Time-domain simulation of quantization noise mixing and charge pump device noise in fractional-N PLLs

In this paper we model phase noise and spurious tones (spurs) for a fractional-N phase-locked loop (PLL) with static phase offset. The phase detector (PD) input-output characteristic around the bias point is approximated by a parabolic function. Using a MATLAB code, phase noise spectrum and fractional spurs are calculated as a function of slope and curvature of the PD characteristic. The dependence of the PLL output spectrum on PD nonlinearity and rms phase error at the PD input is discussed and compared with theoretical results. A close agreement with theoretical predictions is observed.