Highly selective ultra wideband bandpass filters with quasi-elliptic function response

Abstract

This study presents a new type of compact highly selective ultra wideband (UWB) bandpass filter (BPF) that exhibits a quasi-elliptic function response. By using the quasi-lumped element concept, a broadside coupled resonator is designed to implement a transmission zero at lower stopband, and some shunt resonators are adopted to generate transmission zeros at the upper stopband. Hence, very selective performance with a quasi-elliptic function response is achieved. Two UWB filters are designed for the demonstration. The first filter is designed with a passband from 3.1 to 10.6 GHz for the Federal Communications Commission (FCC)-defined indoor limit application, and the second filter is designed with an even wider passband from 3.0 to 16.2 GHz, showing the flexible wide passband performance of the proposed filters. Experiments are carried out to validate the designs. A low cost multilayer liquid crystal polymer technology is deployed to implement the proposed filters. Small sizes for fabricated filters are achieved, which are 8.8 mm×9.0 mm×0.5 mm and 6.4 mm×5.9 mm×0.2 mm, respectively. All the measured filters have excellent performance, including low insertion losses, flat group delay and high selectivity. The proposed new type of UWB BPF is favourable for practical UWB communication and radar systems.