Second-harmonic suppression in DART-SFIT devices

Important SPUDT cell types (DART, EWC) yield large 2^nd harmonic levels. The goal of this work is to reduce the level of 2^nd harmonic in SFIT devices using DART cells by design tools. One-track and two-track solutions of DART-SFIT filters are presented. For suppressing 2^nd harmonic of one-track SFITs these are divided into partial SFITs. The gaps between them are widened by half a DART cell length and opposite finger polarities of the partial SFITs surrounding the considered gap are adjusted. The influence of this method on the 2^nd harmonic level is studied for partial SFITs connected serially and in parallel, resp. Two-track solutions of SFITs are composed of two sub-SFITs arranged in acoustically parallel tracks. For suppressing 2^nd harmonic the sub-SFITs are shifted to each other by half a minimum DART cell length and are additionally sloped. Some combinations of widened gap numbers of 1st and 2^nd SFIT were analyzed in the case of one-track SFIT filters. Test structures were implemented and analyzed. As an experimental result, the combination 2/3 widened gaps in 1st/2^nd SFIT yields the lowest level of 2^nd harmonic. We obtain approximately 10 dB additional 2^nd harmonic suppression in comparison to the case of no widened gaps. The analysis of comparable two-track solutions according to the channel model yields lower 2^nd harmonic levels compared to the one-track solutions.