Implementation of orthogonal frequency coded SAW devices using apodized reflectors

Recently, orthogonal frequency coding (OFC) has been presented as a novel method for coding SAW tags and sensors (Malocha et al.,2004). Orthogonal frequency coding is a spread spectrum technique and has been shown to provide enhanced processing gain and reduced time ambiguity resulting in greater range and increased sensitivity when compared with single carrier frequency devices. The sensor works both as a tag and a sensor with the ability to send back "tagged" sensor information in a multi-sensor environment. The tag information is provided by a series of reflectors which map into a known chip sequence. The time-chip- sequence is coded by differing OFC and PN sequences. Therefore, the implementation of an OFC sensor requires reflectors having differing local carrier frequencies. In the case of narrow fractional bandwidths or high reflectivity (such as on LiNbO₃), it is desirable to adjust the reflectivity per electrode in the various chips. For varying system requirements, the use of weighted reflectors is an option; both apodization and variable weighted reflectors are investigated. Experimental results on cosine weighted apodized reflectors will be compared to COM predictions on YZ LiNbO₃. This paper presents several OFC SAW device embodiments that are employed using wideband input transducers and multiple weighted reflector gratings. The devices operate in the differential mode using gratings on either side of the transducer. The advantages of using the weighted reflector gratings in OFC SAW devices are discussed. OFC temperature sensor device experimental results are presented using weighted reflectors to obtain the proper time coded response and compared to predictions