter window at 4.2 K, the error probability was definitely lower than 3*10/sup -15/ per logic operation, despite experimentation with rudimentary shielding and filtering.>