Configuring the internal baseband source
R&S
®
SMM100A
106
User Manual 1179.1341.02 ─ 05
●
A "Cut Off Frequency Shift" = -1 results in a very narrow-band filter
●
Increasing the value up to 1 makes the filter more broad-band
●
By "Cut Off Frequency Shift" = 0, the -3 dB point is at the frequency determined by
the half of the selected "Sample Rate".
4.4.1.7
Methods for optimizing the crest factor
Communication standards utilizing higher order modulation techniques or using multi-
ple carrier and complex signals consisting of the signals of more than one digital stan-
dard can feature a high crest factor. The signals of some digital standards can have
high crest factors also particularly with many channels and long sequences.
The
crest factor
represents the ratio of the peak voltage value to the RMS voltage
value, i.e. the peak to average ratio (
PAR
). The higher the crest factor and the resulting
dynamics of a signal, the greater the requirement for a power amplifier fed by the sig-
nal to be linear. A high crest factor arises for instance, when in a multi carrier signal the
carriers feature an identical start phase. This is based on the fact that the carriers are
periodically superposed that leads to high peak voltages in relation to the RMS voltage
values.
High crest factors entail two basic problems:
●
The nonlinearity of the power amplifier (compression) causes intermodulation
which expands the spectrum (spectral regrowth).
●
Since the level of the D/A converter is relative to the maximum value, the average
value is converted with a relatively low resolution. This leads to a high quantization
noise.
Both effects increase the adjacent-channel power.
Direct approaches
At the individual signal generation stages, the R&S
SMM100A offers different direct
approaches aimed to reduce the crest factor. While the corresponding parameters are
enabled, the implemented algorithms ensure minimizing the crest factor or achieving of
predefined target crest factor by applying of automatic settings. Methods of reducing
the crest factor differ regarding both the optimization achievable and the time required
for computation.
Common functions and settings in the baseband domain
