ABSTRACT
A phase-locked loop (PLL) is a system that uses feedback to maintai.n an output signal in a specific phase
relationship with a reference signal. PLLs are used in many areas of electronics to control the frequency and/or
phase of a signal. These applications include frequency synthesizers, analog and digital modulators and
demodulators, and clock recovery circuits. Figure 11.1 shows the block diagram of a basic PLL system. The
phase detector consists of a device that produces an output voltage proportional to the phase difference of the
two input signals. The VCO (voltage-controlled oscillator) is a circuit that produces an ac output signal whose
frequency is proportional to the input control voltage. The divide by N is a device that produces an output
signal whose frequency is an integer (denoted by N) division of the input signal frequency. The loop filter is a
circuit that is used to control the PLL dynamics and therefore the performance of the system. The F(s) term is
used to denote the Laplace transfer function of this filter.
