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Q&A: Cables, cable shields for EMC mitigation

23 Dec 2013  | Kenneth Wyatt

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Q; Do we need to use CM chokes for I/O signals or do we also need them power lines?

A: It's always good EMC practice to design in common-mode chokes in both the signal and power lines. I would also recommend adding transient protection devices to guard I/O connections against ESD and other pulse-type signals.


Q: What if my product uses a plastic enclosure? Where do I connect the shield of the I/O cable?A: Using shielded cables with unshielded enclosures can lead to design and performance issue for EMC. Many low cost consumer products can not afford a shielded enclosure, so how do we resolve this? The important thing to keep in mind is that there will inevitably be common-mode noise sources on the PC board. To keep these noise currents off our I/O and power cables, we can either block the currents from getting to the cables with a ferrite choke or divert the noise currents back to their source. Often, a combination of blocking and diversion is the best method. Many higher-end handheld consumer products use a diversion plate under the PC board. This method was described in another blog posting on The Connecting Edge and is merely a metallic plate or metalized film with one end bonded or clamped well to the I/O and power connector ground shells. This offers a low impedance path for the common-mode currents to flow back to the source through distributed capacitance. It also protects sensitive circuitry from external ESD currents injected at the I/O connectors. In addition, it serves as an image plane which helps reduce radiated emissions. To answer the question more directly, the cable shield must be bonded in some way to the digital ground (if a signal or I/O cable) and power ground (if a power cable). Ideally, all I/O connectors and power connectors should be grouped together on one side of the board. If they are spread all around the perimeter, that's often bad news, as any noise sources on the PC board are now potentially driving the midpoint of a dipole antenna!

Feel free to add additional questions related to cables and cable terminations.


About the author
Kenneth Wyatt is an independent consultant and specialist in electromagnetic compatibility (EMC) design, test and troubleshooting. In The EMC Blog, he discusses issues, trends, EMC design, and product troubleshooting techniques. One speciality is his use of do-it-yourself probes and use of low-cost test equipment and measurement techniques for evaluating EMC issues on the workbench.


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