What is (phase lock loop)PLL?, Lock Range & Capture Range?
There is much out there on the basic descripton of a phase-locked loop (PLL).  Basically, a PLL is a phase comparator producing a feedback signal to adjust a independent oscillator to exactly match the frequency of some incoming signal.  The output of the oscillator then is a locally produced copy of the incoming signal.  That by itself may not sound useful, but here are some things that can be done with this:

A little low pass filtering of the oscillator control signal can make the local signal be a "cleaned up" version of the incoming signal.

It is a way of making a low impedance version of a incoming clock without clock skew.  This is useful for distribution of high speed clocks.  If you just took the incoming weak clock signal and ran it thru some gates to clean it up and distribute it, you would get delay in those gates, hence clock skew.

You can put a frequency divider between the oscillator and the phase comparator.  The PLL then becomes a frequency multiplier, since the oscillator must be running at the divide value times the incoming frequency.

The oscillator frequency adjust input signal can be useful.  This is one way of doing FM demodulation, for example.



Capture range and lock range refer to how close the local oscillator frequency needs to be to lock onto the incoming signal.  Since the phase comparator goes thru its full output range over only 1 cycle of mismatch between the incoming signal and the local oscillator, it's output becomes effectively gibberish when the frequency difference is high.  It goes back and forth so fast that the local oscillator doesn't have enough time to adjust.

The capture range is the incoming signal frequency range over which the phase comparator and oscillator can react fast enough so that phase lock is achieved before the phase comparator goes thru another cycle.  This is basically what the input frequency needs to be for a cold start.

Tracking range is the range of input frequencies over which the oscillator can stay in lock once lock was achieved.  This is a wider range than the capture range since the comparator is already in lock, so incoming frequency variations are dealt with incrementally.