[SI-LIST] Re: Which layer is better for GHz signals
- From: steve weir <weirsi@xxxxxxxxxx>
- To: michael.mirmak@xxxxxxxxx, <dbostan@xxxxxxxxx>, <Ravinder.Ajmani@xxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 04 Aug 2005 11:01:30 -0700
Michael, yes, but as you note: coupling is weak so it is not a big
contributor to net reduction in EMI.
The big reduction is due to the signals being complements. If they are
accurate complements then the return currents induced in any antenna, be it
the adjacent return plane, or a distant victim, oppose and the net
observable current can be many times lower than what we would get with a
single ended signal.
The show and tell experiment is to construct a board with two traces
separated by 10H. Measure the EMI with at 3 meters:
A. One signal switching, and the other driven to a hard Vcc or hard Vss level.
B. Both signals switching in phase.
C. Both signals switching complementary.
D. Both signals switching complementary with a defined phase offset.
C. will perform the best.
Regards,
Steve.
At 09:55 AM 8/4/2005 -0700, 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
> > Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
> >=20
> >
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