A spectrum analyzer plots signal strength against frequency. Where an oscilloscope shows the shape of a signal over time, an analyzer separates it into its frequency components, so you see every carrier, harmonic, spur and noise floor as a spike or a hump. It is the tool for checking whether a transmitter is clean.
What it shows
Scope: amplitude vs time. Spectrum analyzer: amplitude vs frequency, so spurs and IMD show up as extra spikes.
A typical analyzer sweeps a narrow filter across the span and plots the power it finds at each frequency. Others, including SDR-based ones, capture a block of samples and use an FFT (see Time domain, frequency domain and Fourier). Either way, the result is the signal's ingredients. Two-tone testing is the classic use: feed an SSB transmitter two audio tones and the analyzer shows the two wanted lines plus any intermodulation products beside them.
Two AF tones in, spectrum analyzer on the RF out. Extra spikes beside the two tones are the intermodulation distortion.
Resolution bandwidth
Illustrative: three tones 2 kHz apart and a very weak one. Narrower RBW separates A from B and lowers the floor, revealing C.
Noise floor-119 dBm
Sweep time (relative)×11
The resolution bandwidth (RBW) is the width of the sweeping filter, and it is the setting that matters most.
A narrow RBW separates signals that are close together and lowers the noise floor: random noise power falls 10 dB for every tenfold narrowing, so weak signals emerge.
But a narrow RBW sweeps slowly, roughly with the square of the narrowing: ten times narrower is about a hundred times slower. Sweeping faster than the filter can settle gives wrong, low readings.
Set RBW as wide as you can while still separating what you need to see.
Dynamic range and overload
An analyzer can only show a weak signal beside a strong one over a limited range, set by its noise floor at the bottom and its own distortion at the top. Drive the front end too hard and its mixer makes harmonics and intermodulation that aren't in your signal. A quick test: add 10 dB of input attenuation. A genuine harmonic falls by the same 10 dB as the carrier, so its level relative to the carrier is unchanged. An internally generated second harmonic falls 20 dB, so it drops 10 dB relative to the carrier.
Connecting a transmitter
Common uses: checking harmonics and spurs (see Harmonics and spurious emissions), two-tone IMD, filter response with a tracking generator, and hunting local noise with a sensitive antenna.