Basic Phase Locked Loop Operation

BASIC    PHASE   LOCKED  LOOP   OPERATION

The basic operation of a PLL circuit can be explained using the circuit . We will first consider the operation of the various circuits in the phaselocked

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. Best operation is obtained if the VCO center frequency, fo, is set with the dc bias voltage midway in its linear operating range. The amplifier allows this adjustment in dc voltage from that obtained as output of the filter circuit. When the loop is in lock, the two signals to the comparator are of the same frequency, although not necessarily in phase. A fixed phase difference between the two signals to the comparator results in a fixed dc voltage to the VCO. Changes in the input signal frequency then result in change in the dc voltage to the VCO. Within a capture-and-lock frequency range, the dc voltage will drive the VCO frequency to match that of the input.

While the loop is trying to achieve lock, the output of the phase comparator contains frequency components at the sum and difference of the signals compared. A lowpass filter passes only the lower-frequency component of the signal so that the loop can obtain lock between input and VCO signals.

Owing to the limited operating range of the VCO and the feedback connection of

the PLL circuit, there are two important frequency bands specified for a PLL. The

capture range of a PLL is the frequency range centered about the VCO free-running frequency, fo, over which the loop can acquire lock with the input signal.

Once the PLL has achieved capture, it can maintain lock with the input signal over a somewhat wider frequency range called the lock range.