[SI-LIST] Re: Relation between slew rate and ISI
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: "Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>, <wolfgang.maichen@xxxxxxxxxxxx>
- Date: Thu, 8 May 2008 08:51:19 -0500
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 <http://www.logicpd.com/>
/ / / / / / / / / / / / / / / / / / / / / / / / /
This message (including any attachments) contains confidential
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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 <http://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.
>
>
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