# Obtaining power spectral density without having knowledge about frequency bandwidth or sampling frequency of the signal in Matlab?

Is there a way to plot PSD of a signal which show true frequency amounts ( not normalized one) without having knowledge about frequency bandwidth or sampling frequency of the signal in matlab ?

To give an example, consider the samples of $\cos(2\pi 100t)$ taken at $f_s=500\,\text{Hz}$, and the samples of $\cos(2\pi 200t)$ taken at $f_s=1000\,\text{Hz}$. They are exactly the same! There is no way to tell the two signals apart just from their samples.
It may be, however, that you have some extra piece of information that may help deduce $f_s$. For example, you may know that the signal's spectrum has a peak, or a particular phase, or some other property at a certain frequency. If you find the normalized FFT of the samples and identify this property, you can extrapolate from there to estimate the actual signal frequencies.