Editing Spectrum analyzer (section)

== Typical functionality ==

=== Center frequency and span ===
In a typical spectrum analyzer there are options to set the start, stop, and center frequency. The frequency halfway between the stop and start frequencies on a spectrum analyzer display is known as the '''center frequency'''. This is the frequency that is in the middle of the display's frequency axis. '''Span''' specifies the range between the start and stop frequencies.  These two parameters allow for adjustment of the display within the frequency range of the instrument to enhance visibility of the spectrum measured.

=== Resolution bandwidth ===
As discussed in the '''operation''' section, the '''resolution bandwidth''' filter or RBW filter is the [[bandpass filter]] in the [[intermediate frequency|IF]] path.  It's the [[Bandwidth (signal processing)|bandwidth]] of the [[RF chain]] before the detector (power measurement device).<ref name=plsa>[http://www.piclist.com/techref/postbot.asp?by=thread&id=%5BEE%5D+TV+Tuner+Based+Spectrum+Analyzer&w=body&tgt=post&at=20120524233824apiclist.com – &#91;EE&#93; TV Tuner Based Spectrum Analyzer] {{webarchive|url=https://web.archive.org/web/20130921055149/http://www.piclist.com/techref/postbot.asp?by=thread&id=%5BEE%5D+TV+Tuner+Based+Spectrum+Analyzer&w=body&tgt=post&at=20120524233824apiclist.com |date=2013-09-21 }}, 2012-05-25</ref>  It determines the RF [[noise floor]] and how close two signals can be and still be resolved by the analyzer into two separate peaks.<ref name=plsa/>  Adjusting the bandwidth of this filter allows for the discrimination of signals with closely spaced frequency components, while also changing the measured noise floor. Decreasing the bandwidth of an RBW filter decreases the measured noise floor and vice versa. This is due to higher RBW filters passing more frequency components through to the [[envelope detector]] than lower bandwidth RBW filters, therefore a higher RBW causes a higher measured noise floor.

=== Video bandwidth ===
The '''video bandwidth''' filter or VBW filter is the [[low-pass filter]] directly after the [[envelope detector]].  It's the bandwidth of the signal chain after the detector.  Averaging or peak detection then refers to how the digital storage portion of the device records samples—it takes several samples per time step and stores only one sample, either the average of the samples or the highest one.<ref name=plsa/>  The video bandwidth determines the capability to discriminate between two different power levels.<ref name=plsa/>  This is because a narrower VBW will remove noise in the detector output.<ref name=plsa/> This filter is used to "smooth" the display by removing noise from the envelope. Similar to the RBW, the VBW affects the sweep time of the display if the VBW is less than the RBW. If VBW is less than RBW, this relation for sweep time is useful:

:<math>t_\mathrm{sweep} = \frac{k \cdot (f_2 - f_1)}{\mathrm{RBW}\times \mathrm{VBW}}.</math>

Here ''t''<sub>sweep</sub> is the sweep time, ''k'' is a dimensionless proportionality constant, ''f''<sub>2</sub>&nbsp;&minus; ''f''<sub>1</sub> is the frequency range of the sweep, RBW is the resolution bandwidth, and VBW is the video bandwidth.<ref>''[https://www.keysight.com/us/en/assets/7018-06714/application-notes/5952-0292.pdf Keysight Spectrum Analyzer Basics] {{webarchive|url=https://web.archive.org/web/20180323154714/http://literature.cdn.keysight.com/litweb/pdf/5952-0292.pdf|date=2018-03-23}}'', p. 36, August 2, 2006, accessed July 13, 2011.</ref>

=== Detector ===
With the advent of digitally based displays, some modern spectrum analyzers use [[analog-to-digital converter]]s to sample spectrum amplitude after the VBW filter. Since displays have a discrete number of points, the frequency span measured is also digitised. '''Detectors''' are used in an attempt to adequately map the correct signal power to the appropriate frequency point on the display. There are in general three types of detectors: sample, peak, and average
*'''Sample detection''' – sample detection simply uses the midpoint of a given interval as the display point value. While this method does represent random noise well, it does not always capture all sinusoidal signals.
*'''Peak detection''' – peak detection uses the maximum measured point within a given interval as the display point value. This insures that the maximum sinusoid is measured within the interval; however, smaller sinusoids within the interval may not be measured. Also, peak detection does not give a good representation of random noise.
*'''Average detection''' – average detection uses all of the data points within the interval to consider the display point value. This is done by power ([[Root mean square|rms]]) averaging, voltage averaging, or log-power averaging.

=== Displayed average noise level ===
The '''Displayed Average Noise Level''' (DANL) is just what it says it is—the average noise level displayed on the analyzer.  This can either be with a specific resolution bandwidth (e.g. −120 dBm @1&nbsp;kHz RBW), or normalized to 1&nbsp;Hz (usually in dBm/Hz) e.g. −150 dBm(Hz).This is also called the sensitivity of the spectrum analyzer. If a signal level equal to the average noise level is fed there will be a 3&nbsp;dB display. To increase the sensitivity of the spectrum analyzer a preamplifier with lower noise figure may be connected at the input of the spectrum analyzer.<ref>''[https://www.keysight.com/us/en/assets/7018-06714/application-notes/5952-0292.pdf Keysight Spectrum Analyzer Basics] {{webarchive|url=https://web.archive.org/web/20180323154714/http://literature.cdn.keysight.com/litweb/pdf/5952-0292.pdf|date=2018-03-23}}'', p. 50, August 2, 2006, accessed March 25, 2018.</ref>