This letter presents a practical solution to realize finite frequency transmission zeros with independent location for wideband evanescent-mode filters. The method consists of using offline resonators located at the input and/or output of the filter. In contrast with the extracted-pole technique commonly employed for narrowband filters to generate a pole-zero pair, each offline resonator is used here in such a way so that the sole transmission zero is actively used in the filtering function. The latter characteristic is what makes this technique practical for wideband filters as it prevents the presence of unfeasibly strong coupling coefficients, as well as avoiding impractical nonresonating node elements. The offline resonator, which we named extracted-zero , can be coupled to the first or last resonator of a wideband bandpass filter, and it is conveniently realized using the same technology as any other resonator of the filter. The extracted-zero can be seen as a first-order notch filter which is directly embedded into the bandpass filter (instead of being cascaded afterward). The feasibility of this solution is validated by the experimental results of a challenging 225–450-MHz bandpass filter (66.6% fractional bandwidth) which employs 17 inline resonators plus 2 extracted-zeros so as to obtain 50 dB rejection below 195 MHz.

中文翻译：

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提取零点：宽带滤波器中传输零点的实用解决方案

这封信提出了一种实用的解决方案，用于为宽带渐逝模式滤波器实现具有独立位置的有限频率传输零点。该方法包括使用位于滤波器输入和/或输出端的离线谐振器。与通常用于窄带滤波器以生成零极点对的提取极点技术相比，这里使用每个离线谐振器的方式是，在滤波功能中主动使用唯一的传输零点。后一个特性使该技术适用于宽带滤波器，因为它可以防止出现不可行的强耦合系数，并避免不切实际的非谐振节点元件。我们命名的离线谐振器提取零，可以耦合到宽带带通滤波器的第一个或最后一个谐振器，并且可以使用与滤波器的任何其他谐振器相同的技术方便地实现。提取的零可以看作是直接嵌入到带通滤波器中的一阶陷波滤波器（而不是之后级联）。该解决方案的可行性通过具有挑战性的 225–450 MHz 带通滤波器（66.6% 分数带宽）的实验结果得到验证，该滤波器采用 17 个在线谐振器和 2 个提取的零点，以便在 195 MHz 以下获得 50 dB 抑制。