Rohde & Schwarz R&S FSP Series, Operating Manual

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R&S FSP Getting Started
Measuring Signals in the Vicinity of Noise
2.10 Operating Manual 1164.4556.12 - 06
Impact of the video bandwidth
The displayed noise of a Spectrum Analyzer is also influenced by the selected video
bandwidth. If the video bandwidth is considerably smaller than the resolution band-
width, noise spikes are suppressed, i.e. the trace becomes much smoother. The
level of a sinewave signal is not influenced by the video bandwidth. A sinewave sig-
nal can therefore be freed from noise by using a video bandwidth that is small com-
pared with the resolution bandwidth, and thus be measured more accurately.
Impact of the detector
Noise is evaluated differently by the different detectors. The noise display is there-
fore influenced by the choice of detector. Sinewave signals are weighted in the
same way by all detectors, i.e. the level display for a sinewave RF signal does not
depend on the selected detector, provided that the signal-to-noise ratio is high
enough. The measurement accuracy for signals in the vicinity of R&S FSP intrinsic
noise is also influenced by the detector which has been selected. The R&S FSP has
the following detectors:
Maximum peak detector
If the max. peak detector s selected, the largest noise display is obtained, since the
R&S FSP displays the highest value of the IF envelope in the frequency range
assigned to a pixel at each pixel in the trace. With longer sweep times, the trace
indicates higher noise levels since the probability of obtaining a high noise amplitude
increases with the dwell time on a pixel. For short sweep times, the display
approaches that of the sample detector since the dwell time on a pixel is only suffi-
cient to obtain an instantaneous value.
Minimum peak detector
The min. peak detector indicates the minimum voltage of the IF envelope in the fre-
quency range assigned to a pixel at each pixel in the trace. The noise is strongly
suppressed by the minimum peak detector since the lowest noise amplitude that
occurs is displayed for each test point. If the signal-to-noise ratio is low, the mini-
mum of the noise overlaying the signal is displayed too low.
At longer sweep times, the trace shows smaller noise levels since the probability of
obtaining a low noise amplitude increases with the dwell time on a pixel. For short
sweep times, the display approaches that of the sample detector since the dwell
time on a pixel is only sufficient to obtain an instantaneous value.
Autopeak detector
The Autopeak detector displays the maximum and minimum peak value at the same
time. Both values are measured and their levels are displayed on the screen joint by
a vertical line.
Sample detector
The sample detector samples the logarithm of the IF envelope for each pixel of the
trace only once and displays the resulting value. If the frequency span of the
R&S FSP is considerably higher than the resolution bandwidth (span/RBW >500),
there is no guarantee that useful signals will be detected. They are lost due to
undersampling. This does not happen with noise because in this case it is not the
instantaneous amplitude that is relevant but only the probability distribution.