[SI-LIST] : Broadside v edge coupled striplines

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From: David Instone (david_instone@uk.xyratex.com)
Date: Thu Jan 18 2001 - 04:47:57 PST


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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