A Novel Microstrip Symmetric Diagonal Cross-Coupling Quadruplet Bandpass Filter Using Even/Odd-Mode Stepped Impedance Resonators
In this paper, a novel symmetric quadruplet coupling scheme for two symmetric/asymmetric transmission zeros (TZs) is introduced, and its new symmetric microstrip filter is proposed for a bandpass filter (BPF) design having high skirt selectivity at passband and high attenuation at stopband. In conventional quadruplets such as box section, two TZs are hard to produce at arbitrary frequencies owing to their coupling schemes and physical structures. The quadruplet employed in the paper is simply realized by a symmetric microstrip structure using even/odd-mode stepped impedance resonators (SIRs) and quarter-wavelength resonators (QWRs), thereby intrinsically generating a negative coupling required for two prescribed TZs. Moreover, the combination of SIRs and QWRs leads to a wide, high attenuation stopband response due to the suppression of direct source/load coupling as well as spurious resonances. The validity of the proposed BPF is proved through design and measurement of a 2-GHz fourth-order BPF with two TZs.