UHF RFID chip impedance and sensitivity measurement using a transmission line transformer

This article describes an alternative procedure for measuring the impedance of an ultra high frequency (UHF) radio frequency identification (RFID) chip and finding its turnon-point. The proposed method is based on measuring the balanced impedance of the RFID chip using a standard vector network analyzer (VNA) and a custom fabricated printed circuit board (PCB) test fixture. The test fixture uses a transmission line transformer to (1) provide a balanced signal to the ports of the RFID chip and (2) achieve a pre-matching to avoid the inaccuracies resulting from measuring high Q components with a VNA. No additional RFID reader is needed, as the turnon-point is extracted from the measured voltage reflection coefficient as a function of frequency and input power. A matching network is designed using a Smith chart approach to prove the applicability of the method by measuring the resulting reflection coefficient. A comparison with the typically used single-ended technique is provided.