Draw and explain the block diagram of transponder/communication channel subsystem.

Subject: Advance Communication System

Difficulty: Medium

Marks: 4 marks

1 Answer

Single conversion transponder:

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  1. In this transponder only a single-frequency translation process takes place
  2. First uplink frequency signal is picked up by the receiving antenna and is routed to LNA (Low Noise Amplifier)
  3. The signal is very weak at this point, so LNA amplifies the signal
  4. Once the signal is amplified, it is translated in correct frequency by mixer.
  5. The output of mixer is then amplified again and fed to band pass filter (BPF1)
  6. BPF1 allows only a desired down-link signal of 4 GHz
  7. At last, the down-link signal is amplified by high power amplifier (HPA) usually TWT (Travelling wave tube)
  8. Again output of BPF2 is fed to the down-link antenna
  9. If common antenna is used for transmission or reception then diplexer is used to share the antenna.


Double conversion transponder:

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  1. First uplink signal is received by the receiving antenna.
  2. LNA amplified the received signal.
  3. Amplified signal first fed to first mixer (1).
  4. The mixer 1 translates the received signal frequency into intermediate frequency (typically 70 and 150 MHz). If output is fed to an IF amplifier.
  5. The output of IF amplifier is fed to another mixer 2.
  6. The mixer 2 translates the signal to the output frequency. BPF1 filters the output signal and eliminates the unwanted output
  7. HPA increases the output signal level.
  8. Again output signal is passed through BPF2 to filter out the harmonics etc.
  9. At last, transmitting antenna sends the signal over the down link.
  10. This transponder provides greater flexibility in filtering and amplification.
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