RFSatellite Modem Loopback Types: RF, IF, and Baseband
This article discusses different types of loopbacks used in satellite modems. These loopback configurations – RF, IF, and baseband – are invaluable for RF engineers to debug and troubleshoot issues at the earth station or VSAT (Very Small Aperture Terminal) side, helping isolate the source of the problem.
VSAT Loopback Types
The following figure illustrates the different types of VSAT loopbacks.
VSAT Loop Back Tests
A satellite modem typically consists of scrambler-descrambler and FEC (Forward Error Correction) encoder-decoder pairs, along with a modulator-demodulator module. Let’s explore each loopback type in detail:
1. Baseband Loopback
The baseband loopback is configured directly within the satellite modem at the VSAT/Earth station. It loops the baseband transmitter’s In-phase and Quadrature (IQ) data back to the receiver section, bypassing the modulator. This allows verification of the following modules:
MUX -> Scrambler -> FEC Encoder -> FEC Decoder -> Descrambler -> DEMUX
In this test, FEC-encoded data is fed back as input to the FEC decoder (e.g., Viterbi or CTC decoder).
2. IF Loopback
Similar to the baseband loopback, the IF (Intermediate Frequency) loopback is configured within the satellite modem. This configuration loops the modulated IF signal back to the demodulator, effectively testing the system without involving the RF components. This is achieved by setting the demodulator IF frequency equal to the modulator IF frequency. The modules tested include:
MUX -> Scrambler -> FEC Encoder -> Modulator -> Demodulator -> FEC Decoder -> Descrambler -> DEMUX
This test loops the IF modulator output directly to the IF demodulator input.
3. RF Loopback
The RF loopback is the most comprehensive, verifying the entire VSAT system as it includes the RF modules in the loopback path. The following modules are involved:
MUX -> Scrambler -> FEC Encoder -> Modulator -> RF Upconverter -> RF Power Amplifier -> Antenna -> Satellite uplink receive signal part -> Satellite downlink transmit part -> Antenna -> LNA (Low Noise Amplifier) -> RF Down-converter -> Demodulator -> FEC Decoder -> Descrambler -> DEMUX
RF Loopback without Satellite
VSAT RF loopback can also be performed without involving a satellite. This is achieved by connecting the RF output of the RF amplifier to the RF LNA input using a Test Loop Translator (TLT). The TLT converts the frequency from 6 GHz to 4 GHz, matching the LNA’s input requirements. Additionally, the TLT introduces a free space path loss of approximately 196 dB.
This test loops the RF output back to the RF input port of the LNA. RF loopback can also be performed bypassing the Power Amplifier (PA) and the LNA. In this setup, the RF ODU (Outdoor Unit) output from the up-converter is connected to the down-converter input after the necessary frequency conversion to 4 GHz, using a TLT or similar method.
RF Loopback with Satellite
In a satellite-based RF loopback, the satellite itself performs the 6 GHz to 4 GHz frequency conversion. This conversion relies on the satellite’s onboard 2225 MHz Local Oscillator (LO). This test confirms that the entire remote station or VSAT site is functioning correctly.
