Characterization of the Individual Short-Term Frequency Stability of Cryogenic Sapphire Oscillators at the $10^{-16}$ Level

We present the characterization of three cryogenic sapphire oscillators (CSOs) using the three-cornered-hat method. Easily implemented with commercial components and instruments, this method reveals itself very useful to analyze the fractional frequency stability limitations of these state-of-the-art ultrastable oscillators. The best unit presents a fractional frequency stability better than 5 × 10
^-16
at 1 s and below 2 × 10
^-16
for τ <; 5000 s.