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

• From: Young <long.0.yang@xxxxxxxxx>
• To: ZHENGGANG CHENG <zhenggang.cheng@xxxxxxxxx>
• Date: Fri, 12 Nov 2010 09:10:48 +0800

```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.
> >
> >
> >
> >
> >
> > 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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