As I reviewed my posts on RFI I realized I left several important
notes out.

Switch mode power supplies (SMPS) are extremely popular because of
the
greater efficiency. As energy costs rise and government mandates force
ever
greater efficiency. SMPS rapidly switch the rectified DC from the AC
mains.

SMPS emit RFI from the rectifier diodes, and from the switching
itself.
Most decent SMPS will have some sort of RFI filtering on both the AC
input and DC output, but sadly even those supplies that meet Class B,
the rating for devices used in residences, are very potent RFI
generators.

Additional filtering, isolation transformers with an integral electro-
static
shield can provide a significant reduction in RFI. But for many users
the
residual RFI will still be too high to be acceptable.

I have mentioned my wife's cursed APAP sleep apnea breathing assist
medical device is the nastiest RFI source other then a electric welder
I have ever dealt with.

The unit has an aux 12V power input for use in mobile or camping
situations.
I managed to reach one of the design engineers for the APAP and he
told me
that the FCC didn't regulate "Medical Devices". But he was very
helpful and sent
me complete specifications and a PDF service manual. He warned me
that the
air pump could draw as much as 1.5A peak current. I powered the APAP
from
a lead acid Gel Cell and the RFI was simply gone. So I built a 12V 5A
linear
power supply, with auto change over to a backup gel cell with a low
battery
audible alarm and one major RFI nightmare was eliminated.

Several friends have built linear DC power supplies for their laptops
in desperation
to quieten down their lap tops.

While I know that SMPS can be built quiet enough to rival linear power
supplies,
I suspect that most of us will never run into these good quality
units.

I am lucky because my Gateway SMPS ceased to be a significant RFI
source
with some additional filtering on the AC input.

Further effort is wasted because the power supply is now quieter then
my PCs.

A friend has a new Panasonic "Toughbook" that is exceptional RFI free.
The unit is
completely encased in metal, and even the SMPS isn't horrible. When
operating from
it's internal battery I could not detect any spurs that I could
attribute to the Toguhbook

Too bad they are so expensive.

The SMPS in almost every modern electronic device defy a simple
listing.
Every modern PC, TV, DVD, VTR, Battery Charger....on and on forever.

This is the reason why such devices are such strong noise sources.

SMPS are omnipresent and will continue to spread. And we will continue
to experience
increasing noise from them.

Another point I neglected is BPL, Broadband over Power Line. This is a
lame brained idea
that proposes to use the power line to carrier high speed data
communications. BPL will
be a serious RFI problem. To save rehashing all the info here, just
Google BPL and RFI.

NATO was concerned enough to write a report detailing the expected
problems to HF comms.
http://ftp.rta.nato.int/public//PubFullText/RTO/TR/RTO-TR-IST-050/$
$TR-IST-050-ALL.pdf

I am certain that I left out at least as much as I included.

Feel free to ask any questions about RFI and I will give it my best
shot.

Terry