  We use two fiber-based femtosecond frequency combs and a low-noise carrier
suppression phase detection system to characterize the optical to microwave
synchronization achievable with such frequency divider systems. By applying
specific noise reduction strategies, a residual phase noise as low as -120
dBc/Hz at 1 Hz offset frequency from a 11.55 GHz carrier is measured. The
fractional frequency instability from a single optical-to-frequency divider is
1.1E-16 at 1 s averaging down to below 2E-19 after only 1000 s. The
corresponding rms time deviation is lower than 100 attoseconds up to 1000 s
averaging duration.
