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

摘要

In this paper, we describe a longitudinally coupled double-mode surface acoustic wave (SAW) filter on a 45° X-Z Li_2B_4O_7 (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 Ω. 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.