Modelling and visualization of "diffraction like" coupling in SAW transversely coupled resonators filters

Generally, transversely coupled SAW resonator filters use the cascade of two sections to obtain the desired number of poles. It is possible to design 3 or 4 pole filters in one section, resulting potentially in lower insertion losses, smaller chip sizes and suppressing the need for an external coupling component. Investigating such devices, we designed a 3-pole filter at 71 MHz and a 4 pole filter at 200 MHz. Unfortunately, unexpected large spurious signals in the high frequency side of the transfer function were observed. These spurious signals could not be described using only the classical waveguiding model. If discrete transverse modes are found by assuming exponential decrease of the energy outside the gratings, a continuum of solutions exists for propagating waves outside the grating (similar to the angular spectrum of waves). By considering the mode shapes, it was shown that a significant part of the source excitation was not coupled to the transverse modes. To take this remaining energy into account, decomposition on the continuum was performed. Our P-matrix based model for transversally coupled structures was improved to take the continuum into account. Comparison between the simulated and measured frequency responses will be shown for different filters exhibiting close agreement and showing the existence of “diffraction like” coupling. Furthermore, visualization of the energy in the filter, using an interferometric optical probe is given.