A design technique of balanced longitudinal coupled mode SAW filters for RF-stage with a desired matching impedance

摘要

A design technique for balanced longitudinal coupled mode SAW filters, in which the matching impedance can be easily controlled, has been developed. This is achieved by adoption of a divided IDT structure for the center IDT of the filter, which has three IDTs. The center IDT is connected to the balanced terminal. The IDTs on the both sides are connected to the unbalanced terminal. The center IDT is divided into three divided IDT portions, which are connected in series/parallel. It can make the impedance higher than that of the original filter. The finger number ratio for the divided IDTs can control the impedance. Applying this technique to a center IDT of 50ohm design, the matching impedance in the range approximately from 50ohm to 200ohm can be realized.