Re: [SI-LIST] : 2.5Gbps across a backplane?

From: Michael Nudelman ([email protected])
Date: Mon Oct 16 2000 - 10:19:26 PDT

BTW,

When Dennis Thomlinson suggested a math description of a impedances of coupled
versus un-coupled pairs, it seems, tha it substituted the real subject, that
was not the coupled versus un-coupled but edge-coupled versus broadside
coupled pairs; what was forgotten is, that the edge-coupled pair is not a
coupled pair with 100 Ohm (for .ex.) impedance - it is two wires with 50 Ohm
each, and just because they are done in a exactly same fashion, they form this
pseudo-diffpair. If in case of broadside coupling you have sizable coupling
between traces, in case of edge-coupling it is next to none, otherwise you
would have to form it from traces with higher than 50 Ohm impedance.

In Hi-Speed connectors, however, you find different couplings - there are
connectors with purely differential coupling, wher a pair of pins forms 100
Ohm pair, or the is a mixed coupling, where pins form 100 Ohm pair, being
coupled to each other AND to a GND blade, over which they run. The question
is, what part each coupling plays in creation of those 100 Ohms.

But, as I said - we do use tha type connector with broadside, and, having
taken care of driver/receiver types and PCB material, we do not have
noticeable problems in that department.

Mike.

The only true diffpar is two wires with no reference planes, forming 100%
coupling impedance equal to char. impedance. Like a twisted pair.

GARY ROSEN wrote:

> I had never heard the term before this thread either. It seems
> obvious that you would get stronger coupling from broadside
> than from edge-coupled (depending on layer thickness), right?
> Are there any second-order effects to be aware of?
>
> - Gary Rosen
>
>
> > Broadside coupling is when diff pair traces are run parallel on adjacent
> > signal layers.
> >
> > As in....
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