The periodogram P = P(f)
expresses how the power of a signal x = x(t)
is distributed across the different frequencies. As such, it can be seen as a function which associates to a frequency f
the squared of the absolute value of the Fourier transform of x
evaluated at f
.
In other words, in terms of your notation, the periodogram of x = x(t)
goes as P(f) = |X|^2(f)
.
As a consequence, the RMS
satisfies
RMS = sqrt(sum(P))/N.
CAVEAT:
I am not quite convinced on your normalization factors. In principle, Parseval's theorem states that the Fourier transform is isometric isomorphism of L^2
to itself. Hence the norm of a signal is preserved once a Fourier transform is done.
Nonetheless, different definition of such transformations can lead to different normalization factors (e.g. your 1/N
). In a nutshell, attention should be paid to that constant.