# Re: [SI-LIST] : Dual Stripline impedance

Mike Degerstrom ([email protected])
Wed, 15 Apr 1998 11:20:04 -0500

Magnus,

I can't give you a formula to predict your odd mode structure. However,
the universal trick for odd mode structures is that you simplify the
structure by drawing a ground plane 1/2 between your 'L1' and 'L2'
conductors. So if you have a formula for an offset stripline, that
is one that is not placed midway between top and bottom ground
planes, then you can solve for Zo. I've seen two definitions
of Zdiff, but I think your definition would utilize the formula:

Zdiff = 2*Zo' , where Zo' is the augmented structure defined above.

You can use an EM solver to predict Zdiff if you have one
available. I think that there are even demo versions that are limited
by complexity and thus should be able to handle your problem.

If you use an EM solver for a two conductor structure, then you
would have to know how to use the results to compute Zdiff and
I can expand on this if you decide to go that route. However, the
same trick described above works, of course, for your EM solver.

I've considered using your structure on a few occasions but always
abandon the approach. Instead I revert to having both the
true and complement conductors on the same plane. With the
latter approach, it is much easier to balance the vias between
the conductors and I don't block orthogonal routing for other
conductors.

A word of caution is to consider the tolerances of the material
dimensions and dielectric properties if you want a well controlled
Zdiff. For example, if L1 is built on a different core than L2 and
the two are laminated together, you typically cannot control the
spacing between L1 and L2 very well.

Mike

On Apr 15, 4:45pm, Magnus Homann wrote:
> 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.

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Mike Degerstrom                       Email:    [email protected]
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