The Spectrum Application (RF Measurements)
R&S
®
FPL1000
393
User Manual 1178.3370.02 ─ 03
7.6.1.5
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. The R&S
FPL1000 automatically selects the
appropriate sweep type. For RBWs lower than 100
kHz and for gated measurements,
an FFT is performed, otherwise a sweep is performed.
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 partial span which is cov-
ered by one FFT analysis depends on the RBW. The partial span cannot be defined
directly, but it can be optimized according to measurement requirements.
Narrow partial spans provide a higher dynamic range, and also allow you to perform
measurements near a carrier with a reduced reference level. With a wide partial span,
the carrier 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 partial span, this is less likely to happen,
so the reference level can be reduced.
For an optimal dynamic range
, the narrowest possible partial span (depending on
the RBW) is used. Furthermore, the autorange function for the internal IF gain calcula-
tion is activated to obtain the best control range of the A/D converter.
On the other hand, the narrower the partial span, the more steps are required to cover
the entire span, thus increasing analysis and calculation time. To
optimize the sweep
rate
, the widest possible partial span (depending on the RBW) is used.
For an optimal sweep rate, it is recommended that you set the "Sweep Time" to
"Auto" , as well.
For general purpose measurements, an "Auto" mode is available, which provides a
compromise between a large dynamic range and a fast sweep
. In this case, a
medium-sized partial span is used.
Optimization for zero span sweeps
For normal sweeps in the time domain (zero span), the optimization mode determines
the selection of the A/D converter prefilter, which depends on the RBW.
In
"Dynamic" mode
, the narrowest possible prefilter is used.
In
"Speed" mode
, the widest possible prefilter is used.
Bandwidth, Filter and Sweep Configuration
