A Low Power All-Digital PLL With −40dBc In-Band Fractional Spur Suppression for NB-IoT Applications

Abstract

This paper proposes a low-power fractional-N all-digital PLL (ADPLL) for the narrow-band Internet-of-Things applications. Multi-step lock controlling and oscillator tuning word coarse prediction algorithms help to accelerate the locking process to less than $20~\mu \text{s}$ . A digital-to-time converter (DTC) is used with a phase prediction algorithm to minimize the detection range of the time-to-digital converter for low power consumption. The $\Sigma \Delta $ dither block is designed to improve the nonlinearity of DTC resulting in a −40-dBc in-band spur suppression. A low supply of 0.6-V digitally controlled oscillator has the frequency coverage of 1.5–2.05 GHz. Fabricated in a 55-nm CMOS, the ADPLL occupies an active area of 0.88 mm2 and consumes 4 mW at 1.8 GHz. With a 24-MHz reference clock, the measurement results show that this work achieves better than −94-dBc/Hz in-band phase noise and −120.5 dBc/Hz at 1-MHz offset of 1.83-GHz carrier frequency. The FoM value is −233.4 dB, and the reference spur is −76.3 dBc.

Keywords

• All digital phase-locked loop (ADPLL)
• digital-to-time converter (DTC)
• time-to-digital converter (TDC)
• digitally controlled oscillator (DCO)
• time amplifier (TA)
• narrow-band Internet-of-things (NB-IoT)