| the distribution as a function of frequency of the power per unit bandwidth of the spectral components of a signal or a noise having a continuous spectrum and a finite mean power
NOTE 1 – The instantaneous power of a signal or a noise is by convention equal to the square of its instantaneous value. This square is proportional to a physical power if the characteristic quantity is a field quantity.
NOTE 2 – The power spectral density is the Fourier transform of the autocorrelation function of the signal or noise. The autocorrelation function of a deterministic signal exists if the signal has a finite mean power. The autocorrelation function of a random signal or random noise exists if it is represented by a second order random stationary function.