T&M VSAT Testing Basics: Module, System, and Field Testing
This article delves into the fundamentals of VSAT (Very Small Aperture Terminal) testing, encompassing both module-level and system-level assessments. It also explores field testing scenarios, outlining common problems and their potential causes and solutions.
Understanding VSAT
VSAT technology is designed to deliver voice and data services in remote, often hilly areas where traditional cable-based solutions like fiber optics are impractical. These systems communicate with satellites orbiting the Earth. VSAT networks are typically deployed in two configurations:
- Star: VSAT terminals communicate via a central hub station equipped with a larger antenna.
- Mesh: VSAT terminals communicate directly with each other, bypassing a central hub.
The primary frequency bands used in VSAT systems are the C band and the Ku band.
Figure-1: VSAT system in star configuration with its sub-systems
As shown in Figure 1, a VSAT system typically consists of the following:
- Indoor Unit (IDU): Primarily includes the MUX/DEMUX (Multiplexer/Demultiplexer) and MODEM.
- Outdoor Unit (ODU): Contains the RF Transceiver, RF Power Amplifier, RF LNA (Low Noise Amplifier), and OMT (Ortho-Mode Transducer).
- Antenna
Types of VSAT Testing
VSAT testing involves several key stages:
- VSAT Subsystem Testing: Examining individual components within the IDU and ODU.
- VSAT Integration Testing: Verifying the interaction between different VSAT subsystems.
- VSAT Testing with Satellite (Self-Loopback): Testing the system by looping signals through the satellite.
- VSAT#1 Testing with VSAT#2: Testing communication between two VSAT terminals.
- VSAT Environmental Testing: Assessing performance under various environmental conditions.
- Performance and Stability Tests: Evaluating the long-term reliability and operational characteristics.
Essential Test Equipment
The following equipment is typically used for 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 (Bit Error Rate) Meter
- Frequency Counter
- Anechoic Chamber
- Environmental Chamber
VSAT Testing at Module Level
Module-level testing involves evaluating the performance of individual components within the IDU and ODU. The following table outlines the key parameters tested for different subsystems:
| Subsystem | Parameters Tested |
|---|---|
| Transceiver | For RF Upconverter: Gain, Gain Response, Spurious Emissions, Harmonics, 1dB Compression Point, Frequency Stability, etc. For RF Downconverter: Gain, Gain Response, Noise Figure, Image Rejection, VSWR (or Return Loss) |
| Amplifier | Gain, Gain Response, Power Output Stability, Spurious Emissions, Harmonics, TOI (Third Order Intercept) Point, VSWR (or Return Loss) |
| LNA | Gain, Gain Response, Gain Stability, Noise Temperature, Dynamic Range, Intermodulation Distortion, VSWR (or Return Loss) |
| Modem | BER (Bit Error Rate) in loopback for long durations, IF loopback test. |
| MUX | MUX/DEMUX should be interfaced with the modem to provide the data interface. Refer to the modem manual for further tests. MUX should also be tested in loopback. Refer to the respective manual for further tests. |
VSAT Testing at System Level
System-level testing focuses on evaluating the overall performance of the integrated VSAT system. Here are some common tests conducted:
-
Functional Testing: All VSAT subsystems are interconnected and tested using a test loop translator in place of a satellite. Normal voice and data communication are assessed. Connections are made as follows:
- MUX (Data Out) to Modulator (Data In)
- Modulator (IF Out) to RF Upconverter (IF In)
- RF Upconverter (RF Out) to RF PA (RF In)
- RF PA (RF Out) to TLT (6 GHz In)
- TLT (4 GHz Out) to RF LNA (RF In)
- RF LNA (RF Out) to RF Downconverter (RF In)
- RF Downconverter (IF Out) to Demodulator (IF In)
- Demodulator (Data Out) to DEMUX
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Testing with Satellite in Self-Loopback: The RF frequencies of the upconverter and downconverter are set to correspond to the same RF carrier. For instance, an upconversion frequency of 6175 MHz and a downconversion frequency of 3950 MHz (calculated as 6175 MHz - 2225 MHz).
-
VSAT#1 to VSAT#2 Testing: Live two-way voice and data tests are conducted with the satellite in the loop to assess communication between two VSAT terminals.
-
Environmental Testing: Individual systems are tested in an environmental chamber according to the QM333 standard. All VSAT systems are connected in loopback mode, and the BER is measured as a critical pass/fail criterion over a short duration (48 hours).
-
Stability Tests: Short-term and long-term stability tests are performed, with BER as the pass criterion. Tests may last for three days or three months, depending on the requirements for type approval and commissioning.
VSAT Testing in the Field
Field testing during installation or maintenance often reveals common issues. The following table outlines potential problems, causes, and solutions:
| Common VSAT Issue | Cause & Solution |
|---|---|
| The frequency output is not stable and varies more than its specification | Cause: This can be due to phase noise or interference from other RF carriers. Solution: 1. Check the phase noise of the RF upconverter and RF PA. 2. Use a spectrum analyzer to check for interfering RF frequencies after switching OFF your RF carrier output. |
| RF output is too low at the output of RF Amplifier or RF transceiver | Cause: This may be due to insufficient RF input or incorrect gain levels. Solution: 1. Ensure the input power level is within the specified limits according to the system budget. 2. Check gain levels; if they are not as specified, there might be an issue with RF chip attenuators, other RF components, or RF cables. 3. Perform chip-level analysis using an open RF probe at various stages in the RF chain. |
| Modem does not lock to the desired IF carrier. | Cause: This might be due to interference, low power levels, or improper modem parameter settings. Solution: 1. Use a spectrum analyzer to check for interference by switching off your carrier. 2. Verify the power level after the RF LNA and downconverter, as per the RF link budget. 3. Ensure modulator settings are compatible with the demodulator of the remote VSAT, and vice versa. If the settings of the remote modem are not known, use trial and error to achieve lock on the local modem. |
When troubleshooting in the field, remember that RF subsystems are often difficult to repair. In many cases, the best course of action is to replace a faulty subsystem with a known good unit.
Conclusion
VSAT testing is essential to ensure reliable performance and efficient communication. Careful analysis at both system and module levels is crucial to identify and rectify problems during installation or maintenance. In many cases, replacing a faulty subsystem with a known good one can quickly restore service.
