Adaptive carrier supression for UHF RFID using digitally tunable capacitors

In UHF (Ultra High Frequency) RFID (Radio Frequency Identification) systems Rx (Receiver) is usually isolated from Tx (Transmitter) by a circulator or a directional coupler. Since tags become more sensitive in order to improve reading distance, the relatively poor isolation of 20dB - 30dB limits the tag to reader link. A way to improve a directional coupler´s isolation is to mismatch the unused port to generate a carrier cancelling signal. In this paper an impedance network using digitally tunable capacitors is proposed. The advantage of this solution is that changes in the system - like antenna characteristics or cable length - can be recalibrated at any time by a controller. Finally an isolation of minimum 47dB for any antenna characteristic (minimum return loss of 18dB) and any cable length was reached. Also an algorithm was implemented on a simple MSP430 controller to find the optimum impedance to minimize carrier leakage.