Satellite Transponder
description
Transcript of Satellite Transponder
Satellite Transponder
Mary Grace C. Raborar
ETEEAP - BSECE
Function Receives transmission from
Earth (uplink). Amplifying the signal with low
noise. Converting the signal to a
frequency to be transmitted to the ground.
Limiting bandwidth to prevent unnecessary signal emission.
Amplifying power to a level for transmission to the ground.
Transmitting the signal to the ground (downlink).
Components
Antenna Low-Noise Amplifier
(LNA) Frequency Converter Filter High Power Amplifier
Solid State Power Amplifier (SSPA)
Traveling-Wave Tube Amplifier (TWTA)
Antenna Primary function
Receive and transmit the telecommunications signals to provide services to its users.
Provide Tracking, Telemetry, and Command (TT&C) functions to maintain the operation of the satellite in orbit.
Beam types Spot beam Area beam (10% of the earth) Global spot beam (42.4% of the
earth) Hemisphere spot beams (20% of the
earth) Shaped beam
Polarization types Circular Polarization Linear Polarization
Low-Noise Amplifier (LNA) At the satellite communications transponder,
the weak RF uplink signal is amplified in a special low-noise amplifier (LNA).
Commonly the LNA has been constructed using a low-noise device such as a tunnel-diode amplifier. Modern C-band and KU-band transponders
tend to use either gallium-arsenide Schottky barrier field-effect transistors (GaAs FET) or high-mobility electron transistors (HMETs).
Ka-band transponders are mainly based on parametric amplifiers.
The total noise at the input of the LNA is a function of bandwidth so, before amplification, the RF uplink signal is band-limited by a band-pass filter.
The gain of the receiving section of the transponder is approximately 60-70 dB.
Frequency Converter A frequency converter is used to
convert the frequency of a received signal into a frequency for use in transmission to the ground. Ex: The Ku-band frequency
converter converts signal frequency from the 14-GHz band to the 12-GHz band.
The main characteristics required for a frequency converter includes Low spurious emissions Linearity Stable local frequency
Types Single Frequency Converter Dual Frequency Converter
Frequency Converter
Filter There are applications
where a particular band, or spread, or frequencies need to be filtered from a wider range of mixed signals.
Filter circuits can be designed to accomplish this task by combining the properties of low-pass and high-pass into a single filter.
The result is called a band-pass filter.
C-band Filter
High Power Amplifier The final stage of the transponder
includes a power amplifier such as a traveling-wave tube amplifier (TWTA) or a transistor solid-state power amplifier (SSPA). Solid-state power amplifiers (SSPAs) are
tending to replace TWTAs for low-to-medium power requirements due to their better linearity and lower power. Solid-state power amplifiers provide output powers of 10-30W with efficiencies of 20-35% with gains of approximately 50 dB.
TWTAs are generally preferred in satellite communications because they provide high gain over a wide bandwidth and have high efficiency and linear amplitude and phase response. In a 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 satellite communications operation, typically 50W is required at C band and 80-120W at Ku band.
SSPA
TWTA
Satellite Frequency Bands Different kinds of satellites use different frequency bands.
L–Band: 1 to 2 GHz, used by Mobile Service Satellites (MSS)
S-Band: 2 to 4 GHz, used by MSS, NASA, deep space research
C-Band: 4 to 8 GHz, used by Fixed Service Satellites (FSS)
X-Band: 8 to 12.5 GHz, used by FSS and in terrestrial imaging, ex: military and meteorological satellites
Ku-Band: 12.5 to 18 GHz: used by FSS and Broadcast Service Satellites (BSS)
K-Band: 18 to 26.5 GHz: used by FSS and BSS Ka-Band: 26.5 to 40 GHz: used by FSS
Uplink/Downlink Frequency In satellite telecommunication, a downlink is the link from a
satellite down to one or more ground stations or receivers, and an uplink is the link from a ground station up to a satellite.
The table below shows the main frequency bands used for satellite links.
The C band is the most frequently used. The Ka and Ku bands are reserved exclusively for satellite communication but are subject to rain attenuation. Some satellites carry transponders for both C and Ku bands.