Current pulse shaping technique for low spur fractional‐N phase‐locked loop

Abstract

Current pulse shaping technique is proposed for low spur fractional-N phase-locked loop, which is realised by a variable amplitude charge pump (VA-CP) and a fixed-position, fixed-width and variable-period pulse generator (FFVPG). The current pulse of VA-CP driven by the FFVPG is shaped to fixed position, fixed width, variable amplitude and variable period for eradicating both reference spurs and fractional spurs. The period of VA-CP's current pulse is T REF during transient and N 1 × T REF during locking status, in which the N 1 should be properly set in order to reduce fractional spurs as far as possible and accelerate locking time. The spur performance of the proposed fractional phase-locked loop is evaluated by analysing the spectrum of tuning voltage when reference frequency is 20 MHz and division ratio is 120.13. Spectre based simulation results show the reference spurs at integer multiplies of 20 MHz is eradicated, and the fractional spurs at 10 MHz and the other fractional frequencies are 19.67 dB and much less than that with the existing technique, which verify the feasibility of this study.