[SI-LIST] Re: FEXT reduction by very small spacings
- From: Hermann Ruckerbauer <hermann.ruckerbauer@xxxxxxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Sun, 16 Aug 2009 22:01:16 +0200
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Hi Steve,
thanks for the fast feedback ... it's sunday afternoon ... do you have
nothing better to do than reading the SI-List e-mails ? ;-)
So far I have done only analysis in the time domain (just a step looking
on FEXT and NEXT). This was not a real project, and I just stumbled over
the issue preparing some training material so I didn't spend too much
time on investigation. I wanted to do some more checks, but when looking
to the paper I was quite sure to see the same effect.
The only check I made was some Q'n'D length variation (100/150/200mm) to
ensure not to have some length resonance. This did not change the
picture. If i can trust the simulator (and I do trust the ADS Multilayer
transmissionline model in this case) I think this effect is true. Maybe
someone can do a short check with another tool ?
I agree, that bot inductive and capacitive coupling will increase with
reduced spacing, but my understanding is, that they do change at a
different rate (maybe an effect of the sidewall capacitance getting more
important once close enough, while you have nearly no change for
inductive X-talk changing spacing from 100 to 300um ?). So far for me
the paper and the simulation result look somehow reasonable, but I'm not
sure how much effort I should spend here ...
regards
Hermann
EKH - EyeKnowHow
Hermann Ruckerbauer
www.eyeknowhow.de
hermann.ruckerbauer@xxxxxxxxxxxxx
Veilchenstrasse 1
94554 Moos
Tel.: +49 (0)9938 / 902 083
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steve weir schrieb:
> The long and short IMO is that the greater the coupling between two=20
> random data sources the greater the impairment. Both capacitive and=20
> inductive coupling increase as spacing decreases. I suspect that the=20
> test method is yielding data that gets misinterpreted. Did you do your=
=20
> sweeps in the frequency or time domain? Structural resonances can lead=
=20
> to very erroneous interpretations of FDM measurements.
>
> Steve.
> Hermann Ruckerbauer wrote:
> =20
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>> 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 too=
k
>> 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 go=
t
>> 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 spa=
cing?
>>
>> The biggest issue I see is the NEXT. It is getting quite large with th=
is
>> 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 contro=
l
>> 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
>> Veilchenstrasse 1
>> 94554 Moos
>> Tel.: +49 (0)9938 / 902 083
>> Mobile: +49 (0)176 / 787 787 77
>> Fax: +49 (0)721 / 151 258 230
>>
>>
>>
>>
>>
>>
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