[SI-LIST] Re: Question on varying the coupling ratio on a differential pair

  • From: Doug <Brooks@xxxxxxxxxxxxxxxxx>
  • To: wcooke@xxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Wed, 17 Jan 2007 09:15:05 -0800

The coupling ratio is an integral (implicit, if not explicit) part of 
the impedance calculation. The more closely coupled the traces are, 
the lower the differential impedance. Therefore, 100 Ohms 
differential impedance is 100 Ohms differential impedance. If the 
coupling ratio changes, something else must also have changed to keep 
the differential impedance constant.

Assuming constant differential impedance, changes in the coupling 
ratio would not cause a reflection. Only impedance discontinuities 
cause reflections.

Doug Brooks


At 02:54 PM 1/15/2007, you 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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