User’s Manual
8
1.7
TWTA TRANSFER CHARACTERISTICS
All TWTA which are employed on Hellas- Sat 2 transponders provide a maximum output power
of 105W.
The OBO denotes the power level available at the output of the TWTA relative to that when the
transponder is saturated. The OBO is therefore very important for link budget calculations as it
provides the available power per carrier for one or more carriers in the down link.
Operation of the TWTA at saturation means that the maximum output power is obtained in the
down link which in turn directly affects the design of the terrestrial receive equipment.
When a number of carriers are simultaneously amplified at different frequencies by the power
amplifier of a satellite transponder or of a transmit E/S, non-linearities of the amplifiers cause
intermodulation, i.e produce unwanted signals, called intermodulation products.
The number of intermodulation products increases very quickly with the number of input
carriers (for example, for 3 carriers, there are 9 products and for 5 carriers there are 50).
However, in most cases only the third-order intermodulation products falling within the
frequency band of the wanted carriers are considered.
To reduce intermodulation products in multicarrier operation (FDMA mode) the TWTA needs to
be driven with a sufficient back-off: i.e an input back-off of about 9 dB corresponding to an
output back-off of about 4.1 dB. In the case of earth station HPAs, an output back-off is usually
required (3 to 8 dB). However the situation can be improved by the utilization of linearizers.
Above limitations do not apply in the case of a single carrier occupying the whole transponder
bandwidth and therefore the TWTA can be driven almost to saturation.
For a Hellas-Sat 2 TWTA, in different operational modes, the following total IBO/OBO values
may be employed:
No of Carriers
1
2
3
4
MultiCarrier
IBO
0 -5 -7.5 -7
-9
OBO
0 -2 -3.1
-3.9
-4.1
Trp 1-12,
Beams F2, S2
C/IM
- 27.7 18.5 18
18
No of Carriers
1
2
3
4
MultiCarrier
IBO
0 -5 -8.5 -8
-10.5
OBO
0 -2 -3.5
-3.4
-4.9
Trp 13-24 & 25-48
Beams F1, S1, S2/F1
C/IM
- 26.5 18 18
18
Table 2: IBO vs. OBO for multi-carrier operation
