Synthesis of the Acoustic Wave H-Ladder Filter Topology

The ladder topology is the workhorse in acoustic wave (AW) filter modules because of the high selectivity with low losses that it provides. However, the main limitation is that the achievable bandwidth and the distance of the transmission zeros (TZs) to the passband are limited to the electromechanical coupling coefficient $k_t^2$ of the resonators. Moreover, the strong requirements in terms of area also demand for more compact solutions. For these reasons, a new compact topology is proposed in which there is more flexibility in the allocation of the TZs. This topology has been named H-Ladder and it is based on the conventional ladder, but it uses double shunt resonators at the input and output ports. However, it requires a proper definition of the transfer function to achieve it.