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What is the most common practice in performing EMC tests for PCB assemblies that will be sold without a housing? In my case it is an FPGA development board. Is it possible to design excessive switching hardware in a way that it passes CE and FCC testing without a metal housing? Or do you usually define a designated case of application with a housing?

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  • \$\begingroup\$ what do pc motherboard suppliers do? \$\endgroup\$ – Andy aka Mar 12 '14 at 12:28
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    \$\begingroup\$ Slight terminology issue: To most people, a "bare" PCB is one that doesn't have any parts on it at all. It might be clearer if you said "unenclosed PCB" instead. \$\endgroup\$ – Dave Tweed Mar 12 '14 at 12:34
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    \$\begingroup\$ With proper grounding and keeping radiated emissions in mind during the whole process, it's not usually the board that radiates but cables that connect to it. There are ways to mitigate that on the board too. Lots of electronic products come in plastic boxes that do little to attenuate emissions from the board. \$\endgroup\$ – Olin Lathrop Mar 12 '14 at 13:25
  • \$\begingroup\$ When you are talking about "excessive switching hardware", it is only the FPGA or external components ? \$\endgroup\$ – zeqL Mar 12 '14 at 18:49
  • \$\begingroup\$ Wouldn't the compliance of an FPGA board depend pretty heavily on the design loaded into the FPGA? \$\endgroup\$ – Chris Stratton Mar 12 '14 at 20:16
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You can make single board products with fast electronics pass CE/FCC in plastic or no housing.

It is easier if you:

  • keep all cables attached in one end only
  • use fewer cables
  • specify a specific simple application for the FCC/CE test
  • don't have things that stick off the board (display, large connectors etc.)
  • make a very low impedance power distribution network (PDN)
  • have good filtering on the interfaces off the board

It is very difficult if you have multiple boards (plug-in, stacked, cable connected etc.). Actually I would specify a Faraday cage type box if you have multiple boards.

If you absolutely must include a display - try to get one with LVDS interface.

If you have a radio as part of your design, some of this becomes a bit more complex.

I suspect many go fairly light on the CE/FCC thing and self-certify with limited time spent on measurements. It is highly unlikely anyone will ever sue you for an eval board that is out of FCC/CE spec and most of these boards runs in fairly limited volumes.

Also: I have seen some outfits in China offer to do very cheap CE/FCC marking for you. You get the paperwork - they get some money. Win-win. :-)

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We have passed the test for FCC and CE compliance without a metal housing. Thus: Yes it is possible. Sure, the EMC behavior strongly depends on the design inside the FPGA, therefore we defined a typical application. For getting a credible result this typical application should be the worst-case (especially in terms of IO pins and cables).

For all who are planning to do such test: Define your typical application and don't forget to bring several high quality cables and PSUs.

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