UHF Band SAW Based RFID Sensor System

Among the various identification systems that are currently in development, Radio Frequency Identification (RFID) systems show the most potential due to their wireless sensing capabilities, making them highly suited for remote monitoring and measurement systems. A RFID system consists primarily of a reader unit and transponder. The interrogation signal emitted from the reader unit is reflected by the transponder, and the information on the transponder is embedded into this reflected signal. This reflected signal is picked up by the reader unit, reading the information on the transponder. By combining an antenna with a surface acoustic wave (SAW) device, a passive RFID transponder that do not require any external power source could be achieved.
In this thesis Abbott’s Coupling Of Mode (COM) theory is used to simulate and analyze the frequency response of the UHF SAW based RFID tag. Furthermore, inverse fast Fourier transform is applied in order to obtain the time domain response by which the coded information is encoded. Using the result of simulation, parameters such as substrate, finger pairs of the InterDigital Transducer (IDT), and apertures in frequency and time domain were discussed. By considering the influences of an external load (sensor) on the boundary conditions of the tag, we can simulate the results of varying load. Finally, different designs of SAW based RFID tags with central frequency of 433MHz were fabricated on the 128°Y-X LiNbO3 using Micro-Electro Mechanical Systems (MEMS) techniques. The experimental result and simulation results are compared and the COM theory proof to be suitable for analysis of SAW based RFID tag. Using these results, guiding rules for the designing of SAW based RFID tags are proposed and a single tag capable of carrying multiple independent codes is designed and fabricated.
In conclusion, this thesis utilized the COM theory to analyze SAW based RFID tag, and designed a novel dual band RFID tag. This dual band RFID tag can reduce cost and increase the amount of information carried by a single tag. Hopefully, this SAW based RFID tag will be widely employed in monitoring and measurement applications.