[SI-LIST] Re: Routing 10G differential lines over Standard FR-4
- From: Mahesh Chandra <mahesh@xxxxxxxxxx>
- To: billw@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 16 May 2005 18:46:15 +0530
For well designed differential pairs do you need to be concerned about vias
to power planes ?
I assumed as long as differential pairs are very closely matched, vias to
power planes was not a concern since the RF return current is in the
differential pair itself and not through the power plane
Mahesh
At 06:32 PM 5/16/2005, Bill Wurst wrote:
>Sunil,
>
>Unfortunately, there are no simple answers at 10Gbps. At that speed,
>there are three main concerns and you have already mentioned two:
>dielectric losses and discontinuities. The third is skin-effect losses,
>which can be mitigated by widening your traces and spaces and increasing
>the dielectric thickness to maintain a 100-Ohm differential impedance.
>
>To be certain of the design, one must develop a link budget and
>determine what margin remains after all losses (including reflections
>from discontinuities) have been accounted for. If your link length is
>short, then it may be possible to ignore dielectric and skin-effect
>losses and use standard FR-4. What defines "short" will depend on your
>design; the dividing line may be on the order of 10cm at this speed.
>
>As you point out, the advantage of microstrip is that no vias are
>required. The disadvantages are that the plating process on the outer
>layers makes it more difficult to control the impedance, and soldermask
>tends to be fairly lossy. Fortunately, at 10Gbps non-TEM modes that
>exist in non-homogeneous dielectrics should not be a problem yet.
>Stripline can be made to work, but attention must be paid to both the
>signal vias and the vias in the reference planes for the return current,
>including the signal via stubs (blind vias or back-drilling can
>eliminate these). A 3D field solver can be most helpful here.
>
>Some useful references:
> High-Speed Signal Propagation by Johnson/Graham (particularly
> Chapters 2, 3 and 6, as well as section 5.5 on vias),
> A SI Introduction to RocketIO Transceivers, and
> Loss Budgeting for RocketIO Transceivers.
>The last two are DVDs available from Xilinx at:
>http://www.xilinx.com/products/design_resources/signal_integrity/index.htm
>and contain much useful information no matter which vendor's parts you
>are using. Scroll to the bottom of the page for more information.
>
>Again, no simple answers but I hope I've been helpful.
>
> -Bill
>
>
> /************************************
> / billw@xxxxxxxxxxx /
> / /
> / Advanced Electronic Concepts, LLC /
> / www.aec-lab.com /
> ************************************
>=================================================
>sunil.mekad@xxxxxxxxx wrote:
> > Hi Everyone,
> >
> > I was planning to route high speed differential traces 10Gbps over
> > standard FR-4 material. Can anyone suggest which FR-4 material can be
> > used (Nelco N4000-6 or Rogers RO4350)? Is it possible to do use standard
> > FR-4 without compromising signal quality.
> >
> > Also is it recommended to route these traces over microstrip rather than
> > striplines traces? If I route it over stripline then it would require
> > vias to be put on every pair of these differential traces .. I am not
> > too comfortable doing that!!
> >
> > Can anyone tell what can be done? Thanks in anticipation,
> >
> > Regards,
> > Sunil
> >
> >
> >
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