PFD Spurs ADF5355

We have our own PCB design - a fixed frequency application. Target 5GHz.

PN measurements have turned out to be in line with simulations, so that s all good.

Icp .9mA, FRAC1=FRAC2=0 for testing, PFD between 15 and 200 MHz.

We are consistently getting strong PFD spurs at Fc +/- n*Fpfd order of magnitude -45dBc (n=1..4) only typically. Several boards have been tested.

- bleed influences somewhat, a few dB, but this looks dramatically below what I would expect from datasheet (around -80dBc).

Our application use a very clean reference Crystek 100M  locked to GPS/10M using an ADF4107. Nothing wrong with that. Using single ended ref on the ADF5355. Using correct init.

I have been reading up on the datasheet and forum, but running out of ideas.

Can you at least confirm that this is not normal behaviour? 

Thanks for any pointers.

Christophe

Spectrum plot below for PFD = 50MHz



update plot
[edited by: xtof huygens at 4:47 PM (GMT -4) on 26 Sep 2020]

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