Period Jitter of Frequency-Locked Loops

This paper presents a spectral analysis of period jitter of frequency-locked loops (FLLs). It is shown that the period jitter of the output clock of the FLL due to stationary noise sources is cyclostationary. It is further shown that the FLL behaves as a time variant loop and there is translation of jitter frequency at the output. These effects cannot be explained from the usual linear time invariant (LTI) models. A z-domain model of the FLL is presented and spectrum, power spectral density, and time averaged power of the single period and long term jitter are derived for various sources of noise in the FLL. The results of the z-domain model are compared with a computer model of the FLL made in MATLAB. A comparison of the z-domain model of the FLL and LTI models is presented and the limitations of LTI models are discussed in the light of the derived results.