Accurate Measurement of Power Transfer to an RFID Tag with On-chip Antenna

Dramatically decreasing the size of an RFID tag is possible by using on-chip antennas, so-called OCAs [1, 2]. For the applications in mind, such as identifying goods in the food supply chain or in medical supply, the systems on chip need to be externally powered. This is challenging due to the small aperture available and because the antenna typically resides on a lossy substrate such as silicon. Accurate measurement of the power inductively transferred to the tag is impossible using bond wires or probes, because both would introduce severe distortions of the electromagnetic field. Questionable results concerning the voltage induced into the on-chip antenna would be such obtained. We present a novel method to accurately measure the voltage induced into an on-chip antenna and demonstrate its application in the UHF band at 850 MHz. To verify the proposed method we have designed and manufactured a system on chip. This chip consists of a loop antenna (similar design as reported in [1] and [2]), a rectifier, and a voltage controlled oscillator [3]. The latter modulates the energy backscattered by the chip with a frequency proportional to the voltage delivered by the rectifier. Hence, by measuring the modulation frequency the exact value of the voltage induced into the chip's antenna can be revealed. In this paper, comprehensive measurements of the inductive coupling between an elaborate reader station and an RFID tag with on-chip antenna will be shown. The silicon is moved in planes above an excitation coil, and the modulation frequency of the signal backscattered is measured. With the relation between the voltage induced into the chip's antenna and the backscattered signal's modulation frequency being calibrated, the energy received by the chip as a function of its position in space is mapped.