A Compact Transformer-Based Fractional-N ADPLL in 10-nm FinFET CMOS

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.