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Journal of Applied Physics

Harmonic Generation by Resistive Feedback from Superconducting Point Contacts

Robert Adde, G. Vernet

Published May 1, 1972• 5 citations
Publisher

Abstract

The rf emission from a superconducting point contact driven with a constant current source allows the direct observation of harmonic generation by purely resistive feedback. The amplitudes of the harmonics are shown to be in agreement with the theoretical values determined from a simple model of the junction.
DOI
10.1063/1.1661510

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Spectral analyses are carried out on the strong RF (1.2-1.4 GHz) voltage fluctuations associated with the sudden onset of DC voltage near the critical current in a "clean" superconducting point contact. The strong RF voltage is concluded to arise from the remarkably efficient frequency conversion effects of weak external low-frequency (below 100 MHz) interference. Strong higher harmonics of intentionally-applied signals of about 250 and 90 MHz are observed up to 2 GHz. The mechanism of efficient harmonic generation is discussed briefly.
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The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb–Nb point-contact junctions at 4.2 °K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1, 0.3, and 9.2 GHz. The point contact is in a nonresonant structure and fed by a constant current source: neither a strong rf coupling of the contact with a cavity nor a stabilizing shunt resistance was proved necessary. The direct observation of frequency conversion was performed with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1600 GHz. The experiments with intermediate frequencies fi of 0.1 and 0.3 GHz indicate that conventional frequency conversion does not occur but that the observed signals are related to noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. The results show that low-frequency current components are induced in the junction, which can give large spurious signals when small i.f. are used.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb-Nb point contact junctions at 4.2 K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1 and 9.2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0.1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9.2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.