I went and picked up one of those little 12V switchmode USB chargers today. To my surprise, when I plugged my phone in, I lost all but the local radio stations.
I stuck it on the scope to see if it was putting some sort of HF crap back up the 12v line, and sure enough, there's a 1.344MHz signal coming out. When I turn the load up to 1A (what it's rated for) the noise shoots up to 500mV p-p! Opening it up, there's not even so much as a bypass cap on the input
What gets me is, this was an Energizer brand charger. I thought I'd actually get my money's worth from a big brand, but the internals of this look exactly like the $5 charger that melted on my bench.
Anyway, I can't take it back to the store now, but I thought it'd be a good opportunity to learn about filtering. Putting 100uf or so on the input cuts the noise in half, but what else can I do?
Surely this violates some sort of FCC regulation? I'm sure a device wiping out the FM band would make them a bit unhappy...