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.