A mixed transverse modes and angular spectrum of waves model for the analysis of SAW transversely coupled resonators filters

Transversely-coupled SAW resonator filters commonly employ two or more sections in cascade to obtain the required number of poles in the response. It is possible design 3 or 4 pole filters in a single section, resulting in potentially lower insertion loss, smaller chip size, and elimination of the need for an external-coupling component. A 71 MHz 3 pole filter and a 200 MHz 4 pole filter were designed using this approach. Unfortunately, unexpectedly large parasitic coupling was observed on the high-frequency side of the transfer function. This coupling could not be described using only the classical waveguide model. If discrete transverse modes are identified, under the assumption that the energy distribution vanishes outside the gratings, a continuum of solutions exists in the case of propagating waves outside the grating (similar to angular spectrum of waves). By looking at the mode profiles, it was shown that a large part of the source excitation was not coupled to the transverse modes. To take this remaining energy into account, a decomposition of the continuum was performed. The P-matrix based model for transversely coupled structures was improved to take into account this continuum. A comparison of the measured and simulated frequency responses is shown for different filters and different metalization thicknesses. The excellent agreement demonstrates the need to take into account the coupling continuum in a TCF, even for a filter with only 2 cascaded sections.