steve, I have found a document from agilent technology which says that if rise time is greater than 6 times of cable propogation delay then It can be considered as lumped load. Does this mean that I can consider 1/6*Tr delay of cable as lumped load for my approximation. I have provided the link of the document also. I am confused with different conclusions. If you have any link which can provide detail of Tx line characteristic and delay approximation , it would be great. http://www.ewh.ieee.org/r5/denver/rockymountainemc/archive/2005/Feb/High_Speed.pdf Thanks sushil steve weir wrote: > Sushil, that capacitance you are trying to model does not exist in a > vacuum w/o incremental inductance that oppose the changing currents > needed to charge the line. > > For purposes of argument, assume that the propagation velocity is > 6"/ns. Every fs of that 600ps rising edge, the wave propagates 0.006 > mils away from the driver, and does so through an incremental > inductance.equivalent to the partial inductance for that bit of the > transmission line length. So, while you increase the capacitance of > the wire by lengthening it, each incremental bit of capacitance is > decoupled more and more from the driver by the inductance along the > way. Let us assume the inductance is about 6.8nH / ". Would it make > anymore sense for you to ask what the equivalent inductance of the > line should be to get the same loaded wave form as your present > question concerning the capacitance? Both effects are definitely > present and interact with each other. Surely if you break the line > into fs long LC stages you can get an accurate representation of the > behavior. You might be unhappy with the solution time for a 500,000 > stage LC network. Perhaps you can approximate with fewer stages and > still get a decent answer. > > Steve > > > > > At 04:55 PM 4/7/2005 +0530, Sushil Kumar GUPTA wrote: > >> Hi Andrew, >> Perhaps I couldn't explain the problem in >> right way. >> Suppose you have a transmission line (cable) which is >> very long let's say 30ns delay. If we assume cable has no loss then >> whatever waveform you apply, you will get FAR-END wave after 30ns delay. >> I am assuming both end are perfectly matched. >> Now if you cansider the NEAR-END rise/fall time (let's assume >> avg value 600ps) , it will not depend on the length of the cable. So my >> question is that waht will be the approximate value of capacitive load >> at the NEAR-END (this time no cable is used) which will provide the same >> rise/fall time achieved with cable. >> I can state the problem in a different way also. Let's assume >> that IO driver has 600ps rise/fall time with 10pf capacitive load. If I >> increase the capacitive load obviously rise/fall time will increase. >> The PCB trace has 2.72pf/inch capacitive load. If now I connect cable( >> PCB trace) with IO driver, it will see 2.72pf/inch capacitve load. The >> driver has 45Ohm dc impedance w.r.t ground. So when driver starts >> charging PCB trace, every inch of trace will be charged, but only that >> part of PCB trace will impact the rise/fall time which is travelled >> during transition time. So how will I know how many inches to be >> considered. >> Andrew Ingraham wrote: >> >> >> I need to calculate equivalent lumped capacitance seen by >> an IO >> >>driver which is connected to PCB trace (approx 3 Inches) and then >> cable. >> >>The average rise/fall time is 600ps. I have information about the PCB >> >>trace capacitance/inches. Will my assumption be correct if I take >> >>PCB-trace length (for lumped-capacitance calculation) which >> provides 300ps >> >>delay. >> >> >> >> >> > >> >For lumped capacitance, just multiply capacitance/length times the >> length. >> > >> >The rise/fall time doesn't make any difference as far as equivalent >> lumped >> >capacitance is concerned. However, if the rise/fall time is fast >> enough, >> >you shouldn't be using a lumped capacitance in simulations. >> > >> >Are you sure your PCB trace delay is only 100ps/inch? >> > >> > >> > >> >>What would be the best way to simulate IO driver as far as >> rise/fall time >> >>accuracy is concerned assuming lumped model for Tx line. >> >> >> >> >> > >> >For accuracy, best way = abandon the lumped model for the line. >> > >> >What did you mean by "best way" anyway? If you have an I/O driver >> model and >> >a transmission line model, just simulate them and look at the results. >> > >> >Regards, >> >Andy >> > >> > >> >------------------------------------------------------------------ >> >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 FAQ wiki page is located at: >> > http://si-list.org/wiki/wiki.pl?Si-List_FAQ >> > >> >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 >> > >> > >> > >> > >> >> >> >> >> ------------------------------------------------------------------ >> 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 FAQ wiki page is located at: >> http://si-list.org/wiki/wiki.pl?Si-List_FAQ >> >> 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 >> > > -- ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ 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