COMMUNICATIONS SUBSYSTEM
Satellite Communications Glossary
Satellite Communications — Communications Subsystem
Information on Communications Subsystem
The fundamental function of the communications satellite is to provide
the Communications Subsystem, which comprises a number of clusters
of receivers, frequency translators and transmitters called transponders. The total
bandwidth of the satellite is provided by a number of
transponders, each of which has a typical bandwidth of 36
MHz. For example, the Intelsat IX satellite has a total
available bandwidth of 3,528 MHz divided up across 45 C-band
and 16 Ku-band transponders.
First, a band-pass filter identifies the
band of frequencies (channels) appropriate for the transponder. Because the
received signals are very weak after travelling 36 000 km,
the low-noise amplifier increases the signal strength without adding any
more noise—tunnel diode front ends are common. The channels are
then down-converted in frequency to the desired down-link frequency range.
Frequency conversion can also occur in two stages with down-conversion
of the up-link frequency to an intermediate frequency (IF) at
which amplification is conducted and then up-conversion to the downlink
frequency.
Before re-transmission, power amplification is carried out with a travelling-wave
tube amplifier (TWTA). TWTAs are generally preferred to other devices
such as Klystrons because they provide high gain over a
wide bandwidth and have other attractive characteristics such as high
efficiency and linear amplitude and phase response. In a travelling-wave
tube amplifier (TWTA) the signals travel along a wire helix
while electrons in a high-voltage beam travel through the helix
and transfer their energy to the electromagnetic wave in the
wire. For GEO operation, typically 50W is required at C
band and 80–120W at Ku band.
The outputs of all
of the transponders are multiplexed together and then re-transmitted through
the transmit antennas. Before multiplexing, variable power dividers may be
used to allocate the required power to the various coverage
areas.
Other topics in our resources on Satellite Communications related to Communications Subsystem include:
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