[SI-LIST] Re: Question on varying the coupling ratio on a differential pair
- From: Bill Wurst <billw@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 15 Jan 2007 20:23:26 -0500
Wayne,
Even if the differential impedance is 100 Ohms from the source to the
final load and the signal is purely differential (no common-mode
component, originating from either the source or skew within the pair),
there will be reflections. The question is, how significant are they.
To better understand what is happening, you might refer to Dr. Johnson's
second book, High-Speed Signal Propagation - Advanced Black Magic, pp.
417-8, where he describes "the differential U-turn." The current loop
created in the "U-turn zone" has a magnetic field associated with it
which can cause EMI, coupling into nearby conductors, and create an
inductive discontinuity.
If your transition times are slow enough relative to the electrical size
of the "U-turn zone", you won't be able to "see" a reflection.
Regards,
-Bill
/************************************
/ billw@xxxxxxxxxxx /
/ /
/ Advanced Electronic Concepts, LLC /
/ www.aec-lab.com /
************************************
================================================================
cookeway wrote:
> Hi all,
>
> I have a question concerning differential pair coupling ratios that
> is causing some debate here in our design group. If a differential
> pair with an impedance of 100 ohms connects through a backplane
> connector and into a backplane such that the impedance is
> consistently 100 ohms, but the coupling ratio of the return currents
> on the adjacent planes versus the return current using the compliment
> differential signal changes, will there be a reflection on the signal?
>
> For example, if you have a strongly coupled differential pair of 100
> ohms on your motherboard which goes through a connector to connect to
> a loosely coupled 100 ohm differential pair on the backplane, is
> there a reflection? My gut feeling is that the reflections caused by
> the different coupling ratios would cancel each other out causing no
> net reflection. Am I right in thinking this?
>
> Thanks,
>
> Wayne Cooke
> Hardware Engineer, Zeugma Systems Inc.
> Suite 250, 13571 Commerce Parkway, Richmond, BC, Canada, V6V 2R2
> Tel: (604) 247-2828
> Fax: (604) 247-3251
> wcooke@xxxxxxxxxxxxxxxxx
>
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