Software defined radio for passive sensor interrogation

The software defined radio (SDR) provides a unique platform to interrogate passive, wireless sensors. The SDR platform defines many radio functions in software, rather than hardware. This allows for a very versatile interrogation platform that can be quickly reconfigured for diverse scenarios. This paper investigates the Universal Software Radio Peripheral (USRP), a commercial SDR developed by Ettus ResearchTM, to determine its viability to interrogate passive, wireless sensors. In particular, the N200 USRP and the WBX daughterboard are considered because they allow for the greatest possible bandwidth for this platform (40MHz) and frequency tuning that is ideal for surface acoustic wave (SAW) sensor interrogation (50MHz-2.2GHz). In the default operation mode, the USRP continuously streams data to and from the host computer. The host computer generates samples for transmission and processes any received samples. Synchronization of the transmit and receive chains becomes difficult due to latency in the communication medium (USB or Ethernet) between the host and USRP. While this mode is sufficient for most narrowband applications, wideband applications are more difficult to achieve because of the high sampling rates required (even in a baseband system such as the USRP). A prototype transceiver (passive tag reader) is developed by modifying the N200 FPGA to introduce new functionality to the USRP. These modifications include a custom interrogation signal generator (linear chirp) and triggering of the receiver based on the transmission state and desired listening time. These modifications are discussed and the modified transmit and receive characteristics are analyzed. Finally, a passive, wireless SAW OFC sensor is interrogated to demonstrate performance as a passive sensor transceiver.