This letter presents a novel multilayer dual-band balanced bandpass filter (BPD) design by using two perturbed circular patch resonators. TM11 mode and TM21 mode of the resonator with odd-symmetric field distributions are explored to realize the desired differential-mode (DM) transmission and common-mode (CM) suppression. Two circular patches are properly coupled in the back-to-back form to realize a dual-passband balanced response by virtue of coupling apertures etched on the ground. In addition to the internal coupling, the above apertures are also further utilized for the undesired degenerate mode harmonic suppression. Besides, slot perturbations on the patch are introduced to perturb TM21 resonant mode to independently adjust center frequency of the higher passband while the lower passband remain almost unchanged. Thus, two passbands can be flexibly controlled by simultaneously tuning the above slots and size of the patch. For validation, a dual-band balanced BPF prototype is implemented. Results indicate 18% and 26% wide fractional bandwidth centered at 5.5 GHz and 7.5 GHz with better than 20 dB return loss under DM operation and higher than 40 dB CM suppression over an ultra-wide frequency band.

中文翻译：

---

圆形贴片谐振器上多层双频平衡带通滤波器的设计

这封信通过使用两个扰动圆形贴片谐振器介绍了一种新颖的多层双频带平衡带通滤波器 (BPD) 设计。探索了具有奇对称场分布的谐振器的 TM11 模式和 TM21 模式，以实现所需的差模 (DM) 传输和共模 (CM) 抑制。两个圆形贴片以背对背的形式正确耦合，通过在地面蚀刻的耦合孔径实现双通带平衡响应。除了内部耦合之外，上述孔径还进一步用于抑制不希望的简并模式谐波。此外，贴片上的缝隙扰动被引入扰动TM21谐振模式，以独立调整较高通带的中心频率，而较低通带几乎保持不变。因此，通过同时调整上述槽位和贴片大小，可以灵活控制两个通带。为了验证，实现了双频段平衡 BPF 原型。结果表明，以 5.5 GHz 和 7.5 GHz 为中心的 18% 和 26% 的宽分数带宽在 DM 操作下具有优于 20 dB 的回波损耗，并且在超宽频带内具有高于 40 dB 的 CM 抑制。