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Japanese Journal of Applied Physics

Fluxon Feedback Oscillator

Shigeki Sakai, Hiroshi Akoh, Hisao Hayakawa

Published Aug 1, 1984• 21 citations
Publisher

Abstract

A fluxon feedback oscillator which is formed by a ring-like Josephson transmission line with a feedback resistor is proposed. A device with a Josephson transmission line made by the NbN/NbN junction is fabricated, and the successful operation of the device is obtained using the Josephson sampler. The oscillation period can be controlled between 35 and 70 ps. The results agree well with simulation results. Possible applications of the fluxon feedback oscillator are also discussed.

DOI
10.1143/jjap.23.l610

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A fluxon feedback oscillator which is formed by a ring-like Josephson transmission line with a feedback resistor is proposed. A device with a Josephson transmission line made by the NbN/NbN junction is fabricated, and the successful operation of the device is obtained using the Josephson sampler. The oscillation period can be controlled between 35 and 70 ps. The results agree well with simulation results. Possible applications of the fluxon feedback oscillator are also discussed.
Dynamics of fluxons propagating in a Josephson transmission line (JTL) have been studied in real time using Josephson sampling techniques. A JTL is an NbN-oxide-NbN Josephson junction with dimensions of 2.5 × 150 μm2, while Josephson samplers and pulse generators for generating and driving fluxons are fabricated by a Pb-alloy technology. Among the various behaviors, especially, it is found that a propagating fluxon can be made to pause without reflections at any desired position which is created by placing a resistor on the surface of a portion of a JTL. The surface resistor has a length larger than the Josephson penetration depth. A pausing fluxon can be re-driven by supplying a small current to the portion of the surface resistor after more than 100 μs, The experimental results are compared with computer simulations. Reflections of fluxons observed by samplers have also been discussed.
A resistive-coupled fluxon divider consisting of Josephson transmission lines is designed and fabricated. Dynamic behavior of the divider is measured using two Josephson samplers connected to both output transmission lines, and successful operations are observed. It is also clarified that whether or not a fluxon signal can be transmitted to each output transmission line is determined mainly by the value of the bias current applied to each such line. Numerical simulations are made, and the results are in good agreement with the experimental ones.
We have designed ring oscillators to test rapid single flux quantum (RSFQ) circuits with passive transmission lines at high frequencies. To reduce the width of the transmission line, design methods for increasing the characteristic impedance of the transmission line have been investigated and then we designed a 5-stage of Josephson transmission line (JTL) receiver which can transform the impedance from 1 to 4 /spl Omega/ by changing the critical current and the inductance. The receiver, driver, and micro-strip line were inserted in a ring oscillator. The ring oscillator fabricated with the Nb/AlOx/Nb junction process was successfully operated at frequencies ranging from 6 to 10 GHz for line lengths of 0.6, 3, 6, and 9 mm. The usefulness of the impedance transformation was then confirmed experimentally. A propagation delay per unit length of 8.4 ps/mm was also obtained.
The authors propose a new type of sampler design based on shunted junctions ( beta c =1). The sampler is to be used for the investigation of single flux quantum devices (RSFQ) employing similar junctions (Kaplunenko et al. 1989, Filippenko et al. 1991). They measure the time resolution of the sampler and delay time of the shunted Josephson junction array. The circuit was manufactured on a single chip and was DC powered. They employed Nb-AlO x -Nb tunnel junctions of 300 A m -2 critical current density and observed Josephson oscillations of the output junction at frequencies from 6 to 50 GHz. Despite a long plasma period of 20 ps the time resolution was 5 ps. The transmission line of 27 Josephson junctions (critical current 165 mu A each) demonstrated time delay of 0.5 ps mu A -1 .