How is phase noise measured?

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

Phase noise can be measured directly on a spectrum analyzer (ℒ(f) ≈ sideband power in RBW − carrier power − 10·log10(RBW) plus detector corrections), limited by the analyzer’s own LO phase noise and AM noise. Phase-detector methods lock a low-noise reference in quadrature and measure the baseband voltage noise; cross-correlation analyzers use two independent channels and averaging to measure below each channel’s own noise floor.^[1]^[2]

What are the trade-offs of each method?

Method	Strengths	Limits
Direct spectrum analyzer	Simple, fast	Analyzer phase noise, includes AM noise, drift
PLL / phase detector	High sensitivity	Needs a better reference; loop corrections
Delay-line discriminator	No reference needed; good for drifting sources	Lower sensitivity close in
Cross-correlation	Below single-channel floor, separates AM/PM	Long averaging for deep improvement (5 dB per 10× averages)

How is integrated phase noise reported?

As rms phase in degrees or radians over a stated offset range, rms jitter in seconds, or residual FM in Hz. See jitter.^[3]

What is residual (additive) phase noise?

The phase noise a two-port device (amplifier, divider, multiplier) adds to a signal passing through it, measured by splitting a source, passing one path through the DUT, and comparing in a phase detector so the source noise cancels.^[2]

Frequently asked questions

Why can't I measure a great oscillator on a cheap analyzer?

The analyzer's own LO phase noise may exceed the DUT's.

What does cross-correlation remove?

Uncorrelated noise of the two measurement channels.

Is AM noise included in direct measurements?

Yes; direct spectrum methods cannot separate AM from PM noise.

References

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

“Phase noise measurement application notes”. Rohde & Schwarz. application note.
“Phase Noise Measurement Solutions / Phase noise characterization application notes”. Keysight Technologies. application note.
“MT-008: Converting Oscillator Phase Noise to Time Jitter”. W. Kester, Analog Devices. tutorial.

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