All of the SDRAM output drivers I have measured have impedances of around 20 ohms. Adding the 22 ohm resistor is the usual way to use these parts. If you are driving into a matched system having a characteristic impedance of 50 ohms (and end terminated) you will reduce the driver current by 2 dB. Was your reduction in EMI near this level?
The dominant "antenna" can be the SDRAM package, or the PCB connector, or a cable. It all depends on what your system is. The SDRAMs I have measured in many applications all had highest emissions at multiples of the clock frequency, most commonly the odd harmonics. Spread spectrum clocking seems to have the best cost-benefit for reducing emissions. With a 1% spread in your 59.9 MHz system the fundamental will be reduced 5 dB. Each harmonic is reduced by 10LOG(H/F), H being the harmonic freq and F being the fundamental freq. This assumes a 120 kHz measurement bandwidth. Dave Cuthbert Linear Technology From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Thursday, May 03, 2007 7:13 AM To: [email protected] Cc: [email protected] Subject: Emission at SDRAM harmonics and ethernet signal current Dear Experts, In our ethernet device, by introducing series resistors of value 22 ohms in the Tx / Rx signals we noticed reduction in the emission levels. Reduction of emission in the ethernet harmonics (175, 200, 225, 250, 275, 300 & 350 MHz) is logical. At the same time we notice reduction of emission in SDRAM clock harmonics also. (multiples of 59.9 MHz). Why & how this happens and what to infer from this is not clear to us. Sincerely K.Balasubramanian Project Leader - Hardware. - 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 ______________________________________________________________________
