Thickness-shear and thickness-twist modes in an AT-cut quartz acoustic wave filter

• Frequencies and modes are obtained for a monolithic AT-cut quartz filter.
• Trapped modes with vibrations mainly under the electrodes are found.
• The effect of the distance between the two pairs of electrodes is examined.

We studied thickness-shear and thickness-twist vibrations of a monolithic, two-pole crystal filter made from a plate of AT-cut quartz. The scalar differential equations derived by Tiersten and Smythe for electroded and unelectroded quartz plates were employed which are valid for both the fundamental and the overtone modes. Exact solutions for the free vibration resonant frequencies and modes were obtained from the equations. For a structurally symmetric filter, the modes can be separated into symmetric and antisymmetric ones. Trapped modes with vibrations mainly under the electrodes were found. The effect of the distance between the two pairs of electrodes was examined.