Chapter 10 - Are You Locked? A PLL Primer

Abstract

This chapter discusses about the most useful electronics building blocks which known as phase-locked loop (PLL). A large number of electrical engineers are afraid to pronounce this name. But the PLL system is very user-friendly. PLL is a feedback loop which locks two waveforms with same frequency but shifted in phase. The fundamental 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. PLL generates an output frequency which is based on a reference input clock. The output frequency can be either higher or lower than the input. Another important use of PLLs is to clean a noisy reference frequency. It also removes any short-term phase variations while keeping the same long-term precision. Later, one of its important components is explored. One of the major components of a PLL is a voltage-controlled oscillator (VCO). The PLL architecture which is called an integer-N PLL is also discussed. It is a simple and elegant architecture that we can use to generate any frequency.