R&S FSL Measurements on Modulated Signals
1300.2519.12 2.35 E-11
Fig. 2-28 CCDF of white noise
The CCDF trace indicates the probability that a level will exceed the mean power. The level
above the mean power is plotted along the x–axis of the graph. The origin of the axis
corresponds to the mean power level. The probability that a level will be exceeded is plotted
along the y–axis.
4. Bandwidth selection
When the amplitude distribution is measured, the resolution bandwidth must be set so that the
complete spectrum of the signal to be measured falls within the bandwidth. This is the only way of
ensuring that all the amplitudes will pass through the IF filter without being distorted. If the
resolution bandwidth which is selected is too small for a digitally modulated signal, the amplitude
distribution at the output of the IF filter becomes a Gaussian distribution according to the central
limit theorem and so corresponds to a white noise signal. The true amplitude distribution of the
signal therefore cannot be determined.
5. Selecting the number of samples
For statistics measurements with the R&S FSL, the number of samples N
Samples
is entered for
statistical evaluation instead
of the sweep time. Since only statistically independent samples
contribute to statistics, the measurement or sweep time is calculated automatically and displayed.
The samples are statistically independent if the time difference is at least 1/RBW. The sweep time
SWT is, therefore, expressed as follows:
SWT = N
Samples
/ RBW
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