Multi-track low-loss SAW tags with flexible impedance matching for passive wireless sensor applications

This paper presents recent results on a NASA program to build a low-loss, multi-sensor, SAW temperature sensor system. Multi-track CDMA tags have been previously studied, which helps to balance the tag reflectivity from chip-to-chip. Normally the IDT's beam extends over all tracks and the bandwidth is the same for all tracks, and there is no significant advantage over a single-track using this approach. Wideband tags using orthogonal frequency coding (OFC) can use multi-frequency chips subdivided into multi-tracks with low loss operation. Each track has one or more chips, with each chip having a different chip frequency. The track-transducer is then designed to operate only over the required frequency bands; making each non-interacting track low loss. The overall transducer embodiment is now tailored for optimum performance for loss, coding and chip reflectivity. If all tracks are electrically in parallel, the overall transducer Q remains the same as a short wideband IDT, but the electrical reflection coefficient is chosen for minimum loss or matching. Measured parallel track OFC Sn response was measured where the reflection coefficient is nearly optimized for minimum unmatched loss. In comparison, for a short wideband transducer of equivalent bandwidth, the reflection coefficient is close to unity with large unmatched loss. An analytic, synthesis model and the coupling-of-modes model (COM) were developed for predictions. Experimental devices were fabricated on YZ LiNbO
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at 250, 500 and 915 MHz for evaluation. Several frequency, track number and chip configurations were designed and fabricated having various beam widths. The Sn response illustrates the rotation on the Smith chart around the constant Q arc. Current designs indicate that an unmatched sensor loss is approaching 10 dB, which is 20-40 dB less loss than typical CDMA performance. This paper will present the approaches, analysis, design, layout, and results of several multi-track OFC devices, having several differing operating frequencies and embodiments. The experimental results agree remarkably well with analysis and predictions and confirm the advantageous of the multi-track approaches. The results of this work will ultimately produce lowloss SAW sensor platforms, which lead to greater sensor system range and performance.