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Superconductor Science and Technology

Development of a rapidly tunable microwave source

K Wan, A. K. Jain, Linus A. Fetter, W Zhang, Siyuan Han, James E. Lukens

Published Nov 1, 1991• 3 citations
Publisher

Abstract

Frequency tuning rates have been measured for coherent oscillators using Josephson junctions coupled in a lumped circuit configuration. Coherent operation of the array was maintained while modulating the frequency between 12 GHz and 18 GHz at frequencies up to 3.6 GHz. The array can achieve coherent power output in less than 1 mu s as the bias current is stepped up to switch the junctions from the zero to finite voltage state.

DOI
10.1088/0953-2048/4/11/025

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Frequency tuning rates have been measured for coherent oscillators using Josephson junctions coupled in a lumped circuit configuration. Coherent operation of the array was maintained while modulating the frequency between 12 GHz and 18 GHz at frequencies up to 3.6 GHz. The array can achieve coherent power output in less than 1 mu s as the bias current is stepped up to switch the junctions from the zero to finite voltage state.
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Josephson junction arrays have long been suggested as efficient, tunable sources for upper microwave and mm-wave frequencies. Based on the fundamental properties of the junction and the ability to combine the power output of many junctions using an array, the circuit concept is quite promising. The challenge is to phase lock the junctions so that power adds coherently and a number of demonstrations with Nb junction arrays have been performed doing this. This program was intended to begin using YBCO junction arrays to demonstrate source potential at 77K. Edge junctions have been used in arrays with up to 60000 junctions and the ability to couple power off-chip has been demonstrated directly for frequencies of at least 70-160 GHz. Power outputs have approached 1 micro W (in relatively narrow bands) and tunability has exceeded 20 GHz. Some novel array architectures have been built, including one with improved linewidth per unit junction, and new techniques developed for extracting junction statistics from array spectra. These techniques are used as feedback for process optimizations now in progress.
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We have investigated a new type of microwave circuit for the generation of frequency-scaled Josephson voltages useful for the representation of highly accurate reference voltages. The circuit consists of a superconducting microstrip resonator and series arrays of Josephson junctions integrated within its rf current antinodes. The circuit is designed to operate at low frequencies of about 10 GHz to simplify the microwave techniques. At a resonator circuit with four Josephson junctions driven at 10.9 GHz we have obtained stable overlapping Shapiro steps up to 4.6 mV at a step width of about 70 µA. A more advanced double-resonator circuit with altogether 216 junctions allows to generate scaled reference voltages up to 0.16 V at steps of about 25 µA in width and at a microwave frequency of 11.1 GHz.
A submillimeter wave source based on a new design using Josephson junction arrays has been developed and tested. The maximum rf power, delivered to a 68Ω load and detected on chip, was 47 μW at 394 GHz. Significant power was detected at a number of frequencies from 300 to 500 GHz where the power was 10 μW. The observed power at the designed operating frequency near 400 GHz is consistent with all 500 junctions in the series biased array delivering current in phase to the loads. This is in agreement with simulations of smaller arrays of the same design. The linewidth, inferred from the measured resistance at the point of maximum power, with T=4.2 K, is less than 1 MHz. The minimum inferred linewidth near 400 GHz, at somewhat lower power, is about 100 kHz.