How does a spectrum analyzer work?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

A spectrum analyzer displays signal power versus frequency. A classic swept-tuned analyzer is a superheterodyne receiver whose LO sweeps across the span; the IF resolution bandwidth (RBW) filter sets frequency resolution and noise floor, and a video bandwidth (VBW) filter smooths the detected trace. Modern signal analyzers digitize the IF and use FFTs for narrow RBWs.^[1]^[2]

What sets the displayed noise floor?

The displayed average noise level (DANL) is approximately −174 dBm/Hz + NF_analyzer + 10·log10(RBW), adjusted for detector and averaging effects (log averaging reads noise about 2.5 dB low). Reducing RBW by 10× lowers DANL by 10 dB; reducing input attenuation or enabling a preamplifier lowers it further.^[1]

How do RBW and sweep time interact?

For swept analysis, sweep time scales roughly as k·Span/RBW² with k about 2–3 for analog filters, so narrow RBWs over wide spans are slow; FFT modes are faster for narrow RBW.^[1]

How is dynamic range optimized?

Input mixer level trades noise against internally generated distortion: second-harmonic distortion rises 1 dB per dB of mixer level and third-order 2 dB per dB relative to the signal, while relative noise falls 1 dB per dB. The optimum mixer level for a given measurement is where the curves intersect. Test by adding attenuation: if a distortion product changes level, it is generated internally.^[1]^[2]

Which detectors are used?

Positive peak, negative peak, sample, average (RMS power, voltage or log) and, for EMC, quasi-peak per CISPR 16. Use RMS average for noise-like signals and channel-power measurements.^[2]^[3]

Frequently asked questions

What is RBW?

The bandwidth of the IF filter that determines frequency resolution and noise level.

Why does noise read low with log averaging?

Averaging logarithmically detected noise underestimates its power by about 2.5 dB.

What is phase-noise-limited dynamic range?

Close-in measurements limited by the analyzer's own LO phase noise.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Spectrum Analysis Basics (Application Note 150)”. Keysight Technologies. application note.
“Fundamentals of Spectrum Analysis”. C. Rauscher, Rohde & Schwarz. 2001 and later editions.
“CISPR 16-1-1: Specification for radio disturbance and immunity measuring apparatus and methods — Measuring apparatus”. IEC / CISPR. current edition.

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