Multi-mode synthesis of RSPUDT filters

The influence of parasitic surface transverse waves on responses of RSPUDT based filters is studied. Both experimental and simulated results show that parasitic STW generation deteriorates out-of-band rejection for RSPUDT filters on quartz substrate. Some filter structures providing partial reduction of STW distortions are considered. As a fundamental solution to the problem, a multi-mode synthesis is presented. This approach accounts for STW generation in the synthesis of RSPUDT. Experimental results well agree with filter simulations and high shape factor is achieved.