Rohde & Schwarz R&S FSP Series, Operating Manual

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R&S FSP Getting Started
Measurements on Modulated Signals
2.44 Operating Manual 1164.4556.12 - 06
2.5.4 Amplitude distribution measurements
If modulation types that do not have a constant envelope in the time domain are
used, the transmitter has to handle peak amplitudes that are greater than the aver-
age power. This includes all modulation types that involve amplitude modulation,
QPSK for example. CDMA transmission modes in particular may have power peaks
that are large compared to the average power.
For signals of this kind, the transmitter must provide large reserves for the peak
power to prevent signal compression and thus an increase of the bit error rate at the
receiver.
The peak power, or the crest factor of a signal is therefore an important transmitter
design criterion. The crest factor is defined as the peak power / mean power ratio or,
logarithmically, as the peak level minus the average level of the signal.
To reduce power consumption and cut costs, transmitters are not designed for the
largest power that could ever occur, but for a power that has a specified probability
of being exceeded (e.g. 0.01%).
To measure the amplitude distribution, the R&S FSP has simple measurement func-
tions to determine both the APD =
A mplitude P robability D istribution and CCDF =
C omplementary C umulative D istribution F unction.
In the APD display mode, the probability of occurrence of a certain level is plotted
against the level.
In the CCDF display mode, the probability that the mean signal power will be
exceeded is shown in percent.
2.5.4.1 Measurement Example – Measuring the APD and CCDF of white noise gener-
ated by the R&S FSP
1. Set the R&S FSP to its default state.
➢
Press the PRESET key.
The R&S FSP is in its default state.
2. Configure the R&S FSP for APD measurement
➢
Press the AMPT key and enter - 60 dBm .
The R&S FSP’s intrinsic noise is displayed at the top of the screen.
➢
Press the MEAS key.
➢
Press the SIGNAL STATISTIC ! softkey.
To measure the R&S FSP’s intrinsic dynamic range for W-CDMA adjacent chan-
nel power measurements, a filter which suppresses the adjacent channel power
is required at the output of the transmitter. A SAW filter with a bandwidth of 4
MHz, for example, can be used.
In the literature, APD is also used for the probability of amplitude violation. This is
the complimentary function to the APD function of R&S FSP. The term PDF
(=Probability Density Function) which is frequently used in the literature corre-
sponds to the APD function of R&S FSP.