Beneficial effect of diffraction on the frequency response of a 250-MHz SAW mobile IF resonator-filter with long IDTs and triple longitudinal modes on quartz

Diffraction theory is applied to model the frequency response of a low-loss (~2 dB) 250-MHz SAW mobile IF resonator-filter (BW ~0.2%) with long-pair IDTs and triple longitudinal modes on ST-quartz, as applicable to circuitry such as for the Japanese Personal Handy Phone (PHP). In a simplified approach, a parabolic diffraction approximation is applied, with the IDT acoustic aperture conceptually sub-divided into parallel channel strips of equal widths. An anisotropy parameter value A_d =-0.22 for ST-quartz is used in the parabolic approximation. Frequency responses of individual segments are summed, using quadratically-shifted wave vector values in the parabolic diffraction approximation, together with Fraunhofer and phase corrections. Excellent agreement is obtained between theory and reported experimental frequency responses