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]>
