Download Noise Figure for an equivalent temperature of 75 K

CHAPTER 1
Part 2  Noise
Objectives
To differentiate the types of noise
 To calculate the thermal noise
generated by a resistor
 To calculate the signal-to-noise ratio
(SNR) and noise figure for an amplifier

Lecture overview
Types of noise
 Thermal noise
 Signal-to-noise ration (SNR) and noise
figure

Introduction
Noise can be defined as
 undesired random variations that interface with
the desired signal and inhibit communication.
Where does noise originate in a communication
system?
 Channel @ transmission medium
 Devices @ Equipments
Cont’d...
Noise Effect
 One of the main limiting factor in obtaining
high performance of a communication
system.
 Decrease the quality of the receiving signal.
Block Diagram of Communication
System With the Existence of Noise
Noise, interference and
distortion
◦ Noise
 Refers to random and unpredictable
electrical signals produced by natural
process.
 Superimposed on information bearing
signal, the message partially corrupted
or totally erased.
 Can be reduced by filtering but can’t
totally eliminated.
◦ Interference
 A contamination by extraneous signals
from human sources (e.g. from other
Tx, power lines, machineries)
 Often occurred in radio system whose
Rx antenna intercept several signals at
the same time.
◦ Distortion
 The signal perturbation caused by
imperfect response of the system to
the desired signal.
 Disappear when the signal is turnedoff.
 Can be corrected by the equalizers.
Noise Remedies?
REDUCE BANDWIDTH
INCREASE TRANSMITTER’S
POWER
LOW NOISE AMPLIFIERS
Types of NOISE
NOISE
INTERNAL
EXTERNAL
THERMAL NOISE
-transistor
-diode
-resistors
MAN MADE NOISE
-automobile engine
-electric motor
-computer
SHOT NOISE
-electronic system
-equipment
SPACE NOISE
-solar noise
-sky noise
FLICKER NOISE
-tubes
ATMOSPHERIC NOISE
-Noise blanking
-lighting
EXTERNAL NOISE
◦ Noise generated outside the device or circuit.
◦ Three primary sources:
 Atmospheric noise.
 Extraterrestrial noise
 Man-made noise
EXTERNAL NOISE
Atmospheric noise
o Naturally
occurring
electrical
disturbances that originate within
Earth’s atmosphere.
o Is often in the form of impulses that
spread energy throughout a wide
range of frequencies.
EXTERNAL NOISE
Extraterrestrial noise
o Consists of electrical signals that
originate from outside Earth’s
atmosphere.
o 2 categories:
o Solar noise: is generated directly from
sun’s heat.
o Cosmic noise: are continuously
distributed throughout the galaxies.
EXTERNAL NOISE
Man-made noise
o Noise that is produced by mankind.
o Predominant
sources
are
sparkproducing
mechanisms
such
as
commutators in electric motors,
automobile ignition systems, ac powergenerating and switching equipment.
o Is impulsive in nature and contains a
wide range of frequencies that are
propagated through space in the
same manner as radio waves.
INTERNAL NOISE
◦ Electrical interference generated within a
device or circuit.
◦ 3 primary kinds:
 Shot noise
 Transit time noise
 Thermal noise
INTERNAL NOISE

Shot Noise
oCaused by a random arrival of carriers
(holes and electrons) at the output of an
electronic devices such as diode, field-effect
transistor or bipolar transistor.
oRandomly varying & superimposed onto any
signal present.
oSometimes called transistor noise.
INTERNAL NOISE

Transit-time noise:
o Any modification to a stream of carriers as
they pass from the input to output of a device
produces an irregular, random variation.
Thermal Noise
Rapid and random movement of
electrons within a conductor due to
thermal agitation.
 Is present in all electronic components
and communications systems.
 Is a form of additive noise, meaning that
it cannot be eliminated and it increases
in intensity with the number of devices in
a circuit and with circuit length.

Thermal Noise
Since it is dependent on temperature, it
is also referred to as thermal noise.
 Thermal noise power is proportional to
the product of bandwidth and
temperature.

N = KTB
Where
N = noise power (Watts)
B = bandwidth (Hz)
K = Boltzman’s constant (1.38 x 10-23 joules
per kelvin)
T = absolute temperature (kelvin) (room
temperature = 17oC or 290K)
To convert oC to kelvin, add 273oC
EXAMPLE 1
Convert the following temperatures to
kelvin: 100oC, 0oC and -10oC.
Solution:
T = oC + 273oC
T =100 oC + 273oC = 373K
T = 0oC + 273oC = 273K
T = -10oC + 273oC = 263K
NOISE VOLTAGE


From the study of circuit theory, the
relationship between source resistor
and matched load under maximum
power transfer is when RN = RL .
The total of noise source power is N.
Let RN = RL = R
The noise voltage dropped across R is
VN
VR 
2
2
 VN 
 
2  VN 2

Noise power, N  KTB 

4R
R
Thus,
VN  4 RKTB
2
VN  4 RKTB
EXAMPLE 2
For an electronic device operating at a
temperature of 170C with a bandwidth
of 10 kHz, determine:
(a) thermal noise power in watts and
dBm
(b) rms noise voltage for a 100Ω
internal resistance and 100Ω load
resistance.
SIGNAL-TO-NOISE POWER RATIO

Is the ratio of the signal power level to
the noise power level.
S/N = Ps/Pn
where Ps = signal power (watts)
Pn = noise power (watts)
Or
S/N (dB) = 10 log(Ps/Pn)
EXAMPLE 3
For an amplifier with an output signal
power of 10 W and an output noise power
of 0.01 W, determine the signal-to-noise
power ratio and express it in dB.
NOISE FACTOR AND NOISE
FIGURE

Noise factor is a ratio of input signal-tonoise power ratio to output signal-tonoise power ratio

Noise figure is the noise factor stated in
dB
Continue…

When two or more amplifiers are
cascaded as shown in figure below, the
total noise factor is the accumulation of
the individual noise factors
Input
Amplifier 1
F1
Ap1
Amplifier 2
F2
Ap2
Amplifier 3
F3
Ap 3
Output
Continue…
To calculate the total or overall
noise factor:
( Fn  1)
( F2  1) ( F3  1)
FT  F1 

 ... 
A1
A1 A2
A1 A2 ... An 1
EXAMPLE 4
For three cascaded amplifier stages, each
with noise figures of 3dB and power gains
of 10 dB, determine the total noise figure
EQUIVALENT NOISE
TEMPERATURE

Equivalent noise temperature (Te) is a
hypothetical value that cannot be directly
measured
Te = T (F – 1)
Where Te = equivalent noise temperature (kelvin)
T = environmental temperature (reference
value of 290K)
F = noise factor (unitless)
Similarly, the overall equivalent noise temperature, Te
of cascaded amplifiers is given by:
Te 2
Te 3
Ten
Te  Te1 

 ... 
A1 A1 A2
A1 A2 ... An 1
EXAMPLE 5
Determine:
a. Noise Figure for an equivalent
temperature of 75 K (use 290 K for the
reference temperature).
b. Equivalent noise temperature for a Noise
Figure of 6 dB.