An alternative method for suppressing undesired transverse modes in longitudinally coupled SAW resonator filters

As known, perturbing transverse modes arising from the waveguide properties of the interdigital transducers and reflectors can appear in SAW resonator filters. Usually, these undesired modes are suppressed by transducer apodization. This method has the disadvantage of increasing the transducer impedance. We propose a longitudinally coupled resonator filter, the basic elements of which are able to guide the two slowest waveguide modes alone, but to excite and receive the first mode only. The basic elements are arranged side by side forming the complete filter permitting coupling with each other weak enough to fulfill the filter specification. The suppression of undesired modes by the construction is demonstrated by transmission measurements.