RF Signal Measurement
RF Signal Measurement
This domain tests correct measurement of RF levels and quality in a 75-ohm broadband plant. Expect dBmV, dBµV, dBm conversions, signal-level meter setup, digital channel power, MER, BER, constellation, spectrum-analyzer RBW, and test-point correction.
Core rules and concepts
- dBmV is a voltage ratio referenced to 1 mV across 75 ohms; dBµV is referenced to 1 µV. A meter's channel type must match the signal: digital average power is not measured the same way as analog video peak envelope power.
- Digital power is usually set several dB below analog video per plant design; do not compare them without accounting for channel type.
- MER is the ratio of average symbol power to average error-vector power; higher is better. BER should be checked pre-FEC and post-FEC; post-FEC errors near zero are the goal.
- Measure at the correct test point and correct for probe/tap loss. Spectrum analyzers need appropriate RBW, span, and detector settings; too wide an RBW inflates noise.
Worked example
Convert 0 dBmV to dBµV and dBm. 1 mV = 1,000 µV, so 20log10(1,000) = 60 dBµV. Power = V²/R = (1 × 10^-3)²/75 = 1.333 × 10^-8 W = 1.333 × 10^-5 mW. 10log10(1.333 × 10^-5) = −48.75 dBm. So 0 dBmV = 60 dBµV = −48.75 dBm. A +10 dBmV reading is 70 dBµV and −38.75 dBm.
Traps that cost marks
Using analog mode on a digital channel; wrong RBW; ignoring test-point loss; treating a good MER as proof of good BER; measuring upstream without the correct attenuation or at the wrong point.