A Low-Jitter and Low-Fractional-Spur Ring-DCO-Based Fractional-N Digital PLL Using a DTC's Second-/Third-Order Nonlinearity Cancellation and a Probability-Density-Shaping Δ σ M

Abstract

This work presents a low-jitter and low-spur, fractional-N ring-oscillator-based digital phase-locked loop (RO-DPLL). First, to suppress fractional spurs, the probability-density-shaping delta-sigma modulator (PDS-Delta sigma M) is presented. Since the output codes of the PDS-Delta sigma M are designed to have a time-invariant probability density function (PDF), they have spur immunity to any nonlinearity (NL) of the digital-to-time converter (DTC). In addition, by using a special dither consisting of uniform random numbers (URNs) based on the dithered quantization theorems, the PDS-Delta sigma M can also suppress fractional spurs due to the NL of other loop-building circuits. Second, the DTC's second-/third-order nonlinearity cancellation (DST-NLC) technique is presented to reduce the quantization noise (Q-noise), thereby reducing the rms jitter. The proposed RO-DPLL was fabricated in 65-nm CMOS, and it used a 0.146-mm(2) silicon area and 9.27-mW power. At a near-integer-N frequency, i.e., near 5.3 GHz, the measured rms jitter and the fractional spur were less than 365 fs and -63 dBc, respectively.