Enhanced Modeling of Wideband Acoustic Wave Ladder Filters

The synthesis of acoustic wave (AW) ladder filters is usually based on lowpass prototype (LP) approximations, which allows analyzing asymmetric bandpass responses within the normalized domain by incorporating frequency-invariant reactance (FIR) elements and reducing the polynomial order. Although this approach remains effective for narrowband filters, some inaccuracies arise between the normalized and scaled responses as the fractional bandwidth (FBW) increase. These errors are due to a poor modeling of the asymptotes at origin and infinity that are present in the real frequency domain, but are not accounted for in the lowpass prototype approximation. Several Direct Bandpass (DB) approaches have emerged to accurately model filters without approximations. However, current techniques cannot be generalized for AW ladder filters because do not offer a flexible way of characterizing the frequency-dependent phases. This work aims to formulate the DB filtering function that must be accommodated in an AW ladder filter using a Multiband Reduced Chebyshev (MRC) that enables the incorporation of complex reflection zeros (CRZ) to manage the input and output (I/O) phases. To validate the proposed synthesis methodology, a synthesized 3rd-order AW ladder filter is compared to a manufactured LiNbO3 thin-film AW filter, whose FBW is 18%.