RE: [SI-LIST] : Differential Impedance of a Stripline w/ No Planes?

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From: Jian X. Zheng (jian@zeland.com)
Date: Tue May 08 2001 - 15:24:23 PDT


Hi, James:

I would like to give some comments:

1. I am not sure why you use the terminology "edge-coupled differential
stripline". Stripline normally means that you have a trace between top and
bottom ground plane. Do you mean the trace is inside the 62 mils thick
substrate? However, it does not seem that is what it should be. I assume the
traces are on one surface of the substrate.

2. Using our IE3D field solver, I can optimize the dimensions for you:

(a) If you want to keep the gap width to be 6-mils, the trace width will be
25-mils in order to achieve Zc = 100-ohms.
(b) If you want to keep the trace width to be 10-mils, the gap width will be
3-mils.

The above results assume the traces are on top of the one surface.

3. If the traces are inside the dielectrics:

(a) If you want to keep the gap width to be 6-mils, the trace width should
be 8-mils to get Zc = 100-ohms.

Any question, please feel free to let me know. Thanks!

Best regards,

-----------------------------------------------------------------------
Jian-X. Zheng, Ph.D
Zeland Software, Inc., 48890 Milmont Drive, 105D, Fremont, CA 94538, U.S.A.
Tel: 510-623-7162, Fax: 510-623-7135, Web: http://www.zeland.com
---------------------------------------------------------------------

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Rede, James
> Sent: Tuesday, May 08, 2001 2:18 PM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : Differential Impedance of a Stripline w/ No Planes?
>
>
> To the SI Good Guys,
>
> I have been asked to design an edge-coupled differential stripline with no
> planes present
> for FR-4. This design is necessary to pass a Hi-Pot requirement of 1500
> volts.
> The differential impedance is to be 100 ohms for T1 frequencies
> (1.544MHz).
> Four layers are present but no layers are power or ground. Let the board
> thickness
> be 62 mils +/-10%. Use 1/2 oz copper and assume that Er = 4.1.
>
> I cannot use my Hyperlynx field solver because THEY TOOK MY DONGLE.
>
> When I use an impedance calculator found on the web and specify 5-7-6 mils
> (Trace-Space-Trace, TST) and a plane-to-plane separation of 30 mils, the
> differential
> impedance is 100 ohms. If I then widen my plane-to-plane spacing to
> 30000000000 mils,
> I get something like 255 ohms. By trial and error, I was able to
> find a TST
> of 20-6-20
> that would produce a 100 ohm differential impedance at the 30000000000 mil
> plane-to-plane
> separation.
>
> Qs: Is this a reasonable way to approach this problem? Is there a better
> way?
> How should this problem be done, even a field solver will not
> give an answer
> without ground planes present? What about accounting for the
> board thickness
> then air?
>
> J.J. Rede
> Signal Integrity Engineer
> Taqua Systems
> Hyannis MA
>
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