[SI-LIST] Re: Lower epsilon achieves lower crosstalk

  • From: Young <long.0.yang@xxxxxxxxx>
  • To: Stefan Milnor <stefan.milnor@xxxxxxxxxxxxxx>
  • Date: Fri, 12 Nov 2010 14:43:37 +0800

Cited from Eric Bogatin:
"Stripline traces will have very little far-end crosstalk, while microstrip
can have enough to easily exceed noise budgets.
This is a real issue in the decision process. If far-end noise is a problem,
don't use microstrip. But when the trace-to-trace
spacing is less than twice the line width, stripline actually has more
near-end noise than microstrip. For spacings larger
 than twice the line width, stripline has lower near-end noise."

On Fri, Nov 12, 2010 at 14:31, Stefan Milnor
<stefan.milnor@xxxxxxxxxxxxxx>wrote:

>  Other Experts -
>
> Is this correct (that there is no FEXT for the stripline case)?
>
> Does this make stripline routing "better" than microstrip?
>
> ------------------------------
> *From:* si-list-bounce@xxxxxxxxxxxxx on behalf of ZHENGGANG CHENG
> *Sent:* Thu 11/11/2010 5:18 PM
> *To:* Young
> *Cc:* Liu Xike; si-list@xxxxxxxxxxxxx
> *Subject:* [SI-LIST] Re: Lower epsilon achieves lower crosstalk
>
>  Due to Cm/C-Lm/L =0, meaning negative inductive coupling canceled with
> positive coupling at far end.
> Zhenggang
>
> On Thu, Nov 11, 2010 at 5:10 PM, Young <long.0.yang@xxxxxxxxx> wrote:
>
> > Zhenggang
> >        Could you tell us why there is no FEXT in the case of stripline?
> >
> >
> >
> > On Fri, Nov 12, 2010 at 03:25, ZHENGGANG CHENG <
> zhenggang.cheng@xxxxxxxxx>wrote:
> >
> >> Hi Liu,
> >> Xtalk analysis is very different for microstrip and stripline cases, and
> >> it
> >> also depends what xtalk you are talking about such as FEXT or NEXT.
> >>
> >> For stripline, there is no FEXT, and NEXT proportional to Cm/C. So in
> this
> >> case, lower Er, to keep the same impedance, H will be decreased, so Cm/C
> >> will be smaller.
> >>
> >> For microstrip case, FEXT is proportional to Cm/C-Lm/L, and NEXT is
> >> proportional to Cm/C+Lm/L. In your case, Lm/L is becoming larger (Lm is
> >> not
> >> changing, L is decreased due to smaller H), Cm/C may be smaller, then
> FEXT
> >> will become larger due to lower Er.
> >>
> >> Zhenggang CHENG
> >>
> >>
> >> On Wed, Nov 10, 2010 at 10:46 PM, Liu Xike <xike.silist@xxxxxxxxx>
> wrote:
> >>
> >> > We could achieve lower crosstalk by using dielectric with lower
> >> > permittivity, while keeping *Z0* as a constant.
> >> >
> >> >
> >> > I'm new in SI, my understanding about this question is:
> >> >
> >> >
> >> >
> >> > If we use dielectric with lower permittivity while keeping *Z0* as a
> >> > constant.
> >> >
> >> > We need to reduce *h *to maintain a constant *C*, because *C*>
> >> *epsilon***w*
> >> > /*h*, and *Z0*=1/*vp*/*C*.
> >> >
> >> >
> >> >
> >> > Since *Z0* = sqrt(*L/C*), to keep* Z0* as a constant, the stripe line
> >> need
> >> > to adjust it?s current distribution to maintain a constant *L*.
> >> >
> >> >
> >> >
> >> > Considering the loop area decrease because *h* is reduced, the current
> >> > distribution should be more tighter to maintain a constant *L*.
> >> >
> >> >
> >> >
> >> > Finally the tighter current distribution on the returning pass results
> >> in
> >> > the reduction of the cross talk.
> >> >
> >> >
> >> >
> >> > Are these correct? I will appreciate your help
> >> >
> >> >
> >> >
> >> > Best Regards
> >> >
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