Dynamic thermal sensitivity of microwave cryogenic sapphire resonator

TL;DR

This paper reports a memory effect in cryogenic sapphire resonators caused by Cr3+ impurities, leading to hysteresis and dynamic thermal sensitivity that affect frequency stability in cryogenic oscillators.

Contribution

It reveals the influence of paramagnetic impurities on the thermal sensitivity and stability of sapphire resonators, highlighting a previously unreported memory effect.

Findings

Memory effect causes hysteresis in frequency vs temperature behavior.

Thermal sensitivity depends on the rate of temperature change.

Degradation in frequency stability occurs around 10 seconds due to this effect.

Abstract

We have discovered a memory effect in the temperature sensitivity of a cryogenic sapphire microwave resonator, at the heart of the ultra-stable Cryogenic Sapphire Oscillators (CSOs). Such effect is due to the relaxaxtion time of Cr3+ impurities, and results in hysteresis in the frequency vs temperature behavior, These paramagnetic impurities, always present in synthetic sapphire, produce a temperature turning point which is necessary to achieve ultimate frequency stability. The practical implication on the CSO is that the sapphire resonators's frequency depends on the rate of temperature change. This dynamical thermal sensitivity results in a wide bump in the Allan deviation at 10 s integration time, where the frequency stability is degraded. The actual degradation depends on the specie and on the amount of the dominant paramagnetic impurity.