Following are some issues to consider with applying this “adding on fixes” methodology.
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The “ferrite” solution
The sources of emissions/offending signals may be spread out in frequency (some low, some high) and you may not be able to find one solution, like adding a single ferrite choke to a suspect cable, that is able to suppress emissions over both low and high frequencies.
In this situation, you need more than one ferrite choke, and you probably have to select one choke that is made of one type of ferrite material that is only useful for suppressing lower frequency emissions, and another choke made of another type of material that tackles the higher frequency emissions. One problem with this solution is that adding more than one choke may appear “kludgy” and would not likely be accepted by product management or the end customer.
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The “ferrite plus absorber” solution
If the offending frequencies are spread low and high as in the above scenario, an alternative to using two ferrites of differing materials and suppression capabilities would be to use a combination of low-frequency clamp-on ferrite choke for any emissions emanating from cables and a high-frequency RF absorber sheet to suppress emissions at the source, such as those emanating from fast clock chips or microprocessors.
This solution may not appear as kludgy to product management and the customer as the two-ferrite solution described above but may involve more experimentation and design effort to figure out where the best location is to place the absorbing pad to reduce emissions and how to effectively incorporate it on a production basis.
There may also be heat issues to contend with depending on the end-use environment of the product and how hot components may get underneath the absorber pad. Re-doing temperature testing may be in order if this solution works.
Other tests such as vibration, shock, and bump may also be required depending on how the absorber pad solution is implemented. You want to make sure by adding the absorber pad, you don’t create a new problem elsewhere.
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“Other” Solutions
You may be able to incorporate a shield can over offending high-frequency noise source and use a clamp-o ferrite for lower frequency emissions. Reference 2 covers more on the subject of shielding at the PCB level.
Since our original goal was to do as little as possible (not go back into layout), adding a shielding can over offending sources of emissions may not be a viable solution depending on the original design and how much room you have. If room is tight, you may not be able to effectively add a shield can.
- MacArthur, D., “Let’s Talk About Flexible Absorber Sheets,” In Compliance Product Insights, October 2019.
- MacArthur, D., “Let’s Talk About Shielding at the PCB Level,” In Compliance Product Insights, August 2019.
- MacArthur, D., “What Every Electronics Engineer Needs to Know About: Filters,” In Compliance Product Insights, November 2018.
- MacArthur, D., “What Every Electronics Engineer Needs to Know About: Shielding,” In Compliance Product Insights, August 2018.