A highly selective tri‐band bandpass‐to‐bandstop filter with the third band independently tunable

Abstract

AbstractA highly selective tri‐band bandpass (BP)‐to‐bandstop (BS) filter with the third tunable band and the other two fixed ones is proposed in this article. This filter is based on a novel crossed resonator that is composed of a two‐end‐open UIR with an open stub and a short stub connected in parallel on both sides of the resonator center. A PIN diode is utilized to achieve BP‐to‐BS switchable characteristics. Then, varactor diodes loaded at the open stub are employed to obtain the independently tunable third band. In addition, due to the symmetry of the resonator structure, the even‐ and odd‐ mode analysis method is used to derive the resonant frequencies. For validation, a tri‐band BP‐to‐BS filter operating at 2.4 GHz (WLAN), 3.3 GHz (WiMAX), and 4.9 GHz (LAA/LTE‐U) is designed, manufactured, and measured that achieves the continuous frequency tuning range (FTR) of the third band from 4.52 to 4.90 GHz with return loss (RL) better than 10.05 dB under the BP characteristic, from 4.55 to 4.90 GHz with the rejection level (RL) higher than 15.53 dB under the BS characteristic. The measured results agree well with the simulated ones.