Adaptive Digital Pre-Emphasis for PLL-Based FMCW Modulators

The direct frequency modulation of a phase locked loop suffers from limited modulation bandwidth. To overcome this limitation, the modulation signal can be pre-emphasized by means of a high-pass filter. Unfortunately, the incorrect equalization of the PLL transfer function causes modulation error. This paper introduces a new method to adaptively match the transfer functions of the PLL and the pre-emphasis filter over environmental and process variations. The technique is verified using a time-domain model of a digital PLL designed for the generation of chirp signals for FMCW radar sensors. The new adaptive digital pre-emphasis technique enables the generation of highly-linear fast chirps with significant reduction of the idle time.