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-<inline-formula> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> digital phase locked loop (DPLL) supporting Bluetooth low energy (BLE) frequency modulation with an frequency-shift keying (FSK) error between 2.4&#x0025; and 3.3&#x0025;. 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-<inline-formula> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> and fractional-<inline-formula> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> channels. This work was fabricated in the 40-nm CMOS technology occupying a 0.09-mm² area. A fractional spur of &#x2212;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.