the basic operation of a PLL circuit can be explained using the circuit of 13.25 as reference. WE will fist consider the operation of the various circuits in the phase-locked loop when the loop is operating in lock (the input signal frequency and the VCO frequency are the same). When the input signal frequency is the same as that from the VCO to the comparator, the voltage Vd taken as output is the value needed to hold the VCO in lock with the input signal. the VCO then provides output of a fixed-amplitude square-wave signal at the frequency of the input.