RE: [SI-LIST] : Hopefully not a controversial question ...

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From: George Borkowicz (jbor@nortelnetworks.com)
Date: Fri Feb 25 2000 - 06:36:04 PST


Doug,

This technique is closely related to the system level concepts of single
point hierarchical grounding. In our company we have first implemented it
in 1970s with the introduction of the DMS switch. This was done to mitigate
all the possible issues mentioned by Michael and more (AC/DC power return
interference, lightning/ESD susceptibility etc.). To this day this system
is deployed in a "single point grounded" environment and requires that all
(I mean ALL) metallic interfaces enter the "island" through the "bridge"
which here is called a "ground window" and also hosts a single ground point
(in this case a big chunk of copper). This discipline has served us well
over the past couple of decades (look at the stock) despite difficulties in
enforcing and policing system isolation in everyday life. As Michael
mentioned, on a board level one has even more toys available in a sandbox
and partial/controlled isolation can be used successfully to compromise
between the desired effect and the routing or SI (length, X-talk) problems.
Good field simulator seems essential in this approach.

Just chatting...

George Borkowicz
tel: 613-763-2682
jbor@nortelnetworks.com

> -----Original Message-----
> From: Doug McKean [SMTP:dmckean@corp.auspex.com]
> Sent: Thursday, February 24, 2000 2:05 PM
> To: SI Discussion Group
> Subject: [SI-LIST] : Hopefully not a controversial question ...
>
> Moating: What have people discovered (in general)
> about this technique over the years?
>
> At another company, I was in a position where
> a S/N ratio was critical with CATV circuitry
> as was defined by the FCC. Moating did quite
> a nice job meeting that criteria versus not
> moating. The specific requirement by the FCC
> I can't recall, but the moat achieved something
> on the order of -10dB to -15dB more than not
> moating. Two spins of boards were done for
> comparison.
>
> Buuut, a very specific case under very specific
> conditions. So I don't think it's a hard fast
> rule that has to be followed all the time.
>
> Anyone up for some general comments?
>
> Regards, Doug McKean
>
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