Martin,
Filtering the power line is the right thing to do. The broadband
switching noise from the power supply gets onto the power mains and
radiates from them. (It seems there's been a similar discussion about
BPL lately....). However, all the ferrite clip-on does is try to force
the high frequency transients to be differential on the power line.
This reduces the radiation some, but doesn't really reduce the noise
power on the line much. Corcom makes power line filters that will
help. You can get their products from Digi-Key or Newark Electronics.
Another option is to replace your computer power supply with an
"Instrument Grade" unit. This will cost more than an odd integral
multiple of the price of the average low budget computer power supply,
but you will be amazed at the reduction in noise. I've had good luck
with the brand "Enermax". There are others. ( I have no personal
interest in either Corcom or Enermax)
I work at Georgia Tech Research Institute, where just about every
employee has a computer on their desk, and where there are network and
email servers, routers, uninterruptible power suppplies, etc.
everywhere. The noise from all the switching supplies radiating from
the power lines in the buildings has gotten so bad that we have had to
build screen rooms with power line filters in order to prevent
corruption of critical signal measurements by power line noise! Our
office building is within metro Atlanta, but it is just about impossible
to listen to any local AM broadcast radio station in any of our
buildings because of this power line noise problem.
I learned by disassembling several low cost computer power supplies
that the foreign manufacturers simply do not install the filtering
components required to keep the power lines clean. Looking at the
circuit board of one of these cheap supplies, you'll usually find
several silkscreened input circuit component locations marked "L"
bypassed with a wire, and items marked "C" left open. While the
original design included these filter items in order to get US FCC type
acceptance, they are left out of the mass produced product to save a few
cents per supply, and no one ever checks when thousands of these come on
shore in shipping containers..
Bob
WB4TGG
Martin wrote:
Hello elecrafters,
today i wandered through the house carrying the battery-powered K2
with a vhf antenna connected to ŽsniffŽ for the noise i have on 80 &
160m. On 160m, snap-on ferrites on 2 network-cables completely
removed the carriers. On 80m the problem seems to be more
difficult: The noise is produced by one of 3 Computers in my buro,
even when it is switched off. The other computers are quiet, also the
one in the shack.
The noise becomes a lot stronger when the computer is on, but only
completely disappears when it is disconnected from the mains.
Its a raspy carrier slowly sweeping up and down the band.
Any ideas where to start? A snap-on ferrite on the power-cord
helps a little, but not much.
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