Self-Tuning NFC Circuits

Abstract: Contactless automatic identification procedures which are called RFID systems (Radio-frequency Identification) have become very popular in recent years for transferring power and data. With the development of RFID technology, the demand of easy transmitting of short data packages has made NFC (Near-field Communication) technology wildly used especially in mobile applications. The communication between a mobile and a tag is achieved through a magnetic field generated by the mobile’s NFC interface. In order to get a maximal power transmission, the tag circuit is designed to operate at the resonance frequency of 13.56 MHz, which is equal to the operation frequency of the mobile’s NFC interface. As mutual inductances provided by different kinds of mobiles exist divergence, optimal power transfer cannot be reached every time. This thesis focuses on the optimization of power transfer during the communications between tags and mobiles with uncertain NFC coils. By incorporating a self-tuning parallel variable capacitance compensation circuitry the resonance frequency of an NFC tag circuit can be self-tuned to 13.56 MHz to ensure an optimal power transmission. This thesis presents both theoretical and experimental analysis of this improved self-tuning NFC circuitry in detail and demonstrates that by digitally tuning a parallel capacitor circuit, the energy transferred to an NFC tag can be optimized when facing different kinds of NFC-enabled mobile phones.