In this paper, we introduce an all-digital phase-locked loop (ADPLL) for Bluetooth low energy (BLE) that eliminates the need for a crystal oscillator (XO) other than a 32.768-kHz real-time clock (RTC) already present in wireless systems. Specifically, we propose to replace the conventional channel settling with a band settling that would be carried out only once per global device power up. The ADPLL locks to the center of the Bluetooth band (2440 MHz) upon system power-up and jointly performs an instantaneous channel hopping and Gaussian frequency shift keying (GFSK) modulation in a two-point manner to overcome the narrow PLL bandwidth (BW) due to the 32.768-kHz reference. Extensive calibrations linearize the effective cubic digitally controlled oscillator (DCO) transfer function to achieve a precise frequency range of hopping and modulation. Realized in 16-nm FinFET, it consumes <1 mW at ≤0.45 V, while achieving best-in-class performance and <100-ns hopping time.

The current paradigm of frequency synthesis for short-range wireless transceivers, such as BLE, is to use a crystal oscillator (XO) in the tens-of-MHz range as a frequency reference (FREF) to phase lock an RF oscillator [1-4]. This ensures a sufficiently wide PLL bandwidth of tens to hundreds of kHz to quickly acquire a new channel and to suppress lower-frequency phase noise (PN) of the RF oscillator. The latter requirement can be alleviated by substantially lowering the flicker PN of a digitally controlled oscillator (DCO) thus allowing to freeze its tuning word updates during receive (RX) packets and further directly FM-modulating the DCO during transmit (TX) packets [1]. However, an all-digital PLL (ADPLL) is still needed just to quickly settle the DCO to each new channel.