Rohde & Schwarz FSW13, User Manual

Page: 251 / 1150

Measurements
R&S
®
FSW
251 User Manual 1173.9411.02 ─ 19
5.8 Statistical Measurements (APD, CCDF)
To measure the amplitude distribution, the R&S FSW has simple measurement func-
tions to determine both the Amplitude Probability Distribution (APD) and the Comple-
mentary Cumulative Distribution Function (CCDF). Only one of the signal statistic func-
tions can be switched on at a time.
● About the Measurements ......................................................................................251
● Typical Applications .............................................................................................. 251
● APD and CCDF Results ........................................................................................252
● APD and CCDF Basics - Gated Triggering ...........................................................254
● APD and CCDF Configuration .............................................................................. 255
● How to Perform an APD or CCDF Measurement ..................................................261
● Examples .............................................................................................................. 262
● Optimizing and Troubleshooting the Measurement .............................................. 265
5.8.1 About the Measurements
The probability of amplitude values can be measured with the Amplitude Probability
Distribution function (APD). During a selectable measurement time all occurring ampli-
tude values are assigned to an amplitude range. The number of amplitude values in
the individual 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.
Only one of the signal statistic functions can be switched on at a time. When a statistic
function is switched on, the R&S
FSW is set into zero span mode automatically. The
R&S FSW measures the statistics of the signal applied to the RF input with the defined
analysis bandwidth. To avoid affecting the peak amplitudes the video bandwidth is
automatically set to 10 times the analysis bandwidth. The sample detector is used for
detecting the video voltage.
Statistic measurements on pulsed signals can be performed using a gated trigger. For
details see
chapter 5.8.4, "APD and CCDF Basics - Gated Triggering" , on page 254.
5.8.2 Typical Applications
Digital modulated signals are similar to white noise within the transmit channel, but are
different in their amplitude distribution. In order to transmit the modulated signal without
distortion, all amplitudes of the signal have to be transmitted linearly from the output
power amplifier. Most critical are the peak amplitude values. Degradation in transmit
quality caused by a transmitter two port network is dependent on the amplitude of the
peak values as well as on their probability.
If modulation types are used that do not have a constant envelope in zero span, the
transmitter has to handle peak amplitudes that are greater than the average power.
This includes all modulation types that involve amplitude modulation, QPSK for exam-
Statistical Measurements (APD, CCDF)