PhaseNoise

Phase noise is the rapid, random fluctuation of the phase of a periodic signal around its ideal phase. In RF, microwave, and optical systems these fluctuations show up as a broadening of the carrier spectrum rather than as changes in amplitude. Phase noise is commonly quantified by the single-sideband phase-noise spectral density L(f) in dBc/Hz, representing the noise power in a 1 Hz analysis bandwidth at an offset frequency f from the carrier, relative to the carrier power. It is related to the underlying time-domain phase fluctuations, and thus to short-term frequency instability and linewidth.

Measurement and interpretation of phase noise are typically performed with phase-noise analyzers or high-performance spectrum analyzers, often using a clean reference signal, IQ demodulation, or a phase detector. Results are plotted as L(f) versus offset f, spanning a range from a few hertz to several megahertz. Lower phase-noise values indicate a more stable carrier and a narrower spectral line, which is crucial for coherent demodulation and timing accuracy.

Common sources of phase noise include intrinsic oscillator noise (thermal and flicker), AM-to-PM and PM-to-AM conversion, and environmental factors such as temperature fluctuations, mechanical vibration, and supply voltage noise. Nonlinear upconversion of near-DC noise and noise from other system components can also contribute.

Impact and mitigation: Phase noise degrades performance in communications systems, especially those using high-order modulation or coherent detection, and can affect radar, timing, and synchronization tasks. Mitigation approaches include selecting low-noise oscillators (crystal, dielectric resonator, or PLL-based sources), improving environmental control and power supply purity, and optimizing phase-locked-loop bandwidth and design to suppress close-in noise.