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.