[SI-LIST] Re: AW: Re: Reduction of crosstalk by using stitching vias between traces

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 06 Jan 2012 09:59:51 -0500

Gert

Just one "slight" correction.  Crosstalk between traces is matter of 
distance.  Crosstalk between vias may not be, and quite likely is not in 
a real design.  Poor localization of structures (i.e. - bad use of 
ground vias) can cause very interesting crosstalk results.

Scott

Scott McMorrow
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On 1/6/2012 4:19 AM, Havermann, Gert wrote:
> Yes, x-talk is a matter of distance, and yes, talking about distance without 
> relation to the tracewidth isn't accurate. That's why I also stated a 
> distance in factors of tracewidth (the tracewidth is always related to the 
> reference plane distance). There are quite good rules of thumb:>5x tracewidth 
> distance between differentialtraces "removes crosstak" This works fine for 
> most applications.
>
> I don't like using Microstrip for Datarates exceeding 6GBps, but in some 
> cases it is absolutely ok (and even better than stripline). If you have very 
> short traces, Microstrip is better because you don't need to worry about via 
> stub effects and reflections. The longer the trace, the more preferable it is 
> to route it as stripline. Crosstalk and EMI are other factors to prefer 
> stripline.
>
> BR
> Gert
>
>
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> -----Ursprüngliche Nachricht-----
> Von: Pehr Andersson [mailto:andersson.pehr@xxxxxxxxx]
> Gesendet: Donnerstag, 5. Januar 2012 19:48
> An: Havermann, Gert
> Cc: si-list@xxxxxxxxxxxxx; Rick Brooks (ricbrook)
> Betreff: Re: [SI-LIST] Re: Reduction of crosstalk by using stitching vias 
> between traces
>
> If we have in between differential traces, a single trace and stitch that one 
> with thru hole vias connecting reference planes, then it's a guard trace, how 
> much reduction in crosstalk could we then expect?
>
> Also how wide should this single "guard trace" be, if the drill hole diameter 
> of my stitching vias are 10 mils, is it OK if the trace is>
> 12 mils wide? (The gap is 30 mils wide)
>
> So basically if I understood correctly only means we have for reduction of 
> crosstalk is to increase the spacing between the pairs and also how close are 
> the traces to the reference planes also affects crosstalk, but closeness to 
> reference plane isn't something that I could change, since then the 
> differential impedance changes, and I have to use smaller traces, which could 
> be problematic from manufacturing point of view? Thinner traces will be more 
> susceptible to impedance variations.
>
> Is it recommendable to use microstrip traces for high-speed signaling where 
> data rate is>  10Gb/s?
>
> Since stripline is what is usually seen for high-speed Ethernet (10GBase-KR)
>
> Thanks and best regards! //Pelle
>
>
>
>> 2012/1/5, Havermann, Gert<Gert.Havermann@xxxxxxxxxxx>:
>> Pelle,
>>
>> the effect is minimal. I'd guess, that you will reduce the x-talk of
>> the microstrip by 2%,  and the stripline by 1% (attention: I'm talking
>> about total reduction. I'm not saying stitching brings you down to 1% 
>> x-talk!!!).
>>
>> Just for reference, 4 mil of additional separation between the
>> differential pairs gives you approx, 5-10dB lower crosstalk (5dB
>> microstrip, 10dB stripline depending on the topology). Having a
>> separation of>3x the tracewidth in stripline already gives you approx 30dB 
>> isolation (2% x-talk).
>>
>> Distance is much more effective for isolation than stitching vias.
>>
>> BR
>> Gert
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