Beyond Conventional Transmission Zero Allocation: Synthesis of Acoustic Wave Ladder Filters via Out-of-Band Complex Reflection Zeros

Traditional acoustic wave (AW) ladder filters suffer from the rigid even allocation of extremely close transmission zeros (TZs) on both sides of the passband. Therefore, out-of-band (OoB) complex reflection zeros (CRZs) are employed in this paper to break the TZ allocation rule of conventional AW ladder filters. With the degree of freedom provided by the CRZs in synthesis, TZs can be placed either above or below the passband, regardless of whether the AW resonators are series- or shunt-connected. Moreover, by regulating the CRZs, the uniform electromechanical coupling coefficients of all the resonators can still be maintained. In this way, AW ladder filters can generate flexible TZs at desired frequencies to satisfy asymmetric and diverse specification requirements. Moreover, the filter footprints are not enlarged with the OoB resonators. One AW filter is fabricated, which can achieve significant near-skirt selectivity and stopband attenuation simultaneously for the Band 3 receiver channel application.