Dispersion curves for STX Quartz and for STX+25 Quartz are calculated by modified FEMSDA program. All the basic COM parameters are extracted from calculated dispersion curves. In particular, the phase velocity in the center of the stopband, reflectivity, transduction coefficient per electrode and directionality are determined for 0.1 les w/p les 0.9 and 0 les h/p les 0.1, where w and h are width and thickness of electrodes respectively and p is the spatial period of the grating. It is shown, in particular, that the maximum of the transduction coefficient stays near w/p = 0.5 for all the h/p values, and the maximum of the reflectivity for short-circuited aluminum grating shifts from w/p = 0.3 to w/p = 0.9 when h/p changes from 0 to 0.1. Amplitude-frequency characteristics of some concrete devices are calculated