RMS - the Root of the Mean of the Square. So first u Square the signal. Say sinus(t) become Sinus(t)^2

Then ud take the mean.The mean can be done by summing op your hole signal, and deviding with the length. If your signal is periodic u can jkust take the mean over 1 periode since this will be the same.

Finally u take the root.

Thank you Laurits.

I agree with you and I know this but I am talking about RMS value from spectral density of a signal, I mean how to calculate this value in frequency domain.

" I mean how to calculate this value in frequency domain "

I fear, that depends on what you're heading for, and on what exacly is found in your frequency domain (e.g. one spectrum or several ordered (time alingned) spectra).

RMS is defined in/for time domain, so I'd suppose, the only proper way to find your RMS value is to transfer you signal from frequency (back) to time domain and then compute RMS.

Hi,

I will answer my question about converting voltage spectral density SD to RMS noise voltage.

Vrms= SQRT(integral (SD^2 *df)) and it can be calculated otherwise.

Interested researchers could find the details in the attached file.