Abstract
A conventional surface acoustic wave (SAW) resonator filter requires reflectors with many grating fingers at both sides of the interdigital transducers (IDTs). On the other hand, a small-sized low-loss resonator filter without grating fingers can be realized by utilizing the reflection of a Bleustein-Gulyaev-Shimizu (BGS) wave or a shear horizontal (SH) wave at the edge of a substrate. As a result, the authors have developed longitudinally coupled resonator filters using the reflection of BGS and SH waves at the edges of a substrate. Longitudinally coupled resonator filters on piezoelectric ceramic substrates for the first intermediate frequency (IF) stage have been realized to be low-loss, small-sized and extremely wide bandwidth (fractional bandwidth of 15%) filters as compared to the ordinary resonator filters, and they can have various kinds of bandwidths (2 to 15%) by employing ceramic substrates with various electromechanical coupling factors. On the other hand, the edge-reflection-type resonator filters on LiTaO3 single crystal for various first IF stages have also been realized to be low-loss and small-sized filters. In spite of being a wire-bonding type of IF filter, the package size of this filter is the same as that of the radio frequency (RF) SAW filter.
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