Question

Explain the need of double conversion transponder in
first place?

Answer 1

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⠀⠀⠀⠀⠀⠀ answer

1. These transponders normally employ a double frequency conversion scheme.
2. It is easier to design filters, amplifiers and equalisers at an intermediate frequency (IF) such as 1100MHz than at 14 or 11 GHz, so the incoming 14 GHz carrier is translated to an IF around 1GHz.
3. The input demultiplexer is employed to separate the bandwidth into individual transponder channels whose bandwidth depends on the satellite’s mission.
4. The amplification and filtering are performed at 1 GHz and a relatively high level carrier is translated back to 11GHz for amplification by the HPA.
5. Filters must provide good rejection of unwanted frequencies, such as intermodulation products, and also have very low amplitude and phase ripple in their pass bands.
6. A filter will be followed by an equaliser that smoothens out the amplitude and phase variations in the pass band. Phase variations across the pass band produces group delay distortion which is troublesome with wide band FM signals and high speed phase shift keyed data transmission.
7. The each downlink channelized signals are then combined by the output multiplexer for transmitting to the earth station.

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