基于短路/开路枝节线的小型化超宽带滤波器

Compact Ultra-wideband Bandpass Filter Based on Open and Short-ended Stubs

介绍了一种基于并联枝节线单元的超宽带滤波器。该滤波器由3种基本元件组成:短路枝节线、开路枝节线和连接线。其中,每个短路枝节线和开路枝节线组成一个枝节线对,在枝节线对间利用连接线实现强电磁耦合。在设计中,这3种基本元件的电长度是相同的。通过合理选择各元件的特征阻抗,就能实现带宽在100%左右的超宽带滤波器。此外,该超宽带滤波器还具有较宽的高端阻带。文中首先基于理想传输线模型对该滤波器进行了仿真研究;然后在全波仿真的基础上,利用微带线实现了一个三阶的超宽带滤波器;最后利用平面印制板技术对滤波器进行了实物加工。测试和仿真结果吻合良好。实验结果表明,该滤波器的中心频率为1.775 GHz,相对带宽为97.5%。通带内匹配优于-15 dB,最小插入损耗为0.25 dB(在1.3 GHz处)。第一个寄生通带的频率高于6 GHz,是通带中心频率的3.4倍左右。

An ultra-wideband bandpass filter based on shunt stubs is proposed.The filter is composed of three kinds of unit,e.g.,short-ended stub,open-ended stub and connecting line,respectively.Every two open-and short-ended stubs are combined into a stub-pair.Between every two stub-pairs,a section of connecting line is inserted to realize the electromagnetic coupling.In the filter,all the units are equal in electrical length.By selecting the characteristic impedances of the units properly,a bandpass filter with about 100％ fractional bandwidth can be implemented.Besides,the filter also has a wide upper stopband.The filter is at first analyzed by using the ideal transmission line model.Then a 3-staged ultra-wideband filter is designed and implemented on a microstrip substrate based on full-wave simulation.Finally,filter samples are fabricated by using PCB process.The measured results agree well with the prediction,which shows the bandpass filter achieves a 97.5％ bandwidth at the central frequency of 1.775 GHz.The minimal insert loss within the passband is 0.25 dB at 1.3 GHz while the matching is better than-15 dB.The first spurious frequency is higher than 6.0 GHz,which is about 3.4 times of the central frequency.