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Using the excellent filter calculation tool that can be found here:

I calculated a 9th order?Chebyshev low pass filter?


As you can see the tool can be told to use standard E12 components.
The calculated values fitted nicely in an assortment of SMD L and C components I bought some time ago.
With these low values it would be best to put the filter on a PCB but I did not want to make one so I re-used a bias-T module I had from some previous experiment.
The result is a nice small filter?

.Next step is to see if this indeed eliminates the aliases seen when measuring the ADF4351 signal generator

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HBTE Files section:?/g/HBTE/files

Erik, PD0EK

And here are the measurements.
First 128MHz from a SI5351 through a -30dB attenuator


The fractional spurs around 128MHz are visible but the signals at 27.66MHz and 155.66MHz should not be there.
Lets see what happens when you add the 250MHz low pass filter



This is already much better. The signals at 27.66MHz and 155.66MHz where clearly aliases as they are now gone.
The advantage of a low-pass filters is it does not increase measurement time.
But if you have a lot of time you can add spur reduction with averaging. Lets see what that does:



This look as is expected.
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HBTE Files section:?/g/HBTE/files
Erik, PD0EK