Imagine we have a time-domain signal which is a voltage signal. Now if we sample this at the correct rate, and taking the Fourier transform gives us the magnitudes of various frequency components present in the signal (within the context of sampling rate). Taking the modulus squared of this quantity will give us power/Hz, correct?
On the other hand, taking the Fourier transform of the autocorrelation function (rather than the direct transform as discussed earlier) gives the PSD in power/Hz as well.
So what is the difference between these two? Why/when should I do one over the other? Which method a digital scope use for its FFT function?