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Re: Laptop TEMPEST (fwd)
At 08:30 AM 2/9/98 -0600, Jim Choate wrote:
>> FCC RF/EMC testing is well nigh useless for TEMPEST protection.
>
>I disagree, it would give you a gross baseline on the total emissions
>between monitors and laptops. That field strenght measurement would at least
>allow you to calculate radiuses of equal strength to calculate approximately
>how far the emissions are from each class of device for equal probabilities
>of detection. One of the specific goals is to measure how effective the
>device is at effecting other co-located devices (such as seeing ghost images
>on other monitors or causing static in paging equipment).
FCC specs aren't real tight.
My old laptop*, which passed FCC specs (at least when it was new,
before being hauled around on airplanes and trains, dropped a few times,
having cooked parts replaced, and generally getting treated like a laptop),
broadcast enough emissions that its video would show up on my parents' TV
when used 2-4 meters away. The video sync wasn't right, so there were
several hard-to-read images of the screen on the TV, but it was obviously
emitting enough that a properly tuned receiver could read it.
On the other hand, it doesn't bother my TV from 8-10 meters away,
but I've got cable, while my parents use Real Rabbit-Ear Antennas and
some kind of antenna-booster widget.
I have heard that passive-matrix emits less, but I don't know
if that's true from a TEMPEST standpoint or only noise output.
It is a bit easier to put a laptop into an RF-shielded enclosure,
but enclosures that worked fine for harmonics of 4.77 MHz machines
don't necessarily block harmonics of 477 MHz machines :-)
especially critical are the penetrations used to get air and
fiber-optic connections through, which are basically waveguides -
once the wavelength of the signal is short enough, they lose.
___
* AT&T Globalyst 250P, which is a NEC Versa Pentium-75 with a Death-Star
painted on it. It has the expensive active-matrix 24-bit-color 640x480
screen (sigh - I'd rather have had the cheaper 800x600 8-bit-color :-).
A couple years old, and the case was a bit dented so perhaps it had been
more radio-tight when it was new.
Thanks!
Bill
Bill Stewart, [email protected]
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