PCB structures for common mode suppression on differential microstrip lines

Common mode noise on differential microstrip lines can be suppressed by PCB embedded filters. These filters are basically resonators, and only the common mode signal can couple to them. Though such structure suppress common mode signal only in narrow bands because of the resonant nature, they can be combined to produce broadband filtering effect. The dimension of such filters could be further reduced by adding lumped elements. In this paper, design principles of the PCB filters are given, and examples are given to demonstrate these rules. At last, a new PCB filter structure is proposed with an electrical size of only 0.04λ×0.067λ which is capable of suppressing the higher order harmonics of common mode signal.