Re: [SI-LIST] : Effect of low Zo for unterminated lines

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From: Tadashi ARAI (arap@jdc.paso.fujitsu.co.jp)
Date: Wed Oct 20 1999 - 22:36:37 PDT


Hi SI-Gurus,

>From the result I have calculated with HP AppCAD, with 5.65mil of
dielectric hight and 0.01mil of trace thickness, it can be 29 ohms with
6mil width trace of STRIP-line. Dielectric is FR4 (er=4.6).

The point is that "STRIP-LINE", maybe.

//// /// // / / / / / / / / / /
Tadashi Arai//Platform Developing Dept.,Desktop Prd Div. Fujitsu Limited
arap@paso.fujitsu.co.jp TEL:+81-42-370-7617 Inagi-shi, Tokyo, Japan
/ / / / / / / / / / / // ///
On Tue, 19 Oct 1999 09:11:40 -0700
Mail from hycheng@3pardata.com described as below:

> are you sure you have a 29ohm line. its very hard to believe
> you can achieve 29ohm with 6 mil traces on PCB.
> chris
>
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Chris Bobek
> > Sent: Monday, October 18, 1999 5:24 PM
> > To: Si-list
> > Subject: [SI-LIST] : Effect of low Zo for unterminated lines
> >
> >
> > Hi,
> >
> > Because of several constraints on our stackup and line width, a 6mil
> > stripline trace yields a Zo = 29ohms. When I simulate a 29ohm,
> > unterminated line on our board (@ 10MHz), I see a pretty nice waveform,
> > albeit with slower rise/fall times (which is ok for the nets in
> > question). When I increase the Zo to 60 ohms, for example, I see a lot
> > of ringing on the same net.
> >
> > >From a theoretical point of view, my guess is that this happens because
> > as you widen the trace, there is more capacitance, which slows down the
> > rise/fall times. Is this a correct assumption? Are there other factors
> > as well that are going on?
> >
> > What I am most concerned about is the effect of a low Zo net on its
> > driver. For example, if I have an LVCH245 driver (3.3V) driving a 29ohm
> > line, will the low Zo "hurt" the driver? Or, is the only negative
> > effect the slower rise time? This question is a general question for
> > most ICs.
> >
> > Thanks for your help,
> >
> > Chris
> >
> >
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> >
>
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>

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