Meaning
Frequency fluctuations in the output of a local oscillator or carrier generator represent the rapid, short-term random variations in the phase of a waveform. This carrier phase noise is measured in the frequency domain as the power density within a one-hertz bandwidth at a specified offset from the carrier frequency. The metric is a fundamental property of RF and microwave systems, determining how clean a transmitted or received signal will be.
It is distinct from long-term frequency drift, which occurs over hours or days.
Spectral Behavior
Plots of this noise versus offset frequency typically show distinct regions that depend on the physical mechanisms in the oscillator. In the frequency domain, the noise power decreases as the offset from the carrier increases. The behavior of the signal is often represented as a single-sideband plot, where the power density is normalized to the total carrier power.
Noise Source
Physical sources of these fluctuations include thermal noise and flicker noise within the active oscillator circuitry. Flicker noise, which has a one-over-f frequency dependence, dominates at small offset frequencies close to the carrier. In contrast, thermal noise dominates at larger offsets, establishing a flat noise floor that limits the receiver sensitivity.
Performance Impact
High levels of these phase fluctuations in a telecommunication system can degrade the error vector magnitude of modulated signals. For high-order modulation schemes such as 256-QAM, phase instability rotates the constellation points and causes bit errors. In radar applications, this noise can mask weak target returns that are close to the transmitter carrier frequency.
