Automated antenna impedance adjustment for Near Field Communication (NFC)

Near Field Communication (NFC) is a very intuitive way to open a communication link, to authenticate, or to implement a payment by simply bringing two mobile personal devices closely together. NFC is based upon reliable contactless card technology and combines most prominent protocol standards in a specification driven by the NFC Forum. Devices with a NFC interface operate at 13.56 MHz via inductive loop antennas. However, these operate operated in a very different environment than contactless cards. Metal, the presence of several other antennas, and market demands for compact electronic devices are driving requirements for antennas to operate on ferrite foils, resulting in significant tolerances on antenna impedance. In NFC Reader mode, the antennas operate in a resonance circuit, making this de-tuning critical. This paper presents a prototype implementation for automated antenna impedance adjustment based upon Digitally Tunable Capacitors (DTCs). To show the benefit, the paper explains efficiency for contactless power transfer by practical measurements, comparing three scenarios-fixed impedance adjustment, matching with tolerance, and automated readjustment using a DTC.