Unfiltered Switchmode Power Supply Interfence

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Filtered Switchmode Interference

Filtered Power Supply Interference

Computers, Audio Amplifiers, Other Switchmode Supplies. Some equipment needs a smooth constant direct current power flow derived from the Alternating Current main power line. They use filters to supply power to fast switching transistors to regulate voltage. Each time a switch is thrown, transistor or a regular wall switch, a little bit of wide band electromagnetic noise is generated.
You may find this kind of supply in audio amplifiers, computers, tape decks, and other equipment
. This image shows noise from my computer, most of which is from the power supply. To get this image, I had to remove the side of the computer case, and move the rabbit ear close to the computer. The sparkles are probably individual switch pulses, and the fine weave is probably other computer stuff.
My computer spews signal every 40 kHz, multiples of the switch frequency. The computer jams a local AM station station, on 1080 KHz, 200 meters up the road from my house, even though this 10 kW AM station transmits from a tower only 5 kilometers meters from my home. A couple of power strips and some grounding helped with that. To cure this sort of interference, you can turn off the device, or arrange it that you have more distance between the device and your antenna.

Click on image to enlarge.

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FCC Handbook on TV Interference, recently removed from their web site.
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