Born as a specialized subset of the 13.56MHz RFID technology in 2002, Near Field Communication (NFC) technology was first introduced for a mobile payment service commercialized in Japan. Now, integration into credit/debit cards and smartphones is driving rapid growth. More than just for payments, NFC is making its way into a broad range of applications thanks to its convenient bi-directional interaction between electronic devices, enabled by a simple “tap.” Indeed, beyond payment or access control, NFC is becoming popular for device pairing (like BT, Wi-Fi pairing), diagnostic readout, parameter programming, and much more.
NFC is built upon the RFID standard backbone, like ISO 15693 and ISO 14443 RF protocols, supported by NFC / RFID tag, Dynamic tag, and Reader Integrated Circuits (ICs) manufactured by ST.
These ISO standards are based on passive RFID technology, operating in the high-frequency (HF) range, at 13.56 MHz.
The reader / tag system is similar to a voltage transformer, where the reader acts as the primary winding and the tag as the secondary winding. As a result, when operating properly, the reader and the tag are magnetically and mutually coupled to each other.
For optimal communication and data transfer Tag Antenna and Reader Antenna design is critical in NFC / RFID applications. Another critical step for optimal reader performance is the tuning of the antenna matching circuit.
Antenna design support for tags and dynamic tags:
The design of an antenna for NFC tags and dynamic NFC tags consists of designing a well-suited antenna loop placed either on or within the support material of the tag label or on the application pc-board. Its impedance matches the device’s internal tuning capacitance value (Ctuning) to create a circuit resonating at ftuning =13.56 MHz. The basic equation of the tuning frequency is:
where Lantenna represents the loop antenna inductance.
ST’s AN2972 & AN2866 application notes and antenna eDesignSuite are available to assist designers to achieve optimum antenna tuning in their Tag designs. ST provides numerical examples for circular, spiral-shaped, and squared antennae, and the online eDesignSuite tool offers simulations to develop even rectangular antennas at 13.56 MHz.
After entering parameters related to the pc-board material and antenna dimensions, the eDesignSuite tool estimates the antenna equivalent inductance by calculating the self-inductance and estimating the stray capacitance of the antenna.
Readers’ antenna matching and board layout support:
For Readers, a challenge in using NFC/RFID technology is not only the antenna design but also the tuning of the matching circuit, which has also to take into account environmental conditions that can degrade the system performance.
ST’s AN4974 application note can guide designers to define the optimum matching circuit when connecting their loop antennas to any of the ST25R391x NFC/RFID Reader series. This application note comes with the ST25R Antenna-Matching Tool software (STSW-ST25R004), which supports the calculation of the matching components and reduces the tuning iteration effort to a minimum. Along with the tool, ST provides an open-source simulator for basic system validation via simulation.
In order to accelerate the NFC/RFID-reader design, help mitigate unwanted emissions, and keep the overall noise floor low, the AN5043 application note can be a guideline.
The note discusses the interference between the ST25R391x NFC/RFID Reader devices and other active components (like receipt printers or displays) to help avoid problems during the layout phase.
In particular, it suggests paying attention to signals propagating along traces and to the spectral content of each of these signals. The signals should be analyzed in both time and frequency domains to track unwanted emissions back to the source, and take appropriate countermeasures.
Proper pc-board layout minimizes many EMI problems and can enable the application to comply with FCC and/or CISPR standards.
The notes and tools, along with the included examples, will help you achieve a solid NFC/RFID design — while also reducing development time and costs.