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May I know, what kind of application usually people use to design PLL system with an active filter? what is the advantage and disad of active filter compare to a passive filter for the PLL system? Thanks in advanced.
If you are talking about the feedback loop filter, an active filter can give a sharper corner rolloff than a passive filter, unless you use both inductors and capacitors.
And inductors generally must be large at a typical desired loop rolloff frequency.
The main disadvantage of an active filter is that it requires an active component.
an active filter can have:
variable cutoff frequency
adjustable slope (as well as other response characteristics)
it can be made without inductors (which are usually relative large and heavy components)
can be made with higher input impedances than a passive filter
as crutschow said, it requires an active component (one or more transistors or op amps)
other cons for active filters:
requires circuit board real estate
often requires careful component layout.