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European Microwave Conference (EuMC)

A New Broadband 50 OHM Coaxial- to 1 OHM Microstrip Transition for Superconducting Thin Film Devices with Low Input Impedance

M. Daginnus, O. Schattat, Johann Heyen Hinken

Published Oct 1, 1991• 0 citations
Publisher

Abstract

We report on the development of a new transition from a 50 Ohm coaxial line to a 1 Ohm microstrip line. The impedance transformation is done with a three step microstrip quarter-wave-transformer, where the width of the strip and the height of the air dielectric between strip and back-electrode are changed step by step. The thin film strip is evaporated on low loss LaAlO3-substrate. The transition has a transmission loss of less than 2 dB between 16.1 and 17.9 GHz, and less than 3 dB between 15.8 and 20.0 GHz.

DOI
10.1109/euma.1991.336382

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We report on the development of a new transition from a 50 Ohm coaxial line to a 1 Ohm microstrip line. The impedance transformation is done with a three step microstrip quarter-wave-transformer, where the width of the strip and the height of the air dielectric between strip and back-electrode are changed step by step. The thin film strip is evaporated on low loss LaAlO3-substrate. The transition has a transmission loss of less than 2 dB between 16.1 and 17.9 GHz, and less than 3 dB between 15.8 and 20.0 GHz.
Three different schemes for coupling to low impedance Josephson devices have been investigated. They all employ superconducting thin-film microstrip circuit techniques. The schemes are: (i) a quarterwave stepped impedance transformer, (ii) a microstrip resonator, (iii) an adjustable impedance transformer in inverted microstrip. Using single microbridges to probe the performance we found that the most primising scheme in terms of coupling efficiency and useful bandwidth was the adjustable inverted microstrip transformer.
We present a 4 : 1 superconducting transmission-line impedance transformer for cryogenic applications. The device transforms 25 Ω in the coplanar waveguide to 6.25 Ω in the microstrip and is designed to operate at 20 mK. Calibrated measurements in a dilution refrigerator demonstrate a -3 dB bandwidth from 1.6 to 13 GHz. In a modified Ruthroff design, a small capacitor is integrated at the input as a direct-current block, making it suitable for biasing and matching to low-impedance active circuits, such as superconducting quantum interference device (SQUID) amplifiers.
We have developed a novel technique for making high quality measurements of the millimeter-wave properties of superconducting thin-film microstrip transmission lines. Our experimental technique currently covers the 75–100 GHz band. The method is based on standing wave resonances in an open ended transmission line. We obtain information on the phase velocity and loss of the microstrip. Our data for Nb/SiO/Nb lines, taken at 4.2 K and 1.6 K, can be explained by a single set of physical parameters. Our preliminary conclusion is that the loss is dominated by the SiO dielectric, with a temperature-independent loss tangent of 5.3±0.5×10−3 for our samples.
Abstract We describe a superconducting (SC) cable-to-cable connection method for thin-film flexible (Nb/polyimide) parallel transmission line cables that exhibits low loss from DC to microwave frequencies. Design, fabrication procedures, assembly methods and characterization results are reported. The connector assembly incorporates a monolithic array of short superconductive bridging traces that function as a ganged electrical splice between two ribbon cables. The assembly is micro-mechanically self-aligned and uses pressure to make simultaneous electrical contacts between multiple 50 μ m wide microstrip signal traces, as well as between ground planes. Each contacting surface comprises thin SC Nb overcoated with ultra-thin Au, providing nearly zero DC resistance and very good RF performance. A simulation model of the microwave performance of microstrip-to-microstrip interconnections at 4.2 K was successfully matched to measurement results. This new cable-to-cable connection approach exhibited an insertion loss <0.12 dB and a return loss >15 dB up to 14 GHz. The connections performed reproducibly throughout multiple thermal cycles and multiple disassembly/reassembly cycles.
We present recent work on design, fabrication, and characterization of thin-film, flexible superconducting stripline cables. Fabrication details, dc, and microwave performance (up to 14 GHz) of cables with Al/Nb/Al conductors and polyimide (PI) dielectric are discussed. Thin Al was used as a barrier layer between Nb and polyimide to protect the Nb superconductivity during subsequent PI curing steps, which were performed at a reduced temperature. The dc performance, including critical transition temperature and critical current, of the stripline transmission lines is presented. We observed Tcvalues of ~8.8 and ~8.6 K for signal traces and ground planes, respectively. Microwave characterization, including cross-talk, was performed on stripline transmission lines and resonators. We observed reasonable impedance match (S11better than -15 dB), low insertion loss (S21better than 0.04 dB/cm) and cross-talk below -60 dB for stripline transmission lines measured up to 10 GHz at 4.2 K. Furthermore, similarly fabricated stripline resonators exhibited quality factors ~7 000 at 10 GHz and 1.2 K for stripline resonators. The results of these experiments provide insight into material stack-ups for robust, multi-layer superconducting flexible cables that can find use in future cryogenic electronics systems.
Measurements at 3 GHz of the microwave properties of a superconducting thin-film microbridge coupled to a narrowly tuned strip-line resonator (coupled Q approximately 10 3 ) indicate that such a device displays the same variations in small-signal impedance as predicted by the resistively shunted junction model of a Josephson weak-link. Whenever this impedance becomes negative the microbridge is found to generate microwave radiation whose frequency is determined not by the junction bias voltage but by the natural resonance of the strip-line.