Spread spectrum orthogonal frequency coded SAW Tags and sensors using harmonic operation

Wideband SAW devices with orthogonal frequency coding (OFC) are expected to be highly advantageous for various applications in NASA's exploration effort. Surface acoustic wave (SAW)-based sensors can offer wireless, passive operation in numerous environments, with various device embodiments used for retrieval of the sensed data information. Single sensor systems typically can use a single carrier frequency and a simple device embodiment because tagging is not required. In a multi-sensor environment, orthogonal frequency coding (OFC) permits the system to both identify the sensor and retrieve the sensed information discriminately. Previous research efforts have concentrated on a relatively lower operational frequency in the 250 MHz range due to available fabrication technology limitations. Using harmonic frequency operation, as shown in this paper, higher frequency devices are possible while continuing to work within the same limitations of contact lithography resolution. This paper presents a preliminary investigation of OFC SAW tags and sensors at a 900 MHz operational frequency range using conventional contact lithography techniques. The higher frequency devices are achieved using second harmonic operation. The objective of this effort is to increase the technology readiness level (TRL) of wideband OFC tags at higher frequencies. Measured device results are presented and compared with coupling of modes (COM) model predictions to demonstrate performance.