Agilent Technologies X Series Manual page 546

Sweep/Control
Backwards Compatibility SCPI
Initial S/W Revision
Modified at S/W Revision
Key Path
Remote Command
Example
Couplings
Preset
State Saved
Initial S/W Revision
More Information
An FFT measurement can only be performed over a limited span known as the "FFT segment". Several
segments may need to be combined to measure the entire span. For advanced FFT control in the
X-Series, you have direct control over the segment width using the
automatic operation, the X-Series sets the segment width to be as wide as possible, as this results in the
fastest measurements.
However, in order to increase dynamic range, most X-series models provide a set of analog prefilters that
precede the ADC. Unlike swept measurements, which pass the signal through a bandpass before the
ADC, FFT measurements present the full signal bandwidth to the ADC, making them more susceptible
to overload, and requiring a lower signal level. The prefilters act to alleviate this phenomenon - they
allow the signal level at the ADC to be higher while still avoiding an ADC overload, by eliminating
signal power outside the bandwidth of interest, which in turn improves dynamic range.
Although narrowing the segment width can allow higher dynamic ranges some cases, this comes at the
expense of losing some of the speed advantages of the FFT, because narrower segments require more
acquisitions and proportionately more processing overhead.
However, the advantages of narrow segments can be significant. For example, in pulsed-RF
measurements such as radar, it is often possible to make high dynamic range measurements with signal
levels approaching the compression threshold of the analyzer in swept spans (well over 0 dBm), while
resolving the spectral components to levels below the maximum IF drive level (about –8 dBm at the
input mixer). But FFT processing experiences overloads at the maximum IF drive level even if the RBW
is small enough that no single spectral component exceeds the maximum IF drive level. If you reduce the
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[:SENSe]:SWEep:FFT:SPAN:RATio <integer>
[:SENSe]:SWEep:FFT:SPAN:RATio?
The behavior of the analyzer when it receives this command is to compute the
"intended segment width" by dividing the Span by the FFTs/Span parameter,
then converting this intended width to an actual width by using the largest
available FFT Width that is still less than the intended segment width. The
"Span" used in this computation is whatever the Span is currently set to,
whether a sweep has been taken at that Span or not.
Prior to A.02.00
A.04.00
Sweep/Control, Sweep Setup
[:SENSe]:SWEep:FFT:WIDTh:AUTO OFF|ON|0|1
[:SENSe]:SWEep:FFT:WIDTh:AUTO?
:SWE:FFT:WIDT:AUTO ON
Pressing Auto Couple always sets FFT Width to Auto.
ON
Saved in instrument state
Prior to A.02.00
control. Generally, in
FFT Width