R&S SMCVB-K167 User Manual page 23

®
R&S SMCVB-K167
If the constellation is changed, the symbol rate is retained. The required transport
stream input data rate changes, keeping the necessary channel bandwidth constant.
Different code rates are possible in the different constellations.
Remote command:
[:SOURce<hw>]:BB:DVBS:CONStel
Roll Off
Sets the roll-off alpha factor value.
The specification of DVB-S defines a roll-off alpha factor of 0.35. The specification of
DVB-DSNG defines a roll-off alpha factor of 0.25 or 0.35.
A smaller alpha value produces a steeper drop of the modulation spectrum but the
stopband attenuation is lower. A larger alpha value produces a flatter drop of the mod-
ulation spectrum. The spectrum is wider but the stopband attenuation is higher.
The 3 dB point of the modulation spectrum is the same for all alpha values. The output
signals are pulse-shaped by an FIR filter, yielding a root raised cosine characteristic.
Remote command:
[:SOURce<hw>]:BB:DVBS:ROLLoff
Code Rate
Sets the code rate.
The available code rates depend on constellation, see the table below. If you change
the constellation and the previously set code rate is not available in the new constella-
tion, an available code rate is automatically selected.
Constellation
QPSK
8PSK
16QAM
Remote command:
[:SOURce<hw>]:BB:DVBS:RATE
User Manual 1179.1058.02 ─ 02
DVB-S Configuration and Settings
on page 54
on page 54
Available code rate
1/2, 2/3, 3/4, 5/6, 7/8
2/3, 5/6, 8/9
3/4, 7/8
on page 54
Coding Settings
23