[SI-LIST] Re: Design rules for estimating crosstalk
- From: Gwen und Wolfgang Maichen <gwmaichen@xxxxxxxxxxxxxx>
- To: andersson.pehr@xxxxxxxxx
- Date: Mon, 23 Apr 2012 16:50:40 +0200
Which equations are you referring to in particular?
The challenge is to get accurate numbers for mutual and self capacitance
and inductance. Only a 2D field solver can give you good numbers for that.
Crosstalk then follows from that through very simple analytical
relationships (which then also correctly predict that FEXT is zero for
symmetric striplines).
Is there a specific reason why you absolutely don't want to use a field
solver? There are some free ones out there that can easily deal with these
calculations (e.g. MMTL/TNT, which I have recommended in the past). For
specific situations you can always fit analytical expressions to these
results. With a bit of care such expressions can be quite general, e.g.
express distance in terms of line widths so you are independent of the
particular trace dimensions. Also most paths in high speed signaling tend
to be 50 Ohm single ended (or 100 Ohm differential), so one less parameter
that you need to vary.
Finally, unless you want to build a coupler with a specific coupling
constant, most of the time the goal is simply to avoid excessive amounts of
crosstak. For that, rules of thumb are widely known, especially if you
assume typical PCB materials (eps_r arounf 2-5, line impedance 50 Ohms):
- NEXT increases for up to one rise time, the stays at a plateau
- FEXT increases for several meters (assuming typical PCB trace dimensions)
- for NEXT < 1% (with coupled lengths larger than half a rise time), trace
separation > 3-4 line widths for striplines and 4-6 line widths for
microstrips.
Wolfgang
On Mon, Apr 23, 2012 at 4:31 PM, Pehr Andersson <andersson.pehr@xxxxxxxxx>wrote:
> Thanks Wolfgang!
> How accurate are the equations that are used for crosstalk analysis and
> that are based on circuit analysis?
> For example with those equations we cannot explain why FEXT = 0 in
> striplines (theoretically)
> What are other alternatives, I have read that modal analysis could be used
> to accurately model crosstalk, but I couldn't find any explanation what it
> is.
>
> Best regards, Pehr
>
>
>
> 2012/4/23 <Wolfgang.Maichen@xxxxxxxxxxxx>
>
> > Hello Pehr,
> >
> > actually crosstalk for striplines decreases exponentially, i.e.
> > exp(-d/const.). This means beyond a certain distance it drops off much
> > faster than the 1/d^4 you assumed.
> >
> > NEXT increases linearly with coupling length for coupled sections with a
> > round trip delay shorter than the signal rise time (this is why in the
> past
> > people assumed ANY crosstalk - NEXT and FEXT - would increase linearly
> with
> > coupled length, because rise times were cery long). Only for lines longer
> > than that there is the plateau in the NEXT.
> >
> > Regards,
> >
> > Wolfgang
> >
> >
>
>
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