[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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