Single conversion transponder:
- In this transponder only a single-frequency translation process takes place
- First uplink frequency signal is picked up by the receiving antenna and is routed
to LNA (Low Noise Amplifier)
- The signal is very weak at this point, so LNA amplifies the signal
- Once the signal is amplified, it is translated in correct frequency by mixer.
- The output of mixer is then amplified again and fed to band pass filter (BPF1)
- BPF1 allows only a desired down-link signal of 4 GHz
- At last, the down-link signal is amplified by high power amplifier (HPA)
usually TWT (Travelling wave tube)
- Again output of BPF2 is fed to the down-link antenna
- If common antenna is used for transmission or reception then diplexer is used to
share the antenna.
Double conversion transponder:
- First uplink signal is received by the receiving antenna.
- LNA amplified the received signal.
- Amplified signal first fed to first mixer (1).
- The mixer 1 translates the received signal frequency into intermediate frequency
(typically 70 and 150 MHz). If output is fed to an IF amplifier.
- The output of IF amplifier is fed to another mixer 2.
- The mixer 2 translates the signal to the output frequency.
BPF1 filters the output signal and eliminates the unwanted output
- HPA increases the output signal level.
- Again output signal is passed through BPF2 to filter out the harmonics etc.
- At last, transmitting antenna sends the signal over the down link.
- This transponder provides greater flexibility in filtering and amplification.