Optimal Design Of RF Circuit In RFID System

RFID (Radio Frequency Identification) is a kind of automatic identification technology which can realize intercommunication by using wireless channel. In recent years, the application of radio frequency identification system becomes wide. And it has come into many aspects of people's daily life. However, comparing home with abroad, the design and application of UHF band and RFID systems are still have a great gap. The main problem is that the distributed effect is serious in RF circuit which working in UHF (Ultra High Frequency) band. And the high frequency characteristic of element, design of layout and the EMC (Electromagnetic compatible) of circuit will direct impact the RF circuit. But, the corresponding technology and produce level is backward relatively in internal.Based on ISO/IEC18000-6 Type B protocol, a kind of 915MHz RFID system is designed and optimized and has passed the checkup; base on that, optimize design is done and the performance of circuit is further improved.First, this paper studies the basic principle of RFID and protocol standards, and according to the requirements, the specifications design and prototype production were completed. The prototype circuit includes two parts including launching and receiving circuit. In transmitter circuit, the frequency source is realized by the method of PLL, and frequency stepping is realized the same time. Modulators used a switching LNA, and it can pre-amplify the signals; the power amplifier is in the method of adjustable gain IC, enhancing the integration of the circuit; the multi-channel reading and writing is implemented by SPDT RF switch. Receiving Circuit adopts the zero-IF, only consists of the mixer, differential amplifiers and voltage comparator, compared to super heterodyne, obviously simplifies the structure of the circuit; to improve the sensitivity of receiving circuit zero IF and the performance of elimination of fuzzy point, adopts the method of two orthogonal fuzzy point demodulation circuit.Then, this paper use the method of software simulation, the model of mixer in zero-IF circuit is constructed and simulated, the parameters of element which can improve the frequency transform gain are obtained, According to the frequency spectrum of FM0 code, the pass band of filter is optimized and the most simple structure is obtained. The relationship between the phase shift of microstrip and mixed frequency and the appearance of fuzzy points is analyzed in theory. The relative position and structure of the most optimized single-ended mixer is obtained.This paper makes a comparison test between the two prototypes. Testing results show that the optimized circuit is superior to the original prototype in reading, writing distance, circuit size, and interference.