From: Larry Miller ([email protected])
Date: Thu Jan 18 2001 - 06:01:54 PST
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: [email protected]
[mailto:[email protected]]On Behalf Of David Instone
Sent: Thursday, January 18, 2001 4:48 AM
To: [email protected]
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 Tel: +44 (0)23-92-496000
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