In my 8/28/11 post, The Four Basic Tenets of EMC Troubleshooting, I discussed the four basic principles, of EMC troubleshooting, the second of which was the Predominate Effect. In this blog I will discuss the significance of this principle.
An emission at any specific frequency is often caused by more than one source, and/or radiation mechanism– one of which is predominate and overshadows the others. If a fix is applied (such as a ferrite added to a cable, or reducing the size of an enclosure aperture) that decreases the magnitude of one of the non-dominant emission sources, and/or mechanisms, and the emission is remeasured–no improvement is seen, since the reduced emission is overshadowed by the magnitude of the predominate emission. This can lead to drawing a wrong conclusion, that is, that the fix had no positive effect. In order to notice an improvement, the predominate noise source must be fixed first. Therefore, in order to see an improvement, multiple noise sources must be fixed in a specific order–a difficult and time-consuming process.
Many times when troubleshooting I have applied a fix that reduced the emission, but not sufficient to bring the product into compliance with the EMC regulations. Then when I propose a second fix the client says: “No that does not work, I already tried that fix.” But the client tried it before I applied my fix, that reduced the emission, therefore, it probably was not the predominate effect at that time. My response is often “humor me and try it again,” and in many cases it is now effective.
There are basically two approaches to overcoming this perplexing Predominate Effect problem:
One is to always revisit old fixes that did not work, after an effective fix has been applied.
The second, and the one that I usually prefer, is to leave all seemingly ineffective fixes in place until the product is compliant, then remove them one at a time. Using this approach the removed fix, if it has an effect at all, will always be the predominate one. Any fix that does not increase the emission when removed, can now be eliminated.
Just understanding the concept of the Predominate Effect will make your troubleshooting less confusing and easier.
In a future blog I will discuss the “Kill it Dead” Strategy of EMC troubleshooting.