Synthesis and Design of a Highly Selective Band-5 SAW Filter Using Cascaded DMS With Nonuniform Polarities

In this article, a novel compact Band-5 RX surface acoustic wave (SAW) bandpass filter with high selectivity is proposed. The filter is composed of two identical double-mode SAW (DMS), which are symmetrically placed and cascaded. Every single DMS consists of five interdigital transducer (IDT) blocks and the polarities between each pair of adjacent blocks are not uniform. The unique arrangement of the polarity sequence introduces transmission zeros (TZs) on each side of the filter passband. Therefore, by exclusively employing DMS rather than a combination of DMS and synchronous SAW resonators, steep near-band skirt and excellent out-of-band rejection performance are achieved. The in-line extracted-pole topology is applied to characterize the resonance and transmission properties of the proposed DMS. The operating mechanism of the dual-track DMS filter is revealed by the transversal coupling routing diagram with three odd-mode and three even-mode resonances. The filter is implemented on a standard Y-X 42 <tex-math notation="LaTeX">∘</tex-math> LiTaO <tex-math notation="LaTeX">_3</tex-math> wafer with a center frequency of 882 MHz, a 3-dB bandwidth of 32 MHz, a measured minimum in-band insertion loss of 1.3 dB, and a better-than-45-dB suppression level within the frequency ranges of dc–0.851 GHz and 0.912–3 GHz.