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Telamon · May 13th 07, 12:09 AM posted to rec.radio.shortwave

In article .com,
wrote:

I hate switch mode power supplies.
I hate wall warts.
And I really really hate switch mode wall wart power supplies.
Linear wall warts are bad enough.

Found this while looking for a better (as in active) RFI probe.
http://www.arrl.org/tis/info/cable-modems.html

The biggest problem I have with the linked article is that, if I read it
right, he connected the low side of the power supply output to the AC
earth ground. This causes the supply to have a specific relationship to
earth ground and might conflict with other equipment the supply, then
modem, then all else connected to it. If some other piece of connected
equipment did the same thing there would be a ground loop that could
cause equipment malfunction. Highly valuable smoke contained in
semiconductors could be released as a result.

He has the right idea as evidenced by the fact that it greatly lessened
the interference but a .1 or .01 uF cap should suffice instead of the DC
connection and allow the power supply output to remain floating relative
to earth ground.

Another lesson to be learned here is that you must filter differential
noise before trying to filter common mode noise. The author would then
find common mode ferrite chokes to help reduce radiated noise.

The reason switching power supplies generate so much interference in the
short wave bands is the way switch mode power supplies work. First the
AC is rectified to a high voltage DC. Usually a couple of FET
transistors form a high voltage oscillator into a transformer primary
for AC input to DC output isolation. The switching supplies usually
operate at a fixed frequency and regulation is achieved thru pulse width
modulation. The feedback circuit is usually another small transformer or
opto-isolator.

This varying width pulse is usually 100 to 500 KHz square wave with fast
edges so the supply conversion is efficient. This ensures that the power
pulse is rich in odd harmonics that extend into the short wave spectrum.

--
Telamon
Ventura, California

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