Толқын фазасының кездейсоқ ауытқуларының жиілік домендегі көрсетілімі

Frequency domain representation of random fluctuations in the phase of a waveform

In signal processing, phase noise is the frequency domain representation of random fluctuations in the phase of a waveform, corresponding to time domain deviations from perfect periodicity (jitter). Generally speaking, radio frequency engineers speak of the phase noise of an oscillator, whereas digital system engineers work with the jitter of a clock.

Historically there have been two conflicting yet widely used definitions for phase noise. Some authors define phase noise to be the spectral density of a signal's phase only, while the other definition refers to the phase spectrum (which pairs up with the amplitude spectrum) resulting from the spectral estimation of the signal itself. Both definitions yield the same result at offset frequencies well removed from the carrier. At close in offsets however, the two definitions differ. The IEEE defines phase noise as where the "phase instability" Sφ(f) is the one sided spectral density of a signal's phase deviation. Although Sφ(f) is a one sided function, it represents "the double sideband spectral density of phase fluctuation". The symbol ℒ is called a (capital or uppercase) script L.

Phase noise can be measured using a spectrum analyzer if the phase noise of the device under test (DUT) is large with respect to the spectrum analyzer's local oscillator. Care should be taken that observed values are due to the measured signal and not the shape factor of the spectrum analyzer's filters. Spectrum analyzer based measurement can show the phase noise power over many decades of frequency; e. g., 1 Hz to 10 MHz. The slope with offset frequency in various offset frequency regions can provide clues as to the source of the noise; e. g., low frequency flicker noise decreasing at 30 dB per decade (= 9 dB per octave). Phase noise measurement systems are alternatives to spectrum analyzers. These systems may use internal and external references and allow measurement of both residual (additive) and absolute noise. Additionally, these systems can make low noise, close to the carrier, measurements.

The sinewave output of an ideal oscillator is a single line in the frequency spectrum. Such perfect spectral purity is not achievable in a practical oscillator. Spreading of the spectrum line caused by phase noise must be minimised in the local oscillator for a superheterodyne receiver because it defeats the aim of restricting the receiver frequency range by filters in the IF (intermediate frequency) amplifier.