Microwave Transmission Technique for Accurate Impedance Characterization of Superconductive Bolometric Mixers

The impedance of the submillimeter superconducting hot-electron bolometer mixer was measured in the range 0.2-4 GHz. A special technique relying on determination of the S ₂₁ transmission parameter of the device was used. Many advantages of the technique (wide frequency range, in situ calibration, low test power, accuracy) are demonstrated. The estimate of the mixer bandwidth from the impedance data using the mixer theory is in good agreement with the results of the direct bandwidth measurements.