| Nowadays， more and more crowded frequency spectrum hinders the developmentof modern wireless communication systems. The demand on sufficient usage offrequency spectrum and compatible of multi-band communication has greatlystimulated the development of multiband communication system. The multibandband-pass filter plays an important role in a multiband transceiver, and many researchesabout it have been carried out. This thesis summarizes the mainly design method andrealization of microstrip multi-passband filter, creationarily improves the design methodof tapped-line coupling structure, which increases designing freedom and enhances thecontrol of multi-passband external coupling coefficient.Novel tapping structure is optimized by genetic algorithm, realized by two-sectionunequal length transmission line and nonuniform transmission line network,respectively. This structure can not only fulfill external couple of multi-passbands, butalso make tap position chosen freely. Thus novel method for increasing frequencyselectivity and spurious suppression is obtained. The locations of transmission zerosvary with the shift of the tap point. High frequency selectivity is achieved by arrangingtransmission zeros near passbands. Spurious suppression is realized by mismatching Qeof image frequencies. Multi-frequency diverse impedances transformation, whichimplemented through nonuniform transmission line, precisely matches external coupleof multiple resonate modes. Multiple passbands are realized by this method. Severalmicrostrip dual-passband filters tapped by two-section unequal length transmission lineand multi-passband filters with nonuniform transmission line impedance transformer aredesigned. The measured responses agree well with the simulation. The feasibility andefficiency of novel tapping method are verified by comparison. |
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