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 addon 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 vertical bar S-ddBA vertical bar > -1 dB and vertical bar S-ddAA vertical bar, vertical bar S-ccAA vertical bar, and vertical bar S-ccBA vertical bar < -10 dB. Good agreements between the simulated and measured results verify the correctness of the design theory.