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

Chris Bobek ([email protected])
Tue, 19 Oct 1999 10:08:52 -0700

This is a little off subject, but I made a slight mistake, the Zo is 33ohms. Here is a stripline portion of my stackup:

1 oz copper plane
4mil FR-4 (er = 4.5)
1 oz signal
4.4mil Prepreg
1 oz copper plane

When I plug those values into Specctraquest, I get 33 ohms. When I use Howard's equations from his book, I get roughly 30 ohms or so (I kinda forget). So, I believe this is correct.

Chris

Dave Hoover wrote:

> 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:[email protected]]
> Sent: Tuesday, October 19, 1999 9:12 AM
> To: [email protected]
> 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: [email protected]
> > [mailto:[email protected]]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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