BGS wave and SH wave resonator filters using edge reflections

Abstract

Small-sized and low loss surface acoustic wave (SAW) filters are required in current handy phones. In particular, such a small filter for a first IF stage is urgently required, because its current size is much larger than that of a RF SAW filter. The size of a conventional SAW resonator filter is large, because it requires reflectors consisting of a lot of electrode fingers on both sides of interdigital transducers (IDTs). A shear horizontal (SH) component of Bleustein-Gulyaev-Shimizu (BGS) wave or SH type SAW has a characteristic of complete reflection at free edges of a substrate with a large dielectric constant. The author attempted to realize small low-loss resonator filters without electrode reflectors by utilizing the above-mentioned edge reflection of BGS or SH waves. Very small and low-loss transversely coupled and longitudinally coupled resonator filters could be realized using the BGS wave on piezoelectric ceramic substrates or the SH wave on single crystal substrates. For examples, insertion losses of 52 MHz IF filter for a spread spectrum (SS) cordless telephone system, 40 MHz for an electronic toll collection system (ETC), and 49.2 MHz clock filter for digital audio broadcasting (DAB) using the BGS wave are 4.5, 4.4, and 2.7 dB, respectively. In spite of wire-bonded type of packages, sizes of the 52 MHz and 40 MHz filters are small; 4.5/spl times/5.2/spl times/1.75 and 3.8/spl times/3.8/spl times/1.5 mm, respectively. Losses of 135 MHz filter for a global positioning system (GPS) and 190 MHz for a wide-band CDMA (Rx) using the SH wave on LiTaO/sub 3/ are 3.3, and 3.8/spl sim/5.0 dB, respectively. The package sizes are also small: 3.8/spl times/3.8/spl times/1.5 and 3/spl times/3/spl times/1.15 mm, respectively. Furthermore, the author attempted to develop global system for mobile communications (GSM) IF filters in 200/spl sim/400 MHz having a small size (3/spl times/3/spl times/1.15 mm), a low loss (3.5 dB), and good temperature characteristics by using SH wave on quartz. In spite of wire-bonding type of IF filters, the filter size (3/spl times/3/spl times/1.15 mm) is the same as that of RF SAW filters.