Sometimes it is possible to tune a low phase noise high stability
OCXO to the desired offset frequency. A frequency synthesizer can
sometimes be used however it may have lowest noise and spurs at some
weird offset frequency like 13.3333 Hz which may not be convenient.
The highest performance usually requires a custom synthesizer
employing a cascaded pair of lower sideband and upper sideband mixers
combined with a a pair of low noise dividers and a low phase noise
crystal oscillator phase locked to the synthesizer output to
eliminate unwanted spurs.