[SI-LIST] Re: Relation between slew rate and ISI
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: "Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>
- Date: Tue, 6 May 2008 16:19:43 -0700
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>
Sent by: si-list-bounce@xxxxxxxxxxxxx
05/06/2008 03:52 PM
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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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