[SI-LIST] Re: Question on varying the coupling ratio on a differential pair
- From: dgun@xxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 26 Jan 2007 11:30:53 -0800
See below link for a paper that discusses the overall impedance of a
cable and how
it relates to the impedance along the length of the cable:
http://www.home.agilent.com/upload/cmc_upload/All/E206COMPTEST_METHOD.pdf
May be somewhat applicable to longer PCB traces with fast signals.
--
Daniel
----- Original Message -----
From: ryansatrom
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Question on varying the coupling ratio on a
differential pair
Date: Wed, 17 Jan 2007 17:08:31 -0000
Hassan-
It has always been my understanding that the instantaneous
characteristic impedance (at any point) of a transmission has only a
slight frequency dependency.
As we all know, Zo =3D sqrt(L/C).
Inductance has a frequency dependence only at low frequencies, where
there is an internal inductance caused by the fields internal to the
conductor. But as frequencies increase, skin effect causes all the
fields to travel on the exterior. At this point, the instantaneous
inductance of the transmission line is for the most part constant.
As for capacitance, I've always thought that it was a function of the
size of the conductors and distance between the two conductors. Is
there any frequency dependency to that?
Now, return loss is measured from input to output of any given
structure. Therefore we don't view return loss as "instantaneous".
So when I think of impedance and return loss I see two different
(although very related) values.
Am I right? Is there something I'm missing?
Regards,
Ryan Satrom
Everett Charles Technologies
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