[SI-LIST] Re: High speed signals go across isolation moat

Zhuyongfa well that makes the problem more complicated. So what we know 
about the stack-up:

1 Top
2 GND
7mils
3 Signal
8 mils
4 Power mixed
5 ? presume signal
6 ? presume signal
7 GND
8 mils
8 ? presume signal
7 mils
9 Power mixed
10 Bottom

I suggest that this would work better:

1 Top
2 Power mixed
11
3 Signal
4 mils
4 GND
4 mils
5 signal
 > 10 mils
6 signal
4 mils
7 GND
4 mils
8 signal
11 mils
9 Power mixed
10 Bottom

All signal layers tightly reference GND, can readily change routing 
layers as needed and will perform well. The distance from GND to any of 
the power layers hasn't changed so your bypass for IC power delivery is 
unchanged. But now the signals will not excite the PDN, making the board 
much quieter for a given bypass strategy.

Steve.

z46147 wrote:
> Steve,
> Adjusting the stack-up is not a good choice.
> Cause layer 2 and layer 7 are solid GND planes, and layer 4 and layer 
> 9 are power planes.
> If we move layer 3 close to layer 2, then layer 9 will be close to 
> layer 8 too. Then high speed signal traces on layer 8 will go across 
> isolation moats on layer 9.
> Thanks and regards.
> Zhuyongfa
> HUAWEI TECHNOLOGIES CO.,LTD. huawei_logo
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> Address: Huawei Industrial Base
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>     ----- Original Message -----
>     *From:* steve weir <mailto:weirsi@xxxxxxxxxx>
>     *To:* zhuyongfa@xxxxxxxxxx <mailto:zhuyongfa@xxxxxxxxxx>
>     *Cc:* si-list@xxxxxxxxxxxxx <mailto:si-list@xxxxxxxxxxxxx> ;
>     duyumin@xxxxxxxxxx <mailto:duyumin@xxxxxxxxxx> ; xiaoji@xxxxxxxxxx
>     <mailto:xiaoji@xxxxxxxxxx>
>     *Sent:* Wednesday, November 14, 2007 6:31 PM
>     *Subject:* Re: [SI-LIST] High speed signals go across isolation moat
>
>     Zhuyongfa, yes layer 4 acts as a reflection plane to layer 3 signals.
>     Your current stack-up is a nearly symmetric stripline. About half the
>     energy will be bound between traces on layer 3 and each of the plane
>     layers 2, and 4. Where single ended or in-phase differential signals
>     cross moats on layer 4, energy will go into exciting the slots.
>     Signal
>     rise-time, crosstalk, and EMI will all be affected. To know the exact
>     extent, you would need to simulate.
>
>     You can fix this by adjusting the stack-up to reduce coupling from
>     layer
>     3 signals to layer 4 etch by moving layer 3 closer to layer 2, and
>     further from layer 4. Do the same thing with layer 8 wrt layers 9 and
>     7. If you use 4 / 11 in place of your current 7 / 8 you will remove
>     almost all of the coupling from layer 3 to layer 4.
>
>     Good luck.
>
>     Steve.
>
>     z46147 wrote:
>     > Content-type: text/plain; charset=gb2312
>     > Content-transfer-encoding: 7BIT
>     > Hi all,
>     >
>     > A ten layers high speed PCB, the second layer is GND, the third
>     layer is signal, the fourth is Power layer.
>     >
>     > GND plane is a full one, never been splited. Several types of
>     voltage are on the power layer, so the power layer is isolated by
>     so many moats.
>     >
>     > Thickness between the second and third layer is 7 mil, while 8
>     mil between the third and fourth layer.
>     >
>     > Some high speed signals traces are routed on the third layer,
>     such as PCI Express and SAS/SATA signals.
>     >
>     > If the high speed signal traces go across isolation moat of the
>     fourth power layer, can it be treated as crossing split reference
>     plane and give rise to signal integrity problem?
>     >
>     > If it will give rise to signal integrity problem, can we use
>     stiching capacitors across isolation moats of the power layer to
>     deal with this issue, while changing the
>     > isolation moats?
>     >
>     > Any thought on this issue would be appreciated.
>     >
>     > Best regards.
>     >
>     >
>     > Zhuyongfa
>     > HUAWEI TECHNOLOGIES CO.,LTD.
>     >
>     >
>     > Address: Huawei Industrial Base
>     > Bantian Longgang
>     > Shenzhen 518129, P.R.China
>     > Tel:+86-755-89653025
>     > Fax: +86-755-89650731
>     > E-mail: zhuyongfa@xxxxxxxxxx <mailto:zhuyongfa@xxxxxxxxxx>
>     > www.huawei.com <http://www.huawei.com>
>     >
>     
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-- 
Steve Weir
Teraspeed Consulting Group LLC 
121 North River Drive 
Narragansett, RI 02882 

California office
(408) 884-3985 Business
(707) 780-1951 Fax

Main office
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Oregon office
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http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of 
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