BDS RF Technology and Spectrum Management 1 — Questions and Answers
Question 1: What is composite second order (CSO) distortion in a cable RF system?
- Distortion products that fall at frequencies equal to the sum or difference of two carrier frequencies (Correct answer)
- Distortion products at twice the frequency of a single carrier
- Noise generated by the amplifier's thermal characteristics
- Interference caused by overlapping channel bandwidths
Correct answer: Distortion products that fall at frequencies equal to the sum or difference of two carrier frequencies
CSO distortion products result from second-order nonlinearities in amplifiers, producing beats at frequencies equal to the sum and difference of pairs of channel carriers.
Question 2: What is the standard RF channel bandwidth for a US cable television channel?
- 6 MHz (Correct answer)
- 7 MHz
- 8 MHz
- 12 MHz
Correct answer: 6 MHz
US cable television channels are each 6 MHz wide, matching the legacy NTSC analog TV channel standard and carried forward for DOCSIS and QAM digital channels.
Question 3: Which modulation format offers the highest spectral efficiency for downstream cable broadband transmission?
- QPSK
- QAM-64
- QAM-256
- QAM-1024 (Correct answer)
Correct answer: QAM-1024
QAM-1024 encodes 10 bits per symbol, providing the highest spectral efficiency but requiring the highest CNR, and is supported in DOCSIS 3.1 OFDM channels.
Question 4: What is ingress noise in a cable return path system?
- Noise generated internally by cable amplifier transistors
- Unwanted signals from external sources that enter the coaxial plant through poor connectors or damaged cables (Correct answer)
- Noise floor added by the CMTS upstream receiver
- Distortion products from upstream QAM modems
Correct answer: Unwanted signals from external sources that enter the coaxial plant through poor connectors or damaged cables
Ingress is RF energy from external sources (ham radio, AM broadcast, power lines) that enters the coaxial plant through shielding defects, degrading the upstream return path.
Question 5: What does MER (Modulation Error Ratio) measure in a digital cable system?
- The ratio of the desired carrier power to the noise power floor
- The ratio of the ideal QAM constellation point power to the error vector power, indicating signal quality (Correct answer)
- The number of bit errors per second in the digital data stream
- The ratio of upstream power to downstream power at the CMTS
Correct answer: The ratio of the ideal QAM constellation point power to the error vector power, indicating signal quality
MER compares the power of the ideal QAM symbol to the power of the error vector (deviation from ideal), with higher MER indicating a cleaner, more accurately modulated signal.
Question 6: What is the purpose of a diplexer in a cable distribution amplifier?
- To split the signal equally between two output ports
- To combine or separate the downstream (forward) and upstream (return) frequency bands (Correct answer)
- To equalize the frequency response across the amplifier's passband
- To amplify both upstream and downstream signals simultaneously
Correct answer: To combine or separate the downstream (forward) and upstream (return) frequency bands
A diplexer uses frequency-selective filters to separate the low-frequency upstream band from the high-frequency downstream band, enabling bidirectional signal flow in the same coaxial cable.
BDS RF Technology and Spectrum Management 1
What is composite second order (CSO) distortion in a cable RF system?