[SI-LIST] Re: R: Re: Question on varying the coupling ratio on a differential pair
- From: "OPREA Dorin" <Dorin.Oprea@xxxxxxxxxxxxxxxxxx>
- To: <leeritchey@xxxxxxxxxxxxx>, <gianguida@xxxxxxxx>, <hassan@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 17 Jan 2007 11:42:39 -0600
The characteristic impedance is constant only if the losses are
considered negligible; on the other hand, DK and DF change also with the
frequency and with the environment (temperature, humidity).
May help,
Dorin
=20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Lee Ritchey
Sent: Wednesday, January 17, 2007 12:20 PM
To: gianguida@xxxxxxxx; hassan@xxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: R: Re: Question on varying the coupling ratio on
a differential pair
If these transmission lines are built with any of the commonly available
PCB materials, their impedance will not be constant over frequencies up
to about 2 GHz due to the changing value or er.
This is even true for Rogers 4350.
> [Original Message]
> From: <gianguida@xxxxxxxx>
> To: <hassan@xxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 1/17/2007 10:44:21 AM
> Subject: [SI-LIST] R: Re: Question on varying the coupling ratio on a
differential pair
>
> Hassan,
> I beleive that
> trasmission line in TEM region, such as Microstrip, have=20
> characteristic impedance costant over frequency perhaps this is why=20
> people think in terms of characteristic impedance Giancarlo
>
>
> -----Messaggio originale-----
> Da: si-list-bounce@xxxxxxxxxxxxx per conto di Hassan O. Ali
> Inviato: mer 17/01/2007 17.28
> A: si-list@xxxxxxxxxxxxx
> Oggetto: [SI-LIST] Re: Question on varying the coupling ratio on a
differential pair
> =20
>
> Just to add to the discussion: I don't remember seeing any high-speed
electrical medium=20
> that presents one fixed value of characteristic impedance for all=20
> signal
frequencies=20
> that matter! Why then are people so obsessed with "impedances" as=20
> opposed
to maximum=20
> return loss in a bandwidth of interest?
>
> Best regards.
>
> Hassan.
>
>
> On Jan 17, "Loyer, Jeff" <jeff.loyer@xxxxxxxxx> wrote:
> >=20
> > Bill,
> > I'd like some clarification. Are you saying that, even though you=20
> > keep the impedance continuous, you will see reflections (for a=20
> > 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.=3D20
> >=20
> > Even then, I wonder - if you maintained the same impedance by some=20
> > magic... Say you went from a pcb to a twisted pair and back to a=20
> > twisted pair, all with the same differential impedance. Would that=20
> > ever inherently cause a problem? =3D20
> >=20
> > Regarding the original question, I think that it's usually=20
> > impossible to ensure the same coupling coefficient from driver to=20
> > receiver, and don't think it's necessary. I don't remember a design
> > guide that specifies both odd and even impedances (or differential=20
> > impedance plus coupling coefficient), though they may specify a=20
> > "reference" stackup (but not for the purpose of achieving some
magical coupling coefficient).
> >=20
> > Connectors (and other sources of return current discontinuities,=20
> > such as vias, solder balls, or packages) represent challenges in=20
> > maintaining return paths, but I don't know of a reason why the=20
> > coupling coefficients would be maintained on either side of those=20
> > discontinuities. The connectors (or vias, etc.) already represent=20
> > such a tremendous change in impedance and/or coupling coefficient=20
> > that trying to reduce common mode reflections by maintaining the=20
> > coupling coefficient before and after seems pointless. =3D20
> >=20
> > As always, perhaps I've missed something - I'm open to further=20
> > enlightenment.
> >=20
> > Jeff Loyer
> >=20
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx=20
> > [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=20
> > differential pair
> >=20
> > Wayne,
> >=20
> > Even if the differential impedance is 100 Ohms from the source to=20
> > the=3D20 final load and the signal is purely differential (no=20
> > common-mode=3D20 component, originating from either the source or =
skew
> > within the
pair),=3D20
> > there will be reflections. The question is, how significant are
they.
> >=20
> > To better understand what is happening, you might refer to Dr.=20
> > Johnson's
> >=20
> > second book, High-Speed Signal Propagation - Advanced Black Magic,
pp.=3D20
> > 417-8, where he describes "the differential U-turn." The current
loop=3D20
> > created in the "U-turn zone" has a magnetic field associated with=20
> > it=3D20 which can cause EMI, coupling into nearby conductors, and=20
> > create an=3D20 inductive discontinuity.
