Cryogenic Characterization of Microwave Devices

Microwave technologies of interest are ubiquitous in the defence and security field, but less obvious is the frequent crossing of microwave technologies with cryogenics: from quantum computers, working presently only with superconducting microwave components, to high speed superconducting electronics, to microwave circuits and filters that operate or are envisaged to operate in the outer space, in all cases the capability to test microwave circuitry and devices in a cryogenic environment is an important asset for defense and security applications. In this paper we present examples of testing of microwave passive devices, typically resonators with different quality factors and resonant frequencies in the 8 GHz-28 GHz range, for temperatures ranging from 4 K to 100 K. We tune the quality factor by an order of magnitude by incorporating superconducting walls in the resonators, and by applying a dc magnetic field to partially suppress the superconductivity. We will show how common practices in microwave testing, such as a full calibration of the connecting lines, is not easily applicable to cryogenic environment and specific strategies for data analysis need to be adopted. We show how, in selective devices such as filters (resonators in our case), a very accurate control of influence variables beyond temperature is an unavoidable requirement, but the strategy for nontrivial data analysis in any case takes a very relevant role. We show that a simple, in principle, measurement of the parameters of a resonator can be challenging in a cryogenic environment, and straightforward application of room-temperature procedures can lead to substantial errors.