Neven, It is not unusual to see 2-6 dB variation on emissions due to different SDRAM vendor, considering that each of them could use different die process, different drive strength, and different ODT technology ( or the lack of such ). I have observed this both in SDRAMs and DDRS. Unfortunately we are using different size of memory other than those your application. So I am not sure our observation can be applied to 2MB and 8 MB
The challenges of cheery-picking SDRAMS is that even with the same 8MB part, the vendor may have multiple fabs or different substrate, or they may do a process change ( such as die-shrink ) couple month down the road into your production. Given these factors, you may want to consider whether it is beneficial to improve your designs before it get launched. However, if you do choose to constrain SDRAM vendor to get into production, some simple bench top measurement can give you insight on whether a particular SRAM will likely to generate less EMI than another , with the same PCB design. The following two techniques are quick and quite reliable as a 1st pass look. : 1) measure the Data lines' waveshape, close to uC, during memory read ( ie, when data line were driven by DDR ) , compare rise time and waveshape 2) near -filed probe measurment on DDR part during memory read or write session Of course, this assumes that you have already determined that the SDRAM part itself is the dominate noise source. Hope this helps. Catherine Xu __________________________________ Sr. Hardware Design Engineer, Ph.D. HP IJS - San Diego _________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Friday, February 09, 2007 1:20 PM To: [email protected] Subject: Best SDRAM for Low-Emission? Hello everyone, I have a 2-layer PCB design, plastic enclosure, using 120 MHz SDRAM, two stuffing options: 2MB (16 Mb) in 50-pin tsop package or 8MB (64 Mb) in 52-pin tsop. It is slightly above class B radiated emission at the multiples of 120 MHz. I troubleshot it and I determined the SDRAM is the only source of the failing frequencies, everything else is quiet. I wonder if anyone has looked in how various manufacturers' memory chips perform emission-wise, particularly the type that I am using. I hope to find a manufacturer who makes "quieter" modules, so I do not need to change anything else on my board. If anyone has done such a comparison/investigation, I would appreciate if you would be willing to share, on-line or off-line, the results - which memory/manufacturer works best EMI-wise in such conditions. Regards, Neven - ---------------------------------------------------------------- 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 - 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 ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
