[SI-LIST] Re: Question on varying the coupling ratio on a dif ferential pair
- From: "Clewell, Craig" <Cclewell@xxxxxxxxxxxxx>
- To: "'hassan@xxxxxxxx'" <hassan@xxxxxxxx>
- Date: Wed, 17 Jan 2007 11:45:22 -0500
Ali,
If my understanding is correct I believe that the impedance would be
directly realted to the return loss. If there are differences in the
impedance over frequency you would have larger/smaller reflections, thus
generating a higher/lower return loss. I like to try and make things look
pretty in the time domain first since it is difficult to detect what the
culprit is by just observing the return loss.
Craig
-----Original Message-----
From: Hassan O. Ali [mailto:hassan@xxxxxxxx]
Sent: Wednesday, January 17, 2007 11:29
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Question on varying the coupling ratio on a
differential pair
Just to add to the discussion: I don't remember seeing any high-speed
electrical medium
that presents one fixed value of characteristic impedance for all signal
frequencies
that matter! Why then are people so obsessed with "impedances" as opposed to
maximum
return loss in a bandwidth of interest?
Best regards.
Hassan.
On Jan 17, "Loyer, Jeff" <jeff.loyer@xxxxxxxxx> wrote:
>
> Bill,
> I'd like some clarification. Are you saying that, even though you keep
> the impedance continuous, you will see reflections (for a purely
> differential signal) at points where the coupling coefficient changes?
> I can see a problem where a differential pair passes over a break in the
> return path, but not for a continuous return path.=20
>
> Even then, I wonder - if you maintained the same impedance by some
> magic... Say you went from a pcb to a twisted pair and back to a
> twisted pair, all with the same differential impedance. Would that ever
> inherently cause a problem? =20
>
> Regarding the original question, I think that it's usually impossible to
> ensure the same coupling coefficient from driver to receiver, and don't
> think it's necessary. I don't remember a design guide that specifies
> both odd and even impedances (or differential impedance plus coupling
> coefficient), though they may specify a "reference" stackup (but not for
> the purpose of achieving some magical coupling coefficient).
>
> Connectors (and other sources of return current discontinuities, such as
> vias, solder balls, or packages) represent challenges in maintaining
> return paths, but I don't know of a reason why the coupling coefficients
> would be maintained on either side of those discontinuities. The
> connectors (or vias, etc.) already represent such a tremendous change in
> impedance and/or coupling coefficient that trying to reduce common mode
> reflections by maintaining the coupling coefficient before and after
> seems pointless. =20
>
> As always, perhaps I've missed something - I'm open to further
> enlightenment.
>
> Jeff Loyer
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Bill Wurst
> Sent: Monday, January 15, 2007 5:23 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Question on varying the coupling ratio on a
> differential pair
>
> Wayne,
>
> Even if the differential impedance is 100 Ohms from the source to the=20
> final load and the signal is purely differential (no common-mode=20
> component, originating from either the source or skew within the pair),=20
> 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.=20
> 417-8, where he describes "the differential U-turn." The current loop=20
> created in the "U-turn zone" has a magnetic field associated with it=20
> which can cause EMI, coupling into nearby conductors, and create an=20
> 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 /
> ************************************
>
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
>
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> cookeway wrote:
> > Hi all,
> >=20
> > I have a question concerning differential pair coupling ratios that=20
> > is causing some debate here in our design group. If a differential=20
> > pair with an impedance of 100 ohms connects through a backplane=20
> > connector and into a backplane such that the impedance is=20
> > consistently 100 ohms, but the coupling ratio of the return currents=20
> > on the adjacent planes versus the return current using the compliment=20
> > differential signal changes, will there be a reflection on the signal?
> >=20
> > For example, if you have a strongly coupled differential pair of 100=20
> > ohms on your motherboard which goes through a connector to connect to=20
> > a loosely coupled 100 ohm differential pair on the backplane, is=20
> > there a reflection? My gut feeling is that the reflections caused by=20
> > the different coupling ratios would cancel each other out causing no=20
> > net reflection. Am I right in thinking this?
> >=20
> > Thanks,
> >=20
> > Wayne Cooke
> > Hardware Engineer, Zeugma Systems Inc.
> > Suite 250, 13571 Commerce Parkway, Richmond, BC, Canada, V6V 2R2=20
> > Tel: (604) 247-2828=20
> > Fax: (604) 247-3251
> > wcooke@xxxxxxxxxxxxxxxxx=20
> >=20
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