THEORY OF PLL FRACTIONAL-N FREQUENCY-SYNTHESIZERS

This paper presents an overview of the evolution of frequency synthesi zers based on phase-locked loops (PLLs). The main limitations of the d igital PLLs are described, and the consequent necessity of using fract ional-N techniques is justified. The origin of the typical spurious no ise lines on the sidelobes of the synthesized frequency is explained. It is shown how to eliminate these spurious noise lines by using digit al Delta Sigma modulators to control the frequency division value. Fin ally, the implications of using digital Delta Sigma modulators togethe r with fractional-N PLLs on the output phase noise are analysed.