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

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From: Andreas Lenkisch (Trenew) (a.lenkisch@trenew.de)
Date: Thu Oct 21 1999 - 05:21:48 PDT


Raymond,

a longer trace appears more than a transmission line than a lumped element, it depends from the ratio of signal speed and rise/fall time. When you have nearly the same voltage along the line during output switching you can deal with lumped elements (trace length << speed *trise), otherwise transmission line effects must be taken into consideration.
In this case the output edge will not be smoothed down by capacitive loads, because the driver "sees" only a small part of the distributed line capacitance, the remaining part is decoupled by the distributed line inductance. In this case the output voltage step is reduced in high, but not in edge rate. The step high depends from the ratio of driver output impedance and line impedance

Andreas

----------
Von: Raymond.Leung@qsa.idt.com[SMTP:Raymond.Leung@qsa.idt.com]
Gesendet: Donnerstag, 21. Oktober 1999 05:23
An: si-list@silab.eng.sun.com
Betreff: Re: [SI-LIST] : Effect of low Zo for unterminated lines

I think the main issue as far as the driver is concerned is its reliability at
high current/temperature level. I would stick to the 50PF loading spec.
then there should not be any problem of overheat at high speed or
electromigration at high current level. Thus the estimated max. line length
should be 9.2 inches for 33 ohm and 16.7 inches for 60 ohm case.
Please be reminded the loading of the inputs of the other chips has not
been considered.

Raymond

Chris Bobek <cbobek@cadence.com> on 20/10/99 03:08:52

Please respond to si-list@silab.eng.sun.com

To: si-list@silab.eng.sun.com
cc: (bcc: Raymond Leung/QSA/AU)

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

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: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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> >
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