Small-sized resonator IF filter using shear horizontal wave on heavy metal film/quartz substrate

By composing interdigital transducers (IDTs) and reflectors made of a heavy metal film such as Au, Ta, or W on an ST cut 90°X propagation (direction perpendicular to X-axis) quartz substrate, the authors have succeeded in exciting a new type of shear horizontal (SH) wave. This wave has a large electromechanical coupling factor, a very large reflection coefficient and excellent temperature characteristics. Though a conventional longitudinally-coupled resonator filter using Rayleigh waves on an ST cut X propagation quartz substrate requires more than 300 fingers of Al grating reflector on each side of the Al-IDTs, our newly developed resonator filter, utilizing this SH wave, requires only about 10 grating fingers for the reflector at most due to the large reflection coefficient. The authors applied this SH wave to filters for the first intermediate frequency (IF) stage of global system for mobile communications (GSM), personal handy phone system (PHS) and personal digital cellular (PDC in Japan) system. As a result, we succeeded in developing low loss and small-sized IF filters (3×3×1.03mm³) having a high stop band rejection, a sufficient bandwidth without expansion coil, and excellent temperature properties for the first time.