PCB Structures for Common Mode Suppression on Differential Microstrip Lines

Abstract — Common mode noise on differential microstrip lines can be suppressed by PCB embedded filters. These filters temperature coare 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 0.04 λ×0.067λ which is differentcapable of suppressing the higher order harmonics of common mode signal. Keywords — Common mode filter, differential signal, quarter-wavelength, lower resonant frequency. I. I NTRODUCTION All differential interfaces will carry common mode signals. Common mode signals could originate from drivers, non-identical rise and fall times, unbalanced traces or skew, etc. Common mode signals may couple to structures capable of radiation, such as the outside of cable shields and enclosures, causing EMI problems. These problems can be solved by improving the driver, removing the structures that cause differential to common mode conversion, filtering the common mode current, or reducing the coupling to structures capable of radiating the signal. This paper focuses on structures that filter common mode currents on differential traces. Common mode suppression on differential signals can be subdivided into four classes: active compensation of common mode, which is more suitable for lower frequencies; common mode compensation structures; discrete components for suppression, such as common mode chokes; and common mode suppression structures, integrated into printed circuit boards (PCBs). An example of common mode compensation structures is bended differential lines using compensation capacitors[1] and/or inductors[2] so as to improve the symmetry of the differential pairs. The third class of filters is discrete component such as common mode chokes. A common mode choke maintains the differential mode impedance by creating a highly coupled differential pair that is wound into a coil. Ferrite core might be added to the coil to increase the inductance at lower frequencies. Since a common mode choke is discrete broadband component, it cannot be characterized by its electrical size. There are some other common mode chokes that are manufactured using low--fired ceramic (LTCC) substrates with a small size of 1.2 mm ×2.0 mm [3]; these common mode chokes are able to provide common mode suppression up to 5GHz. So far, it is still difficult to find common mode chokes on-board that perform well above 5 GHz. The fourth class of filters are PCB based structures. These resonant structures have been designed to only disturb the common mode field distribution on the differential traces without significantly affecting the ial mode. These structures have been researched wellin [4] -[8] by Tzong Lin Wu’s group. Usually the dimension of the resonant structures used for common mode suppression depends on the operating frequencies. Furthermore, the dimension of the resonators can be reduced by adding PCB structures that act as capacitors or inductors. The PCB structures for common mode suppression usually form narrow-band filters. However, they can be used for wideband applications by combining multiple narrowband filters [9]. Table.I show some examples of PCB structures used for common mode suppression. Electromagnetic band gap (EBG) structures [8] and defected ground structures (DGS) [4] usually occupy large area, which makes them not suitable in practice. Further, it is not clear how defective ground structures perform while there are other planes below them. A structure using quarter wavelength resonator has been proposed in [10], which requires an additional PCB layer, but has a smaller dimension when compared to DGS[4] and EBG[8] structures.