The Spectrum Application (RF Measurements)
R&S
®
FPL1000
319
User Manual 1178.3370.02 ─ 09
7.6.1.4
Coupling Span and RBW
The resolution bandwidth can be coupled to the span setting, either by a manually
defined factor or automatically. If the span is changed, the resolution bandwidth is
automatically adjusted. The automatic coupling adapts the resolution bandwidth to the
currently set frequency span/100.
With a span/RBW ratio of 100 and a screen resolution of 1000 pixels, each frequency
in the spectrum is displayed by 10 pixels. A span/RBW ratio of 1000 provides the high-
est resolution.
A higher span/RBW ratio (i.e. low RBW values and large frequency spans), however,
results in large amounts of data.
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 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.
Bandwidth, Filter and Sweep Configuration
