Double-Mode Surface Acoustic Wave Filter with Suppressed Longitudinal and Transverse Mode Spurious Responses Using Wave-Shaped Apodized Structure

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In this paper, we describe a longitudinally coupled double-mode surface acoustic wave (SAW) filter on a 45° $X$–$Z$ Li2B4O7 (LBO) substrate for 1 GHz radio frequency identification (RF-ID) systems in Japan. Since LBO has a low dielectric constant, the aperture of an interdigital transducer (IDT) on LBO tends to be wide, resulting in the input and output impedances of 50 $\Omega$. Higher order transverse modes existing in such a wide-aperture transducer generate ripple responses in the pass band of the filter. Generally, to suppress such modes, IDTs are apodized. However, it appears that a conventional apodized structure generates a notch response in the pass band of the filter due to longitudinal modes. We analyze the longitudinal mode response theoretically using a longitudinal mode separation technique and propose a novel-shaped apodized structure, which we call the “wave-shaped apodized structure.” This apodized structure completely suppresses both the notch response due to longitudinal modes and higher order transverse modes. By applying this structure, we successfully develop a practical 953 MHz SAW filter for the above-mentioned RF-ID systems.
2006-08-15