A 10-Bit DAC 3GS/s Interpolating DDFS for Distortion-Limited Long Acquisition Time FMCW Ground Penetrating Radars

This paper discusses the issues of distortion averaging in FMCW ground penetrating radar (GPR) systems with very long acquisition times. Specifically, we compare implementing a long chirp with an arbitrary waveform generator (AWG) approach compared to a direct digital frequency synthesizer (DDFS) approach. We show through simulation and measurement that for long acquisition times, the DDFS approach is superior to the AWG because of the longer chirp times it can support leading to more diversity in the distortion components of the radar chirp. This higher distortion diversity leads to improved autocorrelation sidelobe levels and lower distortion floor, allowing the GPR to detect weaker sub-surface echoes. As part of this analysis, we demonstrate a 6ms chirp time 1.5 GHz Nyquist bandwidth DDFS implemented in 28nm CMOS technology which consumes 270mW when operated at 3 GS/s producing a 0.1-to-1.2 GHz FMCW waveform.