Noise (physics)

Under noise, the physics generally understood a disturbance with a broad non-specific frequency spectrum. It can therefore be interpreted as a superposition of many harmonic waves or with a different amplitude and frequency or wavelength.

During the transmission of message signals, the noise is often the largest source of interference. The noise sources occur throughout the complete transmission system, ie in the transmitter, the receiver and on the transmission on. A distinction is made between the noise power generated by external and internal noise sources. The quality of the signals is indicated in communications with the so-called signal -to-noise ratio ( SNR ).

Research history and physical causes

Noise was as a physical phenomenon, namely as a measurable irregular current fluctuations, for the first time in 1918 described by Walter Schottky ( Annals of Physics 57 (1918 ), 541 ). Power to hear these current fluctuations after amplification over a loudspeaker, so you will hear a typical sound that also gave its name to the phenomenon. Meanwhile, the term "noise " is used according to the above definition of standing very much more general.

Among the pioneers of experimental and theoretical investigation of physical noise processes including, among others, John Bertrand Johnson (1887-1970), who ten years after Schottkys discovery of the thermal noise experimentally verified, and after the Anglo-Saxon literature, the thermal noise is named as Johnson noise, and Harry Nyquist, who is also a conceptual model of the power spectral density of the thermal noise developed ten years after Schottkys first publication.

In Schottkys publication of 1918, another important physical noise source is described, the shot noise. 1925 is J. B. Johnson during a review of Schottkys publication flicker noise.

Since then, a variety of other physical phenomena noise has been detected. Representing be mentioned here in semiconductors and the cosmic background noise, the generation - recombination noise. The latter is received by radio astronomical receiving devices even from those directions, where there are no known cosmic objects. It is everywhere and comes from all directions. After today's performances this noise model is a remnant of the cosmic Big Bang. From the standpoint of radio astronomers, noise may thus act not only as a disturbance ( such as generated by the receiving system noise itself ), but also for example as background radiation be a useful variable.

When noise thermometer noise is also evaluated as a useful variable.

Many physical noise phenomena are still the subject of intense investigations.

The causes of glitter or 1/f-noise can not be explained classically. Quantum theory, a unified representation is possible.

Shot noise or Poisson shot noise arises from the fact that individual carriers whose energy is statistically distributed cross a potential barrier.

Power Spectral Density

Except as provided by physical causes classified to noise even after the parameters of the stochastic processes, or by measurement detectable quantities which describe the noise. The latter includes, for example, the power spectral density, which is the power per ( infinitesimally small ) bandwidth. It is generally dependent on the frequency.

The spectral power density in a broader sense or the mathematical power spectral density is obtained as the Fourier transform of the autocorrelation function of a stationary random process. ( For example, the autocorrelation function of the noise voltage across a resistor ).

The spectral power density in the narrow sense, or the physical power spectral density is obtained as the Fourier transform of the cross correlation function of two random processes. The cross-correlation function has to be a physically meaningful performance of the argument 0. (Example: cross-correlation function of noise power and noise by a voltage across a resistor ).

Noise processes with constant spectral noise power density in the broader sense is called white noise in analogy to white light which contains all spectra (frequencies) of visible light with the same power ( intensity). In reality, noise processes can not exist with constant spectral noise power density, since they would have to carry an infinite power. However, there are physical noise processes whose spectral noise power density is practically constant within a certain more or less wide frequency band in the strict sense. For simplicity, then referred to these processes as white. This includes, for example, the thermal noise and the shot noise power. Frequently, this quasi- white noise comes from the fact that of a Gaussian noise, ie, a noise in which the amplitudes of the individual frequencies are Gaussian distributed, only a section is considered or is relevant, in which the amplitudes can be considered virtually constant. White noise is not self-similar.

A noise process with a power spectral density that is significantly different in a relevant practice for the frequency range from a constant value, called " colored noise ". In contrast to " white noise " but there is no generally recognized as binding definition for different types of colored noise power spectra. Thus one finds, for example, the term " pink noise " both for noise with a spectral noise power density, which is proportional decreases inversely with frequency, as well as for noise processes with a spectral noise power density, which is proportional decreases inversely with the square of the frequency.

To avoid this ambiguity, in scientific publications for processes whose spectral noise power density is inversely proportional to the frequency, the term " 1/f-noise " used. Sometimes the 1 / f ² noise is called in distinction to the " pink noise " than "Red noise" because the amplitudes in the low-frequency range ( the red light) are higher. There would be a shift in the color to red in a light spectrum. Some sources speak in this context of " brown noise".

Examples

For external noise sources:

• Background noise (including thermal noise ) by the origin of the universe
• Cosmic noise - mainly of the fixed stars of the Milky Way ( takes about 1 / f ³ ab)
• Terrestrial atmospheric noise as thermal noise, lightning discharges, sparks brush fire in electrical machines and switching operations
• Contact noise at contact points between electrical conductors and / or semiconductors

Internal noise sources:

• (Also called thermal noise, resistor noise or Johnson noise, Nyquist noise) thermal noise in conductors
• Tube noise in electron tubes

White noise

Pink Noise

Red noise