Re: Electrically Quiet Cases

[mike808@users.sourceforge.net]

> which ones provide the best shielding in terms of containing unwanted RF 
> emissions and electrical noise.

Meaning you want to prevent other devices from being interefered with by
whatever you put into your case, correct?

If so, the case manufacturer can't guarantee anything since they don't know
what you're going to put into it.

> When I look at ads for cases you're lucky if they even mention type A or B 
> FCC compliance.

All that means is that whatever device is *inside* the case will not interfere
with devices *outside* the case. Again, the case manufacturer cannot make any
claims as to that.

That's why only complete systems have FCC compliance certifications.
And many manufacturers claim compliance without certification, and will take 
the risk of simply having to invoke a warrantee clause if there are any
complaints to the FCC about their device. i.e. you complain to the FCC, they
test the device, add shielding and pay for compliance testing and then give you
a free new item to "remedy" your problem with the other device. That way they
don't have to pay any of those costs unless and until someone files a complaint.

Do you know anyone who has ever filed a written complaint with the FCC alleging
you have a non-compliant device?

> This may be because there's not a good way to tell until the system is built.

Bingo.

You may be better off using whatever case you like, and looking for RF 
Shielding devices or put the unit in a Faraday cage.

However, as a rule of thumb, the more metal in the case, the more likelyhood
you will have some dampening of the RF emanating from the stuff inside (or
dampening the RF from the stuff outside from affecting the stuff inside).
You also want to look for those thin foil-like "fingers" or "coil" that serve
to increase the surface area of low-resistance contact between the frame and
various panels of the case.

And if you know the frequency ranges you're talking about, you can probably
construct a Faraday cage tuned to those wavelengths to isolate your devices.

So the key item you're probably looking for and will find is the *weight* of the
case (which tells you roughly how much metal there is, versus lighter plastic),
and the thickness of the *metal* sheets used for the frame and panels. And
perhaps a fully aluminum case, like the Lian Li (and knockoff) models should be
considered.

Mike/

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