Why is fft of one part and the entire signal the same?

Hi

I have a that contains two part. The first part is before I start capturing the signal (turning on the device), so bacially the noise, and the seond part after I start capturing. When I do the fft of the first part alone (as a separate signal) and then the entire signal separately, their fft profiles look the same (same number of spikes at the same frequencies, though with different amplitudes). Forgive my being a newbie on this, but does this make sense? 1. Shouldn't all those big voltage amplitudes contribute to the fft?

2. If the fft of only the noise part and the main signal part look mostly the same, what does this mean physically?

Thank you

Brad

Answer 1

[Star Strider]

You asked me to respond to this (as a Comment to another of my Answers), so I will.

The two Fourier transforms of the various parts of the signal appear to be essentially the same, with the exception that the peak amplitude of what appears to be the fundamental frequency (at about 1.2 Hz) and low-frequency components of the whole-signal Fourier transform have slightly greater additional amplitude. I would expect the fundamental amplitude in the whole-signal Fourier transform to be greater because the spikes are reasonably high compared to the earlier part of the signal, however that could be an artifact of simply having a longer signal to analyse in the second (full-signal) plot.

That is essentially all I can write about it.

[Brad]

Thank you

[Star Strider]

As always, my pleasure!