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Measuring far-out phase noise of square waves

Question asked by jkc on Feb 4, 2015
I am confused about the measurement of far-out/broadband phase noise at large offsets frequencies of a signal with high harmonic content like a square wave. When looking at a ideal spectrum plot of an ideal square wave with phase noise, there is noise that rides on each harmonic and if the noise bandwidth of the phase noise is great enough, then the noise from the 3rd harmonic leaks into the noise that rides on the fundamental and increases the amount of phase noise seen at a particular frequency offset. Using a standard spectrum analyzer appears to suffer from this, but I am unsure about phase noise analyzer boxes. Do these boxes account for broadband noise that leaks over from harmonics?