Question: When I look into a analog signal in frequency domain (FFT), I see harmonics with certain dB. What I also see (which I shouldn’t) is some noise (!!) in between the harmonics with certain dB. What is this noise ?
Answer: Thank you for the question on analyzing frequency content with an oscilloscope. Depending upon your signal, instrument setup and other factors, there are a number of possibilities as to why you are seeing noise between the signal. As a general rule of thumb, to minimize noise coupling into the scope I first try to ensure proper grounding with short lead lengths. Depending on the signal type, you can also perform filtering or post-processing through some averaging techniques to reduce noise. Again, without knowing anything about the signal or your setup, these are some general considerations that may or may not be relevant.
I would, however, recommend a very nice application note on using Tektronix oscilloscopes for FFT analysis. You can download the application note here.
I hope this helps and thanks again for the question.
Scope Guru 