pllJune03_PLL锁相环介绍.pdf

Phase-Locked Loop H R Pota June 6, 2005 1 Introduction Phase-locked loop (PLL) is a feedback loop which locks two waveforms with same frequency but shifted in phase. The funda- mental use of this loop is in comparing frequencies of two waveforms and then adjusting the frequency of the waveform in the loop to equal the input waveform frequency. A block diagram of the PLL is shown in Figure 1. The heart of the PLL is a phase comparator which along with a voltage controlled oscillator (VCO), a filter and an amplifier forms the loop. If the two frequen- cies are different the output of the phase comparator varies and changes the input to the VCO to make its output frequency equal to the input waveform frequency. The locking of the two frequencies is a nonlinear process but linear approximation can be used to analyse PLL dynamics. In getting the PLL to lock the proper selection of the filter is essential and it needs some attention. If the filter design is understood from control theory point-of-view then the design becomes quite simple. In this short note we will discuss only the fundamentals of the PLL and how you can use nonlinear simulation and linearised approximation to get a better understanding of the PLL. Input Phase vp Low-pass volp vo Amplifier Detector Filter cos(ω t + k v dt) 0 o o VCO