1. Phase-Locked Loop Concepts
Phase-locked loop is a feedback loop where a voltage-controlled oscillator (VCO) can
be automatically synchronized (“locked”) to a periodic input signal. The locking
property of the PLL has numerous applications in communication systems (such as
frequency, amplitude, or phase modulation/demodulation- analog or digital), tone
decoding, clock and data recovery, self-tunable filters, frequency synthesis, motor
speed control, etc.
The basic PLL has three components connected in a feedback loop, as shown in the
block diagram of Figure 1: a voltage-controlled oscillator (VCO), a phase detector
(PD) or phase comparator, and a low-pass loop filter (LPF).