A 650 GHz Unilateral Finline SIS Mixer Fed by a Multiple Flare-Angle Smooth-Walled Horn

Abstract

We report the design and successful operation of an superconductor-insulator-superconductor (SIS) mixer operating near the superconducting gap of niobium. A key feature of this design is the employment of a unilateral finline taper to transform the waveguide modes to microstrip signals. This transition is easy to design since it can be rigorously modeled, and also easy to fabricate being a single-layer structure. We will show that unilateral finline mixers have important advantages at THz frequencies since they exhibit wideband operation at both radio frequency (RF) and intermediate frequency (IF), allow elegant on-chip integration of the mixer circuits and result in an extremely simple mixer block that does not require a backshort or any mechanical tuners. The mixer we describe below is fed by a multiple flare-angle smooth-walled horn which exhibits beam pattern characteristic comparable to the conventional corrugated horn and yet is much easier to fabricate.In this paper, we shall present a brief discussion of the testing of the multiple flare-angle horn and detailed description of the design and testing of the mixer, covering 100 GHz bandwidth centered at 650 GHz. In particular, we will present full electromagnetic design description of the mixer chip including the superconducting effects, and the heterodyne properties of the mixer using quantum mixing theory. Mixer performance tests that we carried out from 595 to 702 GHz gave a best receiver noise temperature of 145 K at 600 GHz, corrected for a 75 m beam splitter.Finally, we performed a thorough analysis of the mixer performance, comparing the experimental results with theoretical models. Our investigation demonstrated that unilateral finline mixers fed by a multiple flare-angle horn can yield performance comparable to conventional designs, hence are suitable for large format mixer array at THz frequencies.

Experimental Investigation of a Broadband High-Temperature Superconducting Terahertz Mixer Operating at Temperatures Between 40 and 77 K

“…Generally, they are divided into low-temperature superconducting (LTS) and high-temperature superconducting (HTS) mixers based on the critical temperature of the superconducting materials. Typical LTS devices like superconductor-insulator-superconductor (SIS) mixers [4]- [6] and hot electron bolometer (HEB) mixers [7]- [9] are more sensitive than the HTS mixers, but they operate at liquid helium temperature (4.2 K) range thus requiring expensive and bulky cryogenic facilities. In comparison, the HTS devices operate at higher bath temperatures, where cheaper and portable single-stage cryocoolers can be used.…”

650 GHz SIS mixer fabricated on silicon‐on‐insulator substrate

“…2. The two inductive strips are used to provide a broadband tuning response [3]. A 3‐step quarter‐wavelength transformer is placed before this tuning circuit to match the impedance of this section to the slotline‐to‐microstrip transition.…”

Waveguide-to-Substrate Transition Based on Unilateral Substrateless Finline Structure: Design, Fabrication, and Characterization

“…Furthermore, they are simply grounded to the waveguide through beamleads. In [8], a THz receiver employing a finline mixer is demonstrated. Nevertheless, this approach results in problems related to impedance matching between the waveguide and the substrate, which degrades its performance and prevents its application over large operational bandwidths.…”