Emerson R&S SGT100A User Manual page 136

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R&S SGT100A
About crest factors
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 example, for a multi carrier signal that has carriers with
an identical start phase. 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 digital to analog (D/A) converter is relative to the maximum
value, the average value is converted with a relatively low resolution. This low res-
olution leads to a high quantization noise.
Both effects increase the adjacent-channel power.
Clipping and filtering algorithm
A common and simple approach for achieving a lower PAR is the combination of clip-
ping and filtering.
●
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.
However, signal clipping not only changes the peak value but also the average
value and the effect on the crest factor is unpredictable.
●
Filtering is applied after clipping. The used filters are specially designed so that
they filter out the distortion.
Peak cancellation algorithm
The peak cancellation algorithm uses Blackman windowed sinc pulses and subtracts
them from the original baseband signal wherever the signal amplitude exceeds a
defined threshold. This threshold equals maximum peak of the baseband signal minus
a user-definable crest factor difference ("Desired Crest Factor Delta").
This algorithm often provides better EVM results of the output signal versus out of
band noise introduced by the pulses, compared to the clipping and filtering method.
The cancellation pulse bandwidth is configurable and the cancellation pulse transition
bandwidth. The achieved resulting crest factor can be higher than expected, because
of filters in the signal processing chain increasing the crest factor again after a previous
crest factor reduction.
User Manual 1176.8674.02 ─ 12
Signal generator settings
Baseband
136