Generation of 103.75 GHz CW Source With $5.10^{-16}$ Frequency Instability Using Cryogenic Sapphire Oscillators

We report the generation of millimeter wave signals with frequency instability of a few parts in 10¹⁶. Such performance was achieved at 103.750 GHz by frequency multiplication of two nominally 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 s, resulting in a minimum of frequency instability of 5×10^-16 at 20 s averaging time. Such ultra-stable signal sources operating at frequencies of 100 GHz are very important for many applications including high frequency radio astronomy.