Examples of White noise are Thermal noise, and Shot noise. Toggle navigation. Types of Noise sources: There are several types of noise sources in electrical circuits. Thermal or Johnson Noise 2. Shot Noise 3. Shot noise is proportional to the current passing through the devise. However, note that this formula may not be applicable under all circumstances. Flicker noise is more prominent in FETs, and bulky resistors. A particular set of problems arises in multi-level signaling systems e.
In digital systems that use multi-level signaling, you are generally working with razor-thin noise margins, and thermal noise will already increase the BER value for each symbol. Add to this intersymbol interference, and different symbols can become indistinguishable.
In wireless systems, thermal noise is usually ignored at the transmitter side as the signal chain is run at sufficiently high power, i. On the receiver side, the SNR value of a low-level analog signal is generally low, and the signal must be amplified prior to demodulation. Unfortunately, noise that lies in the frequency band of interest will also be amplified and can become problematic in demodulation and digital conversion.
In these systems, it is important to determine the bandwidth of various noise sources so that you can anticipate how noise affects signals produced and received by different components. With this in mind, the question arises: when does thermal noise become prominent and how does the thermal noise bandwidth affect different components.
Thermal noise will always arise in any component due to the inherent DC resistance in the system. In general, cooling your system or using components with lower parasitic resistance will reduce the thermal noise intensity. Thermal noise has the following characteristics:. Uncorrelated i. This means that thermal noise is uncorrelated over time; the thermal noise you measure does not depend on thermal noise measured at all previous times. Note that this also applies in circuits with feedback e.
Power spectral density : As a result of the uncorrelated nature of thermal noise, power is uniformly distributed at all frequencies below some limit, i. Gaussian distributed : Changes in the amplitude of thermal noise over time follow a Gaussian stochastic process with zero drift i. The standard deviation depends on temperature and frequency, as shown below.
The thermal noise bandwidth depends on the bandwidth of the circuit in which noise is present. In other words, we can calculate the root-mean-squared RMS noise voltage and current in terms of the impedance of a circuit:.
This equation has an important consequence; the resistive portion of the circuit determines the voltage associated with thermal noise. This is the case for a circuit in series with a driver with low output impedance. For an impedance matched driver and load, simply ignore the factor of 4 in the above integral; this tells you the thermal fluctuations seen in the voltage drop across the source or load on its own.
Due to the fact that the RMS voltage noise is only due to the resistive portion of the circuit, the above integral can be solved and written in terms of the Thevenin equivalent resistance. This is certainly applicable in the majority of circuits that operate at room temperature. If we consider this case for a purely resistive circuit, we have the following often-cited result:. But recent progress in nanofabrication technology has revived the interest in shot noise, particularly since nanostructures and "mesoscopic" resistors allow measurements to be made on length scales that were previously inaccessible experimentally.
Shot noise in mesoscopic devices has already proved to be a fruitful playground for theoretical physicists. Recent calculations show that shot noise should exist in mesoscopic resistors, although at lower levels than in a tunnel junction. For these devices the length of the conductor is short enough for the electron to become correlated, a result of the Pauli exclusion principle.
This means that the electrons are no longer transmitted randomly, but according to sub-Poissonian statistics. The sub-Poissonian shot-noise power, S , of a metallic resistor as a function of its length, L , as predicted by theory. Indicated are the elastic mean-free path, l , the electron-electron scattering length, l ee , and the electron-phonon scattering length l ep. The experiments by Steinbach, Martinis and Devoret have confirmed the theory for lengths down to l ee.
The dashed line gives the shot-noise spectral density of a Poisson process as found in, for example, tunnel junctions. Theorists have predicted the shot noise in a metallic resistor as a function of its length see figure. In the so-called ballistic regime the resistor is so short that it does not contain any impurities. The electrons cannot be scattered, so the probability of an electron being transmitted is unity and there is no shot noise. When the resistor is longer than the mean-free path for elastic scattering, the electrons are scattered by impurities in the metal.
The electron motion becomes diffusive but the energy of each electron remains constant. If the resistor is longer than the electron-electron scattering length, the electrons undergo inelastic collisions.
Their total energy is conserved, so they are heated above the lattice temperature. Ioffe Institute in St. Intriguingly, in both of these regimes the shot noise is proportional to the full Poissonian shot noise. Moreover, the constant of proportionality is a simple numerical coefficient that does not depend on the shape of the resistor nor on the material that it is made of. The first experimental observation of sub-Poissonian shot noise in a metallic resistor was made in a collaboration between the University of Utrecht and Philips Research.
We investigated a gate-defined wire in a semiconductor structure, where the wire was 17 micron long. The measurements were in agreement with theory but lacked the precision needed to discriminate between the predicted values.
Indeed, noise experiments are notoriously difficult.
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