Reference modulation for calibrated measurements of tag backscatter

This paper presents an approach for calibrating backscattering measurements from 860-960 MHz Ultra-High Frequency Radio Frequency Identification (UHF RFID) tags. An S-parameter model is formulated to relate diode switch and antenna input circuit parameters with the scattering performance of the calibration device. Measurements of modulated backscattered power agree with the model to within ±0.1 dB. Tag backscatter measurements can then be calibrated by comparing them to the reference signal. In an example testbed, the expanded uncertainty of these measurements is estimated to be ±0.4 dB, compared with uncertainties worse than -0.9 dB, +1.2 dB for methods that calibrate against radar cross section (RCS) standards in the same testbed.