Research And Design Of The NFC Tag Modem Circuit

With the progress of society and increased demand of electronic products, the mobile payment technology has been widely used in people’s daily life. At the same time, the security problem of mobile payment technology has appeared. NFC (Near Field Communication) technology evolved from a non-contact radio frequency identification technology and inter-communication technology, which is compatible with existed contactless smart card and widely used in access control, public transportation and mobile payments. NFC technology is safe, easy and fast which lead to the fast developing all over the world.Firstly, the concept and characteristics of NFC technology is introduced. Then the development process and current situation is presented. After that, the working principle and communication patterns of NFC are analyzed. Furthermore, the structure of NFC tag and ISO14443 protocol are introduced in detail. The heart of this paper is the circuit of modulation circuit and demodulation. For circuit of demodulation, this paper presents some basic theories, like sorts, physical principles and realizing methods, and then, presents and analyzes the circuit structure. Then each module of demodulation circuit is described and simulated. For subcarrier modulation circuit, this paper firstly introduced the concept of load modulation and two different ways to load modulation. Next, this paper presents and analyzes the design of the subcarrier modulation circuit.Compared with conventional demodulation circuit, demodulation circuit of this paper increases the DC bias generation module, which can facilitate the comparison of the envelope signal. In addition, the demodulation circuit also adds a feedback control module, which uses the demodulated output signal for dynamically generating control signal to control the demodulation process and achieves the purpose of reduce the effects of noise. For subcarrier modulation circuit, this design uses dual MOS transistor switch control circuit, which can make the antenna of the NFC tag chip get stronger voltage fluctuations. Therefore, the antenna of reader can couple stronger signal fluctuations. The design of this paper is simulated by Cadence based on SMIC 0.18μm CMOS process. The simulation results show that they can achieve their function.