# RE: 10 Gigabit EMI

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>  In the world of EMI, my personal experience is that the
>  problems are a combination of religion, black magic, and
>  some practical experience (with a little knowledge of
>  Maxwell's equations thrown in). Historically, it has been
>  very difficult to get "experts" to step up to the plate
>  because invariably there are problems in the actual
>  implementations that the "experts" simply can't explain.
>  This is not a dig on the experts, just a point on the
>  extreme difficulty of the problem.

Actually, it is a matter of understanding that not all of the
components in your design, are on the schematic. Once you realize
that, the problems get easier to understand.

>  It is my opinion that the coding will be an important, but
>  second order effect. My opinion is that the rise times are a
>  second order effect. My opinion is that the actual bit rate
>  is a second order effect. I know, I know. You can bring me
>  Maxwell's equations and show me the math that demonstrate
>  the direct relationship between these parameters and the emissions.

Whether a problem is first-order or second-order depends on the
magnitude of your first-order problem. If your box has a TON of
high frequency attenuation, these problems may appear as second-order.

On the other hand, if your box is constrained by manufacturing
technology, time-to-market, etc.. they may become first order.

The basic equation is EMI  =  Source Magnitude * Channel Attenuation

For any given frequency, the Source Magnitude is determined by the
baud rate, rise time, repetitiveness of edges, amplitude of the signal,
and imbalances in the electrical channel.

Channel Attenuation is harder to grasp because there are many paths
for EMI to get out of a box. Ground/Power planes, air, traces, slots,
etc.

One thing for sure... If you have a line-code that is inherently
going to be an order of magnitude lower than its alternative in EMI,
it is much more likely to fall into a second-order bucket than its
alternative.

>  But, in the end, it is the holes is the system that
>  matter!!! It is the little antennas built into the boxes. It
>  is the phase array of optical transceivers.

It might be the fan holes, or the power-cord entry for that matter.
It depends on the knowledge of your system designer. But whatever the
"channel" for propagation, it always comes back to the source. What
is the source and how big is it?

>  Most transceiver companies will claim that the majority of
>  the noise that radiates from the nose of the optical
>  transceiver is generated inside the system, not inside the
>  transceiver. The new "small form factor" transceivers, with
>  ~1/2 the aperture size of the SC, are not inherently going
>  to solve the EMI problem at 10 Gig.

But there are shielding techniques to reduce the impact of these
aperatures substantially. Whatever the shielding requirement is, it
will be based on the spectral content inside the box.

>  In short, from an implementation perspective, we have a very
>  fundamental problem: the way we design systems today as
>  racks of modules with arrays of optical transceivers poking
>  out through the EMI enclosure may simply not hack it.

Maybe.

>  Up to Gig E, we have had the luxury (admittedly, with a fair
>  amount of pain) of having a "convenience based, user
>  friendly" box design. With 10 Gig, have we finally hit the wall?

This might have been said about 10 or 100 or 1000. We will see. Rather
than choose a "spikey" code and roll the dice, I would recommend
using careful consideration for the code.. then roll the dice. :)

I liken EMI design to firefighting.. It is better to prevent fires,
than try to to contain them.

8B10B might be too hot to handle without some improvements.

Dan

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