R&S PR100 User Manual
The R&S®PR100 uses an FFT length N of 2048 points to generate the IF spectrum.
To calculate these points, the 12.8 kHz sampling band in the above example is
divided into 2048 equidistant frequency slices, which are also referred to as “bins”
(see figure "Signal processing for IF spectrum"). The bandwidth BWbin of the
frequency slices is obtained as follows:
Hz
kHzf
BW
s
bin
25.6
===
This means that in the above example only the calculated bandwidth of 6.25 Hz for
each bin has to be taken into account as the noise bandwidth in the calculation of the
displayed average noise level (DANL) in accordance with the formula below (the
effect of the window function (Blackman window) of the FFT is not considered here
for simplicity's sake):
)/log(*10174 HzBWNFdBmDANL
bin
++=
The quantity NF represents the overall noise figure of the receiver. The above
example shows that, due to the use of the FFT, the actual resolution bandwidth
(RBW) to be taken into account in DANL calculation is clearly smaller (i.e. BWbin)
than would be expected for the wide display range of 10 kHz. Another advantage of
the high spectral resolution used in the FFT calculation is that signals located close
together (e.g. f1, f2, f3) can be captured and represented in the IF spectrum as
discrete signals (see figure "Signal display in IF spectrum"). If, on an analog receiver,
a resolution bandwidth equal to the set IF bandwidth were selected (RBW = BWIF
spectrum), a sum signal fsum would be displayed instead of the three discrete
signals f1, f2 and f3.
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