3 Spectrum Analyzer Mode
3.2 Swept SA Measurement
measurements will not fully agree with swept measurements except at extremely
slow sweep rates (note that the meters in the N6141A are zero span measurements
and therefore this statement also applies to the meters).
Because of the faster sweep times and still low errors, SA-Normal is the preferred
setting of Sweep Time Rules.
The third set of rules, Stimulus/Response, is automatically selected when an
integrated source is turned on, such as a Tracking Generator or a synchronized
external source.
Note that there are two types of source-synchronized sweeping, one where the
source sweeps (as with a built-in tracking generator) and one where the source
steps. The former is usually much faster than even general-purpose sweeps because
when sweeping along with a swept source the RBW and VBW filters do not directly
interact with Span. However, sweeping in concert with a stepped source usually
slows the sweep down because it is necessary to wait for the stepped source and the
instrument to settle at each point. The instrument chooses one of these methods
based on what kind of a source is connected or installed; it picks the former if there
is no source in use, which means that by selecting Stimulus/Response rules
manually when there is no source in use, you can achieve faster sweep times than
SA – Normal.
Stimulus/Response auto-coupled sweep times are typically valid in stimulus-
response measurements when the system’s frequency span is less than 20 times the
bandwidth of the device under test. As noted above you can select these rules
manually (even if not making Stimulus-Response measurements), which will allow
you to sweep faster before the “Meas Uncal” warning comes on, but you are then
not protected from the over-sweep condition and may end up with uncalibrated
results. However, it is commonplace in measuring non-CW signals such as noise to
be able to get excellent measurement accuracy at sweep rates higher than those
required for CW signal accuracy, so this is a valid measurement technique.
As always, when the X-series instrument is in AutoSweep Time, the sweep time is
estimated and displayed in the Sweep/Control menu as well as in the annotation at
the bottom of the displayed measurement. Since this can depend on variables
outside the instrument’s control, the actual sweep time may vary slightly from this
estimate.
FFT Width
Displays and controls the width of the FFTs performed while in FFT mode. The FFT
Width is the range of frequencies being looked at by the FFT, sometimes referred to
as the “chunk width” — it is not the resolution bandwidth used when performing the
FFT.
It is important to understand that this function does not directly set the FFT width, it
sets the limit on the FFT Width. The actual FFT width used is determined by several
other factors including the Span you have set. Usually, the instrument picks the
496 Spectrum Analyzer Mode User's &Programmer's Reference
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