### spectral density

(strength of power or energy as a function of frequency)

**Spectral density** is a term used in signal processing and
time series analysis. (The same or similar terms have been used
to describe the distribution of electromagnetic radiation or other frequency-related
phenomena.)
**Power spectral density** (**PSD**, or **power spectrum**)
is the distribution of power
as described by the Fourier transform of the auto-correlation
function of a signal. It shows the strength of the variations
in power as a function of frequency. The **amplitude spectral density**
(**ASD**) is the square root of the PSD, which makes differences less
prominent but is scaled analogous to the data measurements.
**Energy spectral density** (**ESD**) is like the PSD except that
it is for a specific interval of time rather than all time.
The terms *energy* and *power* are, in a sense, "borrowed",
being analogous to how the terms are used for physical energy:
one is an amount and the other a rate of the same concept
over time.

The terms can be used for analysis of a continuous signal, at least
in theory, but modern electronics and data collection is often
digital, recording and analyzing a discrete signal formed by sampling,
taking periodic samples from an incoming signal (such as a voltage
output from a radio telescope antenna over time), thus using
*time series analysis*. In the discrete case, the analysis is on a
sequence of numbers, using computer algorithms.
For example, for the Fourier analysis, the fast Fourier transform (FFT)
(calculating the **discrete Fourier transform**)
is likely to be used.

(*statistics,analysis,function*)
**Further reading:**

http://en.wikipedia.org/wiki/Spectral_density

http://www.fysik.lu.se/fileadmin/fysikportalen/UDIF/Bilder/Signal_analysis_in_astronomy.pdf

http://ui.adsabs.harvard.edu/abs/2002ASPC..278..123H/abstract

http://user.it.uu.se/~ps/SAS-new.pdf

**Referenced by pages:**

antenna temperature

magnetic energy spectrum

Index