WIDEBAND AND UWB BALANCED BANDPASS FILTERS BASED ON BRANCH‐LINE TOPOLOGY

The branch‐line‐like structure is one of the earliest works in the design of wideband differential‐mode bandpass filter with common‐mode suppression. This method has been still widely used because of its simple geometry, easy design rule, and compact size. In another method, a wideband bandpass differential‐mode filter with enhanced common‐mode rejection can be constituted by using the transversal signal interference technique. This design concept stems from the distributed active filter theory. In this chapter, a class of ultra‐wideband (UWB) balanced bandpass filters on microstrip line with good common‐mode suppression is designed and implemented. The so‐called signal interference technique on two parallel transmission lines of different electrical lengths and characteristic impedances is proposed to design a wideband stopband under common‐mode excitation. The chapter also presents a novel method to improve the filter selectivity performance of a wideband filter by stretching a traditional 90° branch‐line stub to 270° in two sides.