FreeLists / si-list / [SI-LIST] Re: Which layer is better for GHz signals

[Dan Bostan <dbostan@xxxxxxxxx>]

Assuming the question was about Gbit Ethernet signals,
for 2 in traces, the number of vias (under a
reasonable number-2-3) is not an issue, as long as the
same number of vias are on both members of the diff.
pair.
/dan



--- Mike Greim <mike.greim@xxxxxxxxxxx> wrote:

> Playing devil's advocate, one might
> make the case for routing on outers 
> as one can avoid the parasitics associated
> with via stubs or may be able to 
> avoid vias altogether.
> 
> Seulement mes deux centimes....
> 
> Best Regards,
> 
> Michael Greim
> Platform Engineering
> Newisys, Inc.
> 512-340-9050, ext 232    (V)
> 512-740-9097                  (C)
> 
> We will either find a way or make one     -Hannibal
> 
> And all this science they don't understand
> is just my job six days a week........
> 
> The time is gone, the email's over
> Thought I'd something more to say.......
> 
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
> Dan Bostan
> Sent: Thursday, August 04, 2005 11:33 AM
> To: Ravinder.Ajmani@xxxxxxxxxxxxxx;
> si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Which layer is better for GHz
> signals
> 
> 
> In general, it is better to use inner layers for
> high
> speed signals, for EMI reasons.
> However, since the GHz traces are differential, the
> radiated field is not as strong as in the case of
> single ended traces.
> From my experience, EMI was not a factor in routing
> such signals.
> Which means, that your other concerns should dictate
> the layer.
> My two cents.
> /dan
> 
> 
> 
> --- Ravinder.Ajmani@xxxxxxxxxxxxxx wrote:
> 
> > Hi All,
> > For my next design I am considering a different
> > approach to route the GHz 
> > signals.  I would like to know the views of other
> > esteemed SI experts on 
> > the following two approaches.
> > 
> > A) Current Design: Differential traces are routed
> on
> > Top and Bottom 
> > layers.  The advantage of this approach is that  I
> > can route one pair 
> > without using any vias, and I get more usable
> board
> > space.  The 
> > disadvantage is that traces on Top and Bottom
> layers
> > have greater 
> > impedance discontinuities due to uneven plating
> and
> > soldermask 
> > application. 
> > 
> > B) Proposed Design:  Differential traces will be
> > routed on the inner 
> > layers.  The advantage here is that the trace
> > impedance will be more 
> > uniform as the trace will be covered with
> dielectric
> > on both sides, and 
> > there will be no plating.  The disadvantage is
> that 
> > I will have to use 
> > two vias on each trace, and I will have less
> routing
> > space.
> > 
> > The reason for considering the new approach is
> > because of the EMI issues 
> > with the previous design traced to common-mode
> > currents.  I am not sure 
> > which design will have less common-mode effect, as
> > one design has 
> > discontinuities due to plating/soldermask whereas
> > the other design has 
> > discontinuities due to extra vias. 
> > 
> > The trace length is about 2 inches.
> > 
> > Regards, Ravinder
> > Server PCB Development
> > Hitachi Global Storage Technologies
> > 
> > Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
> > 
> >
>
------------------------------------------------------------------
> > 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
> >   
> > 
> > 
> 
> __________________________________________________
> Do You Yahoo!?
> Tired of spam?  Yahoo! Mail has the best spam
> protection around 
> http://mail.yahoo.com 
>
------------------------------------------------------------------
> 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
>   
> 
> 

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 
------------------------------------------------------------------
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