A good reply. Has a good technical basis, not opinion. Lee Ritchey leeritchey@xxxxxxxxxxxxx Why Wait? Move to EarthLink. > [Original Message] > From: Larry Smith <Larry.Smith@xxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Date: 8/4/2003 9:10:14 AM > Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator > > Robert - Let me address the subject of capacitor placement with a quick > war story. Several years ago, we were having an EMI problem at about > 900 MHz. We determined the capacitor value that made a low impedance > at that frequency while mounted on our power planes, I think it was > about 27pF or so. We placed several of them on the PCB just a few > inches away from our microprocessor in the decoupling pad sites that > were available. The result was that EMI at 900 MHz got worse! Some > simple calculations showed why. We had placed relatively low impedance > capacitors 1/4 wavelength from the source. Because of transmission > line phenomenon, a low impedance becomes a high impedance 1/4 > wavelength away. This was in the very early days of power plane > modeling. Our primitive power plane models predicted this effect quite > nicely. That convinced us that we had to use PDS tools to place caps > that resonate at frequencies that translate to wavelengths that are > comparable to the PCB size. > > That was back in the days when we used power and ground planes spaced > about 14 mils apart with two signal layers in-between. Since then, we > have learned the value of thin power plane dielectrics and always place > power and ground planes next to each other in the stackup, somewhere > between 2 and 4 mils apart. This has a number of advantages including > increased capacitance between planes, lower spreading inductance, lower > power plane impedance, increased noise damping and better return path > for high frequency signals traveling in reference to those planes. > Several papers have been written from authors at Sun which fully > describe this. Many EMI problems have been solved by thin power plane > dielectrics. > > With a good power plane stackup, it may not be necessary to use many > (or any..) capacitors below 1000pF. As Ray mentioned below, the power > integrity problem is usually limited to less than 100 MHz by the chip > package inductance. If it is still necessary to use low valued > capacitors for the signal integrity (return current) or EMI problems, > then position on the power plane does become an important issue. > > regards, > Larry Smith > Sun Microsystems > > > Delivered-To: si-list@xxxxxxxxxxxxx > > From: "Robert Sefton" <rsefton@xxxxxxxxxxxxx> > > To: <si-list@xxxxxxxxxxxxx> > > Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator > > Date: Sun, 3 Aug 2003 10:27:54 -0700 > > MIME-Version: 1.0 > > Content-Transfer-Encoding: 8bit > > X-Priority: 3 > > X-MSMail-Priority: Normal > > X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165 > > X-archive-position: 7707 > > X-ecartis-version: Ecartis v1.0.0 > > X-original-sender: rsefton@xxxxxxxxxxxxx > > X-list: si-list > > > > Ray - > > > > 5.5" is a pretty loose requirement. That seems consistent with > > Lee's (and UMR's) statement that "location of the capacitors is > > relatively unimportant" for decoupling. But what about EMI? Does > > placing the decaps closer help with EMI, even if it doesn't > > improve the PDS performance? Doesn't seem like it would. > > > > Thanks, > > Robert > > > > ----- Original Message ----- > > From: "Ray Anderson" <Raymond.Anderson@xxxxxxx> > > To: <si-list@xxxxxxxxxxxxx> > > Sent: Friday, August 01, 2003 12:50 PM > > Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator > > > > > > > With all due respect to the UMR authors, I think the location > > > of a decap on a set of power planes DOES matter. > > > > > > There is an inherent time delay associated with the current flow > > > from the location where a decap is placed to the location where > > the > > > chip to bypassed is located. > > > > > > The decaps job is to provide current to the load quickly enough > > > that a voltage collapse on the planes doesn't occur. If the > > decap > > > is too far away from the current consuming load it is possible > > > that load's need for a pulse of current will be over before the > > > current can get there. > > > > > > At low frequencies you can place a decap just about anywhere on > > a plane > > > and it will function just fine. At higher frequencies the decap > > must > > > be closer to the device to be bypassed. The concept of > > "effective bypassing > > > radius" says that a decap must be within say 1/6 to 1/10 > > wavelength > > > at the frequency of interest to be effective. (the exact number > > is > > > debatable). The frequency of interest is the resonant frequency > > of > > > the mounted decap (where the caps ESR is lowest), and the > > distance is > > > a function of the speed of propagation with a given dielectric > > between > > > the planes (around 180ps/inch for FR4). > > > > > > You need to consider the role each component of a PDS (power > > distribution > > > system) plays in maintaining a low impedance across a wide > > bandwidth. > > > A typical PDS is composed of a VRM, bulk capacitors, ceramic > > decaps, and > > > the power planes and last but not least the package housing the > > silicon. > > > > > > The VRM provides a low impedance from DC to perhaps a few > > hundred kHz, > > > the bulk capacitors contribute their impedance profile from a > > few > > > hundred kHz up to about a MHz. From a Mhz to maybe 120MHz or so > > the > > > ceramic decoupling caps provide low impedance. Above that > > frequency > > > the distributed capacitance provided by the planes provides the > > requisite > > > low impedance to the PDS. When the individual impedance profiles > > of > > > each of the constituent parts are superimposed, the resulting > > composite > > > profile provides a low impedance over the full frequency range > > that the > > > PDS was designed for. Note that above the package resonance > > frequency > > > (which usually is from about 50 to 100 MHz for typical packages) > > decoupling > > > on the board is ineffective and the necessary decoupling must be > > provided > > > for either in the package or on the silicon. However, at > > frequencies > > > above the package resonance frequency the discrete decaps and > > distributed > > > plane capacitance can be most useful for EMI purposes. > > > > > > To summarize, at low frequencies, the decoupling capacitors can > > be placed > > > almost anywhere withing reason and will be effective. As the > > frequency > > > of interest increases, the allowable distance decreases. As an > > example, > > > at 1 MHz the wavelength (on FR4) is around 214 meters. So > > lambda/10 is > > > around 21 meters. Hence, if your decap which is effective at 1 > > MHz is > > > within 21 meters of the consuming device, then the decoupling > > will be > > > effective. Conversely, if you are trying to decouple at device > > with > > > a decap that is effective at say 100 MHz then the wavelength of > > interest > > > is about 1.4 meters. Lambda/10 is then .14 meters (approx 5.5 > > inches). > > > In this case if the decap is appreciably more than 5.5 inches > > from the > > > currewnt consumer then the delay will be such that the decap > > will not be able to > > > provide current quickly enough to be effective. > > > > > > -Ray Anderson > > > Sun Microsystems Inc. > > > > > > > > > Lee Ritchey wrote: > > > >> If you remember the UMR paper on power bus decoupling, it > > made a clear case > > > >> that the location of the capacitors is relatively > > unimportant. > > > > > > > > > ------------------------------------------------------------------ > > 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 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 > > > > > > ------------------------------------------------------------------ > 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 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 > ------------------------------------------------------------------ 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 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