# Re: [SI-LIST] : Differential impedance

Danwei Xue ([email protected])
Thu, 12 Nov 1998 10:17:57 -0800

I read Doug's article, one thing not clear is why Z11 = Z0? I don't think they are equal.

Z0 is the impedance of the single transmission line, while Z11 is the impedance of one of the lines of the two
coupled transmission lines. Z11 should be definted as the impedance with respect to BOTH ground and the
other line. Do you think Z0 will not change with another line near it? No, definitely not!

Therefore, I believe they are not equal.

Danwei

Doug Brooks wrote:

> I wrote a column that (I hope) explained Differential Impedance
> in plain english. It was in the August issue of PC Design Magazine,
> page 34. A copy of the article is available on our web page at
>
> Doug Brooks
>
> At 07:04 PM 11/11/98 -0500, you wrote:
> >To calculate the differential impedance of a transmission line pair, I've
> >always used
> >1) Zdiff = 2 * Zoo, where Zoo is the odd mode impedance of the pair. This
> >formula is used by the pc board manufacturers.
> >I was speaking to someone last week who has 30+ years in the industry and
> >said this formula is wrong, that the real differential impedance should be
> >calculated as
> >2) Zdiff = square root of (Zoo * Zee), where Zee is the even mode
> >impedance.
> >
> >If I compare 1) to 2), I get quite different values of differential
> >impedance. Does anyone have comments on this?
> >
> >Thanks, Fabrizio Zanella
> >EMC corporation
> >[email protected]
> >
> >
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> .
> ****************************************************
> Doug Brooks, President [email protected]