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NASA STI/Recon Technical Report N

Superconducting Microwave Transmission Lines

C.P. McClay, Steven Soares, Peter S. Weitzman

Published Oct 1, 1991• 0 citations
OpenAlex

Abstract

Superconducting microwave transmission lines can be designed to have lower loss, lower dispersion, and lower phase velocity than conventional metal lines. These properties make superconducting transmission lines attractive for use in many devices and systems such as filters and analog to digital converters. The problem with designing microwave circuits which utilizes these lines is that accurate circuit models do not exist. This paper present models for the microwave transmission line parameters (phase velocity, attenuation, and characteristic impedance) of superconducting lines as a function of temperature and geometry. An experiment to verify these models is also presented.

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Contents: Superconducting Analog-to-Digital Converter Work at MITRE; Development of a Rapidly Tunable Microwave Source; Design and Evaluation of a Josephson Array Oscillator; Obtaining High-Accuracy Measurements from Low-Accuracy Measurements; Superconducting Microwave Transmission Lines; High Performance, Superconducting Analog-to-Digital Converter; Edge Sharpening with Josephson Junction; Design Evaluation of a Subranging Superconducting Analog-to-Digital Converter; Feasibility Study of a Superconducting Sigma-Delta Analog-to- Digital Converter, and VHDL Simulation Study of Superconducting Sigma Delta Modulators.

•Sep 1, 1991•0 citations
Superconducting microwave transmission lines can be designed to have lower loss, lower dispersion, and lower phase velocity than conventional metal lines. These properties make superconducting transmission lines attractive for use in many devices and systems such as filters and analog to digital converters. The problem with designing microwave circuits which utilizes these lines is that accurate circuit models do not exist. This paper present models for the microwave transmission line parameters (phase velocity, attenuation, and characteristic impedance) of superconducting lines as a function of temperature and geometry. An experiment to verify these models is also presented.
It is pointed out that superconducting microstrip transmission lines operating in the kinetic-inductance limit have the potential to reduce the size and weight of microwave circuits by several orders of magnitude. Since the phase velocity of these guided wave structures is up to a hundred times less than in free space, the electrical length of distributed elements is similarly reduced. Simple transmission lines have been fabricated from both low- and high-transition-temperature superconductors while more complicated circuits, such as filters, with a good match to 50- Omega coax have been fabricated with the low-transition-temperature superconductors. Also, a novel coupled-line topology, compatible with these structures, has been studied analytically.>
Since superconducting quantum computers use microwave signals to control and readout their qubits, microwave transmission lines and circuits that can be used under cryogenic conditions are required. For microwave transmission lines, thermal insulation is essential to enhance wiring density and to prevent parasitic heating that can lead to increased computational errors. Thus, here we design and characterize a scalable, thermally insulated microwave transmission line. The transmission line designed for superconducting qubit readout at 5 GHz band achieves both thermal insulation and low loss, by employing resonator-based impedance matching in a microstripline structure. A flexible cable including eight planar transmission lines was designed, fabricated and characterized using S-parameter measurement, demonstrating low crosstalk below 40dB and signal attenuation of -23.5±0.07dB; this coincides with electromagnetic simulations. These microwave properties remained stable across a wide temperature range from room temperature down to 4 K, including steep gradients. The integration of chip attenuators further reduces thermal noise and protects sensitive cryogenic components. Our approach offers a promising solution for compact, high-density microwave wiring using flat cables under thermal gradients, necessary for large scale quantum computing.
In this thesis we will describe the theoretical and experimental studies of a TEM on-chip superconducting transmission line with a wave impedance as high as 20 kΩ\mathrm{k}\OmegakΩ, phase and group velocity of waves simultaneously reduced by a factor of 100 in a broad range of frequencies from 0 to about 10 GHz\mathrm{GHz}GHz. A conventional microwave coaxial transmission line gets its inductance and capacitance from magnetic and electric fields stored in the space between its inner and outer conductors. This in turn limits its impedance to around 50 Ω\OmegaΩ and group velocity of waves very close to the speed of light in vacuum. In this work we are able to increase the impedance by over two orders of magnitude and reduce the group and phase velocity of waves by over two orders of magnitude as well, by constructing a coplanar transmission line out of a pair of long Al/AlOx/Al Josephson tunnel junction chains. A Josephson junction gets its inductance not from the magnetic energy but rather from the much larger kinetic energy of tunneling Cooper pairs, which is unrelated to the electromagnetic properties of vacuum. In this work we present a design of such a transmission line and low-temperature measurement of its dispersion relation. We then study and characterize the disorder present in the circuit parameters of our system and using this, we conclude that for frequencies up to 12 GHz, there is no evidence of Anderson localization of waves, even for chains exceeding 30,000 junctions. Low dissipation and absence of localization make this transmission line ideal for use in various experiments where high impedance can enable strong coupling between light and matter.
A digital-to-analogue converter based on series arrays of high-temperature superconducting Josephson junctions can work properly when the microwave bias current distribution in large arrays is uniform. Unfortunately, the considerable microwave attenuation per overdamped junction with normal resistances RN0.1 results in a strong total attenuation of the microwave signal in a traditional layout. Also, the meander geometry of the array, necessary when using bicrystal junctions, disturbs the uniformity of the rf current, especially in the millimetre waveband. To overcome these drawbacks we coupled the meander array inductively to a parallel transmission line. The mechanism of microwave coupling in which both external and mutual frequency locking takes place was studied numerically. A circuit with a coplanar waveguide feed line was fabricated and successfully tested at an operating frequency of 30 GHz. A microstrip line with a small attenuation for pumping of junction arrays at frequencies up to 120 GHz was demonstrated.
Abstract At present, microwave transmission lines are characterized by a strong frequency dependence of loss in the subterahertz range. This work is aimed at development, research, and optimization of superconducting integrated circuits designed for matching the impedances of a long Josephson junction oscillator (a so-called “flux-flow oscillator”) and a superconductor–insulator–superconductor (SIS) detector in the subterahertz frequency range. The goal of this study is to improve and approve the numerical calculation methods, which make it possible to describe correctly experimental superconducting structures in a wide frequency range. Numerical calculations of integrated circuits have been performed in order to optimize the topology and parameters of transmission lines. The main parameters of the transmission lines and their influence on the signal propagation are determined. The results of optimization of integrated matching circuits in the range of 450–700 GHz have been experimentally confirmed. Optimization and improvement of transmission lines allow one to design new-generation integrated superconducting detectors and investigate tunnel SIS junctions more thoroughly (including shunted ones) and the properties of heterodyne oscillators based on long Josephson junctions.
Contents: Superconducting Analog-to-Digital Converter Work at MITRE; Development of a Rapidly Tunable Microwave Source; Design and Evaluation of a Josephson Array Oscillator; Obtaining High-Accuracy Measurements from Low-Accuracy Measurements; Superconducting Microwave Transmission Lines; High Performance, Superconducting Analog-to-Digital Converter; Edge Sharpening with Josephson Junction; Design Evaluation of a Subranging Superconducting Analog-to-Digital Converter; Feasibility Study of a Superconducting Sigma-Delta Analog-to- Digital Converter, and VHDL Simulation Study of Superconducting Sigma Delta Modulators.