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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
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.
電壓方均根的雜訊與 4kBTR 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
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
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
4 nV
運算放大器 的背景雜訊
雜訊電壓頻譜
Noises of superconducting device
YBa2Cu3Oy
~2, 3, 4, 5 µm
~170 nm
Grain boundary
Geometrical configuration of dc SQUID
Noises in superconducting devices
Flux noise, S1/2 (/Hz1/2)
1000
100
white noise
1/f
10
1
10
100
Frequency, f (Hz)
1000
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
B (Tesla)
10
Earth field
Environmental
fields Urban noise
Car @ 50 m
Flux-gate
magnetometer
T
10
10
10
10
nT
10
10
SQUID
Transistor
chip @ 2 m
Transistor
die @ 1 m
SQUID
10
pT
10
10
10
10
-4
-5
-6
-7
Biomagnetic
fields
-8
-9
Lung particles
-10
Human heart
-11
Fetal heart
Human eye
-12
Human brain ()
-13
-14
-15
Human brain (response)
without flux dam
with flux dam
Without flux dam
1/2
S (0/Hz )
1000
100
1/2
With flux dam
10
20 fT/Hz1/2
0.1
1
10
100
f(Hz)
The noise power spectrum density of SQUIDs magnetometer
with and without flux dam.
(c)
(b)
 = (n+1/2) 0
 = n 0
Ib
Bias current, Ib
V
Voltage, V
Voltage, V
(a)
Ib
Magnetic flux, /0
Schematic of dc SQUID Electronics
Pick-up coil
IB
Input coil
Modulation
coil
Amplifier
Lock-in
Detector
Integrator

Rf
Oscillator
Vo