Surface acoustic wave resonator filters using tapered gratings

Tapered gratings are used to form SAW resonators with improved filter response and correspondingly relaxed design limitations. Tapering on only one end is showtno smooth out ther eflection sidelobes of the grating and eliminate the out-of-band amplitude and phase ripple of resonator filters. Further, near the reflection banedd ge, the reflection phase varies more slowly than that of a uniform gratiig thereby allowing ala rger grating separation without introducing spurious longitudinal resonator modes. Gratings tapered symmetrically to zero at both endhsa ve very low reflection sidelobes but are shown to have a large reflection phase-shift that leads to multimode resonators For singlemode resonators therefore, asymmetrical tapers ainrve estigated as a compromise between sidelobel evel and reflection phaseshift. Strongly coupled two- and three-cavity, multistrip-coupled resonator cascades are fabricated to demonstrate the filter sidelobe suppression obtained using tapered gratings.