10.8 A wideband fractional-N ring PLL using a near-ground pre-distorted switched-capacitor loop filter

Ring PLLs play an important role in mobile baseband applications. In cases where fine frequency resolution and low jitter are both needed, wideband fractional-N PLL architectures with quantization noise (Q-noise) cancellation are preferred. Phase interpolators (PI) are widely used in recent literature [1-3]. Although the Q-noise is reduced, decreasing supply voltages severely limit the linearity and noise of PI. In this paper, we present a switched-capacitor loop filter (SCLF) to solve this problem. The developed SCLF with correlated double sampling (CDS) keeps the output of the charge pump (CP) near-ground to improve its noise, linearity, and power-supply rejection ratio (PSRR). Furthermore, a calibrated 2b ADC is used to realize type-II operation.