EMC Basics #7: An introduction to troubleshooting EMI problems

[Editor's note : we are pleased to continue our series on the vital and sometimes unappreciated topic of electromagnetic compatibility (EMC), presented by well-known expert Daryl Gerke of Kimmel Gerke Associates. Note that there are links to all previous entries at the end of this item.]

Time to switch gears for a while, and look at troubleshooting EMI problems. We'll examine four key EMI problems — emissions, ESD (electrostatic discharge), RFI (radio frequency interference), and power disturbances. We'll look at these problems in two contexts — the EMI test lab, and the engineering lab. We'll also discuss specific troubleshooting techniques.

Troubleshooting consists of trying to isolate a problem and the underlying causes, and then applying appropriate fixes. Often times, we are acting like a medical doctor to diagnose an EMI illness.

Diagnosis is important — don't just start throwing solutions at the problem. The medical profession has a saying for this —“Prescription without diagnosis is malpractice.” I think this applies to EMI problems, too.

To continue with the medical analogy, doctors use a methodology known as differential diagnosis. That means ruling things in, and ruling things out. In simple terms, diagnosis is often a process of elimination. Or at least, a process of playing the odds.

Diagnosis involves several stages: looking at clues, examining the equipment, and perhaps gathering additional information (usually through tests.)

 The first step is to look at the clues. For example:

     • What are the symptoms? Resets? Lockup? Bizarre readings?

     • How bad is the problem? Small outage? Damage? Catching on fire?

     • Is there an obvious cause and effect? In the test lab, this may be very obvious. In the field, this may be unknown, so you may have to speculate.

     • What are some key parameters? Frequencies? Amplitudes? Dimensions?  Impedances?

The next step is to examine the equipment. For example:

    • How does the electrical design look? Multilayer or two layer boards? Layout? Etc.?

    • How does the mechanical design look? Metal enclosure or all plastic? Seams? Penetrations?

    • What about cables and connectors? Shielded? Filters?

    • And what about the power interface? Filters? Transient protection?

At this point, one should make a preliminary diagnosis. If the data is still fuzzy, you may need additional testing. The tests can either be monitors, or failure forcers.   Both can provide critical information.

By the way, it is OK to change your diagnosis as you proceed — doctors do this all the time. More important, don't fall in love with your initial diagnosis, but keep an open mind as new data becomes available.

Once comfortable with a diagnosis, you are finally ready to try fixes (prescriptions.) Install, test, and observe. If nothing happens, try another fix. And so on. Keep notes as you go along so you can backtrack.

By the way — don't try only one fix at a time, but rather stack them up. To change analogies, EMI problems are often like a leaky boat. If you have five holes in the boat, but you only apply one patch one at a time, you'll never get dry.

A final admonition — at this stage, don't worry about the practicality of your fixes. The initial goal is to find a fix – any fix. Once you find that first fix, you can always try for a better one.

Over this next series, we'll examine various EMI problems. We'll look at the symptoms, and we'll discuss various troubleshooting tests. We'll also include recommended fixes. Stay tuned…   

Previous entries in the series

EMC Basics #1: Welcome!; and Clocks: critical circuits for EMC

EMC Basics #2: Resets as Critical Circuits

EMC Basics #3: Voltage regulators as critical circuits

EMC Basics #4: Analog devices as critical circuits

EMC Basics #5: I/O as critical circuits

EMC Basics #6: Looking at circuit board “stackup”

Also relevant to this topic:

Debugging: The 9 Indispensible Rules for Finding Even the Most Elusive Software and Hardware Problems (Chapter 5, Part 3 of 3) (and see its preceding sections, which are linked within)

About the author

Daryl Gerke , an EMI/EMC consultant since 1987, along with business partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel have been chasing EMI problems for over 80 years (combined, of course.) He is a published author and columnist, and their EDN Designer's Guide to EMC (1994) is still in relevant and in demand. He can be reached via or his other blog at

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