Editing Spectral density (section)

=== Electrical engineering ===

[[File:Spectrogram-fm-radio.png|thumb|right|Spectrogram of an [[FM radio]] signal with frequency on the horizontal axis and time increasing upwards on the vertical axis.]]
The concept and use of the power spectrum of a signal is fundamental in [[electrical engineering]], especially in [[communication systems|electronic communication system]]s, including [[radio communication]]s, [[radar]]s, and related systems, plus passive [[remote sensing]] technology. Electronic instruments called [[spectrum analyzer]]s are used to observe and measure the '''''power spectra''''' of signals.

The spectrum analyzer measures the magnitude of the [[short-time Fourier transform]] (STFT) of an input signal. If the signal being analyzed can be considered a stationary process, the STFT is a good smoothed estimate of its power spectral density.