Quote:
Originally Posted by Electronic M
Modern wall warts and 120V to USB chargers use switch mode power supplies and regulators instead of the 60Hz transformers and linear regulators they used 20 years ago.
Switch mode supplies typically use 17KHz square waves. As Fourrier proved mathematically a square wave is made up of sine waves... Specifically one at the fundamental and an infinite series of multiples of the fundamental each a higher frequency than the last. As frequency increases amplitude decreases, but radios are sensitive to very low amplitude.
The makers of these devices could keep the square waves from interfering by filtering and or shielding the supply to keep these signals from conducting down cables and or radiating, but many makers, especially China, simply don't care enough to do it.
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I wasn't talking about a wall wart style charger, I was talking about the type that you can charge up and take with you to charge a phone or wireless earbuds when you don't have an AC wall wart to use, like the one pictured below.