Active loop filter or Passive loop filter?

[GDF]

We can see there are two classificatons of loop filter in PLL design.
We often use the passive loop filter for our design, but the active filter
still exists. I don't know why we use active filter, what's the advantage
of it? Anybody has the experience of this?

 

[flatulent]

1. It can place the poles and zeros closer to DC than a passive filter, given the constraints on the component values.

2. It can be used to increase the loop gain of the overall circuit.

 

[eng_Semi]

One of the great advantages of the active filter is when the charge pump has a low compliance range, and u need a larger one to let ur VCO work on the desired range. So u use an active filter to eliminate the dependence between the charge pump compliance range and that of the VCO control voltage.