An Injection-Locked Ring-Oscillator-Based Fractional-N Digital PLL Supporting BLE Frequency Modulation

Abstract

This article presents an injection-locked (IL) ring-oscillator-based fractional-N digital phase locked loop (DPLL) supporting Bluetooth low energy (BLE) frequency modulation with an frequency-shift keying (FSK) error between 2.4% and 3.3%. As the fractional spur cannot be suppressed by IL-DPLL, this work proposes a random edge injection (REI) to reduce the spur. This technique also speeds up the convergence time of gain calibration of the digital-to-time converter (DTC). Furthermore, the proposed background calibration schemes allow the DPLL to achieve stable performance across all BLE channels, including both integer-N and fractional-N channels. This work was fabricated in the 40-nm CMOS technology occupying a 0.09-mm(2) area. A fractional spur of -44 dBc and a reference spur of are achieved while consuming 2.76 mW when REI is activated. The background calibrations also ensure stable performance across BLE channels.