Orthogonal frequency coded surface acoustic wave passive remote sensing

Surface acoustic wave (SAW) devices are attractive for sensing of the physical, gas, liquid or biological environment because the device has many parameters which can be adjusted for various applications. The temperature coefficient of delay can provide temperature sensing, coupling to liquids, gases or applied films can be controlled by choice of mode, and putting the device under stress or strain can measure pressure or vibration. In addition, there are many substrate choices to attempt to optimize a given measurand. Finally, substrate choices allow sensing from cryogenic to high temperatures (1000 degrees C), which has the potential for use in a wide spectrum of space applications. This paper will describe a novel SAW sensor platform for sensing of various measurands which is passive and wireless, and has several levels of coding available for providing identification. The paper will describe the concept of orthogonal frequency coding used in the device identification and its advantages in communications and sensing, and the mechanism for implementation using reflectors in a SAW sensor. Results of measured SAW device performance versus the coupling of mode (COM) model predictions show excellent correlation for a 250 MHz device on a lithium niobate substrate. The approach presented uses frequency selective reflectors in a differential delay line to measure temperature; measured from cryogenic temperatures to 150 degrees C with the same device. A discussion of substrate materials and device parameters will be presented to exemplify the versatility and practicality of these devices to a wide range of sensor applications.