Chris, What are the values of the CM and differential mode chokes?
Dave Cuthbert Micron Technology From: [email protected] [mailto:[email protected]] On Behalf Of Chileshe, Chris Sent: Friday, June 03, 2005 6:50 AM To: [email protected] Subject: Common mode filtering on 4-wire serial interface Group, Simple query. I have a product which features two PCBs connected via 4-wire serial (RS-485) interface. The signal assignments are Vcc, GND, A, B I have been performing radiated immunity tests and have had good results over most of the band between 10KHz and 1GHz. I have had minor susceptibility at around 800MHz and major susceptibility at around 18MHz. My product currently features differential line filtering (chip ferrite) for the Vcc and Gnd lines and a 2-wire common mode choke on A and B. The circuits are identical on both boards so A and B exit one board through a CM choke and enter the second through a CM choke. The ferrite/cap filters are mirrored so Vcc and GND exit one board through chip beads and enter the second through chip beads (I have an illustration I can send if required) I have traced some of the susceptibility to the 4-wire harness ( 27cm long). I am thinking about using a common mode choke on the Vcc and Gnd lines. My questions are; Is there an established EMI filter strategy for 4-wire interfaces similar to that described above? Would I be better off using a single 4-way CM choke or two 2-way CM chokes? Any advice or pointers to sources of information most appreciated Rgds - Chris p/s I have googled! 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/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc 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/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
