The Merging of SI, PI, and EMI

Kenneth Wyatt, Principal Consultant

Wyatt Technical Services LLC

During a recent DesignCon conference last January 2018, I was honored to be a part of an opening keynote panel discussion on trends in signal integrity (SI), power integrity (PI), and EMI in the next five years. Others in the panel included Steven Sandler (managing director, Picotest), Istvan Novak (senior principal engineer, Samtec), Eric Bogatin (adjunct professor with the University of Colorado and Dean of the Teledyne LeCroy Signal Integrity Academy), and Alfred Neves (chief technologist, Wild River Technology). One of the trends I pointed out was the gradual merging of SI, PI, and EMI.

Many of us in the industry feel we can no longer partition these three disciplines into separate categories. All three interact with each other and it gets worse as frequencies increase. If you blow one, you’re likely to screw up the other two. Interestingly, the IEEE EMC Society has already incorporated SI/PI into their annual EMC symposia for the past several years.

As we approach data rates of 40 GHz, and higher, we need to understand the physics of the board and channel design. But there are obvious intersections between the disciplines. For example, failing to consider the return path for signal and power currents can affect all three disciplines. For example, failing to flatten out the impedance of your power distribution network can create resonances and result in peaks in the EMI emissions.

As more manufacturers are incorporating wireless connectivity into new or existing products, the issue I’m running into lately is self-interference (or also called “platform interference), from DC-DC converters and digital control, desensing cellular modem receivers and other wireless receivers, such as GPS (References 1 and 2). If circuit boards lack adjacent return planes for all signal and power layers, then conducted and radiated noise can couple into the receiver and can lower receiver sensitivity by 10 to 20 dB…more than enough to fail the cellular providers requirements for total isotropic sensitivity (TIS). And there you go…a product you can’t ship (reference 3).

This may come as a surprise, but with the exception of those already understanding the issues in the SI and PI world, most product designers don’t seem to understand the basic concept of how signals, that is, energy in the form of electromagnetic waves, moves in transmission lines. If they did, I wouldn’t be receiving so many calls for help in passing EMI limits. All you need to do is look at their layer stack-up and you just have to say to yourself, “Well, of course this product has EMI issues! There’s no defined path for return current!” and the result is cross-coupling of signals, SI and PI issues, and resulting board radiation.

Steve Sandler and I recently coauthored an article on PI and EMI issues with voltage regulators that helped explain some of these issues (Reference 4). Because PI, SI, and EMI are not generally taught in colleges and universities, we educators need to really focus on the basics of all three disciplines and how we can better connect to those product designers in the trenches. I really feel this is a competitive issue for companies. That is, companies who understand SI, PI, and EMI, as well as PC board design, and "get it right" the first time, will be able to launch product weeks or months ahead of competitors.

References

1. Wyatt, Platform Interference – Measurement and Mitigation, https://interferencetechnology.com/platform-interference-measurement-mitigation/

2. Wyatt, Ten Tips to Minimize EMI from On-Board DC-DC Converters, https://interferencetechnology.com/ten-tips-to-minimize-emi-from-on-board-dc-dc-converters/

3. Wyatt, Design PCBs for (Low) EMI (in 3 linked parts), https://www.edn.com/electronics-blogs/the-emc-blog/4461633/Design-PCBs-for-EMI--part-1--How-signals-move

4. Sandler & Wyatt, Top Three EMI and Power Integrity Problems with On-Board DC-DC Converters and LDO Regulators, https://interferencetechnology.com/top-three-emi-power-integrity-problems-board-dc-dc-converters-ldo-regulators/

Kenneth Wyatt is principal consultant of Wyatt Technical Services LLC, as well as past senior technical editor for Interference Technology Magazine (2015 to 2018). He is based in Colorado and has worked in the field of EMC engineering for over 30 years with a specialty is EMI troubleshooting and pre-compliance testing. He trains and speaks internationally, is widely published, and is the co-author of the popular EMI Troubleshooting Cookbook for Product Designers. He may be contacted through his web site, http://www.emc-seminars.com.

Great info , good thanks .

Malika

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