Interrogation of orthogonal frequency coded SAW sensors using the USRP

The universal software radio peripheral (USRP) is a versatile software defined radio (SDR) platform, developed by Ettus Research™, which is intended for a wide variety of applications ranging from communication links to RADAR. We have investigated another application of the USRP by implementing a transceiver capable of interrogating passive, wireless surface acoustic wave (SAW) sensors centered at 915MHz. Interrogation of wideband orthogonal frequency coded (OFC) SAW sensors imposes strict requirements on the timing and synchronization of the transceiver. In the standard mode of operation, samples are generated and streamed between the USRP and host computer, introducing latency and bandwidth limitations due to the sampling bus. To achieve the performance required for this application, the USRP FPGA has been modified to introduce new functionality. Extraction of the sensor temperature is accomplished with a custom matched filter correlator. The system is capable of interrogating multiple sensors and can quickly reconfigure the USRP. Demonstration of the USRP wireless sensor system is achieved by interrogating wireless SAW OFC sensors at 915MHz and extracting the sensor temperature.