Spectral Breathing and Its Mitigation in Digital Fractional-N PLLs

Abstract

Although digital phase-locked loops (PLLs) offer several advantages over their analog counterparts, they suffer from a major disadvantage that is rarely mentioned in pub- lished articles. The disadvantage, known as spectral breathing, is caused by component mismatches among the frequency control elements within a PLL’s digitally controlled oscillator (DCO). The mismatches introduce DCO frequency modulation nonlinearity which fluctuates and, therefore, causes erratic variations in the PLL’s measured phase noise spectrum as the DCO’s free-running frequency drifts. The phenomenon is called spectral breathing because the measured phase noise spectrum tends to slowly swell and contract over time as if taking breaths of air. During these breaths, the PLL’s phase noise often becomes severely degraded. This article presents an experimental demonstration of the spectral breathing phenomenon and its solution in a digital fractional-N PLL. The demonstrated solution is a multi-rate dynamic element matching technique and a mismatch-noise cancellation technique that together eliminate spectral breathing.