Mixed orthogonal frequency coded SAW RFID tags

Orthogonal frequency coded (OFC) SAW radio-frequency identification (RFID) tags are currently being explored as a multi-sensor platform because of their passive spread-spectrum operation, low loss, and resilience in harsh environments. Ongoing research continues to search for robust device embodiments that increase the number of identifiable codes, in the presence of intersymbol interference, while maintaining reasonable device lengths. This paper presents a technique that shortens the SAW response length while preserving code diversity and bandwidth by utilizing a multi-track SAW configuration. These new devices allow the time response of multiple OFC chips to overlap and yield a mixed-frequency chip having the sum of the chip bandwidths but shorter overall time response. The theoretical development is presented and examples are discussed for these new mixed orthogonal frequency coded (MOFC) SAW devices. Experimental results for MOFC sensors, fabricated on YZ-LiNbO₃, with a 7% fractional bandwidth and five chip frequencies in three cells, provide a good contrast to similar OFC designs. Experimental results are presented for the simultaneous operation of eight wireless temperature sensors-four OFC and four MOFC-in a 915-MHz wireless correlator receiver system, highlighting the ability of these devices to operate in the same system.