Spectrum Analyzer Primer: DANL & Phase Noise

Key points

• ▹DANL (displayed average noise level) sets the smallest signal you can see.
• ▹Narrower resolution bandwidth lowers the noise floor but slows the sweep.
• ▹Phase noise limits how close to a large signal a small one can still be seen.
• ▹Input attenuation protects the front end, but too much raises the effective noise floor.
• ▹Handheld analysers trade some performance for portability and field use.

Comparison

Key spectrum analyzer settings and their trade-offs

Frequency range and preselection

Choose a maximum frequency that covers your highest signal of interest with margin — analyzers in this class span roughly 9 kHz to 40 GHz. Above a few GHz, preselection (a tracking filter at the input) matters because it rejects images and spurious responses, giving cleaner, more trustworthy measurements.

DANL and sensitivity

Displayed average noise level (DANL) is the analyzer's own noise floor and sets the smallest signal you can see. Lower DANL means better sensitivity. A built-in preamplifier lowers the effective noise floor further for weak-signal work, at some cost to large-signal handling.

Sensitivity interacts with resolution bandwidth: narrowing the RBW lowers the noise floor but slows the sweep. Understanding this trade helps you judge whether a given analyzer can find the signals you care about in a reasonable time.

Phase noise, RBW and swept versus real-time

Phase noise determines how well you can resolve a small signal close to a large carrier. Resolution bandwidth (RBW) and video bandwidth (VBW) set frequency resolution and smoothing. A conventional swept analyzer suits most steady-state measurements; a real-time analyzer captures short, intermittent or frequency-agile signals that a swept unit would miss.

Tracking generator and handheld versus bench

A tracking generator turns the analyzer into a scalar measurement system for filters, cables and insertion loss. Handheld analyzers trade some performance for portability and are ideal for field service and coverage checks; bench units offer the best performance for the laboratory. State your band, required DANL, phase-noise needs, RBW range, and whether you need a tracking generator, real-time capture or a handheld format.

Frequently asked questions

How much DANL do I need?

Enough to see your smallest signal above the noise floor with margin, after accounting for RBW. If you must find very weak signals, prioritize low DANL and a preamplifier.

Do I need a real-time analyzer?

Only if your signals are short, intermittent or frequency-agile. For steady or slowly changing signals, a swept analyzer is simpler and more economical.

Is a handheld analyzer good enough?

For field service, coverage checks and many routine measurements, yes. For the lowest noise floor and best phase-noise performance, a bench unit is better. Match the format to where and how you measure.

What is a tracking generator for?

It outputs a signal that follows the analyzer's sweep, letting you measure scalar transmission — filter response, cable and insertion loss — with a single instrument.

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