On the Influence of Input Phase on the Allocation of Transmission Zeros in Acoustic Ladder Filters

This work presents a novel concept in the design of acoustic wave ladder filters: using a series resonator to place a transmission zero below the passband. Moving transmission zeros from above to below the passband, or vice versa, is feasible by careful consideration of the filter reflection phase and allows to go beyond the traditional ladder concept in the design of acoustic wave filters with alternating transmission zeros. The method is faced from a rigorous and accurate synthesis perspective and provides the key aspects that lead to the novel solution for performing alternative responses. Filter examples with evenly distributed coupling coefficient among resonators are provided and compared with a traditional filter alternating zeros above and below the passband. The proposed approach can be specially exploited to enhance filter selectivity or improve out-of-band rejection, while also allowing to keep appropriate impedance ratios among all resonators.