Synthesis of Double-Mode SAW Filters: Bridging Reactance Models and Physical Structures for Advanced RF Applications

This work presents a synthesis methodology for double-mode SAW (DMS) filters linking reactance-level synthesis with the physical behavior of the structure. By combining the coupling-of-modes (COM) model with trisection and extracted-pole topologies, the method provides a direct mapping from coupling matrix (CM) parameters to geometric variables such as the IDT gap ( $\text{L}_{\text{T}}$ ), grating pitch ( $\text{P}_{\text{m}}$ ), and aperture (W). A key contribution of this work is the explicit connection established between the synthesis formulation and the physical geometry of the DMS structure, enabling a clear interpretation of how geometric parameters-particularly the IDT gap and aperture-govern the resulting filter performance. The methodology further enables the incorporation of gradient-based optimization tools, yielding reproducible and robust designs, and supports fine-tuning for manufacturability to ensure practical layouts while preserving the desired transmission response. Validation is demonstrated through simulations and experimental measurements for a 605-MHz stand-alone filter fabricated on 42°Y-cut LiTaO3, where synthesized, COM-simulated, and measured responses show good agreement for the baseline implementation. The approach enables reproducible and physically interpretable DMS designs adaptable to a wide range of bandwidth requirements.