[SI-LIST] Re: High speed signals go across isolation moat
- From: steve weir <weirsi@xxxxxxxxxx>
- To: zhuyongfa@xxxxxxxxxx
- Date: Wed, 14 Nov 2007 02:31:04 -0800
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
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