SBCA System Troubleshooting & Maintenance 4 — Questions and Answers
Question 1: A technician measures an LNB output and finds the signal is 6 dB weaker than expected. If the cable runs are within spec, what is the most likely remaining cause?
- Incorrect polarity selection on the IRD
- LNB noise figure degradation or internal gain reduction due to component aging (Correct answer)
- Satellite transponder beacon power reduction
- Azimuth misalignment of 0.5 degrees
Correct answer: LNB noise figure degradation or internal gain reduction due to component aging
A 6 dB drop with cables in spec and proper alignment most likely indicates LNB internal component degradation reducing conversion gain.
Question 2: When troubleshooting a satellite system using a spectrum analyzer, what does a signal with symmetrical sidebands spaced at equal intervals from the carrier indicate?
- A healthy QPSK modulated carrier
- Intermodulation distortion products from amplifier nonlinearity (Correct answer)
- Adjacent satellite interference
- Phase noise from the LNB oscillator
Correct answer: Intermodulation distortion products from amplifier nonlinearity
Symmetrical intermodulation products (IMD) at equal spacing are characteristic of nonlinear amplifier distortion, often caused by an overdriven amplifier stage.
Question 3: What is the correct procedure when a DiSEqC 1.0 switch stops responding to commands from the IRD?
- Replace the switch immediately without further testing
- Verify 13/18V switching voltage is present at the switch input, check cable continuity, then test with a known-good IRD (Correct answer)
- Increase IRD output power above specification
- Reprogram the DiSEqC switch firmware via USB
Correct answer: Verify 13/18V switching voltage is present at the switch input, check cable continuity, then test with a known-good IRD
DiSEqC faults are often caused by insufficient voltage at the switch or cable issues, so systematic verification starting with power and continuity is the correct first step.
Question 4: A broadcast facility notices periodic audio/video dropouts occurring every 24 hours at exactly the same time. What is the most likely cause?
- Daily solar interference patterns
- A scheduled automated process or equipment cycle causing interference (Correct answer)
- Atmospheric temperature inversion at the same time each day
- Satellite orbital drift occurring daily
Correct answer: A scheduled automated process or equipment cycle causing interference
Perfectly periodic dropouts at fixed times are characteristic of a scheduled process such as a backup, reboot cycle, or automated maintenance task causing interference.
Question 5: What does an excessively high noise temperature (Tsys) in a satellite receive system indicate?
- The system is receiving too much signal power
- Additional noise sources are degrading receiver sensitivity beyond design specifications (Correct answer)
- The uplink power control is set too high
- The satellite transponder EIRP has increased
Correct answer: Additional noise sources are degrading receiver sensitivity beyond design specifications
High Tsys means the system is adding more thermal noise than designed, reducing sensitivity and degrading C/N ratio on received signals.
Question 6: During preventive maintenance on a satellite uplink facility, which safety precaution is MOST critical before working on the HPA (High Power Amplifier)?
- Verify the uplink is transmitting at minimum power
- Lockout/tagout the HPA power supply and confirm RF output is at zero before physical access (Correct answer)
- Notify the satellite operator of upcoming maintenance
- Install an attenuator on the output port
Correct answer: Lockout/tagout the HPA power supply and confirm RF output is at zero before physical access
HPAs produce dangerous RF radiation and high voltages; lockout/tagout and confirming zero RF output are mandatory safety steps before physical access.
Question 7: A receive-only satellite system works perfectly in clear weather but suffers complete signal loss (not just degradation) during heavy rain. What is the most likely explanation beyond simple rain fade?
- The dish is undersized creating a very small rain fade margin, causing complete signal loss (Correct answer)
- The LNB power supply fails in humid conditions
- The IRD software crashes during high BER events
- Rain is physically blocking the dish aperture
Correct answer: The dish is undersized creating a very small rain fade margin, causing complete signal loss
When rain causes complete outage rather than gradual degradation, it indicates the link margin is so small that moderate rain exceeds available fade margin.
SBCA System Troubleshooting & Maintenance 4
A technician measures an LNB output and finds the signal is 6 dB weaker than expected.
If the cable runs are within spec, what is the most likely remaining cause?