FreeLists / si-list / [SI-LIST] Re: PCB layer stackup

["Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>]

Aren't we beating this topic a little hard?


> [Original Message]
> From: Doug Smith <doug@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Cc: <leeritchey@xxxxxxxxxxxxx>; ChandraKanth Gajawada
<chandrakanth.gajawada@xxxxxxxxx>
> Date: 1/9/2007 11:24:04 AM
> Subject: [SI-LIST] Re: PCB layer stackup
>
> Hi Lee and the group,
>
> Isn't it more complicated that that? In the plane:
>
> If the spacing of the diff pairs is somewhat wider than the distance 
> to the plane (conservative assumption to make the point) and I take 
> the current right under one of the diff wires, I will see the current 
> from the above trace returning on the plane. Same thing happens on the 
> other wire. Just like Lee says.
>
> But, the net current in the plane is still the common mode component 
> of the diff signal if I look at a width of the plane somewhat wider 
> than the pair spacing.
>
> If the diff pair spacing is much closer than the plane (I don't think 
> this is very common but a good thought experiment) then the 
> differential plane current will be reduced under each trace and mutual 
> inductance between the two diff traces will cause them to carry more 
> of the other's return current.
>
> In reality, the two cases above get muddled and some fraction (may be 
> small) of the return current may flow on the other diff trace and some 
> (maybe most in many cases) will be in the plane.
>
> For any specific case, it would be easy to calculate, simulate, or 
> build a scale model (like Howard Johnson does) and measure the currents.
>
> Doug
>
> Lee Ritchey wrote:
>
> > The return current for one member of a diff pair is not flowing in the
> > other wire.  It is flowing in the planes over which it travels.  Doesn't
> > matter what the name of the planes are.  EM fields have not been taught
how
> > to read yet, so they don't know what you named the planes and don't
care!
> > 
> > 
> > 
> >>[Original Message]
> >>From: ChandraKanth Gajawada <chandrakanth.gajawada@xxxxxxxxx>
> >>To: <si-list@xxxxxxxxxxxxx>
> >>Date: 1/9/2007 8:47:52 AM
> >>Subject: [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
> >>>Minneapolis, MN 55401
> >>>T // 612.436.5178
> >>>F // 612.672.9489=20
> >>>www.logicpd.com
> >>>/ / / / / / / / / / / / / / / / / / / / / / / / /
> >>>This message (including any attachments) contains confidential
> >>>information intended for a specific individual and purpose, and is
> >>>protected by law. If you are not the intended recipient, you should
> >>>delete this message and are hereby notified that any disclosure,
> >>>copying, or distribution of this message, or the taking of any action
> >>>based on it, is strictly prohibited.
> >>>
> >>>
> >>>
> >>>-----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
> >>>
> >>>DISCLAIMER:
> >>>Unless indicated otherwise, the information contained in this message
is
> >>>privileged and confidential, and is intended only for the use of the
> >>>addressee(s) named above and others who have been specifically
> >>>authorized to receive it. If you are not the intended recipient, you
are
> >>>hereby notified that any dissemination, distribution or copying of this
> >>>message and/or attachments is strictly prohibited. The company accepts
> >>>no liability for any damage caused by any virus transmitted by this
> >>>email. Furthermore, the company does not warrant a proper and complete
> >>>transmission of this information, nor does it accept liability for any
> >>>delays. If you have received this message in error, please contact the
> >>>sender and delete the message. Thank you.
> >>>
> >>>------------------------------------------------------------------
> >>>To unsubscribe from si-list:
> >>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >>>
> >>>or to administer your membership from a web page, go to:
> >>>http://www.freelists.org/webpage/si-list
> >>>
> >>>For help:
> >>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >>>
> >>>List FAQ wiki page is located at:
> >>>               http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >>>
> >>>List technical documents are available at:
> >>>               http://www.si-list.org
> >>>
> >>>List archives are viewable at:    =20
> >>>               http://www.freelists.org/archives/si-list
> >>>or at our remote archives:
> >>>               http://groups.yahoo.com/group/si-list/messages
> >>>Old (prior to June 6, 2001) list archives are viewable at:
> >>>               http://www.qsl.net/wb6tpu
> >>>=20
> >>>
> >>>------------------------------------------------------------------
> >>>To unsubscribe from si-list:
> >>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >>>
> >>>or to administer your membership from a web page, go to:
> >>>http://www.freelists.org/webpage/si-list
> >>>
> >>>For help:
> >>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >>>
> >>>List FAQ wiki page is located at:
> >>>               http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >>>
> >>>List technical documents are available at:
> >>>               http://www.si-list.org
> >>>
> >>>List archives are viewable at:
> >>>               http://www.freelists.org/archives/si-list
> >>>or at our remote archives:
> >>>               http://groups.yahoo.com/group/si-list/messages
> >>>Old (prior to June 6, 2001) list archives are viewable at:
> >>>               http://www.qsl.net/wb6tpu
> >>>
> >>>
> >>>
> >>
> >>
> >>------------------------------------------------------------------
> >>To unsubscribe from si-list:
> >>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >>
> >>or to administer your membership from a web page, go to:
> >>http://www.freelists.org/webpage/si-list
> >>
> >>For help:
> >>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >>
> >>List FAQ wiki page is located at:
> >>                http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >>
> >>List technical documents are available at:
> >>                http://www.si-list.org
> >>
> >>List archives are viewable at:     
> >>            http://www.freelists.org/archives/si-list
> >>or at our remote archives:
> >>            http://groups.yahoo.com/group/si-list/messages
> >>Old (prior to June 6, 2001) list archives are viewable at:
> >>            http://www.qsl.net/wb6tpu
> >>  
> > 
> > 
> > 
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > 
> > or to administer your membership from a web page, go to:
> > http://www.freelists.org/webpage/si-list
> > 
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > 
> > List FAQ wiki page is located at:
> >                 http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> > 
> > List technical documents are available at:
> >                 http://www.si-list.org
> > 
> > List archives are viewable at:     
> >             http://www.freelists.org/archives/si-list
> > or at our remote archives:
> >             http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> >             http://www.qsl.net/wb6tpu
> >   
> > 
> > 
>
> -- 
> -------------------------------------------------------
>      ___          _       Doug Smith
>       \          / )      P.O. Box 1457
>        =========          Los Gatos, CA 95031-1457
>     _ / \     / \ _       TEL/FAX: 408-356-4186/358-3799
>   /  /\  \ ] /  /\  \     Mobile:  408-858-4528
> |  q-----( )  |  o  |    Email:   doug@xxxxxxxxxx
>   \ _ /    ]    \ _ /     Website: http://www.dsmith.org
> -------------------------------------------------------
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List FAQ wiki page is located at:
>                 http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
>                 http://www.si-list.org
>
> List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu