Inconstant Output Noise of SIS Mixers and Its Implication in Input Noise Measurement

In the assessment of a superconductor-insulator-superconductor (SIS) mixer, the breakdown of the overall receiver noise into intrinsic mixer noise and noises from other components is indispensable. The input noise due to losses in front of the mixer is conventionally measured by assessing the receiver noise as a linear function of mixer conversion loss at low local oscillator power and extrapolating the line to an infinite gain. However, it has been observed that this method tends to overestimate the input noise. To address this, the underlying theory of the input noise measurement method was revisited. It was found that the product of mixer gain and noise, i.e., the output noise of the mixer, is not a constant with respect to LO power. Therefore, extrapolating the linear relation to a large gain, where the input noise is inferred, becomes problematic. Numerical simulations were conducted to validate this analysis. Finally, a two-step measurement approach is introduced, in which an attenuator is inserted in front of the SIS mixer at the 4-K cold stage, to obtain the actual RF input noise.