By the way, we use internally developed Method of Moments field solvers
to do our computations. Any number of field solver tools should be able
to handle this geometry.
Regards,
- Michael Baxter
p.s. A simple formula for predicting differential impedance for coupled
pairs is: Z,diff = 2*Z0[(1-kb)/(1+kb)] where Z0 is the single-ended
impedance and kb is the backward crosstalk coefficient. As the coupling
goes to zero, Z,diff = 2*Z0 as it should for uncoupled pairs.
At 11:33 AM 4/15/98 -0400, you wrote:
>Homann,
>
>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
>
>
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| Michael Baxter e-mail: baxter@nesa.com |
| NESA, Inc. http://www.nesa.com/ |
| 636 Great Road Tel +1.978.897-8787 |
| Stow, MA 01775 USA Fax +1.978.897-5359 |
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+