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Physical Origin of Noise
Noise is called fundamental when it arises from the particle
nature of light and matter and can never be totally eliminated, while non-fundamental
(or excess) noise is due to imperfect components and instrumentation and
can at least theoretically be eliminated. Non-random noise is never fundamental.
Thermal noise is generated by the thermal (Brownian) motion of electrons
or charged particles in electrical components (e.g. resistor, or capacitor)
and disappears only at an absolute temperature of zero Kelvin.
Quantum noise (also called shot noise) is caused when, e.g. electrons
or charged particles move across a junction. These events are quantized
and therefore Poisson-distributed.
Flicker noise is formed at transformation points or readout systems
and can cause drifts. When the magnitude of the noise is inversely proportional
to its frequency, then one speaks of 1/f noise. If the variance is proportional
to the signal, this type of noise is also called proportional, multiplicative
Environmental or interference noise is frequency-dependent. Often
it is present only at certain discrete frequencies, e.g. by picking up
electromagnetic radiation (50/60 Hz + harmonics from AC power).
Impulse noise originates, e.g. from turning instruments on/off and can
cause so-called spikes. A source of this type of noise often encountered
in labs is refrigerators. They create a large surge of electrical current
when they are switched on, and create high voltages on the power line when
the motor is switched off.
Quantization noise is due to the finite resolution of analog-to-digital
Since an instrument consists of a lot of different components, which
individually introduce noise into the signal, the resulting measurement
is contaminated by noise which may show complex properties. Fortunately,
either one component is dominant so that the other contributions can be
neglected, or the central limit theorem
applies. In the second case, the noise in the system shows normal distribution.
Last Update: 2002-Nov-03