Accurate Characterization of High- $Q$ Microwave Resonances for Metrology Applications

Microwave resonators are widely adopted as high sensitivity sensors in both applied and fundamental metrology, to measure a number of different physical quantities, such as temperature, humidity, pressure, length and material properties. High sensitivity, and thus potential high measurement precision and accuracy, can be achieved by resorting to high-quality-factor ( $Q$ ) resonators. Nonetheless, in order to accurately measure a high- $Q$ resonance and obtain low measurement uncertainty, as required by metrology applications, the entire measurement set-up must be carefully designed. This papers presents an overview of resonance frequency measurements for metrology applications, illustrating the various aspects and issues to be dealt with when pursuing highly accurate measurements, as well as of the most relevant achievements in this field.