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22 DESIGN007 MAGAZINE I MAY 2021 Feature Article by Kenneth Wyatt WYATT TECHNICAL SERVICES Introduction e three top product failures I see con- stantly in my consulting practice are radiated emissions, radiated susceptibility, and elec- trostatic discharge. Aer reviewing and test- ing hundreds of products over the years, I've come to the conclusion that products fail these tests for five common reasons (somewhat in order of incidence): 1. PCB design—Poor layout and layer stack-up. 2. Cable shield termination and pigtails— Cable shields are not terminated to enclosure or lack of common mode filtering for unshielded products, plus shield pigtails used. 3. Gaps in the return path—High-frequency clocks or signals crossing gaps in the return path. 4. Power distribution design—Poor power distribution network (PDN) design. 5. Shielding design—Apertures or slots in the shielded enclosure that are too long. 1. PCB Design e single most important factor in achiev- ing EMC/EMI compliance revolves around the printed circuit board design. It's impor- tant to note that not all information sources (books, magazine articles, or manufacturer's application notes) are correct when it comes to designing PCBs for EMC compliance— especially sources older than 10 years or so. In addition, many "rules of thumb" are based on specific designs, which may not apply to future or leveraged designs. Some rules of thumb were just plain lucky to have worked. PCBs must be designed from a physics point of view and the most important consid- eration is that high frequency signals, clocks, and power distribution networks (PDNs) must be designed as transmission lines. is means that the signal or energy transferred is propagated as an electromagnetic wave. PDNs are a special case, as they must carry both DC current and be able to supply energy for switching transients with minimal simultane- ous switching noise (SSN). e characteris- tic impedance of PDNs is designed with very low impedance (0.1 to 1.0 ohms, typically). Signal traces, on the other hand, are usually The Top Five Reasons Products Fail EMI Testing

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