Impedance measurement of a UHF RFID transponder by evaluating the rectified voltage

A main task in RFID chip design is the determination of its input impedance. Standard methods involving the use of a state-of-the-art network analyzer require expensive equipment. Here a new approach is proposed to determine the impedance by measuring the voltage output of the rectifier stage of an 868 MHz RFID tag. The tag is connected to several antennas with different impedances and for each the voltage after the rectifier is measured. These values are used to determine the input impedance.