An Improved Wideband Balanced Filter Using Internal Cross-Coupling and $3/4\lambda$ Stepped-Impedance Resonator

In this letter, an improved wideband balanced filter with increased selectivity for differential-mode (DM) response and enhanced rejection for common-mode (CM) response is proposed. Prototype-based on Optimum Distributed Highpass Filter, a λ/4 internal cross-coupling section is introduced. Meanwhile, 3λ/4 stepped-impedance resonators (SIRs) are also presented as the first and third branch lines to jointly enhance CM rejection with five transmission zeros (TZs). Moreover, λ/4 parallel-coupled lines are accepted as the vertical parts of 3λ/4 SIRs to increase DM selectivity. For demonstration, the proposed balanced filter is designed, simulated, and fabricated. Measured and simulated results are in agreement with each other approximately, indicating that favorable DM transmission with 3 dB fractional bandwidth of 59% is covered by acceptable CM suppression with fractional bandwidth of 118% more than 18.2 dB. Compared with other published works based on branch-line structure, the proposed filter in this letter features the multiple transmission poles (TPs) and TZs in DM response, as well as the maximum amount of TZs in CM response.