Definition: | for a quantity having a continuous spectrum and a finite mean power, limit, at any frequency, of the quotient of the power within a frequency band containing that frequency by the bandwidth when the bandwidth tends to zero Note 1 to entry: The instantaneous power of a quantity is by convention equal to the square of its instantaneous value. This square is proportional to a physical power if the considered quantity is a field quantity. Note 2 to entry: The power spectral density is the Fourier transform of the autocorrelation function.
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