Spectrum Measurements
R&S
®
ESR
312
User Manual 1175.7068.02 ─ 12
6.1.1.10
Calculating Signal Amplitude Statistics
To measure the amplitude distribution, the R&S
ESR has simple measurement func-
tions to determine both the APD = Amplitude Probability Distribution and CCDF =
Complementary Cumulative Distribution Function.
To determine the amplitude distribution
► To activate and configure the measurement of the amplitude probability distribution
(APD), press the "APD" softkey (see
To activate and configure the measurement of the complementary cumulative dis-
tribution (CCDF), press the "CCDF" softkey (see
Only one of the signal statistic functions can be switched on at a time. When a statistic
function is switched on, the R&S
ESR is set into zero span mode automatically. The
R&S
ESR measures the statistics of the signal applied to the RF input with the defined
resolution bandwidth. To avoid affecting the peak amplitudes the video bandwidth is
automatically set to 10 times the resolution bandwidth. The sample detector is used for
detecting the video voltage.
About the Statistical Measurements
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, e.g. from the out-
put power amplifier. Most critical are the peak amplitude values. Degradation in trans-
mit 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 zero span envelope, 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-
ple. 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 excee-
ded (e.g. 0.01 %).
Measurements
