Synthesis of Ladder-and DMS-Type Acoustic Wave Filters and Duplexers in Bandpass Domain

In this article, a general synthesis method for acoustic wave filters and duplexers with predetermined spectrum mask and technical requirements is developed and demonstrated. The acoustic wave devices, which can include both the synchronous ladder-type resonators and double-mode surface acoustic wave (SAW) (DMS) structures, are directly synthesized and designed in the bandpass domain. At the first, the generalized Chebyshev filtering polynomials with complex coefficients are derived by the direct synthesis technique (DST). Subsequently, the element values of both the resonating nodes (RN) and the non-resonating nodes (NRN) of the bandpass-modified in-line extracted-pole topology are systematically extracted. Finally, the nodal representations of both the synchronous resonators and the DMS are replaced with their respective equivalent circuits, followed by performance optimization based on the synthesized dimensions. For demonstration, a ladder-type SAW filter and a Band-8 RX filter using mixed ladder-DMS structures, are synthesized, designed, and fabricated. Additionally, a Band-8 duplexer with its TX path consisting of ladder-type resonators and RX path composed of mixed structures is also synthesized and simulated. The obtained measured and simulated results closely align with the predefined transmission responses. The proposed bandpass-domain synthesis technique exhibits desirable characteristics including time efficiency, high accuracy, the capability of synthesizing mixed structures, and flexibility in controlling the input-output phase.