Sources of electronic noisesThe two most commonly encountered types of noise are thermal noises and shot noises.

Thermal noise arise from the random velocity fluctuation of the charge carriers (electron and/or holes) in a resistive material.

The mechanism is sometimes said to be the Brownian motion of the charge carriers due to the thermal energy of the materials.

The thermal noise is often referred to as Johnson noises (or Nyquist noise) in recognition of two early investigators.

The thermal noise is usually expressed as

Sv(f) = 4kBTR (V2/Hz)

where k is the Boltzmanns constant (1.38x10-23 J/K), R is the resistance of the conductor, T is the absolute temperature, and Sv is the voltage noise power spectral density.

4kBTR

Shot noise occurs when the current flows across a barrier. It was first discovered by Schottky. It is often found in solid-state devices when a current passes a potential barrier such as the depletion layer in p-n junction.The stream of charge carrier fluctuates randomly about a mean level. The fluctuations (shot noise) are due to the random, discrete nature of the tunneling process. The shot noise has a constant spectral density of

Si(f) = 2eIDC (A2/Hz)

where e is the electronic charge and Idc is theaverade current.

In many devices, however, there is additional noise which varies with frequency as 1/f-, where usually lies between 0.8 and 1.2. This is commonly known as 1/f noise or flicker noise or excess noise.

The fourth types of noise is sometimes found in transistor and other devices. It is called burst noise or random telegraph noise. It consists typically of random pulses of variable length and equal height.

External noises due to interference from electrical or magnetic disturbances are a separate topic.

Circuit diagram of a noise measurement system

(SR760)

OP

4 nV

Noises of superconducting device

Geometrical configuration of dc SQUID

1/fwhite noiseNoises in superconducting devices

Noises in superconducting devices Possible sources of low-frequency 1/f noise: Critical current fluctuationResistance fluctuation, orMotion of flux line Possible sources of white noise: Thermal noise

Weak Magnetic Fields

The noise power spectrum density of SQUIDs magnetometer with and without flux dam.With flux damWithout flux dam 20 fT/Hz1/2

Schematic of dc SQUID ElectronicsRfOscillatorLock-in DetectorIBVoIntegratorAmplifierModulation coilPick-up coilInput coil

Thanks for your attention!

Hong-Chang Yang