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I am undecided whether or not to add a common mode choke in my product that needs to undergo EMC testing for CE marking. Let me describe my case... My device is split into 2 parts, connected with each other with a 2 meter cable. The overview is the following: enter image description here

For better comprehension, here the detailed sub device: enter image description here

I got different opinions about adding a common mode choke very close to the connector. What's your opinion on this? I do not expect UART frequencies higher than a few kHz. My main goal is to, control the emissions to pass the EMC test.

Thank you:)

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    \$\begingroup\$ @Saadat thank you for your answer. Do they inject noise even if the connectors are not exposed and they are "locked" in the enclosures? \$\endgroup\$
    – mdir
    Commented May 4, 2023 at 8:42
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    \$\begingroup\$ "needs to undergo EMC testing for CE marking" doesn't mean jack. What is the product, what are the requirements, how high radiated susceptibility tests do you plan for, in which frequency span? Also what are the baudrates, radiated emissions may or may not be a walk in the park... not so much if you are running at speeds past 1Mbit baudrates. \$\endgroup\$
    – Lundin
    Commented May 4, 2023 at 9:20
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    \$\begingroup\$ @mdir Conducted susceptibility/emissions testing is done on the "DUT" in the form it will used in the real world. They will not open up the enclosure if that's not how the product will be used. So if this is a battery application with no supply cable then conducted tests don't apply. ESD testing will be done similarly, applied to critical points on the enclosure (basically the test engineer will be as evil as possible and look for exposed metal parts like screws, then shoot contact charges there) \$\endgroup\$
    – Lundin
    Commented May 4, 2023 at 9:22
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    \$\begingroup\$ Just adding a choke doesn't do much. You would also need filtering capacitance. There's also no termination resistance shown. Note that all concerns can be sidestepped by using a shielded cable, and you could remove the RS-485 interface and pass plain logic signals instead. The TVS also doesn't seem necessary. \$\endgroup\$
    – Tim Williams
    Commented May 4, 2023 at 9:27
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    \$\begingroup\$ @TimWilliams "Note that all concerns can be sidestepped by using a shielded cable" Aka hiding your EMC problems under the carpet. Have you ever used shield cables? They are approximately 10 to 100 times more expensive than ordinary ones. "you could remove the RS-485 interface and pass plain logic signals instead" That's plain awful advise, the differential signal is what makes RS-485 rugged. "The TVS also doesn't seem necessary" It is mandatory in pretty much every serious application for use in the real world. Not necessarily on the CAN lines but on the supply. \$\endgroup\$
    – Lundin
    Commented May 4, 2023 at 11:45

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I got different opinions about adding a common mode choke very close to the connector. What's your opinion on this?

It really depends on the frequency but in general if you put the choke in the middle of the cable there will be more of the cable that could couple to other things via cross capacitance.

On the board side it's desirable to put the common mode choke as close to the connector as possible to reduce capacitive coupling from the traces to ground as much as possible if the ground is also part of the issue.

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