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

  • From: "Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>
  • To: "wolfgang.maichen@xxxxxxxxxxxx" <wolfgang.maichen@xxxxxxxxxxxx>
  • Date: Tue, 6 May 2008 17:16:20 -0700

Thanks Wolfang. That helps me concenptually justify what is happening in my
simulation.
I found that reflections are the cause of fast case being worse than slow at
the point where I'm tapping the signal.
I'm still finding it hard to wrap my head around the fact that faster the
rise time, more pronounced the reflection is. How do you explain that
formula ?
Reflection due to parasitics is caused due to discontinuity in the line
impedance. So, everytime a signal hits a parasitic, based on the impedance
of the parasitic, there is a positive or negative reflection and the
amplitude of the reflection is proportional to the discontinuity in the
impedance. Why should this amplitude be any different for different slew
rates ?


Your input is much appreciated.

On 5/6/08, wolfgang.maichen@xxxxxxxxxxxx <wolfgang.maichen@xxxxxxxxxxxx>
wrote:
>
>
> 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
>    To
> si-list@xxxxxxxxxxxxx  cc
>   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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