=There really ARE Class B computers, but you
have to work to make them, and you have to look
to FIND them. I've worked on some of them.
Between CRT's and LCD's, the generators are
different, but the levels are not necessarily
lower for an LCD.
A CRT has a fairly powerful, high frequency
amplifier driving its cathode. This has to
turn pixels on and off rapidly; the FCC H
pattern is designed to generate a periodic
signal as the electron beam is swept across
the screen and turned on and off. Sometimes
this current is not properly handled, and shows
up as a conducted emission, or even is radiated
and makes the system it's used with fail radiated
or conducted emission tests. I've even seen
failing levels on an EUT that was powered down, if
a particularly energetic monitor was located nearby!
An LCD screen doesn't have an electron _beam_
switching on and off -- it has other problems. It
has a backlight, whose switching power supply may
cause problems. It has pixel drivers which do turn
on and off as pixels are addressed, and chip and
pixel select is always being addressed, whether a
pixel is turned on or not. Large video memories may
radiate on their own, as well; you are at the mercy
of silicon builders as to how fast an edge they use.
And turning pixels on and off still takes SOME energy,
even if you don't make them glow with it. The folding
screen typically has a fairly small connection to the
main board, thus an impedance between it and the power
supply, and it is not unusual to see an open laptop
working as a dipole fed at the screen/body connection.
Plus, an emission generated on the >main< board may be
radiated by this dipole, especially if it is not well
decoupled where it is generated.
At a former employer, we tested a laptop that was many
dB over the limit. The OEM apologized; they had - they
said -- accidentally shipped us one without conductive
shielding paint inside. That model laptop, WITH
conductive coating, later DID test Class B compliant.
Cortland
ka5s
====================== Original Message Follows ====================
>> Date: 28-Jun-01 17:29:23 MsgID: 1078-56808 ToID: 72146,373
From: George Stults >INTERNET:[email protected]
Subj: LCD Monitors etc.
Chrg: $0.00 Imp: Norm Sens: Std Receipt: No Parts: 1
Hello All,
I have theorized that a Liquid Crystal Display (LCD) monitors should have
less radiated emission than a comparabe CRT (same viewing area) since there
is no high voltage tube and high powered oscillators, etc. I am wondering
if anyone can confirm or deny this rumor based on actual testing. Also If
there are folks in the crowd who sell LCD monitors, I would like to hear
from you offline.
And to follow up an earlier question that I asked the group about quiet
test equipment, I did an experiment as follows: I purchased and tested a
Fujitsu LifeBook B-2175 (500 MHz laptop). As a 'Best Case' I tested it
stand alone, battery operated, no cables, just running MS Word with an
'HHHHHHH' pattern on the screen (I'm guessing that the pattern made no
difference on the LCD screen). In that configuration, it was 4.4 dB over
class B. I think the test site claims an measurement uncertainty of 4 dB.
Anything else I did, connecting cables etc, made it worse. The lap top in
question has a magnesium case and gives the appearance of quality
construction. Given the 80/20 rule, the production process could be
compliant and I wouldn't know any different, but I wonder if there is such
a thing as a Class B laptop in the real world?
Best Regards
George Stults
WatchGuard Technologies Inc.
====================== End of Original Message =====================
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