Hi Ken, This is a tough question to answer since the coupling mechanism is different product by product in reality. As you mentioned it is typical to tie down USB connector shell to the digital return plane, and I think that is the best option. Of course the cable shield could act as antenna if the digital return plane is contaminated for some reasons. Therefore, maximum efforts are needed to keep the digital return plane as clean as possible. Isolating ground and placing the bead between doesn't seem to be a good option since there could be some voltage drop across the bead at high frequency.
Thanks, Ben From: Ken Wyatt [mailto:[email protected]] Sent: Thursday, September 10, 2015 12:03 PM To: [email protected] Subject: [PSES] Best design practices to keep CM currents off a USB 2/3 cable given an unshielded enclosure? Hi All, It seems I'm always running into client products with USB 2.0 (or 3.0) connectors mounted on circuit boards that are enclosed within unshielded cases. I understand that CM chokes can be added to the data lines. However, given a circuit board with common-mode noise running all around, these CM noise currents can couple directly to the connector shield and thence out along the attached USB cable, creating EMI. Typically, the USB connector shield is soldered directly to the digital return plane. Is isolating the connector ground from the digital return plane and inserting a ferrite bead between the two the answer? Let's assume a large ferrite choke around the cable is not an option. Any other thoughts as to what can be done at the board-level to reduce common-mode currents from getting on the cable shield? I've already done a literature search and found nothing helpful. Thanks so much! Ken _______________________ I'm here to help you succeed! Feel free to call or email with any questions related to EMC or EMI troubleshooting - at no obligation. I'm always happy to help! Kenneth Wyatt Wyatt Technical Services LLC 56 Aspen Dr. Woodland Park, CO 80863 Phone: (719) 310-5418 Email Me!<mailto:[email protected]> | Web Site<http://www.emc-seminars.com> | Blog<http://design-4-emc.com/> The EMC Blog (EDN)<http://www.edn.com/blog/The-EMC-Blog> Subscribe to Newsletter<http://www.emc-seminars.com/Newsletter/Newsletter.html> Connect with me on LinkedIn<http://www.linkedin.com/in/kennethwyatt> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]<mailto:[email protected]>> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe)<http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]<mailto:[email protected]>> Mike Cantwell <[email protected]<mailto:[email protected]>> For policy questions, send mail to: Jim Bacher <[email protected]<mailto:[email protected]>> David Heald <[email protected]<mailto:[email protected]>> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
