3 Spectrum Analyzer Mode
3.2 Swept SA Measurement
Remote Command
[:SENSe]:SWEep:FFT:WIDTh:AUTO OFF | ON | 0 | 1
[:SENSe]:SWEep:FFT:WIDTh:AUTO?
Example
:SWE:FFT:WIDT:AUTO ON
:SWE:FFT:WIDT:AUTO?
Couplings Pressing "Auto Couple" on page 2017 always sets FFT Width to Auto
Preset
ON
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 X-Series, you have direct control over the segment
width using the FFT Width control. Generally, in automatic operation, X-Series sets
the segment width to be as wide as possible, as this results in the fastest
measurements.
However, to increase the 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 in 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 instrument 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 width of an FFT, an analog filter is placed
before the ADC that is about 1.3 times as wide as the FFT segment width. This
spreads out the pulsed RF in time and reduces the maximum signal level seen by the
ADC. Therefore, the input attenuation can be reduced, and the dynamic range
increased without overloading the ADC.
498 Spectrum Analyzer Mode User's &Programmer's Reference
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