PLL

A phase-locked loop (PLL) is a control system that generates an output signal whose phase is related to the phase of a reference signal. It is widely used to stabilize, synthesize, and synchronize frequencies in communications, electronics, and timing circuits. A typical PLL comprises a phase detector, a loop filter, a voltage-controlled oscillator (VCO), and a feedback path that feeds a divided version of the VCO output back to the phase detector.

Operation centers on locking the VCO to the reference. The phase detector compares the reference phase with the phase of the feedback signal and produces an error signal proportional to the phase (and sometimes frequency) difference. The loop filter shapes this error signal, which then adjusts the VCO control input. When the system is locked, the VCO frequency is a fixed multiple of the reference, and the phase error remains constant. The loop’s bandwidth, capture range, and hold-in range determine how quickly lock is achieved and how well the loop tolerates disturbances.

Key components include the phase detector (analog or digital), the loop filter (passive or active), the VCO (or a numerically controlled oscillator in digital implementations), and the feedback divider. The divider ratio sets the synthesized output frequency relative to the reference. In many designs, fractional-N division enables fine frequency stepping beyond integer multiples.

Variants of PLLs encompass analog PLLs, digital PLLs (DPLLs), all-digital PLLs (ADPLLs), and software PLLs (SPLLs). Digital implementations may use NCOs and fully digital phase detectors and filters.

Applications span frequency synthesis for radios and oscillators, clock generation and distribution in microprocessors, data recovery in communication links, and jitter reduction in high-speed systems.