Doug, Yes, we agree that the charge that makes up the transition must be (and is) within a given radius, assuming that we have distributed PDN and not a hypothetical lumped source. My point was that when we have a matched transmission medium, this radius simply does not matter, because looking back from the load we have a frequency independent resistive source impedance. This means charge will move towards the load as needed, and the disturbance is I(t)*R. Regards, Istvan Novak SUN Microsystems Doug Brooks wrote: > Istvan, > > With all due respect, I would modify your argument a little bit. In very > simplistic terms, suppose we need x amount of charge to transition from a > zero to a one in one ns. That amount of charge (I suggest) must be within > 6 inches of the need (what I think we are referring to as the service > radius). If not, it takes a little longer to reach the logical one state. > I look at it, not from the standpoint of a dip in the rail, as much as the > ability to satisfy the rise time requirement (unless you are referring to > a dip in the rail that occurs during the rise time itself.) In the > slightly longer term, the charge will replenish fairly quickly, but not, > perhaps fast enough to meet the rise time requirement. > > Doug Brooks > > > > >> Andreas, >> >> Yes and no. It is true that charge moves with finite speed, so for any >> given time >> duration the charge has to come from locations closer than the ratio of >> distance >> over speed. BUT the whole notion of service radius is based on the >> assumption >> that as you deplete the charge available in the immediate vicinity of >> the active >> device, you have to wait for replenishment, otherwise you get a big dip >> on the >> supply rail. >> >> Having a matched >> transmission medium to deliver power to the active device, the charge >> moves >> without interruption, and as you deplete the planes close to the device, >> it gets >> replenished on the fly from areas further away, so the service area >> concept is >> pretty much meaningless in this scenario. Current flows without >> interruption. >> The bucket brigade of infinitesimally small inductive and capacitive >> elements >> of the transmission line transmits the power continuously. If the load >> current >> changes, for any I(t) time function of load current, the transient noise >> at the load >> point will be I(t)*Zo, where we assume that Zo is the resistive and >> frequency >> independent characteristic impedance of the transmission medium. This is >> a >> very simplistic one-dimensional model, but it gives a good insight of >> why the >> service radius matters only on PDNs where the network is not matched. >> >> Regards, >> >> Istvan Novak >> SUN Microsystems >> >> >> >> >> >> Andreas.Lenkisch@xxxxxxxxxx wrote: >> >>> Istvan, >>> I'm wodering a little about your comments to the service radius. >>> Independant if the impedance is resistive, we have still a propagation >>> time which would limit the service radius from my understanding. >>> Do I'm wrong? >>> >>> regards >>> Andreas >>> >>> >>> >>> Istvan Novak <istvan.novak@xxxxxxxxxxx> >>> Gesendet von: si-list-bounce@xxxxxxxxxxxxx >>> 11.03.2008 13:14 >>> >>> An >>> Joel Brown <joel@xxxxxxxxxx> >>> Kopie >>> si-list@xxxxxxxxxxxxx >>> Thema >>> [SI-LIST] Re: Questions about interplane capacitance >>> >>> >>> >>> >>> >>> >>> Joel, >>> >>> Just one quick comments to the good summary from Steve: >>> >>> While considering planes and bypass capacitors in terms of effective >>> capacitances and inductances is a >>> valid approach, we need to keep in mind that focusing on the capacitive >>> or inductive nature of parts >>> without looking at the wider picture misses a very important and useful >>> class of solutions, namely that >>> of matched transmission lines. As it was pointed out earlier several >>> times on the SI list, the best >>> (self) impedance for a power distribution network is a resistive one, >>> neither capacitive, nor inductive. >>> We can get resistive impedance from a matched transmission line, >>> regardless of its capacitance and >>> inductance, and in such cases the notion of 'service area' of parts >>> become meaningless: you can put >>> bypass components further away from the active devices without >>> sacrificing performance. >>> >>> Regards, >>> >>> Istvan Novak >>> SUN Microsystems >>> >>> Joel Brown wrote: >>> >>> >>>> Interplane capacitance is frequently cited as the only effective bypass >>>> capacitance on a PCB at frequencies above 200 MHz. >>>> I am currently working on a design which brings up some questions >>>> >>>> >>> regarding >>> >>> >>>> interplane capacitance. >>>> >>>> 1. Power planes normally carry "standard" voltage rails that are used >>>> throughout a board such as +5V and +3.3V. >>>> High speed ICs usually have core voltages that are local to the IC and >>>> >>>> >>> are >>> >>> >>>> provided by a local regulator which converts the standard rail to the >>>> >>>> >>> core >>> >>> >>>> voltage (example 3.3 to 1.8V). >>>> The local core voltage is distributed on a plane area that is local to >>>> >>>> >>> the >>> >>> >>>> IC and therefore is small in area (0.25 sq in or less) which results in >>>> >>>> >>> a >>> >>> >>>> very small amount of interplane capacitance. >>>> Is this very small amount of capicitance effective for bypassing the >>>> IC? >>>> >>>> >>> I >>> >>> >>>> am sure it depends somewhat on the current waveform being drawn by the >>>> >>>> >>> IC >>> >>> >>>> but this can only be estimated because semiconductor manufacturers do >>>> >>>> >>> not >>> >>> >>>> provide current consumption profile as a function of frequency. To make >>>> matters worse, some ICs have several different VCC pins which the >>>> manufacturer recommends connecting to separate networks of bypass caps >>>> >>>> >>> and >>> >>> >>>> ferrite beads. This cuts the power distributuion up even more resulting >>>> >>>> >>> in >>> >>> >>>> less (practically zero) interplane capacitance. It is somewhat ironic >>>> >>>> >>> that >>> >>> >>>> the the voltages such as +5V and +3.3V which are required at points >>>> >>>> >>> across >>> >>> >>>> the whole board and therefore have the most interplane capacitance are >>>> >>>> >>> also >>> >>> >>>> the voltages which have least requirement for interplane capacitance >>>> >>>> >>> because >>> >>> >>>> they do not directly supply high speed rails. >>>> >>>> 2. There has been a lot of emphasis on reducing the mounted inductance >>>> >>>> >>> of >>> >>> >>>> bypass capacitors. Even with this reduced inductance they are still >>>> only >>>> effective up to several hundereds of MHz at which point the interplane >>>> capacitance becomes the only bypass capacitance mechanism. However >>>> there >>>> >>>> >>> is >>> >>> >>>> inductance between the connection of the IC to the planes. This >>>> >>>> >>> inductance >>> >>> >>>> consists of vias and package inductance. I did look for some numbers >>>> for >>>> package inductance and did not find much, it seems to be a closely held >>>> secret. Also it is unknown how much bypass capacitnace is internal to >>>> >>>> >>> the IC >>> >>> >>>> package. Just for example if we assume 250pH for the vias and 500 pH >>>> for >>>> >>>> >>> the >>> >>> >>>> package, then the impedance at 500 MHz would be 2.36 Ohms. This seems >>>> >>>> >>> rather >>> >>> >>>> high for the interplane capacitance to be of much benefit. >>>> >>>> In summary how much interplane capacitance is needed to be beneficial, >>>> >>>> >>> and >>> >>> >>>> why is it beneficial given the inductance in the vias and package? >>>> >>>> Thanks - Joel >>>> >>>> >>>> >>>> ------------------------------------------------------------------ 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 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