A Visit to the EMC Lab: What to Do if Your Product Fails EMC Testing

Imagine: Your product is going through electromagnetic compatibility (EMC) testing, and you’ve been under extreme pressure just to get the product ready for evaluation at the EMC lab. You think you’ve done it! The product is working, and there you are, at the lab, with a product that functions fine… but it’s not passing EMC testing.

Unfortunately, many of us don’t have to imagine, as this experience can be all too common during the EMC testing phase of electronic product design. Now you’re standing in the lab wondering, likely with a measure of dread: “How is this going to change the product schedule?” And of course: “How am I going to fix it?”

Fear not. It’s time to pivot into problem-solving mode—right there, right then, before you even leave the EMC lab. In this third installment of the “Visit to the EMC Lab” miniseries, I’ll show you what you can consider if your product fails EMC testing on the first go.

What to do if Your Product Fails Immunity Testing

If your product fails EMC immunity testing, don’t leave the lab just yet! Here are five things you can do on-site to improve the chance of passing or gain significant learnings for next time.

• Double-check your product setup. The smallest of changes can have a big impact, so confirm everything is in order: Solid grounding, power connections, and all cables plugged into sockets securely.

• Confirm functionality. Run any self-diagnostic, if available, to ensure your product is operating at 100% without exposure to interference.

• Review coupling mechanisms. Consider if the lab’s coupling mechanism without the interference applied is degrading the normal function of a product. Under certain conditions, you can renegotiate the coupling mechanism used.

• Request a retest. Although you can’t change your result that day without modifying the product, a retest can help identify marginality. If the product passes the second time, consider subjecting it to a slightly higher exposure to reproduce the original results. This way you’ll know how much you need to modify the design (or, in the best-case scenario, how little!).

• Request thresholding. Like retesting, slowly increasing the interference can help you find out how much lower the exposure level needs to go to recover from failure. Thresholding can show whether there is hysteresis involved or if it’s a marginal failure, which can help you determine the path ahead.

What to do if Your Product Fails Emissions Testing

I always make sure to arrive at the EMC lab prepared for a possible emissions testing failure. This means bringing emergency supplies such as ferrite clamps, and copper tape, and knowledge of your product’s specifications, including schematics and layouts. The tools and methods below can help you mitigate points of failure on-site or at least identify them so you know where to focus design efforts later.

• Consider your cabling orientation. Cables make great antennae. You may try S-bending or changing the orientation of your cables to see if it affects the results.

• Apply ferrite clamps. In a pinch, adding some ferrite clamps can help correct cabling and identify the source/scope of the problem. Start by positioning ferrites close to the electronics under test (EUT) but be prepared to slide them. Ferrite clamps may not cure the issue, but pay close attention to small degrees of change; good or bad is equally valuable information.

• Compare polarization results. Consider the orientation of the product and cabling with the receiving antenna. If your product fails in the vertical orientation, immediately compare the same failing region in a horizontal position. This will give you an indication of the orientation of the inadvertent offending antenna in your product.

• Check frequencies. Are the frequencies appearing on the EMI receiver related to the clock and crystal frequencies of the product? Have these details already to hand, including their harmonics, to let you quickly focus on the sources and coupling paths. If the frequencies are not directly accounted for, consider oscillations or intermodulation products.

• Tune programmable options. If available and feasible, leverage the product’s programmable options to disable suspected circuitry or communications to see if this resolves the issue.

EMC Failure Is a Learning Opportunity

After failing either immunity testing or emissions testing, the best thing you can do is come away from the EMC lab with as much information as possible. Your greatest advantage is to know the product, what the test is exercising, and what the failing mechanism is associated with. It’s a personal choice whether you prefer to charge on with further testing (as long as the product isn’t damaged) or focus on fixing specific, identified failures. Do bear in mind that a subsequent change to the product can require the entire suite of tests to be repeated.

Either way, don’t be afraid to ask lab staff for their opinion. They may not know your product as you do, but they know these tests inside out and can give you better insight into how they can impact the product. Use this to your advantage.

Finally, take heart and remember: Smoke is not necessarily bad news. I’ve been there! It gives you a big hint about where to focus your efforts. Often, EMC transient tests can have an impact on the device. Join me in the next blog to understand more about transient testing and how to mitigate against it.