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

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, via regulating the OoB CRZs, the TZs far from the passband can be produced while still maintaining the uniform electromechanical coupling coefficients (keff2) of all the resonators. 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 to validate that the proposed theories can achieve significant selectivity and attenuation simultaneously for the Band 3 application.