Ultra Wide Band Surface Acoustic Wave (Saw) Rf Id Tag And Sensor

A new and novel spread spectrum approach employing orthogonal frequency coding (OFC) for a SAW sensor/tag is presented. This approach provides ultra wide band operation, multi-device interrogation, and reduced loss as compared to conventional SAW CDMA approaches. A traditional SAW CDMA tag consists of an input transducer that launches a surface wave on the substrate towards an array of single frequency collinear reflectors that reflect the wave back to the input after providing ID coding within the reflections. The OFC embodiment uses an appropriate wideband input transducer that launches a SAW towards a collinear multi-frequency reflector bank which encodes the ID information. The sensor information is encoded by interaction of the SAW with delay path changes due to temperature and other measurands. This novel OFC SAW approach enables multiple coded chips to have high reflectivity since the orthogonal frequency chips appear nearly transparent to others; yielding low reflector loss, minimal inter-chip distortion and both phase and frequency encoding. The multi-frequency OFC SAW tag/sensor presented in this paper uses an encoding technique similar to Orthogonal Frequency Multiplexing (OFDM) in terms of its implementation. Each device is built with a unique code defined by the random sequence (chips) of the set of frequencies of the element of the reflector array. The precise OFC conditions yield chip orthogonality in both frequency and time, and enhanced time domain cross correlation properties between chips of differing frequency. The time ambiguity in the autocorrelation due to the OFC is significantly reduced as compared to a single frequency tag having the same code length due to the increased processing gain of the time-bandwidth product. This paper will discuss the OFC SAW temperature sensor design parameters and the transceiver system approach. A prototype wireless transceiver at 250 MHz with approximately 69 MHz bandwidth, and consisting of an RF transmitter, a transmit antenna, a receive antenna, and a receiver was built for the interrogation and detection of an OFC SAW temperature sensor device. Measured results of this new OFC wireless temperature sensor are found to be in very good agreements with results obtained from the thermocouple. This novel OFC RFID SAW embodiment can be used for passive RFID tags and a host of potential wireless-coded sensor systems. © 2009 IEEE