雜訊測量及分析實驗. sources of electronic noises the two most commonly encountered types of...

25

Post on 17-Jan-2016

224 views

Category:

Documents

TRANSCRIPT

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

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

where k is the Boltzmann’s 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.

• 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

OP 放大器之電路模型與雜訊分析

4 nV

Noises of superconducting device

Geometrical configuration of dc SQUID

YBa2Cu3Oy

STO

~2, 3, 4, 5 µm ~170 nm

Grain boundary

1 1 0 1 0 0 1 0 0 0

F req u en cy , f (H z)

1 0

1 0 0

1 0 0 0

Flu

x no

ise,

S

1/2 (

/

Hz1/

2 )

1/f

white noise

Noises in superconducting devices

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

10-4

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

Lung particles

Human heart

Fetal heartHuman eye

Human brain ()

Human brain (response)

B (Tesla)

Earth field

Urban noise

Car @ 50 m

Transistorchip @ 2 m

Transistordie @ 1 m

Flux-gatemagnetometer

SQUID

T

nT

pT

SQUID

0.1 1 10 100

10

100

1000S1

/2( 0/

Hz1

/2)

f(Hz)

without flux dam with flux dam

The noise power spectrum density of SQUIDs magnetometer with and without flux dam.

With flux dam

Without flux dam

20 fT/Hz1/2

Bias current, Ib

Vo

ltag

e,

V

= n 0

= (n+1/2) 0

M agnetic flux, /0

Vo

ltag

e,

V

Ib

V

Ib

(a)

(b)

(c)

Schematic of dc SQUID Electronics

Rf

Oscillator

Lock-in Detector

IB

Vo

IntegratorAmplifier

Modulation coil

Pick-up coil

Input coil