Optimizing Superconducting Matching Circuits for Nb SIS Mixers Operating Around the Gap Frequency

Magnetic field dependence of the coupling efficiency of a superconducting transmission line due to the proximity effect

“…The antenna and waveguide are modeled as the source impedance Z ant , determined by the structure of the horn and waveguide, the mounting position, and the substrate thickness. 8 The junction is characterized by the normal state resistance R n and the geometric capacitance C j ͓with junction impedance Z j = R n / ͑1+ j R n C j ͔͒. The TL is represented as the inductance L and the impedance transformer Z tran , which are used to tune out C j and provide the match between Z ant and Z j .…”

“…Independent measurements have shown that both T c and N of the Nb film with thickness less than 20 nm decrease. 8 Therefore, we use the additional conjecture that the Nb topwire forms a proximity bilayer consisting of Nb͑480 nm͒ / Nb ‫ء‬ ͑20 nm͒ bilayer. Using the measured data on 20 nm Nb film, ␥ is calculated to be 0.64 and ␥ B is simply taken as 1.0.…”

Bandwidth Limitations of Nb/AlN/Nb SIS Mixers Around 700 GHz

“…Apart from the quality of the materials used for the microstripline and the position of the SIS device in the waveguide, determined by both the mounting of the chip and the thickness of the substrate [3], the critical current density of the tunnel barrier itself is the dominant parameter in the bandwidth.…”

“…The latter is needed to match the antenna impedance to the SIS junction impedance [3], which is characterized by the junction's normal state resistance in parallel with its capacitance . Although the specifications for Band 9 can be met with aluminum oxide tunnel barriers, an intrinsically wider band coverage would be beneficial.…”

“…High critical current density aluminum nitride devices have a low time constant, which would enable a desirable larger bandwidth, provided the matching circuit is not limiting [3].…”