vivkr
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Hi Aaron,
It has been quite a while since I did PLLs, but from what I recall, the ripple at reference frequency is typically determined by the loop parameters of the PLL (the chargepump current and the passive elements, C1, C2 and R in the standard scheme). You mention that you changed the filter cap by just a few fF. This should normally not cause a dramatic change. I imagine that you have too little of C2 (the cap directly tied between the control voltage node and ground). All this assuming that you are using one of the conventional chargepump PLL schemes.
Perhaps, you could post a snap of your filter and a couple of snaps of the ripple, and perhaps someone could help then.
Regards Vivek
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