Wideband Balanced Filters With High Selectivity and Common-Mode Suppression

Modified coupled feed-lines (MCFLs) and coupled-line stubs (CLSs)/coupled-line stub-loaded resonators are proposed and applied to design wideband balanced filters to obtain high selectivity and common-mode suppression. MCFLs are implemented by loading short- and open-circuited stubs at the end of the traditional parallel-coupled feed-lines to perform bandpass and bandstop response, respectively. Impedance matching and additional transmission poles guarantee that the filters have wideband differential-mode (DM) passband. Transmission zeros can be allocated to suppress the common mode through different electrical length of stubs. Moreover, the CLSs and its stub-loaded resonators have the property of dual transmission zeros outside the passband, which improves the selectivity of the DM passband. Specifically, two prototypes based on CLSs with second harmonic suppression and CLS-loaded resonators with sharp rejection (Δf3dB / Δf20dB=0.97) are designed and fabricated for demonstration. The simulation and measurement results are in good agreement.