Hi Bill, What you say may be true. I often use memory sticks to terminate unused USB ports during testing, and on several occasions I have come across high emissions caused by the sticks.
Regards Ravinder Ajmani HGST, a Western Digital company 5601 Great Oaks Pkwy San Jose, CA 95119-1003 [email protected]<mailto:[email protected]> From: Bill Owsley [mailto:[email protected]] Sent: Friday, March 20, 2015 10:52 AM To: [email protected] Subject: Re: [PSES] USB and radiated emission issues Have you determined that it is the stick and not the host? If the problem is just this brand of stick, then there are some layout issues that can be re-done to reduce or eliminate the issue. As James mentions, if there is an imbalance in the differential signal, that will generate a radiated problem. So correct the differential signal layout and increase the decoupling on the power+return. If you have a USB cable that you can open up so the pairs can be "sniffed" that may indicate which is the main source, signal or power, or both. The common mode choke with more decoupling on the power rail might help. ________________________________ From: "Pawson, James" <[email protected]<mailto:[email protected]>> To: [email protected]<mailto:[email protected]> Sent: Friday, March 20, 2015 9:22 AM Subject: Re: [PSES] USB and radiated emission issues Hi Amund, a quick brain dump for you If the emissions are caused by imbalance in the differential signal then a Common Mode Choke (CMC) will probably help. Pick one with a good impedance at the frequency of interest, 480MHz in this case but you can also expect to see emissions at 960MHz and possibly 240MHz as well. The two main performance graphs are common mode impedance (higher is better) and differential mode impedance / insertion loss (lower is better) against frequency. There are wirewound and multilayer ceramic CMCs made by Murata, Laird, Wurth, TDK and probably others. The datasheet generally states if the part is suitable for USB 2.0. Ceramic parts usually give better high frequency performance from the datasheets I’ve looked at but are slightly more expensive. I’ve seen some manufacturers with combination ESD protection and CMC and some with CMC built into the USB connector. You’ll obviously need to measure signal integrity before and after adding the chokes to ensure your eye diagram looks OK. Hope this helps James From: Amund Westin [mailto:[email protected]] Sent: 20 March 2015 12:10 To: [email protected]<mailto:[email protected]> Subject: [PSES] USB and radiated emission issues Got some serious radiated emission issues from a USB 2.0 stick (high-speed 480Mb/sec). Spectrum shows the 480MHz way over the EN55022 limit line. We’ve been told to implement a common mode choke between the USB IC and the input/output port. That means on the D+ and D- transmission lines. http://www.coilcraft.com/0805usb.cfm Anybody who have experience with such design? #Amund - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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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]>
