I don't deny what you guys are saying. Wolfang, my simulations are totally in sync with what you are saying. Like I said, these simulation are to prove that we have to let go off this technology of probing at any place other than at the receiver. This is the first time I posted on this site and I must say I'm totally impressed by the number of responses. Thanks guys ! Preethi On 5/8/08, Curt McNamara <CurtM@xxxxxxxxxxx> wrote: > > In your case this may not be a good plan. If the signal edge were perfect > and the logic analyzer thresholds set exactly to your receiver it could be > OK. However we have been talking about reflections which cause edge > discontinuities. These could overlap either switching threshold or occur in > the transition region between the defined Vih and Vil. Any of these > situations could cause sampling errors. > > > > And as Wolgang has noted, the signal may be good at the receiver yet look > bad in the line, or be bad at the receiver yet look good in the line. > > > > > Curt > > > > Curt McNamara, P.E. // principal electrical engineer > Logic Product Development > 411 Washington Ave. N. Suite 400 > Minneapolis, MN 55401 > T // 612.436.5178 > F // 612.672.9489 > www.logicpd.com > / / / / / / / / / / / / / / / / / / / / / / / / / > This message (including any attachments) contains confidential information > intended for a specific individual and purpose, and is protected by law. > If you are not the intended recipient, you should delete this message and > are hereby notified that any disclosure, copying, or distribution of this > message, or the taking of any action based on it, is strictly prohibited. > > > > > > > > *From:* Preethi Ramaswamy [mailto:preethi.gowtham@xxxxxxxxx] > *Sent:* Wednesday, May 07, 2008 7:34 PM > *To:* wolfgang.maichen@xxxxxxxxxxxx > *Cc:* Curt McNamara; si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx > *Subject:* Re: [SI-LIST] Re: Relation between slew rate and ISI > > > > Totally agree. From these simulations trying to prove that at higher speeds > this kind of probing is not viable. Just need to clarify that, this is not > scope probing, this is logic analyzer probing. So I really don't care much > about how the signal looks. All I care is if the bit is sampled correctly as > a 1 or a 0. > > On 5/7/08, *wolfgang.maichen@xxxxxxxxxxxx* <wolfgang.maichen@xxxxxxxxxxxx> > wrote: > > > Probing in the middle of the line can have its pitfalls when trying to make > sense of the observed waveform (and relating those to the "true" waveforms > seeing at the receive)r. One such issue arises when the receiver does not > provide perfectly matched termination (i.e. receiver impedance different > from line impedance, but also - which is always the case - when the receiver > has some input capacitance). In this case - if your probe is further away > from the receiver than a small fraction of a rise time - you will observe > two distict partial partial transitions, while at the receiver you would see > both lumped together at the same time (i.e. only a single transition). > > Extreme case as an example (easy to simulate e.g. in PSpice): 50 Ohm driver > driving a 50 Ohm transmission line with a 1V step, receiver is completely > unterminated (high impedance). The driver is thus launching a 500mV step > into the line (voltage divider - 50 Ohm driver + 50 Ohm transmission line). > The receiver will see a full 1V step (500mV incident + 500mV reflected), > but a probe looking at the middle of the line will see a 500mV step, and > then (one round-trip delay to the receiver and back later) as second 500mV > step on top of it. > > Trouble is, if you are "somewhat" close to the receiver those two > transitions may partially overlap and you may think the waveform fidelity is > poor, when in reality all is fine at the receiver end. > > Wolfgang > > > > > *"Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>* > Sent by: si-list-bounce@xxxxxxxxxxxxx > > 05/07/2008 04:12 PM > > To > > "Curt McNamara" <CurtM@xxxxxxxxxxx> > > cc > > si-list@xxxxxxxxxxxxx > > Subject > > [SI-LIST] Re: Relation between slew rate and ISI > > > > > > > Hi Curt > For my application, I'm trying to tap the signal to feed to a probe. > The best point to place a probe is at the receiver but it isn't always > possible. > Hence, I'm trying out a few options, one of them being tapping the signal > before the receiver. > Preethi > > On 5/7/08, Curt McNamara <CurtM@xxxxxxxxxxx> wrote: > > > > This may have been covered already by Wolfgang's excellent responses: > > The wave arrives at the load and any reflected energy is almost no prop > > delay away. > > In contrast, at other points on the line the reflected energy from the > > load (or other discontinuity) arrives 2 x tpd later (where tpd is the > delay > > from the measurement point to the reflection). > > > > This is one reason we may not care about intermediate points on the line: > > at the load we need to see signal quality and establish that there are no > > voltage levels beyond our limits. At the source signal quality may not be > as > > critical, but checking voltage levels is important. > > > > Why would we care about intermediate points? They could affect crosstalk > > or emissions. Any other perspectives on this? > > > > Curt > > > > > > Curt McNamara, P.E. // principal electrical engineer > > Logic Product Development > > 411 Washington Ave. N. Suite 400 > > Minneapolis, MN 55401 > > T // 612.436.5178 > > F // 612.672.9489 > > www.logicpd.com > > / / / / / / / / / / / / / / / / / / / / / / / / / > > This message (including any attachments) contains confidential > information > > intended for a specific individual and purpose, and is protected by law. > If > > you are not the intended recipient, you should delete this message and > are > > hereby notified that any disclosure, copying, or distribution of this > > message, or the taking of any action based on it, is strictly prohibited. > > > > > > > > -----Original Message----- > > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > > On Behalf Of Preethi Ramaswamy > > Sent: Tuesday, May 06, 2008 5:52 PM > > To: si-list@xxxxxxxxxxxxx > > 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 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 > > > > > > > > > ------------------------------------------------------------------ > 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 > > > > > ------------------------------------------------------------------ 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