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How to Select Absorbing Materials for Retrofitting


You’re the compliance lead for your company’s latest, newest, fanciest widget, just about to be released to production, with anticipated sales in the millions. The product has an extremely high profit margin and customers from all around the world are waiting in line to make a purchase.

The device is in the final stages of official compliance testing, and it must pass all compliance tests before it can be legally sold. On the last day of testing, one day before production release, it fails radiated emissions testing at 800 MHz, 3 dB over the Class B limit line. You contemplate certifying the device as Class A; however, this is legally not an option. It’s a device that can be sold as residential, commercial, light-industrial in addition to the heavy-industrial environment, so it must pass Class B levels.

So, what are your next steps? The pressure is on high, full-throttle! The beads of sweat are starting to form on your forehead. What do you do to solve this problem in the shortest amount of time possible? Don’t even think board spin. A board spin this late in the game will take too long and is therefore off the table as a solution.

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Best Possible Option

You determine that you can likely suppress the 800 MHz offending emission emanating from a printed circuit board (PCB) using a flexible ferrite RF absorber sheet. The problem is that you need to find the right one and you don’t have a lot of time to do it. The performance of various flexible absorber sheets is all over the map. A trial-and-error approach could take days and consume valuable resources before you find the right absorber to solve the problem. You know that most flexible absorber sheets work well in the single GHz and higher region, but your problem is just below a gig.

Luckily for you, you are smart and you’ve already thought through this problem well ahead of time. You had recently obtained sample flexible RF absorber sheets from several reputable vendors and characterized their performance (insertion-loss) following the method described by Ken Wyatt in reference 1. You have this data neatly tucked away on the network, easy to find and use in case of an emergency just like this.

You look through the data you had collected for these sheets and locate the absorber material that provides the highest attenuation at around 800 MHz. You install the absorber onto the bottom of the PCB, re-test emissions on the product, and find out that you now have 6 dB of margin at 800 MHz. The product ships on time, having passed all required compliance tests! And, to top it off, you’re now considered a big hero too! Management just loves you, and so do your customers. You saved the day. This is awesome!

Second Best Possible Option

The second-best option is similar to the best possible option identified above however, this time, you only ordered and received sample flexible RF absorber sheets from several reputable vendors. You have several sheets at your desk ready to go, but you have not yet characterized their performance. Other more pressing projects took the time you were going to use for this task. Fear not! The insertion-loss measurement described by Ken only takes about an hour. This is nothing in the big scheme of things, and soon you have the data you need to choose the correct RF absorber, install it, perform re-test of emissions, obtain passing results, and still become a hero, just only about an hour later than had you already collected the insertion-loss data.

Last Option

This option is not ideal. You don’t already have data characterizing the insertion-loss flexible RF absorber sheets. In fact, you don’t even have any flexible RF absorber sheets in-house. You ask around your company, and nobody else even knows what a flexible RF absorber sheet is. You have to perform an internet search, find what might look like acceptable material, order that material, wait for shipping (you’ll probably want to pay for over-night shipping in this instance), get the material in, probably forego the simple insertion-loss measurement and go the trial-and-error route due to pressure from upper management. After several days of troubleshooting, trying to find the material that will squash the 800 MHz, you may or may not find a suitable material. You repeat this lengthy process until, by some EMC miracle, you happen to find a flexible RF absorber sheet that works. By this time, management is unhappy with you, customers have lost faith in your company’s ability to deliver product on-time, and you have grown a few more grey hairs due to the extra stress involved. You kick yourself because you should have known better and gone with Best Possible Option route or even the Second-Best Option route.

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If you think you’re ever going to need to use flexible RF absorber sheets to help your product pass emissions, even at some later date, it might be a good idea to have already identified and obtained the material and have characterized its insertion-loss performance. By having a solution ready to go at a moment’s notice, you may end up saving yourself and your company a lot in the long run. You may possibly even be considered a hero someday.

References and Further Reading

  1. Insertion-loss measurements of ferrite absorber sheets, EDN, July 25, 2019.
  2. Let’s Talk About Flexible Absorber Sheets, In Compliance Magazine, October 2019.
  3. What Every Electronics Engineer Needs to Know About: Absorbing Materials, In Compliance Magazine, October 2018.

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