I have never really understood what the frequency axis meant when we plot the Power Spectral Density(PSD).
Does it correspond to frequency as we get after we take the Fourier Transform of a time domain signal. This doesn't make sense to me because the frequency here(when we do FT of a time domain signal) corresponds to the time variable "t".
To find the PSD we take the Fourier transform of the Autocorrelation function which is a function of either time difference $$\tau = t_2 - t_1$$ i.e. $$R_{xx}(\tau) $$ (for the Wide Sense Stationary case) or $$R_{xx}(t_1,t_2)$$ for other cases.
Here the autocorrelation is a function of either time difference or two time variables. If this is the case then how can we say that the PSD gives the power distributed in the frequency domain?
How do I understand this. If someone could give both a mathematical explanation and an intuitive explanation I would be really grateful.
Thank you.