The IF Output Impedance of SIS Mixers

In this paper, we describe a measurement setup to characterize the intermediate frequency (IF) impedance of cryogenically cooled superconductor-insulator-superconductor (SIS) mixers. A setup based on a commercial vector network analyzer (VNA), a circulator and a low noise amplifier (LNA) has allowed to increase the dynamic of the VNA and to perform accurate one-port measurements across a high dynamic range of the SIS mixers IF band. The mixers were biased in three different regions of their unpumped IV curve to obtain three known impedances which are close to the classical calibration SOLT standards (Short, Open, Load, Thru): the reference plane was located at the SIS junction itself and the calibration procedure allowed to calibrate out all IF circuitry of the measurement setup, including the on-chip SIS mixer intrinsic capacitance and inductance without requiring to thermally cycle and open the cryostat to locate and measure the different calibration standards. Thus, the devised method provides a simple and direct measurement of the SIS mixers IF impedance which can be used for quick, accurate and highly repeatable IF characterization of any type of millimeter and sub-millimeter wave SIS mixers. The measurement method is described and the experimental results of the IF output impedances of 3 mm band DSB and SSB mixers with 4 GHz wide IFs (across 4-8 GHz) are presented and compared with simulated predictions obtained from combining Tucker's SIS mixer theory and accurate electromagnetic modeling of the mixer structure.