VSAT testing basics

This article on VSAT testing basics covers VSAT testing at module level and system level. It describes VSAT testing in the field with common problems and their possible causes/solutions.

About VSAT: The term VSAT refers to Very Small Aperture Terminal. The technology has been developed to provide voice and data services in the remote hilly areas where fiber optic or other cable based systems are not viable solution. The system interacts with satellite placed above the earth. VSAT system is installed in two configurations viz. star and mesh. In star VSATs communicate via Hub station having larger antenna. In mesh VSATs communicate directly without satellite. The major frequency bands used in VSAT are C band and Ku band. The figure-1 depicts VSAT star configuration.

VSAT system testing
Figure-1: VSAT system in star configuration with its sub-systems

The VSAT system consists of Indoor unit, Outdoor unit and antenna. The indoor unit mainly consists of MUX/DEMUX and MODEM. The outdoor unit mainly consists of RF Transceiver, RF Power Amplifier, RF LNA and OMT.

Following are the types of VSAT testing:
• VSAT subsystem testing
• VSAT integration testing
• VSAT testing with satellite in self loop back
• VSAT#1 testing with VSAT#2
• VSAT Environmental testing
• Performance and Stability tests

Following test equipments and tools are usually needed during VSAT testing:
• RF Spectrum Analyzer
• RF Power Meter
• RF SNA (Scalar Network Analyzer) or RF VNA (Vector Network Analyzer)
• Satellite Test Loop Translator
• RF Connectors
• BER Meter
• Frequency Counter
• Anechoic Chamber
• Environmental chamber

VSAT testing at module level

As mentioned module level testing will be done for both IDU and ODU subsystems. During the tests following parameters are tested for various subsystems. The table-1 mentions these parameters.

Transceiver Amplifier LNA Modem MUX
• For RF Upconverter: Gain, Gain Response, Spurious, Harmonics, 1dB compression, Frequency Stability etc.
• For RF Downconverter: Gain, Gain Response, Noise Figure, Image Rejection, VSWR (or return loss)
Gain, Gain Response, Power output stability, spurious, harmonics, TOI (Third Order Intercept) point , VSWR (or return loss) Gain, Gain response, Gain stability, Noise temperature, Dynamic range, Intermodulation distortion, VSWR (or return loss) BER in loop back for long duration, IF loop back test. MUX/DEMUX should be interfaced with modem in order to provide Data interface. Refer Modem manual for further tests. MUX should be tested in loop back. Refer respective manual for further tests.

VSAT testing at system level

Following are the tests usually carried out at system level during VSAT testing:
➨Functional testing: The VSAT subsystems are interfaced and connected with test loop translator in place of satellite and tested. Normal voice and data testing is varried out. Connections should be done as follows:
MUX (DATA OUT)- (DATA IN) Modulator (IF OUT) - (IF IN) RF Upconverter (RF OUT) - (RF IN) RF PA (RF OUT) - (6GHz IN) TLT (4GHz OUT) - (RF IN) RF LNA (RF OUT) - (RF IN) RF Downconverter (IF OUT) - (IF IN) Demodulator (DATA OUT) - DEMUX
➨Testing with satellite in self loopback: The RF frequency in up converter and down converter is set as per same RF carrier. For example, Upconversion frequency=6175 MHz, Downconversion frequency=3950 MHz (i.e. 6175-2225).
➨VSAT#1 is configured to communicate with VSAT#2 and live two way voice as well as data tests will be carried out with satellite in the loop.
➨Different environment tests as per QM333 chart is carried out by keeping individual systems one by one in the chamber. All the VSAT systems are connected in loop back mode and test is carried out with BER as the major pass criteria for short duration of 48 Hours.
➨The short term and long term stability tests are also carried out with BER as the pass criteria for 3 days as well as 3 months depending upon client requirements for type approval and commissioning.

VSAT testing in the field during installation or maintenance phase

Following are the common issues found during VSAT installation and maintenance. The table mentions probable cause of these issues and their solution to rectify the same. As the RF subsystems are difficult to repair in the field, most of the time subsystems are replaced with the healthy one.

VSAT issue
Cause & Solution
The frequency output is not stable and varies more than its specification Cause: It can be due to phase noise or due to some other RF carrier is interfering
Solution: • Check phase noise of RF upconverter and RF PA • Check in the spectrum analyzer after switching OFF your RF carrier output whether other RF frequencies are present in the same band.
RF output is too low at the output of RF Amplifier or RF transceiver Cause: This might be due to RF input is below the requirement or Gain is not as per specification.
Solution: Check input power level is within limit and provide as per system budget, If the problem persists check for gain, if it is not as per specification then there might be issue with RF chip attenuators or any other RF components or RF cables. Do the chip level analysis using open RF probe at various stages in RF chain.
Modem do not lock to the desired IF carrier. Cause: This might be due to two reasons viz. interference or low power level or settings in modem parameters is not proper with respect to opposite modem.
Solution: • Check for interference in the RF spectrum analyzer by switching off your carrier •  Check power level as desired as per RF link budget after RF LNA and after downconverter. • Do the modulator setting as per demodulator of opposite VSAT and demodulator setting as per modulator of remote modem. As opposite VSAT modem (i.e. remote modem) settings are not known, one can do settings as per trial and error and see the situation changes in the local modem i.e. it is getting locked.

One can face any issues during the VSAT maintenance and VSAT installation. The issue need to be throughly analyzed at system and module level in order to rectify the same. If required healthy system is replaced one by one and checked whether the situation or condition improves in order to arrive at conclusion.

What is difference between or comparison between

Following links mention difference or comparison between various equipments:
comparison between VSG and VSA
DIfference between SNA ans VNA
Oscilloscope Bandwidth versus Rise time
Oscilloscope vs Logic Analyzer
Difference between Oscillscope types
Comparison between spectrum and network analyzer