PLL Theory Tutorial

PLL Theory TutorialSheet 1 of 8

PLL Theory Tutorial
J P Silver E-mail: john@rfic.co.uk

1 ABSTRACT
This tutorials discusses the key areas of Phase Locked Loop (PLL) design, covering the main components of the loop ie the phase detector, divider, VCO & loop filter.

Overall transfer function =

G T ( s) 1 + G T ( s ).H(s)

3 LOOP COMPONENTS
3.1 PHASE DETECTOR Phase detectors can be either analogue eg mixer or digital eg D-type flip-flop. When a mixer is used the output consists of the sum and difference frequencies. The sum of the frequencies are filtered out by the loop filter and the remaining difference frequency (otherwise known as the beatnote) when both input frequencies are the same is the phase difference. This beatnote or phase error signal is filtered in the loop filter to produce a DC control volt