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Re: 8b/10b and EMI

Hi Rick,

Hmn.  I'm not sure I understand your position.  If you are saying
that a 10-Gbps system that wasn't even conceived 20 years ago
needs to be built with the same design and manufacturing 
practices that were use at that time, then I think you are incorrect.

However, if what you mean is that it may be necessary change the 
design of an optical module such that it effectively "builds in"
the necessary EMI shielding, then I think you are correct.

This does not require rocket science, just a new way of thinking
about how things are built.  I think it should be able to build
10 Gbps links with costs in the 3x range of present 1 Gbps systems
(I believe that is the present target range for this development).

Will these modules look the same as todays SFF modules?  Probably
not.  But todays SFF modules don't look at all like the duplex
SC modules, or GBICs, or GLMs, or OLC cards either.

I still think Joe's pinhole opening idea is a good one--if its done
correctly.  Just build the optical module so that its media interface
is the same as that of a coupler which buts two ferrules together.
In the module, couple from the laser, through a ball lens, into
a 1-cm fiber that is embedded in a metal ferrule.  If this 
entire assembly is placed into a metal shell, you have no floating 
metal to act as free radiators, you have a standardized fiber-to-fiber
mating interface, and an opening capable of radiation that is
125 microns in diameter.

This is all possible using todays technology.  Will it cost more than
a plastic duplex SC module using an LED driver at 155 MBd?  Absolutely.
I don't think anyone here expects it to cost the same, because it
is not doing the same job.


Ed Grivna

> Dear Ed,
> > Ed Grivna <elg@xxxxxxxxxxx> writes:
> > I think you missed Joe's point.  He makes reference to a 1-mm hole.
> > that is the size hole you would have if you routed only the
> > bare fiber through the hole.  This implies usage of a buried
> > module, or a module with a fiber pigtail.  In either case, the
> > fiber is routed through a metal plate, significantly removed from
> > the LASER diode and driver.  The plate is generally constructed as 
> > a buried wall in the chassis, with a second bulkhead used to mount 
> > connectors.
> I don't think I missed the point.  I was attempting to show that this
> extremely cumbersome technique is not widely used in low-cost systems. 
> Most low-cost modules are not pigtailed.  They have a front-panel
> connectors.  In this environment, it is much more difficult to properly
> shield things. 
> Are you suggesting that we should require a very expensive, telecom
> style of construction for low-cost 10GbE ports?  Remember that many
> systems may be built in metallized plastic packages with front-panel
> mounted non-pigtailed modules. 
> Best regards,
> --
> Rick Walker