The Design And Simulation Of SAW Tags

Surface acoustic wave (SAW) tags have the characteristics of completely passive operation, low cost and high robustness against environmental impacts that make them superior to other techniques when used in radio frequency identification (RFID) systems. SAW tags are soon expected to be produced in very high volumes. This paper has done some research and simulation on the SAW tags, especially argued on lowering the system cost, improving the reflectivity of reflectors and increasing the coding capacity. A SAW tag is designed and the optimization of the structure is discussed at the end of the paper.SAW tags generally consist of three parts:tag antenna, inter-digital transducer (IDT) and code reflectors. A reader emits an interrogation pulse, which is received by the tag antenna, directly connected to an IDT. The IDT transforms the electrical signal into a surface acoustic wave, which is a mechanical wave of particle displacements. The generated SAW pulse then propagates along the surface of the substrate, which is usually made of a strong piezoelectric material such as lithium niobate (LiNbO3). The SAW pulse is partially reflected and partially transmitted by each of the so-called code reflectors, placed at precisely determined positions on the chip. These reflectors usually consist of one or a few narrow aluminum strips. The reflected SAW returning to the IDT thus carries a code based on the positions of the reflectors. When the train of reflected SAWs finally returns to the IDT, the acoustic signal is reconverted into an electrical form due to piezoelectric effect and retransmitted by the tag antenna. The response signal is then detected and decoded by the reader.This paper has presented theory and technology on the SAW tags, including the general framework of the SAW tag, the structure and character of each component, the and the common used coding scheme. First, the IDT model is simulated to determine the propagation characteristics of SAW and the parameters of IDT by finite element analysis (FEA) software through the static, modal and harmonic response analysis. Second, the reflectivity properties and the dispersion characteristic of the reflectors are studied by using the equivalent circuit model (CM) and coupling-of-mode (COM) theory. It is reasonable to adopt suitable reflector framework through computing out the influence of electrode number and thickness. Third, this paper has also made out the coding scheme with the method of multi-pulse position coding (MPPM) to meet the needs of high capacity encoding. Finally, two kinds of improved structure, single-phase unidirectional transducer (SPUDT) collinear structure and Z-path SAW tags, are proposed in order to further reduce the size and cost of SAW tags.