A fully-integrated Bluetooth synthesizer using digital pre-distortion for PLL-based GFSK modulation

Traditionally, GFSK modulation has been implemented using either I-Q up-conversion or by direct open-loop modulation of the VCO. The up-conversion approach is susceptible to an undesired image signal, and is costly in analog area and power. The open-loop modulation approach is prone to VCO frequency drift and gain errors. The approach described in this paper is a closed-loop modulation of a Delta-Sigma PLL with self-calibrated predistortion filtering. Prior examples of the pre-distortion approach have required either precision loop filter components or custom PLL response tuning. Our approach uses on-chip calibration algorithms which program digital filter coefficients to compensate for PLL loop gain and loop filter variations. This paper describes the fully integrated (including loop filter) 2.4 GHz Delta-Sigma frequency synthesizer, the digital pre-distortion filtering used to cancel the PLL´s response, and the calibration algorithms used to adapt the pre-distortion filter coefficients. The synthesizer is implemented as part of a 0.25 μm CMOS Bluetooth radio, and uses approximately 35 mA on 4 mm² die area.