Simulation and design of wide-band, low-loss longitudinal coupling filters

In general, the pass-band width of double modes SAW filter is narrow and its outside-band rejection is bad. In order to remove these shortcomings, a longitudinal coupling filter with 5 IDT was designed to reduce the filter´s insertion loss, to increase its pass band width more than 50%, and to improve its outside-band rejection. Then, the outside-band rejection and square ratio of filter were more improved by introducing coupling gratings between IDT. Eventually, using two filters of this type in serious, a filter with 3dB pass-band width 5%, insertion loss 0.6 dB, outside-band rejection 40dB, and square ratio 1.578 was designed.