Chirped SAW devices for wireless passive sensors

Chirped structures as sensing elements in a delay line allow for the variation of the sensitivity over a wide range. A sensor system operating at a center frequency of 434 MHz and a system bandwidth of 1.7 MHz will be shown. We designed dispersive inline SAW structures with two chirped and apodized IDTs. Two expanders, one with an up- and the other with a down-chirp, have been built on quartz 37.5° cut. The corresponding compressors (down- and up-chirp) are the sensor elements and have a wider bandwidth to compensate for the frequency shift due to the sensor effect. The compressors have been built on quartz for mechanical sensing and on LiNbO₃, YZ-cut for temperature sensing. To avoid bulk wave conversion we used a modulated IDT with 8 electrodes per electrical period. On quartz we operated these IDTs on the 3rd and on LiNbO₃ on the 5th harmonic which is lower than the onset frequency for bulk wave conversion and gives a suitable impedance level as well. We present theoretical and experimental results for the devices and show results of the corresponding sensor systems for temperature