ABSTRACT
Phase-locked loop (PLL) is one of the most common functional blocks in modern digital communications systems. PLL has been used for demodulating amplitude modulation and frequency modulation signals and also for generating local carriers in radio systems. Noises in phase, frequency, or time matter more in high-speed electronic systems because phase noise and jitter occur only during the brief transient periods while the amplitude noise is suppressed due to nonlinearity or clipping most of the time. Noise in time can be handled as a cycle-to-cycle jitter and defined as a root-mean-square error of its period. Another mechanism that converts voltage noise into phase noise is the voltage-to-phase, voltage-to-frequency, or voltage-to-time conversion. PLL is a negative feedback system in the phase domain, and a voltage-controlled oscillator becomes a key element in the PLL system because it performs such an operation that links voltage and frequency.
