Thousands of electronic parts and devices go into the construction of a satellite. While they all have important functions, non are more relevant than satellite transponders.
A transponder performs the functions of both transmitter and receiver in a satellite. Basically, there are two types of transponders:
- Bent pipe
- Regenerative
The bent pipe transponder receives microwave frequency signals. It converts the frequency of input signal to RF frequency and then amplifies it.
A bent pipe transponder is also called a repeater and conventional transponder. It is suitable for both analog and digital signals.
A regenerative transponder performs the functions of a bent pipe transponder as well as performing the demodulation of RF carrier to baseband, regeneration of signals and modulation.
A regenerative transponder is also called a processing transponder. It is suitable only for digital signals. The main advantages of regenerative transponders are improvement in Signal to Noise Ratio (SNR) and they have more flexibility in implementation.
In general, different frequency values are chosen in a transponder for both uplink and down link in order to avoid the interference between the transmitted and received signals.
Essential parts of a transponder include:
- Duplexer is a two-way microwave gate. It receives uplink signal from the satellite antenna and transmits downlink signal to the satellite antenna.
- Low Noise Amplifier (LNA) amplifies the weak received signal.
- Carrier Processor performs the frequency down conversion of received signal (uplink). This block determines the type of transponder.
- Power Amplifier amplifies the power of frequency down converted signal (down link) to the required level.
Most communication satellites are essentially radio relay stations in orbit and carry dozens of transponders, each with a bandwidth of tens of megahertz. Most transponders operate on a bent pipe principle, sending back to Earth what goes into the conduit with only amplification and a shift from uplink to downlink frequency. However, some modern satellites use onboard processing, where the signal is demodulated, decoded, reencoded and modulated aboard the satellite. This is the more complex regenerative transponder.
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