Meaning
Frequency domain representation of the noise power in a signal per unit of bandwidth describes the distribution of phase fluctuations. This phase noise spectral density is used to quantify the frequency stability of an oscillator at specific offset frequencies from the carrier. It is a standard parameter for evaluating the performance of frequency synthesizers and RF systems.
The measurement shows the noise behavior in a single-hertz bandwidth, plotted as a function of the offset frequency.
Measurement Unit
The standard unit for expressing this noise distribution is decibels relative to the carrier per hertz. This value represents the ratio of the noise power in a one-hertz bandwidth at a given offset to the total power of the carrier signal. A lower decibel value indicates a cleaner signal with fewer phase fluctuations.
Oscillator Noise
Analysing the noise profile across different offset frequencies reveals the physical sources of noise in the generator. Close to the carrier, flicker phase noise dominates, which is caused by the active devices and resonator materials. At larger offsets, the noise floor is determined by thermal noise, which remains constant across all frequencies.
System Impact
High noise density at large offset frequencies can cause reciprocal mixing in receiver systems, where a strong out-of-band signal mixes with the noise of the local oscillator to produce noise in the intermediate frequency band. This effect reduces the signal-to-noise ratio of the receiver and can block weak signals. In radar systems, low noise density is critical to ensure that small targets are not masked by the phase noise of the transmitted signal.
