Generation of 100 GHz with parts in 1016 frequency stability using cryogenic sapphire oscillators

We report on the generation of millimetre wave signals with frequency instability of a few parts in 10¹⁶. Such performance was achieved at 103.750 GHz by multiplying frequencies of two almost identical 12.969 GHz oscillators based on liquid helium cooled sapphire dielectric resonators. The multiplication stages were shown to only add a small amount of noise at averaging times less than 10 seconds, resulting in a minimum of frequency instability of 5×10^-16 at 20 seconds averaging time. Such ultra-stable signal sources operating at frequencies of 100 GHz are very important for many applications including high frequency radio astronomy.