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.
1) Frequency Standards Stability Analyzer for the Deep Space Network
2) An Improved Offset Generator Developed for Allan Deviation Measurement of Ultra Stable Frequency Standards