Noise generator

A noise generator is a special form of signal generator and a form of an electronic circuit which produces noise as random jitter. Noise generators are used in various fields, such as in the measurement technique for generation of test signals, in the electronic music and speech synthesis, and as a basis for the generation of random numbers, such as those required in the cryptography.

Noise sources

For the generation of noise are various noise sources, which themselves differ inter alia, on the nature of the spectral distribution of the noise, the bandwidth of the noise signal, the technical feasibility and the avoidance of predictable signal components, such as interference of periodic signals such as the grid frequency. Some noise sources, such as the radioactive decay of unstable atomic nuclei, are theoretically usable as an ideal source of noise, but have in practice because of the dangers associated with no meaning.

Thermal noise sources

Thermal noise sources, such as in special heated resistors tapped noise voltage supply normally distributed white noise. Since the thermal noise voltage gives only very small signal voltages, for example at room temperature of 23 ° C in a resistance of 1 kOhm and a bandwidth of 1 kHz, an effective voltage of 100 nv, and this noise voltage by periodicals and unwanted signal components such as variations in the supply voltage is superimposed, provide good thermal noise sources in practice electrically complex constructions dar.

Diodes

Technically easier to produce than thermal sources of noise are noise sources based on diodes, which are referred to in this application also as a noise diode. The frequency range may be appropriate in generator circuits from 20 Hz to 40 GHz and is generally not spectrally white, but is described by the so-called shot noise.

Alternatively, but with the same principle, also the base -emitter junction of the bipolar transistors may be used instead of the diode in the generator. The oldest design of diode noise sources are diode vacuum tube with a tungsten filament, which are operated in saturation.

The signals are then amplified and spectrally filtered in filters depending on the application and the noise signal processed via analog -to-digital converter for further signal processing. This can be prepared in good approximation in technical areas used frequency and spectrally white noise.

Gas discharge lamp

For the technically very high frequency noise sources in the field of microwave frequencies up to 200 GHz gas discharge tubes are used which are housed for decoupling of the noise signal in a waveguide. Applications of these noise generators are in the range of high frequency metrology.

Glow discharge

For very low frequency noise in the lower Hz range, the glow discharge, for example in the form of neon lamps, apply.

Noise generators in audio engineering

White noise contains for each frequency interval the same power density at which output through a speaker over the entire frequency range, an identical volume or acoustic energy results (dB / Fq). Pink noise is spectrally weighted and can be obtained from white noise through filters. Pink noise contains at acoustic output per note interval the equal loudness (dB / semitone). This means that pink noise sounds a bit dull as white noise. White noise is used for example in speech synthesis for the replication of the sibilant sound, pink noise, for example, for a surf sound.

To obtain realistic sounds, the basic forms of white or pink noise must be filtered or according, this corresponds to a spectral shaping of the noise signal, eg with one or more voltage-controlled filters.