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IEEE Transactions on Instrumentation and Measurement

A cryogenic system for RF measurements on superconducting vortex flow transistors

David P. McGinnis, James B. Beyer, J.E. Nordman

Published Jun 1, 1988• 0 citations
Publisher

Abstract

A scheme for measuring the RF properties of the superconducting vortex flow transistor (VFT) is presented. This scheme features a low-crosstalk RF launcher with solderless contacts, a cryogenic system that permits room temperature access in close proximity to the VFT, and a circuit configuration that allow vector S-parameter measurements at power levels below -90 dBm.>

DOI
10.1109/19.6065

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A scheme for measuring the RF properties of the superconducting vortex flow transistor (VFT) is presented. This scheme features a low-crosstalk RF launcher with solderless contacts, a cryogenic system that permits room temperature access in close proximity to the VFT, and a circuit configuration that allow vector S-parameter measurements at power levels below -90 dBm.>
Vector S-parameter measurements of the superconducting vortex flow transistor (VFT) are presented. The measurements were obtained for frequencies up to 100 MHz on VFTs that had a calculated transmit-time cutoff frequency of 5 GHz. An equivalent circuit model that includes calculations of the VFT transresistance, input inductance, and feedthrough capacitance is derived from these measurements. The measurements are limited to an upper frequency of 100 MHz due to crosstalk in the low-impedance system.>
A niobium-based superconducting amplifier consisting of a series array of 60 vortex flow transistors (VFT) was fabricated and tested. Each VFT consisted of a long Josephson junction biased in the flux flow state, magnetically coupled to a current based on a distributed amplifier configuration. The transresistance of the amplifier in a 50- Omega environment for frequencies up to 1 MHz was measured. The transresistance of the array was less than the expected value calculated from measurements obtained on a single VFT. The difference was probably due to nonuniformities in the fabrication process, which caused some of the junctions not to be biased in the flux flow state or receive the same amount of magnetic field from the control line. However, measurements of the array showed that a large number of VFTs could be connected in series and simultaneously be biased into the flux flow state.>
The dynamics of fluxons in the vortex flow transistor (VFT) has been much studied. The fluxon transit time determines the fundamental speed limit of operation. Since fluxons can travel at the velocity of electromagnetic waves in the junction, the VFT has the potential for operating in high frequency systems ( ≈ 100 GHz). However, owing to the low input and output impedance of the VFT, use of the device in a conventional circuit would be quite limited. A distributed amplifier configuration consisting of many VFTs has been proposed to remedy the problems of low impedance levels. However, the realization of such an amplifier circuit at microwave or millimetre wave frequencies depends on obtaining a circuit model. In this paper, microwave superconducting VFT distributed amplifiers using the balanced control technique is reviewed. This kind of amplifier has the advantage that the capacitive feedthrough effect is decreased to a negligible extent. This is the major limiting factor for high frequency applications. The self-field effect which makes the current step inclined is reduced by injecting bias current only around the region of one end of the junction thus obtaining steeper I- V characteristics. With the asymmetric geometry, the slope of the current step is about one hundred times steeper than those obtained with the conventional overlap geometry. Owing to the diamagnetic behaviour of the Josephson junction, a little of the magnetic field induced by the current in the control line is allowed to penetrate the junction. Thus, the transresistance rm of a VFT is very small. Some methods for maximizing rm and minimizing the output impedance r0 also appear in this review. The feasibility of fabricating VFT distributed amplifiers using low-Tc superconducting material has been demonstrated. The power gain of the amplifier can be as high as 15 dB with a flat frequency response.
We are developing a new instrument, the RF Josephson arbitrary waveform synthesizer (RF-JAWS), for communications metrology and quantum information applications. An important aspect of the RF-JAWS design is the accurate and traceable characterization of its superconducting devices. In this article, we present a procedure for characterizing microwave superconducting devices in a cryogenic RF probe station via a vector network analyzer (VNA) calibrated with a custom cryogenic calibration kit colocated with the superconducting device under test (DUT) in the cryogenic environment. By de-embedding lossy and dispersive RF interconnects linking the superconducting DUT to the measurement apparatus at room temperature, we characterize the DUT exactly at the cryogenic on-wafer reference plane. More importantly, we operate our VNA with an external modulated source and our procedure features metrology-grade multiline thru-reflect-line calibration and absolute power and phase corrections, as opposed to the more common relative scattering-parameter correction. In addition, we apply anX-parameter model to account for impedance mismatch in cryogenic Josephson microwave sources. Our techniques are also suitable for cryogenic characterization of microwave superconducting devices for solid-state quantum computers and could help to optimize the quantum-classical interfaces in these systems.
Josephson flux-flow transistors based on 0-32/spl deg/ [001]-tilt YBa/sub 2/Cu/sub 3/O/sub 7/ bicrystal junctions have been fabricated with Au loops as the control current lines. In order to measure the S-parameters of the transistors, no matching elements are implemented and 50 Omega coplanar lines are used as the input and output ends of the transistors. The rf properties of the transistors have been modeled on an HP microwave CAD design system, with the junction parameters taken from our dc measurements. Simulations indicate that the transistors can have a maximum stable gain of 20 dB at 7 GHz and 6 dB around 35 GHz under matched conditions. An 'on-wafer' transmission-reflection-load (TRL) method is used for calibrating the S-parameters of the transistor. A low temperature microwave probe station has been built: a pair of microwave probes inside a cryostat are moveable in the xyz-directions. Together with a Wiltron 360B vector network analyzer, this setup provides a convenient way for doing on-wafer characterization of the transistors in the frequency range from 40 MHz to 60 GHz, and in the temperature range from 20 to 80 K.>
Microwave two-port S-parameter measurements and modeling were performed on superconducting flux flow transistors. These transistors, based on the magnetic control of flux flow in an array of high-temperature superconducting weak links, can exhibit significant available power gain at microwave frequencies (over 20 dB at 7-10 GHz in some devices). The input impedance is largely inductive, while the output impedance is resistive and inductive. These devices are potentially useful in numerous applications, including matched amplifiers.>