[SI-LIST] Re: FEXT reduction by very small spacings

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Hermann Ruckerbauer <hermann.ruckerbauer@xxxxxxxxxxxxx>
  • Date: Sun, 16 Aug 2009 12:16:21 -0700

The long and short IMO is that the greater the coupling between two 
random data sources the greater the impairment. Both capacitive and 
inductive coupling increase as spacing decreases.  I suspect that the 
test method is yielding data that gets misinterpreted.  Did you do your 
sweeps in the frequency or time domain?  Structural resonances can lead 
to very erroneous interpretations of FDM measurements.

Steve.
Hermann Ruckerbauer wrote:
> Content-Type: text/plain; charset=ISO-8859-15
> Content-Transfer-Encoding: 7bit
> Hello all,
>
> for a training I wanted to show some simulation with FEXT and NEXT
> comparison. So I did some simulation sweeps with different spacings.
>
> On a normal Microstrip trace I saw, that going to wider spacing it took
> a quite large spacing to reduce FEXT (as expected). But when reducing
> the spacing below 100um the FEXT got smaller quite fast. Based on the
> waveform my guess was, that the capacitive coupling got larger quite
> fast and compensated the inductive coupling, which is normally causing
> the neg. peak (for a rising edge) at the far end of the line. So far I
> have not seen this effect in literature, but from Agilent support I got
> a hint to an IEEE Paper which shows this behavior:
>
> IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 43, NO. 4,
> NOVEMBER 2001
> Far-End Crosstalk Reduction in Double-Layered Dielectric Interconnects
> Talgat R. Gazizov, Member, IEEE
>
> The author closes with the statement, that he hopes the paper can be
> used to find a way how to utilize this effect. And that's my question
> too. Has somebody tried to cancel out the FEXT by reduction of the spacing?
>
> The biggest issue I see is the NEXT. It is getting quite large with this
> small spacings, so even when your system is only reacting on FEXT,
> because you have e. g. a parallel memory bus you need a good source
> termination to ensure, that the reflected NEXT is not hitting the
> receiver after it's reflected at the driver.
> Additionally it might be necessary to exactly control the routing for
> each trace seperatly. In simulation it is easy to "route" two signals
> side by side with always the same distance, but for a real system this
> will be a real problem, and it might be necessary to separately control
> the routing for each from the 64/72 bits of the memory bus.
>
> I guess it might be difficult to use this effect, but it should be
> doable ... what is your opinion?
>
> regards
>
> Hermann
>
> EKH - EyeKnowHow
> Hermann Ruckerbauer
> www.eyeknowhow.de
> hermann.ruckerbauer@xxxxxxxxxxxxx
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>
>
>
>
>
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