Adaptive impedance matching system for downlink of passive semi-ultra wideband ultra-high frequency radio frequency identification tag

The input impedance of ultra-high frequency radio frequency identification tag varies with the received power on the chip. It will induce impedance mismatch between the receiver antenna and microchip, thus drastically affect the performance of communication. In this paper, a low cost and fully integrated automatic impedance matching system was presented to solve this problem. It consists of two control loops for independent control of the real and imaginary parts of impedance. The first control loop realizes resistance correction using a parallel LC tuning network, whereas the second control loop achieves reactance compensation using a series LC tuning network. In both loops, the mismatch information is detected for direct control of the variable elements, varactors, which are tuned in a sequential manner. For unambiguous control of the resistance correction, the sign of the intermediate reactance is used as a secondary control criterion to enforce operation into a stable region. The functionality of the proposed automatic matching system was verified for different input impedances of a specifically semi-ultra wideband ultra-high frequency radio frequency identification tag as the available input power varies. The results indicate that all matched impedances are clustered around the target impedance 50 + j0?O after acquisition of both loops. Copyright (C) 2012 John Wiley & Sons, Ltd.