Ultra-Wideband (UWB) Ring Resonator Bandpass Filter With a Notched Band

This letter presents a novel ultra-wideband (UWB) bandpass filter (BPF) with a notched band using a ring resonator. Bandwidths of the ring resonator with two stepped-impedance stubs are calculated by examining the relations between resonant frequencies and the characteristic impedance of the ring. Stepped-impedance ports are used to obtain improved return losses in a high-frequency band. Interdigital-coupled feed lines with an asymmetric structure embedded in the stepped-impedance ports are developed to avoid wireless local-area network (WLAN) signals at 5 GHz band. Measured results show that 3 dB fractional bandwidths (FBWs) are 102.8% and 103.3% with and without the notched band, respectively. Measured insertion loss of the notched band within the UWB is 11.14 dB at 5.41 GHz.