Harmonic operation of STW filters

The physical mechanism causing the propagation loss of surface transverse wave (STW) devices during harmonic operation is modeled and analyzed. It is found that as the mode frequency is pushed beyond the fundamental, the dominant Bloch component of the STW solution passes through the higher spatial harmonics. In these frequency regions, the lower harmonic components which contribute (perhaps weakly) to the total solution can be untrapped bulk modes which radiate energy away from the surface component into the substrate. As the periodic surface perturbation is increased by increasing the thickness of the trapping metal grating, the coupling to these nondominant radiating components is enhanced, leading to greater propagation loss. The physical reason why these nondominant lower spatial harmonics are untrapped while the dominant mode is trapped is also explained by the model. Experimental results supporting the space harmonic model are presented. They demonstrate that third-harmonic transmission is an extremely critical function of metal thickness, and that high coupling and high acoustic transmission cannot be achieved at the same time at harmonic frequencies