A 72-fs-Total-Integrated-Jitter Two-Core Fractional-$N$ Digital PLL With Digital Period Averaging Calibration on Frequency Quadrupler and True-in-Phase Combiner

This work presents a low-jitter and low out-of-band noise two-core fractional- $N$ digital bang-bang phase-locked loop (PLL). Two novel techniques are introduced to efficiently suppress the quantization noise (QN) of the digitally controlled oscillator (DCO) and to achieve an optimal trade between power consumption and PLL noise. The digital period averaging technique, working in background of the main system, enables the use of a low-power xor-based quadrupler for clocking $\Delta \Sigma$ modulator dithering the DCO tuning word. The true-in-phase combiner circuit implements a digitally assisted power combination of two PLL outputs, to optimally reduce the impact of the PLL noise sources. The prototype, implemented in a standard 28-nm CMOS process, has a core area of 0.47 mm 2 and synthesizes frequencies from 8.5 to 10.5 GHz while dissipating 36 mW. The measured rms jitter (integrated from 1 kHz to 100 MHz and including spurs) is 72 fs for near-integer channels, with a worst case fractional spur of - 59.7 dBc, while the measured out-of-band noise is -140.7 dBc/Hz at a 10-MHz offset.