[SI-LIST] : method for identifying Xtalk and SSO noise ?

Weber Chuang (WeberChuang@via.com.tw)
Sun, 31 May 1998 14:54:44 +0800

> Dear SI gurus,
> Lately, I met with a problem which might be related to Xtalk and
> SSO noise. And maybe you who are experienced can give me some
> discussions on this topic. I had one chip installed on two
> different boards, one is on an ATX form factor, and the other is on a
> Baby-AT format, The ATX one has more real estate so the min trace
> clearance and max trace length is better maintained, but for the AT
> board, this is not the case, the clearance is shrunk, and length is
> longer, as a result, the AT board is not that stable, we will had it
> re-layouted, but since it is a good chance to learn something from
> this board, so I did some investigations on it, and I have identified
> several worst case signals, which have longer PCB trace and worse I/O
> pad grouping, here are the steps I took to identify the cause of the
> glitch, am I doing it right? Is there anything else I should beware
> of? Because though there are lots of books talks about theories of
> xtalk and SSO noise, but few mention about detailed ways in
> identifying and discriminating them. The steps I took are:
> 1. I choose three signal traces which are adjacent to each other, then
> I measure the glitch on the middle line(both far and near end ), I
> recorded the peak value as well as the waveform shape of the glitch.
> In fact, there are several shapes, and which might be generated by
> Far-end or near-end xtalk(for rising and falling edge) and propagated
> SSO or mixture.
> 2. second, I cut out the middle trace and use a jumper wire to replace
> it, I do this for minimizing the Xtalk, then I re-measure the glitch,
> since a jumper wire has higher impedance than PCB trace, so I can
> expect that there will be more serious over-undershooting, in fact,
> there really is. Besides, I found that the glitch level became
> smaller.
> 3, I built a similar environment in QUAD/xtk and run the simulation, I
> recorded the waveform,
> Then I did a comparison between the datum I got, I found some of the
> measured glitches have similar waveforms with the waveform I saw in
> the simulation, both for near-end and far-end, some glitches have
> different waveforms, I suspect that it is a SSO noise. I have the
> following result:
> 1, SSO noise and Far end noise would have combinatorial effect,
> 2. SSO noise needs time to propagate to the far-end. Same is Far-end
> Xtalk.
> 3, For the waveforms that are similar to simulations, they come from
> Xtalk.
> 4. For those waveforms that are different from simulation, they are
> SSO noise, or maybe they are mixture.
> 5. The near-end and far-end xtalk noise have opposite values, I mean
> one would be positive while another is negative if I show them on the
> same scope, though some skew between them might be
> observed(T_propagation), I saw the same in the simulation.
> 6.The near-end SSO and far-end SSO have unanimous values, I mean one
> would be positive while the other is also positive and vice versa.
> This is the same with what I saw with Hspice. Though some
> transmission line effect needs to be implemented while trying to
> explain the waveform.
> Are my conclusions correct, I will be more than happy to see your
> suggestions or comments. Thanks!
> Best Regards
> Weber Chuang(ChingFu Chuang)
> SI Engineer, System Team.
> VIA Technologies, Inc.
> Taipei, Taiwan, ROC
> http://www.via.com.tw
> Very Innovative Architecture
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