Wireless measurement of temperature using surface acoustic waves sensors

Surface acoustic wave devices can be used as wireless sensor elements (SAW transponders) for measuring physical quantities such as temperature that do not need any power supply and may be accessed wirelessly. A complete wireless sensor system consists of such a SAW transponder and a local radar transceiver, is widely discussed. The antenna of the SAW transponder receives an RF burst in the VHF/UHF band transmitted by the radar transceiver. The reader unit performs a radar measurement of the impulse response of the SAW transponder via a high-frequency electromagnetic radio link. A temperature variation changes the SAW velocity and thereby the response pattern of the SAW device. By analyzing the time delay between backscattered pulses with different time delays we get a coarse estimation of the temperature of the SAW transponder. By using this information the ambiguity of ±2π in the phase differences between the pulses can be eliminated, which provides to an overall temperature resolution of ±0.2°C.