High temperature interrogation unit for wireless SAW-based temperature sensors

AbstractSurface acoustic waves (SAW) have been studied for more than 50 years and are mainly used as frequency filters. Using appropriate design considerations, these devices are sensitive to external conditions including temperature, pressure, strain or chemical/biological mass loading: hence, SAW sensors are implemented and provide unique characteristics [1]. Amongst the original characteristics, the linear behavior of the piezoelectric substrates provides interrogation ranges hardly achieved with silicon-based radiofrequency devices, extreme working temperatures and simple manufacturing conditions with a single cleanroom process step: hence, SAW transducers provide ideal transducers for passive sensors interrogated through a radiofrequency wireless link. This work is devoted to the development of a wireless interrogation system able to operate under high temperature environments (650 to 900°C for the sensors). As wireless passive SAW-based sensors are considered here, the sensor reading allows the interrogation unit (so-called reader) to be far enough (some meters) to avoid facing such temperature conditions. However, the reader must be able to operate at temperatures in the 100-150°C range, beyond the usual industrial grade integrated circuit characteristics. Therefore, the capability of the reader to operate under such conditions and hence to meet the above specifications is considered. The sensor and the interrogation unit are here both submitted to “high” temperature environments, with different meaning whether passive surface acoustic wave (SAW) sensors or reader including CMOS and radio-frequency (RF) components are considered. We qualify the maximum operating temperature of our reader designed using commercial off the shelf (COTS) components with standard temperature requirements. The reader is tested from room temperature up to failure, which is met around 140°C. The cause of the failure is due to a communication chip (RS232-USB converter): the whole interrogation unit is thus expected to reach even higher temperatures without significant performance losses