> >=20
> > If your transition times are slow enough relative to the electrical=20
> > size
> >=20
> > of the "U-turn zone", you won't be able to "see" a reflection.
> >=20
> > Regards,
> > -Bill
> >=20
> >=20
> > /************************************
> > / billw@xxxxxxxxxxx /
> > / /
> > / Advanced Electronic Concepts, LLC /
> > / www.aec-lab.com /
> > ************************************
> >
=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D=
3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D
=3D3D=3D
> >
=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D=
3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D
=3D3D=3D
> > =
=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D
> > cookeway wrote:
> > > Hi all,
> > >=3D20
> > > I have a question concerning differential pair coupling ratios=20
> > >that=3D20 is causing some debate here in our design group. If a=20
> > >differential=3D20 pair with an impedance of 100 ohms connects=20
> > >through a backplane=3D20 connector and into a backplane such that=20
> > >the impedance is=3D20 consistently 100 ohms, but the coupling =
ratio=20
> > >of the return
currents=3D20
> > > on the adjacent planes versus the return current using the
compliment=3D20
> > > differential signal changes, will there be a reflection on the
signal?
> > >=3D20
> > > For example, if you have a strongly coupled differential pair of
100=3D20
> > > ohms on your motherboard which goes through a connector to connect
to=3D20
> > > a loosely coupled 100 ohm differential pair on the backplane,=20
> > > is=3D20 there a reflection? My gut feeling is that the =
reflections=20
> > > caused
by=3D20
> > > the different coupling ratios would cancel each other out causing
no=3D20
> > > net reflection. Am I right in thinking this?
> > >=3D20
> > > Thanks,
> > >=3D20
> > > Wayne Cooke
> > > Hardware Engineer, Zeugma Systems Inc.
> > > Suite 250, 13571 Commerce Parkway, Richmond, BC, Canada, V6V=20
> > >2R2=3D20
> > > Tel: (604) 247-2828=3D20
> > > Fax: (604) 247-3251
> > > wcooke@xxxxxxxxxxxxxxxxx=3D20
> > >=3D20
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject=20
> > field
> >=20
> > or to administer your membership from a web page, go to:
> > <a
href=3D'http://www.freelists.org/webpage/si-list'>http://www.freelists.or=
g
/web
page/si-
> list</a>
> >=20
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >=20
> > List FAQ wiki page is located at:
> > <a
href=3D'http://si-list.org/wiki/wiki.pl?Si-List_FAQ'>http://si-
> list.org/wiki/wiki.pl?Si-List_FAQ</a>
> >=20
> > List technical documents are available at:
> > <a
href=3D'http://www.si-list.org'>http://www.si-list.org</a>
> >=20
> > List archives are viewable at: =3D20
> > <a href=3D'http://www.freelists.org/archives/si-
> list'>http://www.freelists.org/archives/si-list</a>
> > or at our remote archives:
> > <a href=3D'http://groups.yahoo.com/group/si-
> list/messages'>http://groups.yahoo.com/group/si-list/messages</a>
> > Old (prior to June 6, 2001) list archives are viewable at:
> > <a
href=3D'http://www.qsl.net/wb6tpu'>http://www.qsl.net/wb6tpu</a>
> > =3D20
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject=20
> > field
> >=20
> > or to administer your membership from a web page, go to:
> > <a
href=3D'http://www.freelists.org/webpage/si-list'>http://www.freelists.or=
g
/web
page/si-
> list</a>
> >=20
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >=20
> > List FAQ wiki page is located at:
> > <a
href=3D'http://si-list.org/wiki/wiki.pl?Si-List_FAQ'>http://si-
> list.org/wiki/wiki.pl?Si-List_FAQ</a>
> >=20
> > List technical documents are available at:
> > <a
href=3D'http://www.si-list.org'>http://www.si-list.org</a>
> >=20
> > List archives are viewable at: =20
> > <a href=3D'http://www.freelists.org/archives/si-
> list'>http://www.freelists.org/archives/si-list</a>
> > or at our remote archives:
> > <a href=3D'http://groups.yahoo.com/group/si-
> list/messages'>http://groups.yahoo.com/group/si-list/messages</a>
> > Old (prior to June 6, 2001) list archives are viewable at:
> > <a
href=3D'http://www.qsl.net/wb6tpu'>http://www.qsl.net/wb6tpu</a>
> > =20
> >=20
> >=20
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at: =20
> http://www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
> =20
>
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at: =20
> http://www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
> =20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at: =20
http://www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
=20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
http://www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts: