In this article, we introduce a fractional-N all-digital phase-locked loop (ADPLL) architecture based on a single LC-tank, featuring an ultra-wide tuning range (TR) and optimized for ultra-low area in 10-nm FinFET CMOS. Underpinned by excellent switches in the FinFET technology, a high turn-on/off capacitance ratio of LC-tank switched capacitors, in addition to an adjustable magnetic coupling technique, yields almost an octave TR from 10.8 to 19.3GHz. A new method to compensate for the tracking-bank resolution can maintain its quantization noise level over this wide TR. A new scheme is adopted to overcome the metastability resolution problem in a fractional-N ADPLL operation. A low-complexity TDC gain estimator reduces the digital core area by progressive averaging and time-division multiplexing. Among the published fractional-N PLLs with an area smaller than 0.1mm2, this work achieves an rms jitter of 725fs in an internal fractional-N mode of ADPLL's phase detector (2.7-4.825GHz) yielding the best overall jitter figure-of-merit (FOM) of -232dB. This topology features small area (0.034mm2), wide TR (56.5%) and good supply noise rejection (1.8%/V), resulting in FOMs with normalized TR (FOMT) of -247dB, and normalized TR and area (FOMTA) of -262dB.

The current paradigm of frequency synthesis for short-range wireless transceivers, such as BLE, is to use a crystal oscillator (XO) in the tens-of-MHz range as a frequency reference (FREF) to phase lock an RF oscillator [1-4]. This ensures a sufficiently wide PLL bandwidth of tens to hundreds of kHz to quickly acquire a new channel and to suppress lower-frequency phase noise (PN) of the RF oscillator. The latter requirement can be alleviated by substantially lowering the flicker PN of a digitally controlled oscillator (DCO) thus allowing to freeze its tuning word updates during receive (RX) packets and further directly FM-modulating the DCO during transmit (TX) packets [1]. However, an all-digital PLL (ADPLL) is still needed just to quickly settle the DCO to each new channel.

All-digital PLLs (ADPLL) based on a ring-oscillator (RO) provide very fast settling, but they suffer from quantization (Q) noise due to discrete tuning of their digitally controlled oscillator (DCO). Although RO charge-pump PLLs (CP-PLL) do not exhibit Q-noise thanks to their continuous VCO tuning, they are quite slow and require huge VCO gain. We propose a hybrid-PLL in 7nm FinFET CMOS that combines the best advantages of ADPLL and CP-PLL with a periodical phase realignment by the reference clock. It covers 0.2GHz-4GHz with 0.619ps integrated jitter and settles in 0.6us.