Phase noise

Phase Noise ( Phase Noise english ) is viewed over time the difference between the theoretical and actual phase angle or zero-crossing of a harmonic oscillation or periodic signal. It is used for analyzes in the frequency range to evaluate the noise power density of an oscillator, whereas viewed in the time domain of the jitter, which indicates the time variation in the period of the oscillator signal. Phase noise and jitter are different forms of description of the same physical phenomenon.

Do not confuse the concept of phase noise with the phase shift.

Formation

Phase noise is that an oscillator frequency in addition to the intended further comprises, adjacent spectral components. The phase noise is a feature of all oscillators and largely depends on the quality factor Q. Oscillators with a high quality factor usually have lower phase noise than those with little merit.

Representation in the frequency domain

In the case without phase noise, a sine frequency can be expressed with additive noise as:

Here, the term describes the noise floor, which is how the phase noise caused by thermal noise.

By phase noise ( " Rock - formation " ), the sine frequency is expanded spectrally, as shown in the illustration in the form of a power spectral density (PSD ).

The phase noise is described by the term. This is a random change in the phase angle of the sine wave and thus a deviation from the ideal single-frequency oscillation dar. Subject in addition, the amplitude of temporal variation, one speaks of the amplitude noise. In general, the phase noise is the dominant noise effect in oscillator circuits.

Measurement

The phase noise can be measured with a spectrum analyzer when the phase noise of its local oscillator is significantly smaller than the measured phase noise.

It is specified in dBc / Hz ( carrier dB / Hz) at a certain distance to the oscillator frequency. Since there is a noise power density in phase noise, the noise power density must be related to a specific bandwidth, specifying a noise power.

Is for example the output of an oscillator to its frequency 5 dBm, and the noise power is measured with a 1 Hz bandwidth and is a power of -110 dBm as measured at a frequency offset of 100 kHz, in addition to the oscillator frequency, this results in a phase noise of -115 dBc / Hz

Terms of phase noise and jitter

Can phase noise to the cycle -to -cycle jitter are set in the areas of power density spectrum without 1/f-noise and at a uniform rate of change of -20 dBc / Hz per decade over the following equation approximately in terms of occurrence:

This corresponds to the phase noise, which is the oscillator frequency and the offset frequency. The restriction on absent 1/f-noise is related to the nature of the distribution function. In the spectral region where those approximation formula 1/f-noise occurs is not applicable and there are complex relationships between phase noise and jitter.

For example, corresponding to an oscillator with the given oscillator frequency of 150 MHz and a phase noise of -55 dBc / Hz at an offset frequency of 1 kHz, a cycle -to -cycle jitter of 0.97 ps.

Follow

Phase noise in communication technology has the consequence that the selectivity decreases, or it comes to sampling errors, in turn, cause a higher bit error rate. In order to achieve high data transfer rates even from a distance, so oscillators are needed with very low phase noise. In the high-frequency technology, the phase noise often limits the accuracy of measurement systems.