FMCW Based Readout System Accuracy Enhancement Techniques for Surface Acoustic Wave RFID sensors

Surface acoustic wave (SAW) RFID sensors provide fully passive access to identification in harsh environments. The rising demand of higher bit counts in excess of 256 bits require very precise tag readout techniques. Standard FMCW based systems with swept voltage controlled oscillators (VCO) can not provide the necessary stability in frequency ramp linearity and phase noise. This article describes the benefits of a direct digital synthesis (DDS) based low noise direct conversion FMCW system and shows the influence of DDS spurious noise generation on the baseband noise floor. Two techniques of receiver system layout improvements are shown with baseband noise reduction ratios of up to 15 dB. Followed is a discussion of the improvements of the DDS system over the standard VCO technique.