Thanks for the elaborate explanation Wolfang. Regards Preethi On 5/6/08, wolfgang.maichen@xxxxxxxxxxxx <wolfgang.maichen@xxxxxxxxxxxx> wrote: > > > The source of your confusion is that you can get reflection at an > impedance change, and you can get reflection at a parasitic element. In the > former case you have constant, homogeneous line impedance Z1 before the > change, and constant, homogeneous line impedance Z2 after the change, so you > get a constant sized step back (rho=(Z2-Z1)/(Z2+Z1)), i.e. reflection > amplitude form this particular discontinuity will not change over time, > unless the incident signal changes again). On the other hand, a parasitic > per definition is a localized deviation, i.e. spatial extent of the order of > your signal rise time or less. In this case you see a "lumped, total > effect", and the longer your signal rise time, the more this effect gets > smeared out in time (smaller nut wider). You can picture a parasitic as a > short section of transmission line with an impedance that is either higher > (inductive) or lower (capacitive) than the line impedance before and after - > and with a prop delay shorter than your signal rise time; in this picture > you actually see TWO reflections (from the discontinuity at the beginning > and the discontinuity at the end) lumped together, which explains the > different behavior - if your signal rises very slowly, then both > discontinuities see almost the same signal level (and same change) at every > instant, and their responses mostly cancel - thus a long reflection (approx. > as long as the rise time), but very shallow. On the other hand, a faster > rise time means the signal level at the second discontinuity is lagging the > first one quite significantly, and the composite reflection becomes larger - > but on the other hand, the game is more or less over after the signal rise > time (plus prop delay through the short section), so the reflection is > shorter as well. > > Yet another way to explain this is that the parasitc has a frequency > dependent impedance; e.g. a parasitic capacitance C will have an impedance > of Zc=1/(2*pi*C*f), thus since Zc varies with frequency, your reflection > amplitude will vary with frequency. On the other hand, a homogenous, > lossless transmission line has an impedance that is independent of > frequency, hence no influence of the rise time (remember, rise time is > proportional to 1/frequency). > > Hope that helps > > Wolfgang > > > > > > > *"Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>* > > 05/06/2008 05:16 PM > To > "wolfgang.maichen@xxxxxxxxxxxx" <wolfgang.maichen@xxxxxxxxxxxx> cc > si-list@xxxxxxxxxxxxx, si-list-bounce@xxxxxxxxxxxxx Subject > Re: [SI-LIST] Relation between slew rate and ISI > > > > > Thanks Wolfang. That helps me concenptually justify what is happening in > my simulation. > I found that reflections are the cause of fast case being worse than slow > at the point where I'm tapping the signal. > > I'm still finding it hard to wrap my head around the fact that faster the > rise time, more pronounced the reflection is. How do you explain that > formula ? > Reflection due to parasitics is caused due to discontinuity in the line > impedance. So, everytime a signal hits a parasitic, based on the impedance > of the parasitic, there is a positive or negative reflection and the > amplitude of the reflection is proportional to the discontinuity in the > impedance. Why should this amplitude be any different for different slew > rates ? > > > Your input is much appreciated. > > On 5/6/08, *wolfgang.maichen@xxxxxxxxxxxx* <wolfgang.maichen@xxxxxxxxxxxx>< > *wolfgang.maichen@xxxxxxxxxxxx* <wolfgang.maichen@xxxxxxxxxxxx>> wrote: > > What your worst case will be depends on a variety of factors. > > If your main limitation is the drive signal rise time, then yes, slower > rise time will degrade ISI as well as your vertical eye opening. Same goes > if your path is dominated by skin effect and/or dielectric loss. > > On the other hand, if you have capacitive or inductive parasitics in the > path, they will cause reflections, which will get more pronounced (larger > amplitude but shorter duration) with faster signal rise times. As a rough > rule of thumb, > > reflection = (approx) Tc/Tr x 100% > > where Tc is the time constant of the parasitic (Zo x C / 2 for capacitive, > 2 x Zo x L for inductive), and Tr is the signal rise time (assuming linearly > rising edge from 0% to 100%). A slower rise time will smear out the > reflection in time, so it's longer but not as high. If the reflection > happens to come back at the middle of the data eye then it will reduce your > vertical eye opening. On the other hand, even in this case rise time has > only second order influence on data dependent jitter (ISI) - shorter rise > times create larger reflections, but at the same time shorter rise times > reduce the timing hit of a given size reflection, so in first order > approximation that is a wash. > > Wolfgang > > > > > > > *"Preethi Ramaswamy" > <**preethi.gowtham@xxxxxxxxx*<preethi.gowtham@xxxxxxxxx> > *>* > Sent by: *si-list-bounce@xxxxxxxxxxxxx* <si-list-bounce@xxxxxxxxxxxxx> > > 05/06/2008 03:52 PM > > To > *si-list@xxxxxxxxxxxxx* <si-list@xxxxxxxxxxxxx> cc > Subject > [SI-LIST] Relation between slew rate and ISI > > > > > > I'm looking for some information on the relation between slew rate and ISI > effect on high speed memory data signals. > From my SI simulation I'm observing that a signal looks much worse with a > higher slew rate than a lower slew rate. But the point I'm tapping is not > at > the receiver but a point before the receiver. I don't expect the signal to > look as good as at the receiver but I was hoping that the trends match. At > the receiver itself, the fast corner signal looks better than the slow > corner signal. The bus is properly terminated. > Looking at the waveform, I see that in the fast corner case, whenever > there > is a 1010 pattern, the signal is not reaching its intended Vhigh and Vlow > level. A similar thing happens at the slow corner but the signal swing is > much better. Hence, the eye diagram in the fast corner is almost closed > whereas the slow corner looks open. > > While searching some artciles on this, I found the opposite stated. It > said > that ISI effects are more pronounced in the slow corner case and Crosstalk > is more pronounced in the fast corner case. > > Any experience or insight into this topic would be very helpful. > > > ------------------------------------------------------------------ > To unsubscribe from si-list:* > **si-list-request@xxxxxxxxxxxxx* <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*<//www.freelists.org/webpage/si-list> > > For help:* > **si-list-request@xxxxxxxxxxxxx* <si-list-request@xxxxxxxxxxxxx> with > 'help' in the Subject field > > > List technical documents are available at: > *http://www.si-list.net* <http://www.si-list.net/> > > List archives are viewable at: > *//www.freelists.org/archives/si-list > * <//www.freelists.org/archives/si-list> > or at our remote archives: > * > http://groups.yahoo.com/group/si-list/messages*<http://groups.yahoo.com/group/si-list/messages> > Old (prior to June 6, 2001) list archives are viewable at: > > *http://www.qsl.net/wb6tpu*<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 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