Common Measurement Settings
R&S
®
FSW
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User Manual 1173.9411.02 ─ 43
A higher span/RBW ratio (i.e. low RBW values and large frequency spans), however,
results in large amounts of data.
8.5.1.5
How Data is Measured: the Sweep Type
In a standard analog
frequency sweep
, the local oscillator of the analyzer sweeps the
applied signal quasi analog from the start to the stop frequency to determine the fre-
quency spectrum.
Alternatively, the analyzer can sample signal levels over time at a defined frequency
and transform the data to a spectrum by Fast Fourier Transformation (
FFT
). Although
this measurement method requires additional calculations, it can provide results much
faster than the frequency sweep, in particular for small RBWs.
Which sweep mode is appropriate for the current measurement depends on the span,
RBW, VBW and "Sweep Time" settings. By default ("Auto" sweep type), the R&S
FSW
automatically uses the sweep type with the highest sweep rate depending on these
measurement settings.
Restrictions for FFT mode
FFT mode is not available when using 5-pole filters, channel filters or RRC filters, or
the quasi peak detector. In this case, sweep mode is used.
The same applies when an external generator is active (with the optional External Gen-
erator Control).
Optimization
In FFT mode, FFT analysis is performed to determine a spectrum of frequencies. Sev-
eral analysis steps are required to cover the entire span. The subspan which is cov-
ered by one FFT analysis depends on the RBW. The subspan cannot be defined
directly, but it can be optimized according to measurement requirements.
Narrow subspans provide a higher dynamic range, and also allow you to perform mea-
surements near a carrier with a reduced reference level. With a wide subspan, the car-
rier and the useful signal are likely to be measured at the same time, in which case the
powers of both signals are summarized, so the reference level must be high enough to
consider this factor. With a narrow subspan, this is less likely to happen, so the refer-
ence level can be reduced.
For an optimal dynamic range
, the narrowest possible subspan (depending on the
RBW) is used. Furthermore, the autorange function for the internal IF gain calculation
is activated to obtain the best control range of the A/D converter.
On the other hand, the narrower the subspan, the more steps are required to cover the
entire span, thus increasing analysis and calculation time. To
optimize the sweep
rate
, the widest possible subspan (depending on the RBW) is used.
For an optimal sweep rate, it is recommended that you set the "Sweep Time" to "Auto",
as well.
Bandwidth, Filter and Sweep Configuration
