I just got HID lights installed on my truck, but when I turn the lights on, my FM signals (near channel 6 OTA) on the truck radio degrade - local stations are weaker/have static. The lights have a digital ballast. Got to be some type of interference... ?
Anything with switching power supplies cause noise. How well the noise is controlled is determined by the designer (if they even bother) and some other physical properties. The most efficient way to change a dc voltage (say 12 volts) to another voltage (say 450 volts) is to chop it by switching it on and off usually at some frequency near or above the limit of our hearing; that is about 15 thousand times a second (15 kHz) or more. After we have an AC voltage (similar to your household supply), we can transform its voltage with a transformer. For the designer, a higher frequency means that a smaller transformer can be used (saves money and space) but the trade off causes more interference and finally the circuit components start to reduce efficiency. This on and off action is known as a square wave or better labeled a pulse.
So how does this relative low frequency affect our much higher frequency devices? Well, a perfect square wave is made up of an infinite number of frequencies. If the switching frequency (fundamental frequency) is 15 kHz at an amplitude of 12 volts then you will see a harmonic at 3 times that frequency (45 kHz) with one third the amplitude (4 volts), also at 5 times the fundamental frequency at one fifth the amplitude, 7 times at 1/7th, etc... to some natural limit (won't get into that). For the mathematically inclined check out the Fourier analysis of a square wave.
I apologize if that was two minutes of your life you will never get back.
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