When to enable low spur mode of the ADF4351

For some time I have been fine tuning my spectrum analyzer. Did some improvements like replacing the cavity filter with a much narrower version so the 10.7MHz second IF no longer creates spurs.
And I have replace the Ceramic 30kHz resolution with a 6 pole crystal filter with 40kHz bandwidth but much better form factor.
The total looks now like this with shielding removed:




The phase noise picture overall looked ok apart from some annoying spurs between 30kHz and 100kHz and many small spurs above 200kHz:



After trying many things I switched on the low spur mode of ADF4351 first and second LO and that did make a difference:



Although the phase noise around 30kHz went up with 5dB most of the spurs there did disappear. Did not yet look into the spur at 150kHz
For the many spurs above 200kHz, these are caused by the PC and its USB peripherals as the unconnected Audio input has low noise floor but as soon as connected to the Spectrum Analyzer the noise goes up 20dB.
One small step completed, many to go, the journey is the goal.