Configuring the internal baseband source
R&S
®
SMM100A
107
User Manual 1179.1341.02 ─ 05
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.
–
Scalar | I | + | q |
The clipping limit is related to the absolute maximum of all the I and Q values | I
| + | q | . The I and Q components are mapped separately, the angle changes.
However, signal clipping not only changes the peak value but also the average
value and the effect on the crest factor is unpredictable.
Common functions and settings in the baseband domain