Configuring the Internal Baseband Source
R&S
®
SMW200A
251User Manual 1175.6632.02 ─ 16
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 result-
ing dynamics of a signal, the greater the requirement for a power amplifier fed by the
signal 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 SMW 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.
The provided crest factor reduction methods include:
●
Internal calculation of optimized carrier phases for the individual carriers in a multi
carrier signal
●
Automatic calculation of the carrier start phases in a multi-carrier continuous wave
signal
Applying clipping and filtering
Another common and simple approach for achieving a lower PAR is the combination of
clipping and filtering. In several of the firmware options, like 3GPP FDD or CDMA2000,
the instrument provides the possibility to enable baseband clipping and to select the
baseband filter and, when applicable, to adjust the filter characteristics.
●
Clipping is a technique that applies a wanted distortion to the signal.
The principle includes specifying a threshold, finding out the signal peaks once the
defined limits are exceeded and clipping them off. The level limit is specified as a
percentage of the highest peak value. Because clipping is done before filtering, the
procedure does not influence the spectrum. The error vector magnitude (EVM)
however increases.
The instrument offers two clipping modes:
– Vector | I + q |
The clipping limit is related to the amplitude | I + q |. The I and Q components
are mapped together, the angle is retained.
Common Functions and Settings in the Baseband Domain
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