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Electronics Letters

Noise comparison of biased and unbiased even-harmonic Josephson mixers

A. Yamaguchi, S. Kita, K. Fujisawa

Published Apr 25, 1985• 0 citations
Publisher

Abstract

The excess noise in biased and unbiased even-harmonic Josephson mixers is studied numerically. The results show that the unbiased mixing has much lower excess noise. This is attributed to the nonexistence of noise down-conversions due to the odd-harmonic mixing and the Josephson mixing.
DOI
10.1049/el:19850282

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Noise properties of Josephson harmonic mixers were investigated both by simulation studies and by experiments. It was clarified that the unbiased mixing operation peculiar to even-harmonic Josephson mixers has a lower noise temperature than the biased operation. A minimum noise temperature of 121 K was obained for an unbiased second-harmonic Josephson mixer at 70 GHz. The cause of this low-noise property of the unbiased mixing was attributed mainly to the nonexistence of noise down-conversions by the odd-harmonics of LO and the Josephson oscillation.
Even-harmonic JJ mixers allow unbiased operation, while odd-harmonic ones do not. Phenomenological explanations are presented for this phenomenon. Based on the voltage source model, a simple analysis is given that explains this phenomenon clearly. Furthermore, based on the current source model, this is studied by using a JJ simulator. In the odd-harmonic mixing, the waveform of supplied current has an odd symmetry, therefore, there is no mean junction voltage and no IF output. In the even-harmonic mixing, there is no such symmetry in the waveform of supplied current, therefore, there is a finite mean junction voltage and IF output.
We have studied the JJ harmonic mixing with an analog simulator devised by Magerlein. For even-harmonic mixing, there are two operation modes: zero-bias operation and finite-bias operation. The former is peculiar to even-harmonic mixing, while the latter is common to both even- and odd-harmonic mixing. Zero-bias operation is possible with a supply of LO current that suppresses the zero-voltage step completely, and it has the advantage of eliminating the dc bias supply and its accompanying heating effect and shot noise. For relatively low LO frequency (\varOmegaLO≃0.6), zero-bias operation has a higher conversion efficiency than does finite-bias operation. 2nd harmonic mixing under zero-bias operation has a conversion efficiency nearly equal to the fundamental mixing in the range \varOmegaS\lesssim0.5.
We have carried out a series of experiments at 70.03 GHz on Josephson junction harmonic mixers with harmonic numbers 1–8, using an identical Josephson junction. It was found that Josephson junction harmonic mixers operate differently depending on whether their harmonic numbers are odd or even. With odd-harmonic mixers, only finite-bias operation is possible, while with even-harmonic mixers, finite-bias and zero-bias operations are both possible. It is noted that a 2nd-harmonic mixer has a conversion efficiency equal to or higher than that of a fundamental mixer. The dependence of the conversion efficiency (η) on the harmonic number (N) was also studied, and the following approximate relations were determined: η∼N -1.6 for finite-bias operation and η∼N -1.4 for zero-bias operation.