《Z-path SAW RFID tag》.pdf

208 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 55, no. 1, january 2008 Z-Path SAW RFID Tag Sanna H¨arm¨a, Victor P. Plessky, Senior Member, IEEE, Clinton S. Hartmann, Member, IEEE, and William Steichen Abstract—Surface acoustic wave (SAW) radio-frequency reasonably great number of codes. This corresponds to a identification (RFID) tags are soon expected to be pro- time delay of about 20 · 5 · 25 ns = 2.5 µs. In addition, duced in very high volumes. The size and cost of a SAW an initial delay for environmental echoes of about 1 µs, RFID tag will be key parameters for many applications. Therefore, it is of primary importance to reduce the chip corresponding to 2 mm of chip space on lithium niobate, size. In this work, we describe the design principles of a is also necessary. 2.4-GHz SAW RFID tag that is significantly smaller than In previously reported designs [1], [2], code reflectors earlier reported tags. We also present simulated and exper- are situated on both sides of a bidirectional interdigital imental results. transducer (IDT) as sketched in Fig. 2. In this geometry, The coded signal should arrive at the reader with a cer- the 1-µs initial delay is needed on both sides of the IDT. tain delay (typically about 1 s), i.e., after the reception of environmental echoes. If the tag uses a bidirectional in- For a tag having 14 code reflectors, as the tag studied in terdigital transducer (IDT), space for the initial delay is thi