What is phase noise and how is it specified?

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 is random fluctuation of an oscillator’s phase, seen as noise sidebands around the carrier. It is specified as ℒ(f), the single-sideband power in 1 Hz bandwidth at offset f relative to the carrier, in dBc/Hz; IEEE Std 1139 defines ℒ(f) as half the one-sided phase spectral density S_φ(f).^[1] Leeson’s 1966 model explains its shape: ℒ(Δf) = 10·log10{ (2FkT/P_s)·[1 + (f₀/(2Q_LΔf))²]·(1 + f_c/Δf) }, a common form of the model.^[2]

What does Leeson's model predict?

Close to the carrier, phase noise falls 30 dB/decade (flicker noise up-converted, below the corner f_c).
Inside the resonator half-bandwidth f₀/(2Q_L), it falls 20 dB/decade.
Far out, it flattens to a floor set by thermal noise and signal power.
Higher Q_L and signal power P_s lower phase noise.

^[2]

How does phase noise affect systems?

Reciprocal mixing: a strong adjacent signal mixes with LO phase noise sidebands onto the wanted channel, degrading selectivity.
EVM: integrated phase noise rotates constellation points.
Transmitter noise: falls into adjacent channels and receive bands.
Radar: limits detection of slow targets near strong clutter.
ADC clock jitter: limits SNR at high input frequencies.

How does frequency multiplication change phase noise?

Multiplying a signal by N increases phase noise by 20·log10(N) dB; dividing reduces it by the same amount (down to the divider’s own floor). A 100 MHz reference at −150 dBc/Hz becomes about −130 dBc/Hz at 1 GHz when ideally multiplied.^[3]

How is phase noise measured?

Directly on a spectrum analyzer (limited by the analyzer’s LO), or with dedicated phase-noise analyzers using PLL-based phase detection or cross-correlation. See phase noise measurement.^[4]

Frequently asked questions

Why dBc/Hz?

Power relative to the carrier, normalized to a 1 Hz bandwidth.

Is phase noise the same as jitter?

They describe the same phenomenon in frequency and time domains; see the jitter page.

Does a PLL clean up phase noise?

Inside the loop bandwidth the output follows the multiplied reference; outside, it follows the VCO.

References

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

“IEEE Standard Definitions of Physical Quantities for Fundamental Frequency and Time Metrology: Random Instabilities (IEEE Std 1139)”. IEEE. 2008 revision.
“A Simple Model of Feedback Oscillator Noise Spectrum”. D. B. Leeson, Proceedings of the IEEE, vol. 54, no. 2, pp. 329-330. February 1966.
“Phase-Locked Loops: Design, Simulation, and Applications, 6th ed.”. R. E. Best, McGraw-Hill. 2007.
“Phase noise measurement application notes”. Rohde & Schwarz. application note.

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