AN143: A Simple Method to Accurately Predict PLL Reference Spur Levels Due to Leakage Current

Overview

This application note describes a mathematical model for predicting reference spurs in integer-N Phase Locked Loop (PLL) systems resulting from charge pump or op-amp leakage currents. It explains how these currents cause a voltage drift on the VCO tuning line, which the loop corrects through unipolar pulses, resulting in frequency modulation at the phase/frequency detector (PFD) rate. The document covers the impact of these spurs on system performance, such as adjacent channel interference in wireless communications, and provides calculation methods for both passive and active loop filter configurations using components like the LTC6945 PLL and LT1678 op-amp.

Referenced Parts