Siming, you can read the paper available on the ipblox website: www.ipblox.com/papers.html. It directly addresses your issues. A ferrite bead unless properly compenstated introduces simple L-C resonances. Breaking the board up into smaller sections shifts the modal frequencies, and increases their Q. Combining caps and loads does not reduce the worst-case noise. It improves the average case, provided that by combining resonance issues were not created that did not exist before. The resonance issue cuts both ways which means care must be taken to make sure comparisons are apples to apples. Steve. On 4/1/2011 3:14 PM, Siming Pan wrote: > Hi all, > I have a question here: why could board resonance be the price of using > ferrite beads? I agree that small isolated board would cause severe board > resonances. However, the first modal resonance is always at several MHz. For > board-level PDN design, we usually only care about the inductive curve up to > 100 MHz. Chip with proper on-die decap and package will never see those > board resonances. > I think one advantage of using shared power plane for multiple chips is > that all decaps on this power plane can be shared to low the impedance. > However, the total loop inductance is consisted by two portions. One is the > inductance caused by power vias connecting with decaps and the other portion > is inductance due to IC power pin. Although by combing the power planes the > inductance of decap via can be reduced, the mutual effects caused by mutual > inductance of the IC power pins may worse the situations, even when the IC > noise current of multiple chips is at the similar level. So I always use > ferrite beads to isolate chips and control the PDN impedance for each chip. > Although to use ferrite beads might not be the best solution to optimize > decaps, it is always safe to prevent interferences. > Regards, > Siming Pan > Cisco Systems > > On Fri, Apr 1, 2011 at 1:54 PM, Eric Bogatin<eric@xxxxxxxxxxxxxxx> wrote: > >> Hi folks- >> >> >> I thought you might be interested in the piece I wrote for PCD&F magazine >> based on Steve Weir's DesignCon talk on ferrites. Check it out here: >> >> >> >> >> http://pcdandf.com/cms/component/content/article/171-current-issue/7893-sign >> al-integrity-insights. >> >> >> >> You can add comments to the article on the PCD&F site, if you are so >> inclined. >> >> >> >> I've also been writing a blog for EE Times on SI topics, based on >> interviews >> with experts. >> >> >> >> http://www.eetimes.com/electronics-blogs/4207357/BeTheSignal/ >> >> >> >> If you have a suggestion for a topic or someone for me to interview, drop >> me >> a note- offline, please! >> >> >> >> Additional info on other SI topics is on my web site, www.beTheSignal.com >> <http://www.bethesignal.com/> . >> >> >> >> --eric >> >> >> >> ******************************************************* >> Dr. Eric Bogatin, Signal Integrity Evangelist >> Bogatin Enterprises >> Setting the Standard for Signal Integrity Training >> No Myths Allowed Webinar Series >> Public Classes >> <http://www.bethesignal.com/> <http://www.bethesignal.comee/> >> www.BeTheSignal.com >> EE Times Blog: >> <http://www.eetimes.com/electronics-blogs/4207357/BeTheSignal> >> http://www.eetimes.com/electronics-blogs/4207357/BeTheSignal >> beTheSignal Blog:<http://www.bethesignal.com/blog> >> www.beTheSignal.com/blog >> 26235 W 110th Terr >> Olathe, KS 66061 >> e:<mailto:eric@xxxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx >> v: 913-393-1305 cell: 913-424-4333 f: 913-393-0929 >> skype: eric.bogatin >> *********************************************** >> >> >> >> >> >> ------------------------------------------------------------------ >> To unsubscribe from si-list: >> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >> >> or to administer your membership from a web page, go to: >> //www.freelists.org/webpage/si-list >> >> For help: >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> //www.freelists.org/archives/si-list >> >> Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> >> > -- Steve Weir IPBLOX, LLC 150 N. Center St. #211 Reno, NV 89501 www.ipblox.com (775) 299-4236 Business (866) 675-4630 Toll-free (707) 780-1951 Fax ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu