I would like to know why the slew rate of the op amp is not high enough in the PLL active loop filter，then an AC waveform will be generated on the tuning line?
And how defines the frequency of the waveform?
Since the correction of the pulses from the charge pump are very fast, the opamp to be used should be fast enough to replicate this changes. If you used slower amplifier, it wouldn't replicate the abrupt changes and instead the signal will rise up/go down slowly which might form certain frequency or cause an AC modulation resulting in higher spurs on the tuning line.
We have article that recommend some opamps for high performance PLL with high voltage VCOs on this link
You can also play around with this comprehensive and easy to use PLL tool: ADIsimPLL™ tool which could help you simulating including the active filter topologies.
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