Analysis and design of planar wideband balanced bandpass filters exhibiting >55 dB common-mode suppression

In this paper, a series of microstrip balanced bandpass filters exhibiting >55 dB in-band common-mode suppression is proposed. The proposed balanced filter is based on a nested ring structure, wherein a smaller ring is embedded within a larger one. The primary advantage of this configuration lies in the fact that a bandpass response is obtained for the differential mode, whereas a stopband response is achieved for the common mode within the operating frequency band. Furthermore, since no parallel-coupled microstrip lines are employed, the equalization of even- and odd-mode phase velocities and the fabrication of narrow gaps are inherently avoided. Consequently, an in-band common-mode suppression level exceeding 55 dB over a 20% differential-mode fractional bandwidth is readily achieved merely through the adjustment of the characteristic impedances of each section. Moreover, to broaden the common-mode rejection bandwidth, a quarter-wavelength open-circuit stub has been asymmetrically incorporated at the center of the developed filter. Upon the introduction of this stub, two additional transmission zeros are excited in the common-mode response, thereby significantly extending the common-mode rejection bandwidth with the suppression level maintained above 55 dB throughout. To validate the aforementioned theory, two prototypes have been designed, fabricated, and measured.