> tends to get mirrored onto many frequencies for some reason I'm not familiar with. Play with this for a few minutes: http://www.jezzamon.com/fourier/index.ht

> tends to get mirrored onto many frequencies for some reason I'm not familiar with. Play with this for a few minutes: http://www.jezzamon.com/fourier/index.html http://www.jezzamon.com/fourier/index.html And watch any of these: https://www.youtube.com/results?search_query=vibrating+reed+frequency+meter https://www.youtube.com/results?search_query=vibrating+reed+... Okay, so think of a radio receiver as being sensitive to a particular frequency, because it has a filter in front of it. A vibrating reed of a specific length, if you will. (This is almost exactly true for certain types of crystal and SAW filters, actually.) But when you flip a digital output from 0 to 1 or vice-versa, it changes state very quickly -- the output is essentially a square wave. (Physical limits mean it does have a finite slew rate...) Which means it contains components of every frequency. Think of coming up to one of those vibrating-reed meters and smacking it -- a single impulse excites ALL the reeds. Smack it regularly (change the pin state at a specific frequency) and the reeds that're way off frequency will just sort of bounce around but not be particularly excited. You're creating noise, but it's just noise. But the reeds that're odd multiples of the smacking frequency will vibrate too -- these are the harmonics. They'll get really excited, almost as excited as the fundamental. (The evens essentially cancel out.) Here's what it looks like on the radio spectrum: https://www.youtube.com/watch?v=eC36AqL5mw8 https://www.youtube.com/watch?v=eC36AqL5mw8 Incidentally, this is why so many chips now boast "slew-rate limited" outputs: to intentionally slow down those transitions, to reduce the amount of noise that has to be filtered and shielded in order for the device to pass RF emissions testing.