Test & Measurement World, July/August 2012

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EMC TEST Precompliance testing for radiated emissions Some basic tools can help you identify trouble spots in your lab. BY KENNETH WYATT, WYATT TECHNICAL SERVICES M a ny e n g i n e e r s bring their prod- ucts to the test lab having no idea if it will comply with EMI/EMC stan- dards. Building a precompliance test lab is expensive, but you can still isolate ra- diated emissions with magnetic-field "sniffer" probes on enclosures and cur- rent probes on wires and cables. Engineers at smaller companies often can't afford a full-time compliance engi- neer. Thus, it's often difficult to predict whether a product will pass radiated emissions testing prior to taking the product to a test lab. You can spend a lot of time and money preparing a product, only to have it fail, which will require you to spend even more time and money on a redesign and a second round of testing. Ideally, performing some precompliance tests in your own lab will increase your confidence that a product will pass its compliance tests on the first try. An engineer named Jeremy re- cently sent me several questions about how to prepare for qualifica- tion testing. Jeremy's situation is probably similar to that of many en- gineers. After having been laid off from a large firm where he worked on product design and development, he now works for a much smaller company where he wears multiple hats and has responsibility for EMC (elec- tromagnetic compatibility)—yet he has little equipment available for solving problems. Jeremy is learning the hard way about EMC. He takes products to an EMC lab for testing, only to have them fail. He thinks he has found a so- lution to a problem but is unable to per- form any compliance tests himself; he has to take the design to a test lab, pay $600 to $1600, and then potentially fail the testing again. I think we've all "been there and done that!" My heart goes out to engi- neers in this situation, but the situation isn't hopeless. There are easy ways to perform precompliance tests in your own lab. It's an important and cost-ef- fective set of skills to learn. You'll end up saving your company considerable time and money if you can perform some simple tests on the bench at your own facility. Below, I have listed Jeremy's ques- tions (edited to fit this space) along with my answers. I'm sure many engineers who are new to the EMC field will identify with Jeremy's concerns and can learn from his situation. Making near-field measurements Question: When making near-field measurements with a small loop probe (Figure 1), I don't know whether ev- erything I see with the near-field probe is actually a problem. I want to change as little as possible, because at a small company, it really hurts to scrap ma- terials because of a change to a de- sign. Assume that with a small near-field FIGURE 1. Loop probes can help you locate and identify radiated emissions. probe, I see some amplitude of emis- sion at a chassis seam that is 16-in. long, I see the same amplitude of emission from an air slot that is 2-in. long, and I see the same emission level over the entire length of a 6-ft- long load cable. My gut feeling is that the 6-ft-long cable is radiating from a larger area and therefore must be ra- diating more power, but I'm not so sure about the 2-in. air slot and the Test & Measurement World | JULY/AUGUST 2012 | –24–

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