A Low-Spur Fractional-N PLL Based on a Time-Mode Arithmetic Unit

Zhong Gao, Jingchu He, Martin Fritz, Yiyu Shen, Zhirui Zong, Gerd Spalink, Morteza S. Alavi, Robert Bogdan Staszewski, Masoud Babaie, More Authors

Research output: Contribution to journalArticleScientificpeer-review


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&#x2014;the period of a digitally controlled oscillator (DCO) and the instantaneous time offset between the DCO and reference clock edges&#x2014;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> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula>59 dBc. Under considerable supply or temperature variations, the worst spur still remains below <inline-formula> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula>51.7 dBc without any background calibration tracking.

Original languageEnglish
Pages (from-to)1-20
Number of pages20
JournalIEEE Journal of Solid-State Circuits
Publication statusE-pub ahead of print - 2022


  • Arithmetic
  • Capacitors
  • Clocks
  • Digital-to-time converter (DTC)
  • fractional spur
  • Microelectronics
  • Phase locked loops
  • phase-locked loop (PLL)
  • process voltage and temperature (PVT)
  • Switches
  • time-mode arithmetic unit (TAU)
  • Voltage


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