This tutorials discusses the key areas of Phase Locked Loop (PLL) design, covering the main components of the loop ie the phase detector, divider,VCO & loop filter.

A phase locked loop schematic is shown in Figure 1. Pi(s) represents the phase of the reference oscillator, while Pe(S) is the error phase signal which is filtered and used to drive a VCO. The transfer function of this section is represented by G(S).
A Voltage Controlled Oscillator (VCO) can be swept over the frequency range of interest by a control voltage. Some of the VCO output is fed back and compared with a reference frequency in a Phase Detector (PD). The reference is usually a crystal oscillator but might be the output of another loop for example. The PD generates an error voltage, which steers the VCO to lock it to the same frequency as the reference.
This simple system produces an output on the same frequency as the reference crystal oscillator. In a practical system it is necessary to add a programmable divider, a reference divider and a loop filter.