A broadband filter design for common-mode noise suppression with multilayer mushroom structure in differential transmission line

A new multi-layer mushroom structure to suppress common-mode noise rejection in the differential transmission line is proposed in this paper. It consists of a pair of differential transmission lines above the proposed multi-layer mushroom-like structures. In other to produce a particular suppression band to couple with the higher order mode, the two mushroom-like structures interacted with each other greatly. This technique can successfully generate many close transmission zeros in common-mode within the concern frequencies. In addition, the simulation and measurement results are verified that the proposed filter prototype can efficiently suppress the common mode noise to -10 dB from 3.4 to 13.41 GHz with 117 % fractional bandwidth (FBW) while the electrical size of the prototype is 0.25 × 0.25 λ
_g
, where λ
_g
is the wavelength of the central frequency of its stopband. Furthermore, the insertion loss of differential-mode can be kept less than 2 dB from dc to 15 GHz. To sum up, the proposed common-mode filter has a simple geometric structure, a compact size, and a large common-mode FBW. Most importantly, the proposed filter can keep the good signal integrity of the digital differential signals due to its all-pass characteristic.