Transversely coupled resonator filters

Transversely Coupled Resonator Filters (TCRFs) have become an important type of low-loss filters with narrow bandwidth. Their basic layout consists of two one-port resonators arranged acoustically in parallel. Therefore a mechanism that couples both original resonators with each other is necessary. In today's TCRFs, this coupling is implemented by the waveguide effect. A brief description of the historical background is given. The most important steps were MSC, IDT and U-path coupled resonator filters, strip, slot wave-guides and grating waveguides. In the nineties, modeling goes different ways. On the one hand, plane-wave parameters are used for calculating the properties of waveguide modes. To do this, the stack-matrix approach or a conventional eigenmode analysis is applied. On the other hand, the COM theory plays an important role. For this, the COM parameters for the two fundamental waveguide modes are determined experimentally and put into the TCRF analysis. In recent years, new design features were suggested for widening the bandwidth of TCRFs without changing the waveguide parameters. In this connection, the utilization of higher modes, the principle of combining waveguide and cavity modes and the waveguide coupling of resonators with different properties are considered in more detail. Modeling as well as experimental results are presented. Finally, the advantages and disadvantages of TCRFs are discussed.