Millimetric LNAs for astronomy: Characterization at cryogenic temperature

Radioastronomy receivers employ several stages of amplification to reach an overall gain in the range 80-120dB. Usually, a cascade of different stand-alone amplifiers is adopted in order to obtain a high gain and low-noise front-end, which is responsible for the most part of the noise figure. The assembly of the low-noise amplifiers (LNAs) is a critical process because it can degrade the overall performance (S parameters and noise) because of non-idealities in the connections among the various radio frequency components. In the millimeter domain, this is particularly severe because of the physical dimensions of the devices inside the LNAs and because of the cryogenic working temperature. To assemble a high-performance receiver, an a priori characterization and selection of the best LNAs are mandatory. In this paper, with the help of some practical examples, I discuss the features of a cryogenic millimeter test set aimed at full characterization of LNAs for astronomy.