Wideband Balanced Bandpass Filters With Superior In-Band Common-Mode Rejection

Abstract

In this letter, a series of wideband balanced bandpass filters with superior in-band common-mode (CM) rejection has been designed and implemented, simultaneously exhibiting excellent differential-mode (DM) responses. The developed balanced bandpass filters originate from a traditional branch-line balanced filter. The merit of this topology lies in the DM equivalent circuit showing an intrinsic bandpass response, whereas the CM equivalent circuit presents a built-in bandstop response. By adjusting the coupling coefficient of the parallel-coupled lines and the characteristic impedances of the remaining sections, the developed balanced bandpass filter demonstrates a 37% fractional bandwidth for the differential mode and an in-band CM suppression greater than 50 dB over the 41% bandwidth. Moreover, to broaden the CM rejection bandwidth, a quarter-wavelength open-circuit stub was asymmetrically attached to the center of the developed filter. With the stub introduced, two extra transmission zeros emerge in the CM response, extending the bandwidth over which CM signals are suppressed. To verify the concept, two balanced bandpass filters were designed, fabricated, and measured.