Investigation on the microwave cryogenic sapphire clocks

Standards which are optimized for very short measuring times must operate for periods of a year or more without interruption to be considered for many applications. This is because a short-term frequency standard is typically utilized to remove the short-term variations of an atomic standard. This includes both the Caesium fountain and trapped ion clocks. For the past twenty years, microwave cryogenic sapphire oscillators have served this purpose as a secondary reference standard. The predicted achievable fractional frequency instability of microwave cryogenic sapphire clocks is about 3×10^-18 from 0.001 to 100,000 s integration time. This is about three orders of magnitude higher than any commercially available cryogenic sapphire clocks around the world. This paper presents the preliminary investigation of the current limitations from the state-of-the-art cryogenic sapphire oscillators followed by the key design of a high performance microwave clock based at Huazhong University of Science and Technology in Wuhan, China.