Editing Spectrum analyzer (section)

== Types ==
[[Image:Spectrum Analyser PCB.jpg|thumb|right|The main PCB from a {{nowrap|20 GHz}} spectrum analyser. Showing the [[Distributed element filter|stripline PCB filters]], and modular block construction.]]
Spectrum analyzer types are distinguished by the methods used to obtain the spectrum of a signal. There are swept-tuned and fast Fourier transform (FFT) based spectrum analyzers:

*A ''swept-tuned'' analyzer uses a [[superheterodyne receiver]] to [[Superheterodyne receiver#Local oscillator and mixer|down-convert]] a portion of the input signal spectrum to the center frequency of a narrow [[band-pass filter]], whose instantaneous output power is recorded or displayed as a function of time.  By sweeping the receiver's center-frequency (using a [[voltage-controlled oscillator]]) through a range of frequencies, the output is also a function of frequency.  But while the sweep centers on any particular frequency, it may be missing short-duration events at other frequencies.
*An FFT analyzer computes a time-sequence of [[periodogram]]s.  ''FFT'' refers to a particular mathematical algorithm used in the process.  This is commonly used in conjunction with a [[superheterodyne receiver|receiver]] and [[analog-to-digital converter]].  As above, the receiver reduces the center-frequency of a portion of the input signal spectrum, but the portion is not swept.  The purpose of the receiver is to reduce the [[sampling rate]] that the analyzer must contend with.  With a sufficiently low sample-rate, FFT analyzers can process all the samples (100% [[duty-cycle]]), and are therefore able to avoid missing short-duration events.