Surface acoustic wave RFID tags : ideas, developments, and experiments

Abstract

Radio-frequency identification (RFID) provides an efficient means of labeling and identifying various items. The principal advantage of RFID over the more traditional barcode labeling is that RFID labels are read using a radio signal. Hence, an unobstructed line-of-sight is not needed between the reader and the label. In addition, RFID labels, or tags, can carry a significantly larger amount of information than barcodes. They also are physically robust whereas printed barcodes can easily be smudged or damaged. Furthermore, it is possible to read many RFID tags simultaneously and the presence of a human operator is generally not needed. This dissertation focuses on a type of RFID tag that relies on the surface acoustic wave (SAW) technology. SAW RFID tags are passive devices that reflect the interrogation signal in a form that is modified according to the identification information stored on the tag. In reflector-based SAW RFID tags, the encoding is based on the positions of metallic SAW reflectors on the surface of a piezoelectric substrate. In other words, it is based on the time delays of the reflected signals. The dissertation first discusses the extraction of central SAW reflector parameters and then presents novel SAW RFID tag designs. In the first part of the work, the reflectivity of narrow reflector electrodes on YZ-LiNbO3 substrate is determined. In addition, a new method for extracting the frequency-dependent reflection, transmission, and scattering parameters of short metal reflectors is developed. The second part of the dissertation discusses the design of SAW RFID tags. The main objectives of tag design include the reduction of device size and the enhancement of information capacity. This dissertation presents a Z-path SAW RFID tag that uses two strongly reflecting inclined reflectors to fold the acoustic path. The Z-path SAW tag has a significantly smaller size than previously reported SAW tags. The information capacity of SAW RFID tags is enhanced by combining the conventional time-delay-based encoding with phase encoding. A compact device size and strong resistance to environmental echoes are achieved through the application of ultra-wideband (UWB) radio technology. The feasibility of UWB SAW tags is investigated using simulations and confirmed experimentally. The SAW RFID tags presented in this dissertation are all designed for the 128°-LiNbO3 substrate.