Observation of Frequency Up-Conversion Gain in SIS Junctions at W Band
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Observation of Frequency Up‑Conversion Gain in SIS Junctions at W Band Y. Kozuki1 · Y. Uzawa2 · T. Kojima2 · W. Shan2 · T. Sakai1 Received: 4 August 2019 / Accepted: 29 April 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract We have developed a test setup to measure frequency up-conversion gain in superconductor–insulator–superconductor (SIS) junctions at W band (75–110 GHz). A conventional SIS mixer with Nb/AlOx/Nb tunnel junctions was used as a frequency up-converter. An up-converted signal is measured by a room-temperature downconverter system. We observed distinct intermediate frequency responses to signal inputs from a continuous microwave source. The measured single-sideband up-conversion gain in SIS junctions was clearly positive (> 0 dB) with input frequency less than 2 GHz, which was well-calibrated by using an input from the continuous-wave source. Keywords Frequency up-conversion gain · Low-noise microwave amplifier · Millimeter wave · Nb/AlOx/Nb tunnel junction · SIS mixer
1 Introduction Superconductor–insulator–superconductor (SIS) tunnel junctions have been well developed and widely used as frequency down-converters in practical applications such as radio astronomy and global environmental monitoring, since Tucker [1] described a quantum mechanical treatment of heterodyne mixing due to the quasiparticle currents. In addition to their ultra-low-noise performance approaching the quantum limit, it is also predicted that the SIS mixers exhibit positive gain in the down-conversion process, which was demonstrated by Shen et al. [2] after a short time. This cannot be realized by classical mixers. In principle, the SIS tunnel junctions can also work as frequency up-converters in which a microwave signal is up-converted to a millimeter/submillimeter-wave signal
* Y. Kozuki [email protected] 1
University of Electro-Communications, Chofu, Japan
2
National Astronomical Observatory of Japan, Mitaka, Japan
13
Vol.:(0123456789)
Journal of Low Temperature Physics
with a local oscillator (LO) signal. We have calculated the up-conversion characteristics of SIS mixers based on the quantum mixing theory [3]. The analytical results show that it is necessary to select LO frequencies larger than the voltage scale of the dc nonlinearity of the SIS junction in order to obtain positive conversion gains. So far, there have been a few reports on the possible use of SIS junctions as up-converters for frequency multiplexers, LO sources, and so on [4, 5]. However, those applications do not focus attention on the conversion gain of the SIS junctions, because positive gains are not necessarily required for them. Our proposal is to use SIS junctions as a frequency up-converter in a novel microwave amplifier in combination with an SIS down-converter. Given that both SIS mixers (up- and down-converters) pumped by an LO power have positive conversion gains and were connected in cascade, input and output frequencies are identical with a signal gain. We have demonstrated it by our proof
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