R&S FSL Measurements on Modulated Signals
1300.2519.12 2.33 E-11
The level of the W–CDMA signal at the input mixer is shown on the horizontal axis, i.e. the measured
s
ignal level minus the selected RF attenuation. The individual components which contribute to the
power in the adjacent channel and the resulting relative level (total ACPR) in the adjacent channel are
displayed on the vertical axis. The optimum mixer level is –21 dBm. The relative adjacent channel
power (ACPR) at an optimum mixer level is –65 dBc. Since, at a given signal level, the mixer level is set
in 10 dB steps with the 10 dB RF attenuator, the optimum 10 dB range is shown in the figure: it spreads
from –16 dBm to –26 dBm. In this range, the obtainable dynamic range is 62 dB.
To set the attenuation parameter manually, the following method is recommended:
• Set the RF attenuation so that the mixer level (= measured channel power – RF attenuation) is
between –11 dBm and –21 dBm.
• Set the reference level to the largest possible value where no overload (IFOVL) is indicated.
This method is automated with the Adjust Ref Level function. Especially in remote control mode, e.g.
in production environments, it is best to correctly set the attenuation parameters prior to the
measurement, as the time required for automatic setting can be saved.
Note: To measure the R&S FSL's intrinsic dynamic range for W–CDMA adjacent channel 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.
Amplitude Distribution Measurements
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 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 FSL has simple measurement functions to determine
both the APD = A
mplitude Probability Distribution and CCDF = Complementary Cumulative Distribution
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.
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