[RFI] LCD Monitor RFI

dgsvetan@rockwellcollins.com dgsvetan@rockwellcollins.com
Mon, 19 Nov 2001 09:25:00 -0600

Pete and Group,

Alas, even though LCD displays lack many circuits that are required in
CRT-based systems, the bottom line is LCD displays, in and of themselves,
are not a panacea for computer display RFI in the ham shack.  The drivers
for the display itself are high speed, and you must remember that the
drivers are turning current on and off into a load that looks somewhat
capacitive;  hence, sharp spikes from those charging current pulses.  An
additional source of RFI in LCD displays is the backlight system and the
associated drivers for that.  Combine all of that with a nice, large
aperture (the viewing area), and you have the makings for a good broadband
noise generator.

Adherence to good EMC techniques by the designers and manufacturers can and
will result in producing displays that meet US and European requirements
for emissions.  However, there are still finite emission levels from these
devices, and I can tell you that when commercial LCD displays are
considered for use in aerospace applications (where allowable emission
levels tend to be much lower than FCC/CE mark requirements), those displays
frequently require modifications to the circuit board, wiring scheme, and
display backplane.

If you are considering the purchase of an LCD monitor, I would suggest
taking along a portable receiver to check the levels of the display models;
otherwise, make certain that you can return the unit at no penalty if it
does not work for you.  I have no clue as to any particular make/model that
are better/worse than any others.  As has been posted on this reflector
many times in the past, there are several "screamers" out there among the
CRT types that should be avoided.  We have to assume that the "screamers"
have passed FCC testing, but my guess is that the worse ones are right near
the allowable limits.  Since another way to spell "EMC" is "$" (per EMC
guru Darryl Gerke, K0FBF), equipment designers can not reduce emissions
below any level that manages to pass requirements.  I suspect that the
"good" ones are actually designed with more margin to begin with (which is
actually the least expensive approach, as opposed to last minute fixes
applied to pass FCC).

One last point:  although this is NOT guaranteed to get you a "quiet"
display, it MIGHT help if you get one that not only has the FCC ID label on
it, but the European CE mark, as well.  The reason is that the Europeans
require at least 3V/m RF immunity.  That may reduce RFI to the display from
your rig, but my main point in suggesting that you look for the CE mark is
that the design measures taken to achieve those 3V/m immunity numbers can
also be effective at reducing emission levels.  Like I said, not a
guarantee, but everything helps.   Note that most of the major players sell
one piece of equipment for all markets, worldwide, so finding a CE-marked
piece of computer equipment is relatively easy these days.  Good luck.

73, Dale

Pete Smith <n4zr@contesting.com>@contesting.com on 11/17/2001 08:04:53 AM

Sent by:  rfi-admin@contesting.com

To:   rfi@contesting.com

Subject:  [RFI] LCD Monitor RFI

With the prices plummeting and Gateway standardizing on LCD monitors, it
looks to me as if this may become an economical alternative within the next
year.  My Sony Multiscan E-100 is horribly noisy, and I'm thinking about
replacing it with an LCD because I've never heard of any reports of RFI
from LCDs.  Is this right?  Anyone have a contrary experience, with a
laptop for example?  Are the pure digital LCDs (which I guess require a
special video card) to be avoided, or embraced, from this standpoint?

73, Pete N4ZR

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