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

["Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>]

There are practical reasons why microstrip is preferred. Try to route 150ohm 
differential traces in a lot of stripline layers and maintaining a thin PCB and 
you will know what I mean.
I never have problem with EMI with clocks, diff or single ended pairs on 
microstrip. Like I said many times, maintaining your current return is the key.
________________________________

From: si-list-bounce@xxxxxxxxxxxxx on behalf of Scott McMorrow
Sent: Thu 8/4/2005 10:38 AM
To: michael.mirmak@xxxxxxxxx
Cc: dbostan@xxxxxxxxx; Ravinder.Ajmani@xxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Which layer is better for GHz signals



Michael
Even with minimal differential trace coupling, the radiated fields from
the two sides of a differential pair will cancel each other in the far
field.  Generally, it is the common mode components of differential
signals that cause significant radiation.

regards,

scott

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



Mirmak, Michael wrote:

>Dan,
>
>Isn't the amount of EMI reduction from the use of "differential" traces
>dependent on the amount of coupling between them?  From the PC-based
>designs I have seen recently, the coupling between microstrip traces is
>very weak compared to the coupling to the reference plane.  The traces
>are more-or-less single-ended in this kind of case, with all the EMI
>radiation effects this implies.
>
>Are the traces you have seen very strongly coupled?
>
>- Michael Mirmak
>  Intel Corp.
>  Chair, EIA IBIS Open Forum
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Dan Bostan
>Sent: Thursday, August 04, 2005 9:33 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=20
>>signals.  I would like to know the views of other
>>esteemed SI experts on=20
>>the following two approaches.
>>=20
>>A) Current Design: Differential traces are routed on
>>Top and Bottom=20
>>layers.  The advantage of this approach is that  I
>>can route one pair=20
>>without using any vias, and I get more usable board
>>space.  The=20
>>disadvantage is that traces on Top and Bottom layers
>>have greater=20
>>impedance discontinuities due to uneven plating and
>>soldermask=20
>>application.=20
>>=20
>>B) Proposed Design:  Differential traces will be
>>routed on the inner=20
>>layers.  The advantage here is that the trace
>>impedance will be more=20
>>uniform as the trace will be covered with dielectric
>>on both sides, and=20
>>there will be no plating.  The disadvantage is that=20
>>I will have to use=20
>>two vias on each trace, and I will have less routing
>>space.
>>=20
>>The reason for considering the new approach is
>>because of the EMI issues=20
>>with the previous design traced to common-mode
>>currents.  I am not sure=20
>>which design will have less common-mode effect, as
>>one design has=20
>>discontinuities due to plating/soldermask whereas
>>the other design has=20
>>discontinuities due to extra vias.=20
>>=20
>>The trace length is about 2 inches.
>>=20
>>Regards, Ravinder
>>Server PCB Development
>>Hitachi Global Storage Technologies
>>=20



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