Two-junction tuning circuits for submillimeter SIS mixers

Abstract

Abstract-The capacitance of superconducting tunnel junctions can seriously degrade the performance of quasiparticle (SIS) mixers operating in the submillimeter band, so it is essential to provide a circuit for tuning out this capacitance at the operating frequency. In this article, we present two new tuning circuits for SIS mixers which use a pair of SIS junctions connected by an inductance. Compared to previously proposed tuning circuits, ours have a broader bandwidth, are easier to scale to higher frequencies, and may be easier to fabricate. We have constructed quasi-optical mixers which employ these tuning circuits, using NblAl-OxidelNb SIS junctions defined by optical lithography. The performance of these devices is excellent, giving receiver noise temperatures of 113 K (DSB) at 490 GHz and 230 K DSB at 612 GHz. In addition to demonstrating the effectiveness of our tuning circuit, these results show that quasi-optical mixers can be competitive with or superior to waveguide mixers at submillimeter wavelengths. The mixers continue to perform well at frequencies up to 672 GHz, which is about 95% of the Nb gap frequency.

A lownoise 665 GHz SIS quasi-particle waveguide receiver

“…The transmission line losses are therefore significant, (>_ 40%/wavelength) [16]. In addition analyses by Zmuidzinas [7] and Kooi [11] et al have shown that above 500 GHz it becomes difficult to realize the standard "end loaded" tuning stubs which have been proposed by many authors. At frequencies above 600 GHz the junction is beginning to look like a distributed element rather than a lumped structure, which effects the way SIS tunnel junctions should be modeled.…”

“…This technique has been very effectively used in both open structure devices [7,8] and waveguide mixers [9,10,11,13,14].…”

THz SIS mixers with normal-metal Al tuning circuits

“…Quasi-optical SIS Nb/Al-oxide/Nb mixers with integrated Nb tuning circuits have achieved good performance up to the Nb gap frequency (≈ 700 GHz) and a little beyond [1,2]. The tuning circuits, made from microstrip transmission lines, serve to tune out the junction capacitance at the operating frequency and to match the impedance of the junction to that of the integrated antenna.…”

Characterization of submillimetre quasi-optical twin-slot double-junction SIS mixers

“…To design and simulate our twin-slot two-junction mixers [3], we employ a program that we developed inhouse for microwave circuit analysis. This program allows us to include the effects of lossy Nb microstrip lines, such as are used for the antenna-to-junction transformers, and for the inductor used to tune out the junction capacitance.…”