[SI-LIST] Re: Relation between slew rate and ISI
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: "Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>
- Date: Tue, 6 May 2008 17:30:37 -0700
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
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"wolfgang.maichen@xxxxxxxxxxxx" <wolfgang.maichen@xxxxxxxxxxxx>
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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>
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
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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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