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

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From: Inmyung Song (imsong@internex.co.kr)
Date: Tue May 08 2001 - 16:43:20 PDT


Good morning James.

Just my opinion.
If you don't need, you can ignore my comments.

How about using the power/ground fill up and under the differential patterns?
I guess you defined the power/ground plane infinite distance, right?

By the way, as I know the differential impedance calculations besides characteristic impedance is not same within
pcb manufacturers.

I don't know exactly.

Good luck.

----- Original Message -----
From: "Jian X. Zheng" <jian@zeland.com>
To: "Rede, James" <JRede@taqua.com>; <si-list@silab.eng.sun.com>
Sent: Wednesday, May 09, 2001 7:24 AM
Subject: RE: [SI-LIST] : Differential Impedance of a Stripline w/ No Planes?

> 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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