RE: [SI-LIST] : Broadside v edge coupled striplines

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Fri Jan 19 2001 - 08:52:50 PST


>>Isn't the difference is really in manufacturing, not physics?
That was the conclusion reached in the DesignCon 2000 paper. Works for me...

Larry

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
Aubrey_Sparkman@Dell.com
Sent: Friday, January 19, 2001 7:45 AM
To: ldmiller@rhapsodynetworks.com; ribrooks@nortelnetworks.com;
SI-LIST@silab.eng.sun.com
Subject: RE: [SI-LIST] : Broadside v edge coupled striplines

Thanks for that correction. Isn't the difference is really in
manufacturing, not physics? An EDGE-coupled diff pair is more uniform
because the pattern is etched in the same process. The BROADSICE-coupled
diff pair is etched at two different times and additionally has to be
mechanically aligned for lamination. This adds two additional error terms
to the accuracy of your BROADSICE-coupled diff pair that the EDGE-coupled
diff pair does not have. So IMHO, you should really have a packing density
problem before you consider using BROADSICE-coupled traces.

But if you are doing work for Compaq or Sun, you should use
BROADSICE-coupled diff pairs whenever possible. :-)

Aubrey Sparkman
Signal Integrity
Aubrey_Sparkman@Dell.com
(512) 723-3592

-----Original Message-----
From: Larry Miller [mailto:ldmiller@rhapsodynetworks.com]
Sent: Thursday, January 18, 2001 9:41 AM
To: 'Rick Brooks'; SI-LIST@silab.eng.sun.com
Subject: RE: [SI-LIST] : Broadside v edge coupled striplines

Yes, yes, I did mean EDGE-coupled, Rick. Thank you for the correction.

Larry

-----Original Message-----
From: Rick Brooks [mailto:ribrooks@nortelnetworks.com]
Sent: Thursday, January 18, 2001 7:28 AM
To: Larry Miller
Subject: RE: [SI-LIST] : Broadside v edge coupled striplines

Larry,
don't most people, including the AMP paper you refer to, recommend
edge coupled diff pairs, instead of broadside coupled?

Just curious.
You really are having fun, aren't you?
- Rick

        -----Original Message-----
From: Larry Miller [SMTP:ldmiller@rhapsodynetworks.com]
Sent: Thursday, January 18, 2001 6:02 AM
To: 'David Instone'; SI-LIST
Subject: RE: [SI-LIST] : Broadside v edge coupled striplines

        There have been some pointers to good papers here (look in the
archives)
that indicate that from a signal integrity standpoint (i.e., rise time,
etc.) there is NO difference between edge-coupled and broadside-coupled
pairs IF you can meet the geometry requirements for each.

        However, several studies have shown that broadside-coupled pairs are
more
noise-sensitive due to the propagation mode (even vs odd) differences. Also,

in general, broadside-coupled pairs require propagation mode conversion
going through connectors.

        There is considerable (not always agreeing) literature at the web
sites of
AMP, Vitesse and Teradyne on the topic, as well as those of some of the
"regulars" here such as Doug Brooks (UltraCAD). Searching on DesignCon 2000
will also bring up a lot of papers and studies.

        One of the clearest expositions I have seen is AMP 98060A_5.pdf,
though you
do have to carefully watch their sometimes non-conventional terminology.
This paper was done for a large Telco equipment manufacturer and the results

turned out to be very accurate in practice.

        I think that at bottom most purveyors of high speed connector
systems and
backplane/midplane board manufacturers recommend broadside-coupled
differential pairs.

        Larry Miller

        -----Original Message-----
From: owner-si-list@silab.eng.sun.com
[ mailto:owner-si-list@silab.eng.sun.com
<mailto:owner-si-list@silab.eng.sun.com> ]On Behalf Of David Instone
Sent: Thursday, January 18, 2001 4:48 AM
To: SI-LIST@silab.eng.sun.com
Subject: [SI-LIST] : Broadside v edge coupled striplines

        All,
I would like to know what the advantages and disadvatages are of
differential lines laid out as broadside coupled striplines versus edge
coupled striplines, assuming of course that both types are designed to
have the same differential impedance.

        One obvious advantage of broadside is be the increased routing
density
possible, and the minimising of trace length differencies when routing.
It also seems intuitive that adjacent pairs of broadside coupled lines
will have a net differential cross talk that is less than that of edge
coupled, as the geometry can be arranged such that difference in the
distance between one line of one pair to BOTH lines of the other pair
will less than if they were edge coupled pairs.

        However, I have recently read the following about broadside coupled
lines:
'...the RF characteristics of this design suffer greatly....should only
be used for high density or tight areas'
 and
'One must be careful that the lines are not too capacitive. This could
destroy the rise and fall times at 2Ghz or higher frequencies.'

        The latter statement seems strange, Zo is root(L/C) and is frequency

independent. Now granted that in the real world we have skin losses and
dielectric losses, but for the same Zo do broadside lines have more
frequency dependent loss than edge coupled lines?

        Regards

        Dave Instone. Compliance Engineer
 Storage Systems Development, MP24/22
 Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
Tel: +44 (0)23-92-496862 (direct line)
Fax: +44 (0)23-92-496014
http://www.xyratex.com <http://www.xyratex.com> Tel: +44 (0)23-92-496000

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