[SI-LIST] Re: PCB layer stackup

Hi All,
One basic Doubt,(May not be related to this Subject)

Do LVDS signals needs to refer GND plane (A plane in Specific!)
As differential pair has the other trace as return current,I dont think
they need for a Continous plane underneath it for reference....

Am I true with this Idea,Please clear?

Regards
Chandra kanth


On 1/9/07, Curt McNamara <CurtM@xxxxxxxxxxx> wrote:
>
> I like stack-up 2 with the suggestion of turning one of the internal
> power planes to a ground plane. Here is why:
>
> Many times you will get lower emissions from a product by routing your
> clock traces on the outer layers (assuming they are over a ground
> plane). I first saw this in an IEEE EMC conference paper and experience
> bears it out.=20
>
> The second reason is that your devices (also sources of noise) are
> mounted on the outside of the board. In stack-up 2, this means they are
> directly over the ground layer, also increasing coupling to ground.
>
> So from an EMC perspective stack-up two (with an internal ground layer
> next to the power layer) is best.
>
>                                Curt=20
>
> Curt McNamara // senior electrical engineer=20
> Logic Product Development
> 411 Washington Ave. N. Suite 400=20
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> F // 612.672.9489=20
> www.logicpd.com
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>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Kindt, Jan
> Sent: Tuesday, January 09, 2007 6:26 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] PCB layer stackup
>
>
> Hi,
> =20
> I need to specify a 8layer board. Fastest edges on the board are on the
> 600Mbit LVDS channels : about 500ps. I have two stackups in mind :
> =20
>                  stackup 1              stackup 2
> ---------------------------------------------------------
> L1                routing                routing
> Substrate
> L2                power plane            GND
> Substrate
> L3                power plane            routing
> Substrate
> L4                GND                    power plane
> Substrate
> L5                routing                power plane
> Substrate
> L6                routing                routing
> Substrate
> L7                GND                    GND
> Substrate
> L8                routing                routing
> =20
> Stackup 1 has the advantage that all routing on L5, L6 and L8 is
> referenced to GND. We need to push the LVDS channels over a board to
> board connector. On the board to board connector, there are GND's
> between separate pairs. So we keep the same reference to GND over the
> connector.
> =20
> Stackup 2 is the stackup I would normally use due to it's symmetry.
> Signals on L3 and L6 are referenced to both GND and a power plane. But I
> don't know why I would need to favour stackup 2 over stackup 1.
> =20
> Can you comment om which stackup is most preferred ? Or what are the
> drawbacks/advantages of one stackup to another.
> =20
> best regards,
> Jan
>
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