This type of differential impedance configuration is called Broadside
Coupled Stripline. This model has some benefits and liabilities. The
pair is created by having identical routing paths for the two traces and
placing them on adjacent layers. It is mechanically similar to the dual
stripline model for characteristic impedance and makes constructive use
of the interplay between overlapping circuits. A Broadside Coupled
Stripline is theoretically predictable but has poor controllability in a
fabrication environment. The dielectric thickness variation between
Plane1 & L1 and Plane2 & L2 causes signals to have non-identical
reference plane locations.
The dielectric between the two signals becomes critical. Natural
variation of this dielectric causes a large amount of variation in the
differential impedance along the entire trace and in localized sections
of the trace. This variation is caused by variations in the laminate
composition, changes in trace geometry and pressure distribution during
lamination. Layer to layer shift (variation in registration) causes
variation in the differential impedance because of a change in the
overlap between the circuits. In addition, these will most likely be a
different etched trace width distribution from one layer to the other.
This will result in an effective overlap equaling the width of the
smaller trace.
When you think about using a Broadside Coupled Stripline configuration
the variables noted above must be considered and taken into account when
considering impedance tolerancing.
Regarding differential impedance calculators which can calculate
Broadside Coupled Striplines, I would suggest Polar Ltd. As a possible
source. Polar is a manufacturer of TDR's and has published some
differential impedance calculators in the past. Polar's address is
below.
Polar Instruments Ltd, Garenne Park, St. Sampson, Guernsey,Channel
Islands GY2 4AF, UK.
http://www.polar.co.uk Tel: +44 (0)1481 53081 Fax: +44 (0)1481 52476
Email: polar@itl.net
Steve Silbert
Viasystems. Inc.
steve.silbert@viasystems.com
-----Original Message-----
From: homann@erv.ericsson.se [SMTP:homann@erv.ericsson.se]
Sent: Wednesday, April 15, 1998 10:45 AM
To: si-list@silab.Eng.Sun.COM
Subject: [SI-LIST] : Dual Stripline impedance
Hi all,
We're thinking about using a dual stripline for a differential
pair. The diff. impedance should be 100ohm.
Have anyone used this before? Or seen it in suggested? Come to
think
of it, it does sound like a good idea (not mine). Now, the hard
part
is getting the right geometry... Any help from various programs
or
books? Any references would very helpful, the only one I found
is in
"Transmission Line Design Handbook", and it is not very clear.
Can you
analyze this with a field solver?
The concept goes like this (cut-through view of the board):
Plane
-----------------------
--------
| L1 |
--------
--------
| L2 |
--------
-----------------------
Plane
The Zdiff should be between L1 and L2.
Homann