Integer boundary spur considerations for fractional-N PLL based FMCW radar

Fractional-N phase locked loops (PLLs) offer a low-cost solution to the precision frequency ramp requirements of frequency-modulated continuous-wave (FMCW) radar. In this application, however, the infamous spurious signals generated by these PLLs have not received adequate analysis. In this Letter, the authors investigate the impact of integer boundary spurs on FMCW radar performance. They show that the offset of these spurs is swept during the course of waveform generation, and that undesirable spur chirps manifest in the beat signal as a result. These spur chirps span the radar's intermediate frequency bandwidth and produce Fresnel ripples in the range profile which reduce spurious-free dynamic range. They present simulations and measurements that demonstrate how reduction of PLL bandwidth can be used to suppress these spurii.