SAW sensors using orthogonal frequency coding

This paper presents a spread spectrum approach using orthogonal frequency coding (OFC) for encoding SAW sensors. 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 could 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 band-width, 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.