A Reference-Waveform Oversampling Technique in a Fractional-N ADPLL
Jianglin Du, Teerachot Siriburanon, Yizhe Hu, Vivek Govindaraj, Robert Bogdan Staszewski
Abstract
This article presents a low-power fractional- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> all-digital phase-locked loop (ADPLL) employing a reference-waveform oversampling (ROS) phase detector (PD) that increases its effective rate four times, thus leading to lower jitter and settling time. The proposed ROS-PD adopts a bottom-plate sampling with a voltage zero-forcing technique, which yields high power efficiency and supports fractional phase compensation in the voltage domain through a programmable DAC. The PD output is then amplified by a low-noise gated amplifier and digitized by a low-power successive approximation register analog-to-digital converter (SAR-ADC). Leveraging the benefits of digital architecture, gain mismatches from the waveform estimator are calibrated by means of an LMS algorithm, consequently lowering fractional spurs. The proposed ADPLL is implemented in TSMC 28-nm LP CMOS. The prototype generates a 2.0–2.3-GHz carrier with an rms jitter of 414 fs while consuming only 1.15 mW. This corresponds to a state-of-the-art ADPLL FoM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">jitter</sub> of −247 dB in a fractional- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> mode. Due to the wide (largely linear) monotonic range and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times $ </tex-math></inline-formula> oversampling rate from a 48-MHz reference, without any additional circuitry, the proposed ADPLL can settle within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mu \text{s}$ </tex-math></inline-formula> in face of a 70-MHz frequency step.