From: Douglas McKean ([email protected])
Date: Thu Oct 28 1999 - 13:12:35 PDT
At 08:40 AM 10/28/99 -0500, Mike Mayer wrote:
>
>Part of it is that people want "rules of thumb". Alot of these issues
>are complicated enough that there are few general rules. The other
>thing is, as you point out, the dependence on empirical data. The
>CompactPCI Specification says that "CompactPCI boards shall not
>connect earth ground (on front panel) through a low impedance path to
>logic ground used on board." I seem to recall seeing a paper measuring
>the difference in radiated emissions between boards with logic ground
>connected to chassis ground and with them connected, the conclusion
>being that emissions were lower when they were connected. The
>CompactPCI spec does not say why they require them to be isolated, si
>I don;t know if there is a good reason or if they were just using a
>"rule of thumb".
Rules of thumb are part of the tool kit for engineers.
As long as one is aware from whence they originate,
I have no problem with anyone using them. It's when
the person who isn't even partially aware of the origins
uses them blindly. There have been several debated
here quite extensively. 2W and 20H are just a couple.
Traversing across split grounds is another and there
seem to be pros and cons with that under special
conditions as well with the general consensus being
"don't do it."
The PCI bus being a reflective based system is my own
personal nightmare. I'm not sure who came up with it,
but I can only conclude the very last thing concerned
was EMI. My own experience and talking with people at
the labs tells they can be a common problem. Usually
when confronted with a problem at 132, 198, or 330 MHz
the knee jerk question is "Ya got a PCI bus in there
doncha?" One of just a couple counter examples where
one can have excellent SI and terrible EMI. - Doug
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