Abstract: High performance fractional-N phase-locked loops (PLLs) are essential elements of any advanced electronic systems. In recent years, both analog and all-digital PLLs employing sampling or sub-sampling phase detector have gained popularity and demonstrated below 100-fs integrated jitter and superior figure-of-merit. This talk focuses on this PLL architecture and elaborates the advanced design techniques to achieve low jitter, low fractional spurs, and low power operation.