Ah ok, this is great thanks. Yea so outputting a string of 11001100.. would produce a 20MHz waveform, so, then the 3rd harmonic of that is 60MHz, right, so that

Ah ok, this is great thanks. Yea so outputting a string of 11001100.. would produce a 20MHz waveform, so, then the 3rd harmonic of that is 60MHz, right, so that's what is being picked up? Yea you are right about the harmonics of the ideal square wave, but I was thinking along the lines of a distorted version of the perfect square wave that you get from a typical RF type signal that I've seen on spectrum analyzers many times -- typically you don't have this perfect delta function sitting at your carrier frequency -- it has some extra stuff going on next to it above and below (other than the modulated signal -- not nearly as far away as the F*2 or F/2 type range), but hey I'm not an RF engineer, so I'll just stop speculating in that area. I'm an EE, so I get what you are talking about but thanks for the details--I'm sure plenty of people will benefit from it. Still would love to see a PSD spectrum plot of the output signal from 10MHz to 160MHz or something like that kind of range.