A stable and highly sensitive strain sensor based on a surface acoustic wave oscillator

• A single-mode selection SAW oscillator was demonstrated.
• A cantilever beam with tapered cross section was designed to provide quasi-uniformed strain testing platform.
• The SAW oscillator showed a promising stability (about 0.7 ppm in a temperature control environment).
• High sensitivity and good linearity were observed in the strain test.

A stable and highly sensitive strain sensor based on a surface acoustic wave (SAW) oscillator is presented in this article. A 151 MHz SAW delay line was fabricated on a 128° YX LiNbO3 substrate as the oscillator element. A cantilever beam with a tapered cross section was designed to provide a quasi-uniform strain testing platform and analyzed with a finite element method (FEM) model. Single-mode selection capability was achieved by using a comb configuration, and the triple transition interference (TTI) was suppressed by using a single phase unidirectional transducer (SPUDT). Prior to fabrication, a FEM/boundary element method (BEM) simulation was used to simulate and optimize the parameters of the SAW delay line structure. The RF test results of the fabricated SAW devices agree well with the simulation results. Further, the output frequency fluctuation of the developed SAW oscillator was shown to be only approximately 0.7 ppm in a temperature-controlled laboratory environment for stability tests. The measured sensitivity was evaluated to be 126 Hz/μɛ, and good linearity was observed at strain values ranging from 0 to 400 μɛ.