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Japanese Journal of Applied Physics

Simulator Experiments on Josephson Junction Harmonic Mixers

Y. Ohmae, Kenji Kubo, Sadamu Kita, Kazuo Fujisawa

Published Feb 1, 1981• 2 citations
Publisher

Abstract

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.

DOI
10.1143/jjap.20.423

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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.
Josephson junction (JJ) harmonic mixers with even-harmonic numbers can operate at zero bias voltage. Among them, low harmonic ones are expected to have a good conversion efficiency. Hence, we have done experiments on the 2nd-harmonic mixer at 70 GHz. It is found that mixing behaviors are classified into three types: zero-bias, finite-bias and intermediate operation. It is also found that the conversion efficiency of the 2nd-harmonic mixer is not so much less than that of the fundamental mixer. We obtained a conversion gain of +1.5 dB with the 2nd-harmonic mixer, and +2.0 dB with the fundamental mixer.
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.
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.
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.
Two high-Tc superconducting (HTS) YBa2Cu3O7-δ(YBCO) grain boundary (GB) Josephson junctions in series as a harmonic mixer is investigated. The research shows that the mixer consisting of two series YBCO Josephson junctions has a larger harmonic number than that with only one junction in heterodyne mixing at zero-voltage bias. One of the advantages of the series-Josephson-junction mixer is that the total critical current is improved without sacrificing the sensitivity of the junctions in the array that makes the mixer easier to get more microwave induced-steps required by larger harmonic number. Furthermore, the underdamped YBCO GB series-Josephson-junction mixer operated at zero-bias voltage, where the zero-order Shapiro step exists, achieves mutual phase locking of the junctions in the array, resulting in an excellent mixing performance. Thus, the series-underdamp-Josephson-junction mixer with zero bias is predominant and compact in terahertz (THz) application due to its excellent performance and no dc bias supply.
We have demonstrated fundamental and harmonic mixing using a PbInAu/InOxPbOx/Ag superconductor-insulator-normal metal tunnel junction mixer designed to operate over the 450–550 GHz frequency range. Direct mixing experiments at up to 1.8 times the gap frequency of the tunnel junction have shown that the mixer works in both fundamental and second harmonic modes, with the highest conversion from the fundamental mode obtained at bias voltages near the gap voltage and the highest conversion from the harmonic mode obtained at zero bias. The measured mixer conversion curves are consistent with our five-port model simulations based on Tucker’s quantum mixer theory [IEEE J. Quantum. Electron. QE-15, 1234 (1979)].