Nature of phaselock

A phaselock loop contains three basic components

1. A phase detector (PD).
2. A loop filter
3. A voltage-controlled oscillator (VCO) , whose frequency is controlled by an external voltage.

The phase detector compares the phase of a periodic input signal against the phase of the VCO; output of the PD is a measure of the phase diffrence between its two inputs. The diffrence voltage is then filtered by the loop filter and applied to the VCO, Control voltage on the VCO changes the frequency in a direction that reduces the phase diffrence between the input signal and the local oscillator.
When the loop is locked,the control voltage is such that the frequency of the VCO is exactly equal to the average frequency of the input signal.For each cycle of input there is one, and only one , cycle of oscillator output.
One obvious application of phaselock is in automatic frequency control (AFC). Perfect frequency control can be achieved by this method,whereas conventional AFC techniques necessarily entail some frequency error.
To maintain the control voltage needed for lock it is generally necessary to have a nonzero output from the phase detector.Consequently , the loop operates with some phase error present ; as a practical matter, however this error tends to be small in a well designed loop.