Planar Wideband Differential-Mode Bandpass Filter With Common-Mode Noise Absorption

In this letter, a planar differential-mode (DM) four-port circuit is proposed to design a balanced-to-balanced bandpass filter (BPF) with common-mode noise absorption add-on feature. By using the matrix transformation between the mixed and the single-ended scattering parameters, even(odd-) mode equivalent circuit and the closed-form solutions of the proposed circuit structure are obtained based on the constraint rules. Finally, a microstrip DM BPF is designed and fabricated. The measured (simulated) results show that the relative bandwidth is 55.5% (57.5%) under the conditions of IS
_ddBA
I > -1 dB and IS
_ddAA
I, IS
_ccAA
I, and IS
_ccBA
I <; -10 dB. Good agreements between the simulated and measured results verify the correctness of the design theory.