Radio monitoring, measurement and testing
Radio monitoring, measurement, and testing
This domain tests instrumentation and measurement uncertainty: spectrum analyzers, receivers, power meters, field-strength probes, and conformance tests. You must know what changes the reading and what does not.
Core rules
- Spectrum analyzer noise floor moves with RBW: 10log(RBW2/RBW1). A CW carrier amplitude does not change with RBW if the resolution filter is wider than the signal.
- VBW smooths the trace but does not change displayed average noise floor in the same way as RBW. Detector choice matters: peak captures impulsive emissions, RMS/average suits noise and modulated signals, quasi-peak weights pulse repetition.
- Occupied bandwidth is commonly the 99% power bandwidth. Necessary bandwidth and emission designators are regulatory, not just measurement, quantities.
- Power meters need duty-cycle and crest-factor correction for pulsed or burst signals. Directional couplers add coupling and directivity uncertainty.
- Field strength from received power with a calibrated isotropic antenna: E(dBmicroV/m)=P(dBm)+20log f(MHz)+77.2.
Worked example
A spectrum analyzer with 10 kHz RBW shows a noise floor at -110 dBm. Switching to 1 kHz RBW lowers the displayed noise floor by 10log(10/1)=10 dB, to about -120 dBm, while a stable CW tone stays near the same amplitude. A receiver reading -60 dBm at 100 MHz from an isotropic antenna implies E=-60+20log100+77.2=-60+40+77.2=57.2 dBmicroV/m. Cable loss and antenna factor corrections still apply.
Traps
Reporting a peak detector reading as average power, forgetting RBW correction when comparing noise floors, and applying the field-strength formula without the antenna's calibration. Also watch dBm versus dBmicroV and sign errors in cable-loss corrections.