In my pursuit of getting an atta boy award for the longest thread in Si-list without yelling and shouting. I would like to keep the discussion going. Recalling my claim that power distribution management is really boils down to two common sense principles (I will call this CSP from now on): a) Manage core power distribution with a total system approach. Highspeed on-die, medium speed on package and low speed on PCB. b) Manage your I/O signals with proper reference plane and image current return. The corollary of the above is fancy decoupling caps and thin core PCB power planes are not needed because the problem they try to solve can also be fixed by following the above principles. No doubt there has been many heated debates on CSP for the past few years in this Si-list. And either you buy it or you don't. There is really no point for me to continue the debate. However, there is a persistent point in the argument of needing thin core/fancy capacitors that I have not seen a definitive answer on and I really would like to drill deeper into the discussion. This is the area of "containment" vs. "suppression/ decoupling" when system functionality is not in trouble while failing EMI noise limit is the concern. Here is some quote from members of the list : Ray : >The low-pass characteristics of the package are not >"brick-wall" in nature but have a finite amount of >attenuation. You can't really say that the package >attenuates all the noise emanating inside to >insignificant levels. The noise is attenuated, >but some is still there and can excite resonant >structures. Steve : >There is one point that I disagree on. Even though >the package cuts off with at least a -2 slope, >there is quite of bit of high frequency energy >that still passes between the PWB and the IC. >It is just grossly inadequate to power the IC. >But it has lots of potential to aggravate EMI >problems. Zhangkun : >From my view, I am caring about the EMI of PCB. Very >small common mode noise will give rise to critical >EMI problem. In my experience, the common mode noise >is proportional to the impedance of power delivery >systems. This has been verified by measurement >and simulation. I have to admit I am a system designer, not component engineer like Larry and Ray or EMI engineer like Zhangkun and Istvan. Even though I have worn these hats when needed, at the end of the day, I only try to answer the question : "can my bus/chip/system function given the power distribution system I design ?" And CSP is really guiding me to make sure my core power noise is within limits and at the same time does not generate enough noise back to the system to cause other components to fail noise margin. On the I/O side, CSP guides me to prevent image current mis-management to cause both EMI radiation and SSO noise. But the above principles say nothing about residue noise that is small enough not to create functional problem on the system but large enough to potentially create EMI noise. And I believe that's what Ray, Steve and Zhangkun is arguing above. I still don't think there is an argument that you need fancy caps or thin core just to make your system functional if you follow CSP. So the question is, how does one quantifies when is it time to "suppress/decouple" EMI noise on the system (be that using fancy caps/thin core or just conventional capacitor solutions) and when does one just need containment (by bury noisy power planes inside, ground stitching vias on PCB, metal casing, copper gaskets etc etc). We are talking about over-designing decoupling solutions here, ie like Ray or Steve say, may be you have an on die noise of +/-100mV@400MHz on a 1.5V design, after the noise propagate through the package, it creates a +/-20mV of ripple on the PCB. Clearly not big enough to cause functional failure but "The noise is attenuated, but some is still there and can excite resonant structures." or "It is just grossly inadequate to power the IC. But it has lots of potential to aggravate EMI problems." How low do we need to go ? How can one predict these noise will create EMI problem ? As a server or computer designer, I have the luxury of always having metallic chasis/ gaskets, reasonable cables and connectors. So I always fall back to ignoring the above below functional acceptable level noise and allow my containment solutions to deal with it. The only thing I will do extra is buring potentially noisy power planes inside and stitching ground vias in PCB. And, knock on wood, so far any thing I am responsible seems to be passing EMI just fine if I follow CSP. But I welcome anyone to explain to me how you can quantify the need to suppress these below functional noises in a predictable manner. i.e. I am not looking for an answer like "hey, when you fail EMI, you should look into doing X,Y,Z." Rather I am looking for an answer like "if you do X,Y,Z, my analysis tells me you can throw away your gasket or your metallic chassis." What will be really interesting is something like "if you follow CSP but you don't have fancy caps or thin core planes, my analysis predict your system will fail EMI." You will for sure have my attention then. Any takers ? ------------------------------------------------------------------ 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.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu