FreeLists / si-list / [SI-LIST] Re: Relation between slew rate and ISI

[wolfgang.maichen@xxxxxxxxxxxx]

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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[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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