VNA Measurement of the IF Output Impedance of an SIS Mixer

A new method is presented for measuring the intermediate frequency (IF) output impedance (Z_IF) of a superconducting tunnel junction (SIS) mixer at cryogenic temperatures. As an example, the impedance Z_IF of an atacama large millimeter/submillimeter array band 6 (211–275 GHz) SIS mixer chip was measured at the IF bonding pad. The setup uses a commercial vector network analyzer (VNA) with sensitivity enhancements to increase the dynamic range and allow low-power-one-port reflection coefficient (Γ _if) measurements. A coupler inside the VNA is bypassed and replaced with an equivalent cold coupler inside the cryostat. The bias to the mixer is provided through the IF isolator. A cryogenic low-noise amplifier in the return path to the VNA increases its dynamic range. One-port calibration standards (short, open, and 50 Ω) are used, and the impedance is de-embedded from the calibration reference plane to the IF bonding pad of the mixer chip using the proprietary automatic fixture removal capability of the Keysight Technologies VNA. This approach allows direct measurement of the amplitude and phase of Γ_IF and hence Z_IF, from 2 to 16 GHz with very low power levels at the device under test. The measured results are compared with the predictions made from combining Tucker's quantum mixer theory with electromagnetic model of the mixer.