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MAY 2021 I DESIGN007 MAGAZINE 29 path. Ideally, these stitching vias should be located within 1 to 2 mm of each signal via. Other Tips Other design tips include placement of all power and I/O connectors along one edge of the board. is tends to reduce the high fre- quency voltage drop between connectors, thus minimizing cable radiation. Also, segre- gation of digital, analog, and RF circuits is a good idea, because this minimizes cross cou- pling between noisy and sensitive circuitry. Of course, high-speed clocks, or similar high- speed signals, should be run in as short and as direct a path as possible. ese fast signals should not be run long board edges or pass near connectors. Refer to the end of the article [1-4] for further details on PCB design and how fields move through transmission lines. 2. Cable Shield Termination Cable Penetration e number one issue I find when tracking down a radiated emissions problem is cable radiation. e reason cables radiate is that they penetrate a shielded enclosure without some sort of treatment—either bonding the cable shield to the metal enclo- sure or common mode filtering at the I/O or power connector (Fig- ures 7 and 8). is occurs frequently, because most connectors are attached directly to the circuit board and are then poked through holes in the shield. Once the cable is plugged in, it is "penetrating the shield" and EMI is the usual result. ere are four combinations or cases that must be considered: shielded or unshielded products and shielded or unshielded cables. Power cables are usually unshielded for consumer/commercial products and so require power line filtering at the point of penetration or at the connector of the circuit board. Shielded cables must have the shield bonded (ideally in a 360-degree con- nection) to the product's shielded enclosure. If the product does not have a shielded enclo- sure, then filtering (usually common mode) must be added at the point of penetration or at the I/O connector of the PCB. Figure 8 shows the usual result when connectors simply poke through a shielded enclosure. Figure 7: Penetrating the shield with a cable defeats the shield. This example shows how exter- nal energy sources can induce noise currents in I/O cables, which can potentially disrupt internal circuitry. The reverse is also true, where internal noise currents can flow out the cable and cause emissions failures. (Image Source: Henry Ott) Figure 8: Result of a penetrating cable through a shielded enclosure, because of un-bonded I/O connectors to the shielded enclosure.