Hi Ken, In accordance with the original (1998?) Ott, et al, paper, you need to bond the image plane to the I/O cable shield(s), power return, and Ethernet ground shell at the cable egress. I believe Henry Ott has the original paper downloadable from his web site.
Cheers, Ken Kenneth Wyatt Wyatt Technical Services LLC Woodland Park, CO [email protected] www.emc-seminars.com (Sent from my iPad) On Sep 22, 2012, at 11:42 AM, Ken Javor <[email protected]> wrote: > Am working a small plastic box that is failing FCC Class B radiated emissions > 30 – 400 MHz. Box input power is 12 Vdc; it converts that to 5 Vdc (not > isolated – 5 Vrtn is 12 Vrtn) using a small PCB dc/dc converter. The 5 Vdc > is in turn routed using wire pairs to three load boards with digital > circuitry. Each of these boards is off-the-shelf; i.e., non-modifiable. > > Applied decoupling caps at each board power entry point and at the 12 Vdc > entry point to the dc/dc converter and that helped some, but the box is > generating significant common mode noise current from 30 to 400+ MHz on the > 12 Vdc input, which is causing RE problems. Inserting a common mode choke or > rf bead doesn't help here because the impedance at source and load is very > high (80 cm above ground and the 12 Vdc is floated from ground as well). > > Am wondering if an image plane here is the solution to providing a local > reference for the cm current to locally circulate. If so, is there any > connection between a board 5 Vrtn and the image plane, or is it totally > floated from the culprit PCB? Keep in mind this will not be an extra plane > in the stack up, but a separate foil or similar laid as close as possible to > the culprit PCB. How close is close enough? > > There is one other interface: Ethernet. That also has some cm current. Does > a filtered female Ethernet jack require reference to an image plane, or is > the 5 Vrtn ground plane sufficient (or even necessary)? > > Thank you, > > Ken Javor > Phone: (256) 650-5261 - > ---------------------------------------------------------------- > 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://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]> - ---------------------------------------------------------------- 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://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]>
