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

[Robert Haller <rhaller@xxxxxxxxxx>]

Ravinder,
        Great topic, and lots of good discussion. Things to watch
Return path (as pointed out) , Keeping via stubs short, Dielectric losses (~ > 
2Ghz)
Keep Microstrip short if you mix with stripline - PWB processing on outer 
layers is typically additive, on inner layers subtractive - They do not track 
which creates larger impedance mismatchs.

Regards,
Bob Haller
SiSoft



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
>
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