Dave Platt wrote:
In article t,
W. Watson wrote:


I tried the RS ferrites and they made no change. I even took barbell weights
and put them on the AC line of the PC. Zippo. If I hadn't already spent $60
for the new PSU six weeks ago, I might think of buying a much better one.


Ferrites clamped around the power cable will help with common-mode RF
noise. They won't do anything to help cancel out differential-mode
noise.

Differential mode radiation is almost never a problem in situations like
this. The differential mode component of the noise, by definition, is
from equal and opposite noise currents on the two conductors (of the
power line, for example). For this mode, the conductors comprise a
transmission line, and radiation will be extremely small because the
fields from the two very close and parallel conductors are equal in
magnitude and opposite in direction.

Ferrites are a good suggestion, but the ones commonly used for RFI
suppression are of a ferrite type which doesn't have an awful lot of
attenuation as low as the AM broadcast band. Best would be some
Fair-Rite 70 series, or other ferrite with an initial permeability of
several thousand. Best would be to get a large core and wrap multiple
turns on it with the power cord, as close to the PC as possible. This
way, you get a lot more impedance than clamping cores onto the wire,
since the impedance is proportional to the square of the number of
turns. That is, 10 turns on a single core gives you the same impedance
as 100 of the same cores clamped onto the cable.

It'd probably be beneficial for you to try to get a robust powerline
noise filter. These will filter out both common-mode noise, and
differential noise as well.

A good power line filter isn't a bad idea, but differential mode
filtering won't make any appreciable difference.

Unfortunately, it's possible that the new power supply is radiating RF
directly, rather than feeding it back into the mains. If so, nothing
other than replacing it, or switching to a PC case with better
shielding, is likely to help the problem.

To radiate any significant amount of energy requires some sort of
antenna, so a power supply won't radiate much on its own. The trick,
then, is to prevent the noise from getting from its source to the
antenna. The most likely antenna is the power line, but any other wires
connected to the computer can also serve this function. I'd start by
disconnecting everything from the computer except the power line,
getting the noise down with ferrites or a power line filter, then
connecting one thing at a time and applying ferrites to the other wires
as required. If wires inside the computer are acting as the antenna, the
computer box should contain the noise to a high degree -- if you still
have noise with only the power line connected and filtered, check the
integrity of the computer case. Look for any seams that don't have good
metal-to-metal contact between pieces.

Roy Lewallen, W7EL