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

  • From: "Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>
  • To: "wolfgang.maichen@xxxxxxxxxxxx" <wolfgang.maichen@xxxxxxxxxxxx>
  • Date: Wed, 7 May 2008 07:48:57 -0700

Thanks for the elaborate explanation Wolfang.

On 5/6/08, wolfgang.maichen@xxxxxxxxxxxx <wolfgang.maichen@xxxxxxxxxxxx>
> The source of your confusion is that you can get reflection at an
> impedance change, and you can get reflection at a parasitic element. In the
> former case you have constant, homogeneous line impedance Z1 before the
> change, and constant, homogeneous line impedance Z2 after the change, so you
> get a constant sized step back (rho=(Z2-Z1)/(Z2+Z1)), i.e. reflection
> amplitude form this particular discontinuity will not change over time,
> unless the incident signal changes again). On the other hand, a parasitic
> per definition is a localized deviation, i.e. spatial extent of the order of
> your signal rise time or less. In this case you see a "lumped, total
> effect", and the longer your signal rise time, the more this effect gets
> smeared out in time (smaller nut wider). You can picture a parasitic as a
> short section of transmission line with an impedance that is either higher
> (inductive) or lower (capacitive) than the line impedance before and after -
> and with a prop delay shorter than your signal rise time; in this picture
> you actually see TWO reflections (from the discontinuity at the beginning
> and the discontinuity at the end) lumped together, which explains the
> different behavior - if your signal rises very slowly, then both
> discontinuities see almost the same signal level (and same change) at every
> instant, and their responses mostly cancel - thus a long reflection (approx.
> as long as the rise time), but very shallow. On the other hand, a faster
> rise time means the signal level at the second discontinuity is lagging the
> first one quite significantly, and the composite reflection becomes larger -
> but on the other hand, the game is more or less over after the signal rise
> time (plus prop delay through the short section), so the reflection is
> shorter as well.
> Yet another way to explain this is that the parasitc has a frequency
> dependent impedance; e.g. a parasitic capacitance C will have an impedance
> of Zc=1/(2*pi*C*f), thus since Zc varies with frequency, your reflection
> amplitude will vary with frequency. On the other hand, a homogenous,
> lossless  transmission line has an impedance that is independent of
> frequency, hence no influence of the rise time (remember, rise time is
> proportional to 1/frequency).
> Hope that helps
> Wolfgang
>   *"Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>*
> 05/06/2008 05:16 PM
>    To
> "wolfgang.maichen@xxxxxxxxxxxx" <wolfgang.maichen@xxxxxxxxxxxx>  cc
> si-list@xxxxxxxxxxxxx, si-list-bounce@xxxxxxxxxxxxx  Subject
> Re: [SI-LIST] Relation between slew rate and ISI
> 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><
> *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*<preethi.gowtham@xxxxxxxxx>
> *>*
> Sent by: *si-list-bounce@xxxxxxxxxxxxx* <si-list-bounce@xxxxxxxxxxxxx>
> 05/06/2008 03:52 PM
>   To
> *si-list@xxxxxxxxxxxxx* <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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