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

Dave Hoover (dhoover@mccurdy.com)
Tue, 19 Oct 1999 09:40:36 -0700

All the PCB's I've built using 29 Ohms (GTL and Ram something)
was .018" lines .003" above a plane (microstrip).

??????? .006" was using a high Er' dielectric??????

Dave
-----Original Message-----
From: Chris Cheng [mailto:hycheng@3pardata.com]
Sent: Tuesday, October 19, 1999 9:12 AM
To: si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : Effect of low Zo for unterminated lines

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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