Breaking the Ladder Bandwidth Limitation: Synthesis of Acoustic Wave Ladder Filters adding Parallel Connected Resonators

The ladder topology satisfies the demands of narrowband filters in mobile communications by using material systems based on Aluminum Nitride (AlN), lithium tantalate (LiTaO3) or lithium niobate (LiNbO3) among others. However, the increasing demand for wider bands limits the use of ladder topologies because their achievable bandwidth is constrained by the electromechanical coupling coefficient $k_t^2$ of the acoustic wave resonators. This is caused by the following relationship between both parameters: $k_t^2 \approx 2FBW$, which at the same time prescribes the position of the transmission zeros, restricting the number of frequency responses that can be achieved. To cope with this limitation, material systems based on Scandium Aluminum Nitride (ScAlN) or LNOI/LTOI with a $k_t^2$ greater than 20% have been demonstrated. Nevertheless, the price to pay is a reduction in the quality factor, Q. This work presents the synthesis procedure for pseudo-ladder filter topologies that allow wideband filters to be manufactured with standard technologies by connecting resonators in parallel to the network, overcoming the ladder technological limitation by decoupling the transmission zero of the filter from the resonance of the resonator.