Transponder
- The transponder consist of :
- Receiver
- Down convertor
- Input demultiplexer
- Travelling wave tube amplifiers
- Output multiplexer.
- The uplink signals are first filtered by a waveguide bandpass filter with about 600MHz bandwidth and then amplified by a parametric or GaAs FET low noise amplifier with a noise figure of 2 to 4 dB. The amplified signals are then down converted to 3.7 to 4.2 GHz for C band and 11.7to 12.2 GHz for Ku band. After down conversion the signals are gain amplified by GaAs FET amplifier and passed through a ferrite isolator to the input multiplexer and then transmitted through the antenna.
Single conversion transponder
- The bandpass filter selects the uplink frequencies from 5.925 to 6.425 GHz from the receive antennas and feeds them to the wide band receiver.
- A duplicate or redundant receiver is provided so that if one fails the other is automatically switched in. Although two are provided only one is in use at a given time.
- Then the signal from the band pass filter is given to low noise amplifier (LNA) which amplifies the signal with little noise, this overrides the higher noise level present in the next mixer stage.
- The output of LNA is given to the mixer which also gets signal from local oscillator at 2225 MHz. The mixer performs down conversion from 6 GHz uplink signal to 4 GHz downlink frequency.
- Then band pass filter after the mixer removes the unwanted frequencies from down conversion operation.
- The second amplifier then amplifies the signal to give gain of 60 dB.
- The input demultipexer is used to separate the bandwidth to individual transponder channels.
- Then the individual signals are amplified by the high power amplifier (HPA) after passing through the attentuators to provide the desired level of amplitude.
- The each down link channelized signals are then combined by the output multiplexer for transmitting to the earth station.
Single Conversion Transponder
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