Spectrum Measurements
R&S
®
ESR
313
User Manual 1175.7068.02 ─ 12
The probability of amplitude values can be measured with the APD function (Amplitude
Probability Distribution). During a selectable measurement time all occurring amplitude
values are assigned to an amplitude range. The number of amplitude values in the
specific ranges is counted and the result is displayed as a histogram.
Alternatively, the Complementary Cumulative Distribution Function (CCDF) can be dis-
played. It shows the probability that the mean signal power amplitude will be exceeded
in percent.
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 selected resolution bandwidth 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 thus corresponds to a
white noise signal. The true amplitude distribution of the signal therefore cannot be
determined.
Selecting the number of samples
For statistics measurements with the R&S
ESR, the number of samples to be mea-
sured is defined instead of the sweep time. Since only statistically independent sam-
ples contribute to statistics, the acquisition or sweep time is calculated automatically
and displayed in the channel bar (AQT). The samples are statistically independent if
the time difference is at least 1/RBW. The acquisition time AQT is, therefore,
expressed as follows:
AQT = N
Samples
/RBW
Statistic measurements on pulsed signals
Statistic measurements on pulsed signals can be performed using a gated trigger. An
external frame trigger is required as a time (frame) reference. For details see
Gated Triggering for APD and CCDF"
Result Evaluation
Amplitude Probability Distribution (APD)
As a result of the APD function (Amplitude Probability Distribution), the probability of
measured amplitude values is displayed. During a selectable measurement time all
occurring amplitude values are assigned to an amplitude range. The number of ampli-
tude values in the specific ranges is counted and the result is displayed as a histo-
gram. Each bar of the histogram represents the percentage of measured amplitudes
within the specific amplitude range. The x-axis is scaled in absolute values in dBm.
Measurements
