A Low-Spur Fractional-N PLL Based on a Time-Mode Arithmetic Unit
Zhong Gao, Jingchu He, Martin Fritz, Jiang Gong, Yiyu Shen, Zhirui Zong, Peng Chen, Gerd Spalink, Ben Eitel, Morteza S. Alavi, Robert Bogdan Staszewski, Masoud Babaie
Abstract
This article introduces a low-jitter low-spur fractional-N phase-locked loop (PLL) adopting a new concept of a time-mode arithmetic unit (TAU) for phase error extraction. The TAU is a time-signal processor that calculates the weighted sum of input time offsets. It processes two inputs—the period of a digitally controlled oscillator (DCO) and the instantaneous time offset between the DCO and reference clock edges—and then extracts the DCO phase error by calculating their weighted sum. The prototype, implemented in 40-nm CMOS, achieves 182-fs rms jitter with 3.5-mW power consumption. In a near-integer channel, it shows the worst fractional spur below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 59 dBc. Under considerable supply or temperature variations, the worst spur still remains below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 51.7 dBc without any background calibration tracking.