Quasi-Optical SIS Mixers with Nb/AIO_x/Nb Tunnel Junctions in the 270-GHz Band (Special Issue on Toward Digital and Analog Applications of Superconductors)

概要

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A quasi-optical Superconductor-lnsulator-Superconductor (SIS) mixer has been designed and tested in the 270-GHz band. The mixer used a substrate-lens-coupled log-periodic antenna and a tuning circuit for RF matching. The antenna is planar and self-complementary, and has a frequency-independent impedance of around 114Ωover several octaves.The tuning circuit consists of two Nb/AlO_x/Nb tunnel junctions separated by inductance for tuning out the junction capacitances and aλ/4 impedance transformer for matching the resistance of the two-junction circuit to the antenna impedance. The IF output from the mixer is brought out in a balanced method at each edge of the antenna, and is coupled to a low noise amplifier through a balun transformer using a 180-degree hybrid coupler for broadband IF matching. Double sideband receiver noise temperatures, determined from experimental Y-factor measurements, are about 150 K across the majority of the desired operating frequency band. The minimum receiver noise temperature of 120 K was measured at 263 GHz, which is as low as that of waveguide receivers. At this frequency, measurement of the noise contribution to the receiver results in input losses of 90 K, mixer noise of 17 K, and multiplied IF noise of 13 K. We found that the major sources of noise in our quasi-optical receiver were the optical losses.
社団法人電子情報通信学会の論文
1996-09-25