8.6 Rule 6: Shielding reduces the common mode coupling

Shielding of a critical trace with a second trace which is connected to the ground plane at the source and the load position of the shielded trace reduces the common mode coupling. The reduction is caused by the induced current on the shield, which flows in the opposite direction from the trace wire current and thus partially compensates the common mode coupling from the trace current to the enclosure field. The achievable emission reduction with shielding can be estimated from

$$I_{trace}$$ (8.13)

where I_trace denotes the current on the critical trace and I_shield denotes the current on the shield. According to the same flow direction definition of both currents I_trace and I_shield, there is a positive sign in the nominator of (8.13). The following Figures 8.9, 8.10, and 8.11 show radiated emission reductions achieved by the shielding of traces with different dimensions. A 10mV voltage source with an impedance equal to the characteristic impedance of the trace Z_w drives the trace and the trace is terminated with a 10pF capacitance. The trace is routed straight from (x_s=50mm, y_s=20mm) to (x_s=50mm, y_s=30mm). The trace supply, termination, and routing is kept equal to this definition for all examples. Figure 8.9 indicates an emitted power reduction of about 6dB from one shield, parallel to a trace with 0.2mm width, 0,65mm above the ground plane. A second shield wire on the other side of the trace leads to an emission reduction of about 13dB, as depicted in Figure 8.10. However, the current magnitude difference of the shield current and the critical trace current increases with increasing trace width and with decreasing trace height above the ground plane. Thus, Figure 8.11 shows an emitted power reduction of only 2dB from one shield, parallel to a trace with 2mm width, 0,65mm above the ground plane.

(a) Radiated power.

(b) Estimated reduction from 8.13.

Figure 8.9: Comparison of the radiated emission without and with one 0.2mm shield trace (width=0.2mm, height above the ground plane=0.65mm) parallel to the trace with a distance of 0.2mm and terminated to ground.

(a) Radiated power.

(b) Estimated reduction from 8.13.

Figure 8.10: Comparison of the radiated emission without and with 0.2mm shields parallel on both sides of the trace (width=0.2mm, height above the ground plane=0.65mm) at a distance of 0.2mm and terminated to ground.

(a) Radiated power.

(b) Estimated reduction from 8.13.

Figure 8.11: Comparison of the radiated emission without and with one 0.2mm shield trace parallel to the trace (width=2mm, height above the ground plane=0.65mm) with a distance of 0.2mm and terminated to ground.

Therefore, coplanar shielding provides only reasonable common mode emission reduction for traces, of which the length-capacitance to ground is strongly influenced by the shield traces. This is only the case for traces with low trace width and large distance from the ground plane. Routing of a trace between two ground planes provides an opportunity for emission reduction, as well as of wide traces. However, each interconnect between two components must have vertical sections which leave the PCB to enable the connection of these components. These vertical interconnects cause common mode coupling which is not reduced by the shield on the PCB. Therefore, interconnects routed between two ground layers and coplanar shielded on the PCB cannot completely eliminate the common mode coupling. C. Poschalko: The Simulation of Emission from Printed Circuit Boards under a Metallic Cover