Full strip reflectivity study on quartz

SAW devices are becoming more and more widely used in low-loss bandpass filtering (SPUDT filters, resonator filters, ...). For these SAW filters, the strip reflectivity is a key element of their design. Due to its very good temperature stability, quartz is the most commonly used substrate for narrow band applications. Due to small electroacoustic coupling, strip reflectivity on quartz is mostly due to the mechanical part of the electrode perturbation. Therefore, only experimental measurements or complex numerical models can be accurate enough to calculate this reflectivity. In this paper, we present a full experimental characterization of strip reflectivity on 38° Y rotated quartz: the metallization thickness ranges from 0.6% λ, to 315% λ, while the mark to period ratio varies from 0.25 to 0.8. A recently developed mixed FEM/BEM model has been used to compare these experimental data with simulations showing a very good agreement. Despite classical model predictions, maximum reflectivity mark to period ratio was found very thickness dependent. This property was used to greatly improve SPUDT designs