[SI-LIST] Re: Routing 10G differential lines over Standard FR-4
- From: steve weir <weirsi@xxxxxxxxxx>
- To: mahesh@xxxxxxxxxx, billw@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 16 May 2005 06:40:30 -0700
Mahesh, most of the return current is not between the members of the pair.
Steve.
At 06:46 PM 5/16/2005 +0530, Mahesh Chandra wrote:
>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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