Hi Geetha, It might be worth checking your measurement setup. If you are using an RF current probe, then you might be seeing the transitions in the DC draw of the device. The Vbus and ground delay in balancing could be something on the order of the time you are mentioning. I would suggest finding a DC sensitive current probe in parallel with the RF probe or a simple low value resitior and a voltage probe to rule this out. Respectfully, Brent DeWitt Hewlett-Packard Vancouver, WA
_____ From: [email protected] [mailto:[email protected]] On Behalf Of Geetha Balasubramanian Sent: Tuesday, February 06, 2007 8:33 AM To: [email protected] Subject: Common mode current in bus powered USB cable I recently tested some (about 5 different types) USB devices (USB 1.1 Full speed devices with data rate 12 Mbps, low bus powered devices) for understanding the common mode current flowing through the cable. To verify and ensure the balance in the current flowing through the Vbus conductor, ground conductor and shield conductor I measured the nett current flowing through these 3 conductors using a current probe. At regular intervals there is about 25 milli amps common mode current flowing in both the directions. The current pulse is of square wave with about 80 nano-seconds duration. Is this common mode current in DC +ve, DC -ve & Shield conductors of an USB cable is a normal behavior or am I missing something in the measurement. The cable contains 2 twisted pairs, one pair is the D+ & D- signal wires and the other pair is Vbus and ground (DC – ve). In all the devices I found that ground and cable shield are directly connected in the device PCB. I request the experts to throw light on this. _____ Check out the <http://us.rd.yahoo.com/ vt=43257/*http://advision.webevents.yahoo.com/mailbeta> all-new Yahoo! Mail beta - Fire up a more powerful email and get things done faster. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc -- No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.432 / Virus Database: 268.17.29/673 - Release Date: 2/6/2007 5:52 PM ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________ -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.432 / Virus Database: 268.17.29/673 - Release Date: 2/6/2007 5:52 PM - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
