Interesting question and of interest to many of us Ken. My thoughts:
- A choke between board ground and the shield isn't really desirable. All it does is create a frequency dependent floating shield. It becomes another high Q resonant element in the radiation system driven by any common mode currents on the contained conductors. If it were a perfect shield/conductor and many skin depths thick, that might help by keeping circulating currents entirely on the inside surface of the shield, but in reality, they will probably make it to the outside surface and you've got a pretty efficient antenna. - Thinking about "common mode current"; Mr. Kirchoff was right. The CM noise currents have to come from somewhere _and_ make it back to there. How? Displacement currents from the simple capacitance of the EUT to our earth reference plane are very real, but in many cases, the other connections, such as the power cable or other signal cables are more significant. These currents are, in turn, created by a common mode voltage driving a poorly defined impedance, so to reduce the radiating current we have some knobs we _may_ be able to turn: o Reduce the common mode drive voltage * Reduce timing skew in the USB differential signals caused by simple propagation delay. * Reduce capacitive imbalance on the differential signal (never use discreet capacitors from each side to ground). * Improve driver bypassing to reduce package derived CM current. o Increase the loop impedance * Add common mode impedance (choke) to the driven pair. * Figure out how to increase the isolation of the "other side" of the radiator from the USB driver through layout or supply (not ground) chokes. o Lastly, try to give the currents an easier what to complete their journey. No general leads here, but generally layout dependent. I'm sure I'm preaching to the choir on most of this, but you did ask. ;-} Best regards, Brent DeWitt From: Ken Wyatt [mailto:[email protected]] Sent: Thursday, September 10, 2015 3:03 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]>
