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 S11 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.