Common-mode suppressed differential transmission lines based on periodic structures

Differential (or balanced) microstrip lines with common-mode suppression in a controllable frequency band are implemented by periodically modulating the common-mode characteristic impedance of the line, keeping the differential-mode impedance unaltered. Due to the Bragg effect associated to periodicity, the common-mode is efficiently rejected in the vicinity of the Bragg frequency, whereas the structure is transparent for the differential signals. As compared to other approaches for common-mode suppression in differential lines, the periodic variation of the common-mode characteristic impedance reported here is simple and common-mode suppression does not required ground plane etching. The designed and fabricated prototype device reveals that significant common-mode rejection level and bandwidth are achieved by means of a four-cell periodic structure.