Saw Sensors Using Orthogonal Frequency Coding


The SAW sensor offers advantages in that it is wireless, passive, small and has varying embodiments for different sensor applications. In addition, there are a variety of ways of encoding the sensed data information for retrieval. Single sensor systems can typically use a single carrier frequency and a simple device embodiment, since tagging is not required. In a multi-sensor environment, it is necessary to both identify the sensor as well as obtain the sensed information. The SAW sensor then becomes both a sensor and a tag. For multi-sensor systems, many embodiments use coded reflectors having a single carrier frequency. In previous work, several authors have shown that there are advantages of using spread spectrum techniques for device interrogation and coding, such as enhanced processing gain and greater interrogation power. This paper will present a spread spectrum approach using orthogonal frequency coding (OFC) for encoding the SAW sensor. The encoding technique is similar to M-ary FSK in terms of its implementation where a transducer or reflector is built with the desired code. It is shown that 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. The OFC approach is general and should be applicable to many differing SAW sensors for temperature, pressure, liquids, gases, etc. Device embodiments are shown, and a discussion is provided for device design considerations such as the number of chips used, chip length, transducer fractional bandwidth, and chosen piezoelectric material. Measured device results are presented and compared with COM model predictions to demonstrate performance. Devices are then used in computer simulations of multiple transceiver designs and the results are discussed. © 2004 IEEE.

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Publication Title

Proceedings of the IEEE International Frequency Control Symposium and Exposition

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Document Type

Article; Proceedings Paper

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Socpus ID

28744442389 (Scopus)

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