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Re: 8b/10b and EMI

Dear Joe,

> Joe Gwinn writes:
> Yes.  All it takes is a small enough hole, and even 10 gbaud will be
> confined.  If the transmitter and receiver are correctly designed, the
> photons will pass through a 1-mm or 2-mm diameter hole in a metal wall,
> which will cut off everything below a few hundred gigahertz:  (3*10^8
> m/s)/(10^-3 m)= 3*10^11 Hz= 300 GHz.

I don't think it is so simple.  Your analysis might be correct for pure
wave propagation, but it is common for the voltage waveform on the laser
diode to capacitively coupling through the dielectrically loaded hole
onto the fiber ferrule.  Many fiber connectors use metal ferrules, which
make a dandy little antennas. 

It is quite difficult to make a well grounded 2-mm hole.  Usually the
hole in the main panel is much bigger, accommodating the entire module
dimension, and a metal shim in the module acts as the 2-mm shield.  For
this shield to work, it needs a good seal to the front panel.  Many
vendors try to do this with spring loaded flanges, etc., but some
customers prefer to keep system-ground separate from chassis ground. 

This causes many module vendors to capacitively couple the occluding
shim rather than actually connecting it to the system ground.  Of course,
the coupling cap has a series inductance, and really doesn't do much good
above a 1G or so.

Even at 1Gb/s, the various ground loops intrinsic to this fiddly game
make it difficult for most designs to solidly meet EMI regulations
under a wide range of construction practices.

Combine this with the fact that radiation from a current loop
is proportional to (Iloop*Area*frequency^2), and you have a problem
that is 100x more difficult at 10G than 1G.  Anyone who had a mild
headache at 1G will definitely be doing some tough engineering at 10G.

Rick Walker