In this paper, a balanced tri-band band-pass filter (BPF) with the third independently tunable passband and the other two fixed ones based on a novel multi-stub loaded resonator (MSLR) is presented. Due to the symmetrical circuit structure, the even-and odd-mode methods are applied to analyze the differential-mode (DM) and the common-mode equivalent circuits of the proposed resonator, and the correspondingly resonant characteristics can be obtained. With the approximate design of the MSLR, the first three DM resonant frequencies are employed to form the three DM passbands. Then, the external coupling structure can be controlled by coupled lines to meet the required specifications. Furthermore, the frequency tuning of the third DM passband can be realized by adjusting the varactors loaded at the end of the resonators. Finally, the source and load coupling is introduced to improve the frequency selectivity. For verification, a balanced tri-band BPF with the tunable third DM passband was designed, manufactured and tested. The fabricated filter has central frequencies of the first two fixed bands located at 2.4 and 2.95 GHz, the third one tuning from 4.17 to 4.41 GHz, and the correspondingly fractional bandwidths (FBWs) are 12.3%, 3.5% and 1.9% ± 0.89%. The experimental results are in good agreement with the simulated ones.

中文翻译：

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一种使用短线加载谐振器的具有高选择性的紧凑型可调平衡三频带通滤波器

在本文中，提出了一种基于新型多短截线加载谐振器 (MSLR) 的具有第三个独立可调通带和其他两个固定通带的平衡三频带通滤波器 (BPF)。由于电路结构对称，采用偶模和奇模方法对所提出的谐振器的差模(DM)和共模等效电路进行分析，可以得到相应的谐振特性。通过 MSLR 的近似设计，前三个 DM 谐振频率用于形成三个 DM 通带。然后，可以通过耦合线控制外部耦合结构以满足所需的规格。此外，可以通过调整谐振器末端加载的变抗器来实现第三DM通带的频率调谐。最后，引入源和负载耦合以提高频率选择性。为了验证，设计、制造和测试了具有可调第三 DM 通带的平衡三频 BPF。制造的滤波器的前两个固定频段的中心频率位于 2.4 和 2.95 GHz，第三个从 4.17 到 4.41 GHz 调谐，相应的分数带宽 (FBW) 分别为 12.3%、3.5% 和 1.9% ± 0.89%。实验结果与模拟结果吻合较好。相应的分数带宽 (FBW) 分别为 12.3%、3.5% 和 1.9% ± 0.89%。实验结果与模拟结果吻合较好。相应的分数带宽 (FBW) 分别为 12.3%、3.5% 和 1.9% ± 0.89%。实验结果与模拟结果吻合较好。