In article djt_d.14823$Fy.7702@okepread04,
Ken Bessler wrote:

Tonight I looked at my CPU temp and it was a tad higher
(127 deg F) than normal so I decided to turn on a box
fan pointed at the left side of the tower case. To my shock,
the QRM faded away! I switched the fan on and off several
times to prove my theroy. Then I made the mp3 recording you
can download he

http://members.cox.net/kg0wx/index.html

Listen closely to the sound - at 2.2 seconds into the rec-
ording, you can hear the "tick" that is me turning on the
fan to "low". Within a few seconds, the QRM is gone!

Anybody know where I should look to find this and kill
it?

Most modern PCs use a multi-stage power regulation system. The main
power supply isolates and steps down the mains power to create several
DC voltages (+5 and +12 in particular) which are routed to the
motherboard. Then, an on-board regulator further reduces the voltage
being fed to the CPU to meet the CPU's requirements.

Modern CPUs such as the Intel P4 family and the AMD Athlon consume a
large amount of current (tens of amperes) at low voltage (1.2 - 1.8
volts, roughly speaking). Modern AGP graphics cards also operate on
low voltages (AGP 4x is 1.5 volts, AGP 8x is 0.8 volts if I remember
correctly) at high amperages.

To create such high currents at such low voltages in an efficient
manner, single- or multi-phase "bucking" voltage regulators are used.
These are switching regulators, which step down the main voltage (most
commonly from the +12 supply) and step up the amperage. They
typically operate at switching frequencies ranging from the high tens
of kHz up to the low MHz range.

I suspect that the QRM you are hearing is from the fundamental or
harmonic of one of these switching-regulator oscillator rates.
Heating and cooling of the CPU and motherboard are probably causing
the switching oscillator to drift a bit.

As to how to get rid of it... you're already on the right track, I
think... shielding and filtering. Make sure that your monitor and
power (and other) cables have ferrites on them, as close as possible
to the PC case. Make sure you've got a "tight" PC chassis, with metal
shielding in every possible location (including screw-secured plates
over any unused PCI slots, snap-in metal shields for any unused
hard-drive bays, etc.). I recommend a chassis which is actually all
metal, rather than one of the new lighter-weight plastic chassis with
an anti-EMI coating on the inside. Don't run your PC with the side
panels off - put 'em on and fasten the screws.

As for the gamer-style transparent acrylic PC chassis, forget 'em - I
have serious doubts as to whether a PC build in one of these can pass
the FCC Part 15 tests, let alone be RF-clean enough to use in a ham
shack.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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