Characterization of a set of cryocooled sapphire oscillators at the 10−16 level with the three-cornered hat method

In this paper, we present the characterization results of three Cryogenic Sapphire Oscillators (CSO) by using the three-cornered hat method. The three-cornered hat method permits us to extract the individual frequency instabilities. Thus this powerful tool helps us to choose the best mechanical and thermal CSO configurations. We tested two frequency counters requiring two different data processing and get almost the same results. The three CSOs reach a frequency instability better than 7×10-16 between 1 s and 3,000 s integration times. The Allan deviation of the best CSO reaches a noise floor around 1.5×10-16 at 200 s integration time. Although, the new CSO incorporates a Kyropulos instead of a HEMEX sapphire resonator, it presents the almost the same frequency stability than the two other ones.