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Satellite modem loopback types-Baseband,IF and RF loopback

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This page describes loopback types used in satellite modem. It compares RF loopback, IF loopback and baseband loopback types.

The loopback types mentioned here is very useful for RF engineers to debug/troubleshoot if there are any problems/issues at the earth station/VSAT side. It helps segregate the problem. Figure depicts all the types of VSAT Loopbacks in three different colors. As depicted Satellite modem consists of scrambler-descrambler, FEC encoder-decoder, modulator-demodulator module pairs inside it.

baseband loopback

Baseband loopback is done by appropriate setting in satellite modem at VSAT/Earth station. This loops back the baseband transmitter IQ data back to receiver part without interfacing to Modulator IC. This way following modules can be verified.

MUX ->Scrambler->FEC Encoder->FEC Decoder->Descrambler->DEMUX

This test will loop FEC encoded data to the FEC decoder (i.e. viterbi or CTC) input.

VSAT Loop Back Tests

IF loopback

IF loopback is also done by appropriate setting in satellite modem. This loops back modulated IF signal back to demodulator IC and hence will check the system without RF part. Following modules are tested by this. This is done by setting demodulator IF frequency same as modulator IF frequency.

MUX ->Scrambler->FEC Encoder->Modulator->Demodulator-> FEC Decoder->Descrambler->DEMUX

This test will loop IF modulator output to the IF demodulator input.

RF loopback

With RF loopback entire VSAT side can be verified as it brings RF modules also in the loopback test. Following modules are involved in this test.

MUX ->Scrambler->FEC Encoder->Modulator->RF Upconverter-> RF Power Amplifier->Antenna->Satellite uplink receive signal part->satellite downlink transmit part->Antenna->LNA->RF Down-converter->Demodulator-> FEC Decoder->Descrambler->DEMUX

VSAT RF Loopback can also be carried out without satellite in the loop. This is done by connecting RF Output of RF amplifier to the RF LNA input with the help of Test Loop Translator (TLT). TLT does the job of conversion of 6 GHz to the 4 GHz as required by LNA as input. TLT also introduces free space path loss which is about 196 dB.

This test will loop RF output to the RF input port of LNA. RF Loop back can also be done bypassing PA and LNA. In this can RF ODU output from up converter is connected to down converter input after necessary conversion to 4 GHz as mentioned earlier using TLT or by some other means. In satellite based RF loop back Satellite does the job of conversion of 6GHz to 4GHz frequency band. This is achieved as satellite houses 2225 MHz Local Oscillator (LO).

This test helps one confirm that entire remote station site or VSAT site is working fine.


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Design of RF frequency converter
RF Power Amplifier
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satellite Modem design
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