Spectrum Analysis and Troubleshooting
Domain 4: Spectrum Analysis and Troubleshooting
This domain tests whether you can separate RF causes from protocol causes. Spectrum analysis answers what else is on the channel, how much airtime it consumes, and whether the problem is noise, interference, or Wi-Fi contention.
Reading the analyzer
A spectrum analyzer sweeps the band and plots amplitude against frequency. RBW (resolution bandwidth) controls frequency detail: too wide smears narrow interferers; too narrow slows the sweep. The noise floor is the analyzer baseline; a signal must exceed it to be useful. The waterfall/spectrogram shows time on one axis, frequency on the other, and amplitude by color. Duty cycle is the percentage of time energy is present; it is not the same as Wi-Fi channel utilization.
Signatures
Wi-Fi OFDM looks like a flat-topped, roughly 20/40/80 MHz block with sharp edges. DSSS/HR-DSSS is narrower and has a chipping shape. Microwave ovens produce broad bursts around 2.4 GHz with 50/60 Hz modulation. Cordless phones can be narrowband or frequency-hopping. Bluetooth and ZigBee are narrowband and low duty cycle; video cameras can occupy 20 MHz continuously. Radar and DFS events appear as pulsed energy and force channel changes.
Worked example
A 2.4 GHz channel shows -62 dBm Wi-Fi, a -70 dBm noise floor, and a 55% duty cycle of broad energy centered on the channel, with peaks at 60 Hz intervals. SNR is 8 dB, which is marginal, but the bigger issue is the non-Wi-Fi energy. Reasoning: the flat Wi-Fi signal is not the main occupant; the broad, pulsed energy matches a microwave oven. The fix is to change channels or move the AP, not simply raise power.
Traps
Do not equate strong signal with good quality; high noise or high duty cycle can ruin throughput at -55 dBm. Do not read channel utilization from a spectrum analyzer alone; it is an airtime metric from protocol analysis. Do not set RBW so wide that a 5 MHz interferer disappears. Remember antenna gain and cable loss affect amplitude readings, and the analyzer own noise floor can hide weak signals.