Synthesis Methodology for Mixed-topology Acoustic Wave Filters

Ladder topology is an excellent solution for filters based on AW technology, however, the required alternation of series and shunt resonators, or equivalently transmission zeros above and below the bandpass may impose some limitations to obtain greater out-of-band rejection levels. To overcome this limitation, external reactive elements may be used to improve the rejection level by altering the topology and generating an extra transmission zero due to parasitic effects. These elements are usually added to the already synthesized network causing a significant deterioration of the filter performance, requiring an optimization process to restore the in-band response. In this letter, a methodology for the synthesis of fully canonical filters with reactive elements based on a mixed-topology approach is presented. The proposed method combines in-line network with parallel-connected structures to generate ladder-like filters that provides a precise value of the reactive element. In this kind of networks the resonance frequencies of some resonators are not tied to any transmission zero of the filtering function. Thus, the method also allows a number of transmission zeros to be allocated above or below the bandpass, increasing the out-of-band rejection without compromising the feasibility of the filter.