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

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 mm
²
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.