Configuring the Internal Baseband Source
R&S
®
SMBV100B
107
User Manual 1178.4460.02 ─ 03
Cut Off Frequency Shift
The "Cut Off Frequency Shift" affects the cut-off frequency in the way that the filter
flanks are "moved" and the passband increases by "Cut Off Frequency Shift"*"Sample
Rate":
Cut Off Frequency = (1 + "Cut Off Frequency Shift") * "Sample
Rate"
●
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".
3.4.1.7
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.
Common Functions and Settings in the Baseband Domain
