Satellite transponder provides a communication channel between the transmitting & the receiving antenna on the satellite with the base station on the ground. A transponder performs the function of both the transmitter and the receiver in a satellite.

Transponders mainly perform the function of receiving a given signal and then processing, amplifying, and retransmitting the signal to another location or the base station on earth. This is achieved by choosing different frequencies for both uplink (base station to satellite) & downlink (satellite to base station) so that there is no interference between the transmitted and received signals. 

Working of a transponder illustrated by the block diagram

• Satellite Antenna: The satellite antenna is used to transmit and receive the signals.
• Duplexer: It is a two-way microwave gate that receives the Uplink signal from the satellite antenna and transmits the downlink signal to the satellite antenna.
• Low Noise Amplifier (LNA): It is used to amplify a received signal that is weak and increase the signal’s power.
• Carrier Processor: It performs the function of converting Higher frequency signals to a lower frequency which is also known as frequency down-conversion of a received signal (uplink). Frequency down conversion is required to eliminate and avoid interference. 
• Power Amplifier: It is used to amplify the power of the down-converted signal (downlink) which is received by the carrier processor to the required level. Travelling Wave Tube Amplifiers (TWTA) are the most commonly used power amplifiers in satellite transponders.

Types of Transponder

Single Conversion Transponder

Single conversion transponders are also known as bent pipe transponders. It receives a microwave frequency signal. It converts the frequency of the input signal to RF frequency and then amplifies it. As observed in the diagram above, the frequency conversion from uplink to downlink happens in a single stage. Hence, it is known as the single conversion transponder. The single conversion Ttansponder is suitable for both analog & digital signals.

Regenerative Transponder

A regenerative transponder performs the function of a single conversion transponder. i.e., frequency translation and amplification. In addition to these functions, the regenerative transponder also performs demodulation and modulation of a given downlink signal post frequency conversion which provides a significant improvement in the signal-to-noise ratio. However, this type of transponder is only suitable for digital signals.

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