Abstract
A system for wireless interrogation of surface acoustic wave (SAW) based humidity sensor is described. The SAW sensor is a Lithium Niobate (LiNbO3) substrate with metallic interdigital transducers (IDTs) and reflectors etched on it. A microstrip antenna is placed in close proximity to the IDT, such that the antenna can excite the IDTs through the air gap by inductive coupling. The sensor antenna receives the FM signal from the transceiver antenna of the wireless system. The electromagnetic signal received by the sensor antenna is converted by the IDT into a surface acoustic wave that propagates in the LiNbO3 substrate. This surface acoustic wave is reflected by the etched metallic reflectors, reconverted into an electromagnetic signal by the IDT and returned to the transceiver system. The SAW velocity on the LiNbO3 substrate varies as a function of the relative humidity and results in a varying time delay in the reflected signal, which is detected by the wireless system. The resolution and accuracy of such a system are investigated and some experimental data is presented. The operating principle and techniques can also be used for other wireless, passive SAW sensors.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
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