BDS Network Testing and Measurement 1 — Questions and Answers
Question 1: What instrument does a cable technician primarily use to measure downstream signal levels in the HFC plant?
- Digital voltmeter
- Signal level meter (field analyzer) (Correct answer)
- Spectrum analyzer only
- OTDR
Correct answer: Signal level meter (field analyzer)
A signal level meter (SLM) or field analyzer measures downstream RF carrier levels in dBmV, SNR/MER, and BER, providing the primary field measurements for plant verification and troubleshooting.
Question 2: What does a spectrum analyzer display when used on a cable plant?
- A time-domain plot of signal voltage versus time
- A frequency-domain plot showing signal amplitude versus frequency across the spectrum (Correct answer)
- A constellation diagram of a single QAM channel
- A list of all active DOCSIS modems on the node
Correct answer: A frequency-domain plot showing signal amplitude versus frequency across the spectrum
A spectrum analyzer sweeps across a range of frequencies and displays the amplitude of each signal as a function of frequency, allowing technicians to visualize the entire downstream or upstream spectrum at once.
Question 3: What is a QAM analyzer used for in field testing?
- To measure optical power in fiber nodes
- To demodulate and analyze a specific QAM channel, displaying MER, BER, constellation diagram, and error counts (Correct answer)
- To generate QAM test signals for alignment
- To monitor all upstream channels simultaneously from a node port
Correct answer: To demodulate and analyze a specific QAM channel, displaying MER, BER, constellation diagram, and error counts
A QAM analyzer locks onto a specific downstream QAM channel and provides deep signal quality metrics including MER, pre/post-FEC BER, constellation diagram, and codeword error counts to assess channel health.
Question 4: What does a cable technician measure to evaluate the quality of the upstream path at a node test point?
- Downstream carrier levels only
- Upstream noise floor, ingress signals, and upstream modem transmit levels using a return-path sweep or spectrum analyzer (Correct answer)
- The node's optical receive power
- The downstream tilt across the node service area
Correct answer: Upstream noise floor, ingress signals, and upstream modem transmit levels using a return-path sweep or spectrum analyzer
Evaluating the upstream (return) path requires measuring the noise floor, identifying ingress sources, and verifying that upstream modem bursts arrive at the correct level — all visible by connecting a spectrum analyzer or return-path analyzer to the node's test point.
Question 5: What is the purpose of a TDR (Time Domain Reflectometer) in cable plant maintenance?
- To measure optical signal loss in fiber spans
- To locate cable faults (breaks, shorts, impedance changes) by sending a pulse into the coax and measuring the time for reflections to return (Correct answer)
- To align amplifier output levels
- To test cable modem upstream transmit power
Correct answer: To locate cable faults (breaks, shorts, impedance changes) by sending a pulse into the coax and measuring the time for reflections to return
A TDR sends an electrical pulse into a coaxial cable and measures the time and magnitude of reflected pulses, allowing the technician to calculate the distance to faults such as breaks, shorts, or impedance discontinuities.
Question 6: What does a 'sweep' test measure in an HFC plant?
- The number of active subscribers on a node
- The frequency response (amplitude vs. frequency) of a cable segment by injecting test tones and measuring the received levels at the far end (Correct answer)
- The upstream noise level at the headend
- The optical power budget from headend to node
Correct answer: The frequency response (amplitude vs. frequency) of a cable segment by injecting test tones and measuring the received levels at the far end
A sweep test verifies the frequency response of a cable segment by transmitting calibrated carriers across the spectrum and measuring received levels, revealing tilt, roll-off, and the effect of passive and active components.
BDS Network Testing and Measurement 1
What instrument does a cable technician primarily use to measure downstream signal levels in the HFC plant?