Re: Emissions from 8b10b XAUI/HARI
Hi Henning,
your question is more based on transmission line construction rather
than line coding. If the true and complement signals are both
of the EXACT same magnitude, and remain EXACTLY 180 degrees out
of phase with each other at all times, the two lines of the
balanced transmission line are tightly coupled and exactly matched
in all routing, AND (this is the important/impossible one)
both lines consume the itentical piece of space, then yes there
will be no radiated emissions.
OK, now that the theoretical parts are out of the way, I'm a bit
surprised at seeing this question coming from Giga, because I
know that your company has some of the best transmission line
design capabilities in the world.
Getting slightly more realistic, the faster your signalling gets,
the more difficult it is to keep a signal balanced. At each point
that an imbalance is created, you wind up with differential to
common mode conversion, and these common mode signals are the ones
that will give you most of your trouble. Since a ground plane is
also present, that means that the tranamission lines have both odd
and even mode impedances, and to terminate them correctly, you
need to terminate BOTH modes/impedances. This measn that even
though you have a balanced transmission line, it takes more than
just one resistor across the pair to terminate the line.
If you are talking about a cable instead of a circuit board, then
things get worse, because most cable manufactures are willing to
document their balanced or differential impednace, but you'll
be very hard pressed to find ANY that will also document their
even mode impedance.
Regards,
Ed Grivna
Cypress Semiconductor
> From: "Henning Lysdal" <hl@xxxxxxx>
> To: stds-802-3-hssg@xxxxxxxx
> Date: Wed, 15 Mar 2000 13:57:44 +0100
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> Subject: Emissions from 8b10b XAUI/HARI
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> All,
>
> I hope you will have patience with me as I ask a very basic
> electromagnetics question (and remember I'm a chip guy, not a
> board guy):
>
> Assume that you have two transmission lines with a common ground
> plane with a pseudo CML driver. The signal on one will be exactly
> the same as on the other except for a very important sign inversal.
>
> Wouldn't the emissions from one line cancel that of the other
> regardless of the coding???
>
> Henning
>
>
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