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&#x27;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&#x27;ve seen on spectrum analyzers many times -- typically you don&#x27;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&#x2F;2 type range), but hey I&#x27;m not an RF engineer, so I&#x27;ll just stop speculating in that area. I&#x27;m an EE, so I get what you are talking about but thanks for the details--I&#x27;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.