Temperature-dependent performances of Nb SIS mixers at millimeter and submillimeter wavelength

The dependence of the performance of Superconductor-Insulator-Superconductor (SIS) mixers on the ambient temperature is investigated both theoretically and experimentally at millimeter and submillimeter wavelength. We found that the mixers' conversion gain plays a dominant role in the temperature-induced variation of the overall receiver sensitivity. To achieve a straightforward image of how the ambient temperature influences the conversion gain, we introduce a pure shape factor that allows for determining the mixers' conversion gain from the nonlinearity of their IV curves. We measured the performance of an SIS mixer at 100GHz band with the ambient temperature ranging from 3.8K to 8.5K to verify the validity of the quantum mixing theory. The experimental results are found in good agreement with the simulation results. This agreement indicates the validity of quantum mixing theory at a relatively high temperature up to the superconducting critical temperature. The measurement results show that at 100GHz band, the Nb SIS mixer can maintain a reasonable performance at the ambient temperature as high as 80% of the superconducting critical temperature. In addition to the gain and noise, the receiver stability in terms of Allen variance was also measured at various ambient temperatures without any apparent difference found.