Topology Assessment for Dual-Band Filters Based on Acoustic Wave Resonators

This work assesses three different approaches to the design of dual-band filters with single-ended input and output based on acoustic wave (AW) technology together with the filter synthesis methodology behind each structure. First, the straightforward option, already on the market, is to connect two standalone ladder filters in parallel. This work provides a synthesis view to this option and also describes its strong limitations regarding achievable isolation. Nonetheless, departing from the classical ladder, an innovative topology is proposed in which series and shunt resonators of different sub-bands are concatenated forming a double-ladder topology. Although this latter structure is able to achieve better isolation levels, this work describes the particular constraints arising from this innovative solution in terms of response and technology requirements. Finally, the conventional ladder structure with a single signal path is proposed as a strong candidate to accommodate dual-band filtering responses for the first time. All three structures are discussed from a filter synthesis perspective, describing the particular constraints behind each topology. Also, each structure is reviewed and compared in technological terms to highlight its main advantages and disadvantages.