A noval weighting method for low loss SAW wideband filters: topological weighting

There are two kinds of traditional weighting methods for SAW interdigital transducers: apodization and withdrawal weighting. The former has been abandoned in many low loss filters design on account of the additional loss due to the weighting. The latter can be used only for narrow band filters with many fingers. In order to design low loss wideband filters, we propose a new method: topological weighting. We use Green´s function to calculate accurately the COM (Coupling-of-Modes) parameters of the cells with different geometry (different finger widths and different finger gaps) for so called EWC (Electrode Width Controlled) structure, i.e. there are three electrodes in one cell, one of them is applied the voltage and other two are grounded. On the 128° rotated LiNbO₃, we get a filter with ∼6.6% bandwidth, 3.8 dB loss and >41dB sidelobe Suppression by the calculation. In the experiment, because of the bad processing technology and electrode resistance effect, we have the result with loss 5 dB, sidelobe suppression 35.8 dB and the bandwidth of 5.5%. The interdigital transducers have the dimension ∼25×15 wavelengths.