I coupled the 230Vrms AC mains voltage to a scope channel through a 10Vpk-pk step-down transformer. I can imagine the transformer will already filter high freq. noise but I thought I can still see some spectrum for at least narrower bandwidth. So then I saved the time series data using the scope's SAVE function. The scope apparently sampled the input with 250kHz sampling rate.
Here is the time series data plotted in Python:
Here is the FFT:
And below is the zoomed view of the FFT where the horizontal axis is set to linear:
Regarding the last spectrum I have the following questions:
- I marked the harmonics of the fundamental 50Hz in red color. As you see the highest harmonics are at 150Hz, 350Hz, 550Hz, 750Hz. These are the 3rd, 7th, 11th and 15th harmonics. Is that order of significant harmonics something common in the world or completely random?
- I marked the highest non-harmonic FFT components after 50Hz in green. They are 60Hz, 70Hz, 80Hz so on. I'm confused here because I thought the real distortion in mains would come from the harmonics. But here there are 60Hz, 70Hz, 80Hz ect. whose amplitudes' are greater than the harmonics. What could these be? Am I interpreting something wrong here?