雜訊測量及分析實驗

Sources of electronic noises The 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 S v (f) = 4k B TR (V 2 /Hz) where k is the Boltzmann’s constant (1.38x J/K), R is the resistance of the conductor, T is the absolute temperature, and S v is the voltage noise power spectral density.

電壓方均根的雜訊與 4k B TR 正比

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 S i (f) = 2eI DC (A 2 /Hz) where e is the electronic charge and I dc 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

頻譜分析儀的背景雜訊

頻譜分析儀的背景雜訊 (SR760)

OP 放大器之電路模型與雜訊分析

運算放大器 的背景雜訊 4 nV

雜訊電壓頻譜

Noises of superconducting device

Geometrical configuration of dc SQUID YBa 2 Cu 3 O y STO ~2, 3, 4, 5 µm ~170 nm Grain boundary

1/f white noise Noises in superconducting devices

Possible sources of low-frequency 1/f noise: Critical current fluctuation Resistance fluctuation, or Motion of flux line Possible sources of white noise: Thermal noise

Weak Magnetic Fields Biomagnetic fields Environmental fields Lung particles Human heart Fetal heart Human eye Human brain (  ) Human brain (response) B (Tesla) Earth field Urban noise 50 m Transistor 2 m Transistor 1 m Flux-gate magnetometer SQUID TT nT pT SQUID

The noise power spectrum density of SQUIDs magnetometer with and without flux dam. With flux dam Without flux dam 20 fT/Hz 1/2

IbIb V IbIb (a) (b) (c)

Schematic of dc SQUID Electronics RfRf Oscillator Lock-in Detector IBIB VoVo IntegratorAmplifier Modulation coil Pick-up coil Input coil

Thanks for your attention! Hong-Chang Yang