[SI-LIST] Re: Relation between slew rate and ISI

  • From: wolfgang.maichen@xxxxxxxxxxxx
  • To: "Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>
  • Date: Wed, 7 May 2008 16:42:15 -0700

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
> / / / / / / / / / / / / / / / / / / / / / / / / /
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> -----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.
>
>
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