Miscellaneous power adapter wall blocks on a yellow background

EMC Compliance Testing: How to Prepare When You’re Unprepared

Resources are never infinite, and this can be doubly true when it comes to electromagnetic compatibility (EMC) design. Many businesses simply do not have the resources—be it time, funding, or expertise—to prioritize EMC considerations throughout the design process. Sometimes, just getting the product out the door and on its way to the EMC lab for compliance testing can be an accomplishment, let alone designing for each and every test it will be subjected to.  

It is an ill-advised and risky strategy, but if you’ve found yourself en route to the EMC lab with such a product, there are a few things you can do to mitigate that risk. Regardless of the EMC resources available (or not available) to you, you can be more efficient with your time by providing yourself some design options to leverage at the point of compliance testing. 
 

Post Failure Tuning 

Where possible, consider offering yourself flexibility in tuning with optional ferrites in series with, say, a DC supply input. You can buy capacitors on the end of sticks that are designed to be butted to existing populated SMD capacitors or their unpopulated pads. These add some capacitance quickly so you can evaluate the effects. 

Be sure to consider alternative footprints for decoupling capacitors. You don’t necessarily have to populate on first trial, but having prepared footprints for high frequency and low frequency decoupling is great to fall back on.  

Do be aware, however, that the EMC lab will only allow you work directly and live on low voltage circuitry and safer low-voltage tests types. 
 

Wall Warts and Power Leads 

So many products fail because their power source leads make great antennae for unwanted electromagnetic emissions. It can really save the day to have a few “band-aid” solutions in your back pocket the day of testing. 

Wall warts are a great example. The product requirements might have been, simply, a 6Vdc, 100mA mains adapter—BUT as cheap as possible. However, now you’re at the test house and you’re finding out you got what you paid for in that cheap power adapter. If you have a few different vendors’ offerings on hand, you can at least differentiate between them, or maybe even succeed on one.   

Direct mains leads are another example, which can offer additional Common Mode attenuation. They may be purchased as pure, straightforward cabling or they may have ferrites embedded into the cabling. Even if you ultimately don’t intend to pay the extra for cables with ferrites, at least you will know its impact there and then by having an assortment of power leads to play with. 
 

Observations 

If your performance criteria (how you expect your product to behave) is a visual one, like a reset LED coming on or a momentary fault indicator, consider video recording and getting the test frequency list from the lab so you can get back to the frequencies of concern later. Some tests can be over an hour long, which is a long time when you are full of coffee! Gathering key info along the way is one of the quickest ways to get yourself out of the testing room and working toward a solution. 

If the product is damaged, then lining up the frequencies of sweep tests will prove crucial for debugging. A video recording can also be entered as evidence of a successful test.  
 

Spares 

Bring more units than agreed with the test house. Not having enough product is the number one thing that ruins people’s days at the EMC lab. What if you drop the product or plug something in backwards in haste? And that’s not to mention all the ways your device can be damaged by tough testing. Whatever quantity you’ve agreed upon with the test house, bring double if you can! 

If you are to take one thing away from this blog, have many identical spares. If your product is failing the electrostatic discharge (ESD) test at 8KV, for example, it’s so useful to know whether it would pass at, say, 6KV. The lab can help determine this by working their way up from a passing level on spare hardware. This is known as thresholding. 
 

Heat 

Excess power dissipation can manifest as heat dissipation. This is a very common type of failure, so consider bringing a thermometer of some sort; the low-cost laser ones can quickly inform you of overheating devices. It is best to come prepared with known device operating temperatures. In fact, it’s wise to have all your baseline temperature measurements completed before you get to the lab, as time seems to move much faster in there. 

If your product has released smoke, consider asking the lab to test at a lower power on your spare product while you monitor device temperature—another useful application of the thresholding approach. 
 

Communications Connections 

Control of the product is useful, but beware: Ongoing communication with a connected device such as a laptop can inadvertently result in testing your laptop. Fine if the device under test (DUT) is a consumer electronic that’s similar to a laptop. But if your computer is subjected to tests intended for, say, heavy duty industrial automation equipment, you’re going to have a bad day. 

Having the flexibility to reconfigure modes of operation and investigate partial degradation in functionality automatically is powerful. Just make sure the DUT is the device that’s actually being tested.
 

Conclusion 

No matter how prepared you are on the day of testing, it’s best practice to keep the test setup as simple as possible while still reflecting a normal user configuration. If it goes well, and if you have time and reason to do so, then you can explore more complex configurations. But again, beware: if you fail subsequently, it all goes into the same report!