Design Consideration of SAW/BAW Band Reject Filters Embedded in Impedance Converter

This paper discusses design of surface acoustic wave/bulk acoustic wave (SAW/BAW)-based band reject filters composed of the impedance converters, where capacitive elements are replaced with SAW/BAW resonators. First, basic properties of the unit cell are studied. It is shown how basic properties of a unit cell change with the design. It is also shown that when two notches caused by the resonators are placed in proximity, two synergy effects occur: 1) an extra matching point appears on one side of the transition band. This makes the insertion loss at the point smaller and the transition band steeper and 2) the dip level becomes deeper, and the total rejection level improves. Then, two resonators are fabricated, measured, and combined with inductors in circuit simulator to demonstrate functionality of the basic cell design. Finally, the wide rejection band filter is designed by cascading multistages, and effectiveness of the device configurations is demonstrated.