A Synthesis View to Dual-Band Responses with Parallel-Connected Acoustic Wave Filters

This work addresses how to cope with a dual-band filter response through the direct parallel connection of two acoustic wave ladder filters and provides a synthesis view on how to design them individually. Similar to a duplexer, the reflection phase of each filter plays a fundamental role to allow such a connection and, therefore, must be precisely considered at both ports. However, it is not a sufficient condition since, when the filters are connected in parallel, each of them behaves like a source-to-load reactive path on the other. Therefore, their out-of-band rejection suffers a severe degradation if directly connected without any other consideration. This has been observed in a measured filter and this paper provides an explanation of such behaviour. The work also proposes a filter design to overcome such degradation and highlights how filter synthesis can enhance results in a controlled manner.