U18ECI5201
COMMUNICATION ENGINEERING- I
INTRODUCTION
Noise – Types of noise
Dr.S.Sasikala
Department of Electronics and
Communication Engineering
Overview of this
Lecture
• Introduction to Noise
• Types of Noise
• Metrics
Introduction to Noise
Introduction to Noise
➢ An unwanted signal which affects a wanted signal
➢ Noise mainly affects performance of receiving system
➢ It sets a lower limit on the size of signal received.
➢ These unwanted signals arise from a variety of sources
Introduction to Noise
➢ Noise has no pattern, no constant frequency or
amplitude.
➢ It is quite random and unpredictable.
➢ It can’t be eliminated completely but reduced.
➢ Most common examples of noise are:
❖ Hiss sound in radio receivers
❖ Buzz sound amidst of telephone conversations
❖ Flicker in television receivers
Classification of Noise
Correlated Noise
Uncorrelated Noise
Noise exists only when
a signal is present
Noise present all the time
irrespective of the signal
Types of Uncorrelated Noise
External Noise
Internal Noise
External Noise
➢ Noise created outside the receiver
➢ It is produced by the external sources which may occur
in the medium or channel of communication, usually.
➢ This noise cannot be eliminated completely.
➢ The best way is to avoid the noise from affecting the
signal.
➢ External noise can be further classified as:
1. Atmospheric
2. Extraterrestrial
Atmospheric Noise
➢ Atmospheric noise or static Electricity is generally caused by Naturally occurring
electrical disturbances that originate within Earth’s atmosphere such as
lightning discharges in thunderstorms.
➢ Produce Sputtering, Crackling sound in the speaker when there is no signal
present.
➢ Often in the form of impulses that spreads its energy throughout a wide range
of frequencies.
➢ Magnitude of this energy is inversely proportional to its frequency.
➢ Since these processes are random in nature, it is spread over most of the RF
spectrum normally used for broadcasting.
➢ At frequencies above 30MHz, atmospheric noise is relatively insignificant.
Extraterrestrial Noise
➢Extraterrestrial noise consists of electrical signals that
originate from outside Earth’s atmosphere.
➢Also called as Deep-Space Noise.
➢Source – Milky way, Other Galaxies, Sun
Generated from distant stars
Generated from sun’s heat
Extraterrestrial Noise
Solar Noise
➢ Generated directly from sun’s heat.
➢ Even in normal conditions, constant radiation produced
due to high temperature of sun which can disrupt smooth
electronics communications.
Cosmic Noise
➢ Generated from distant stars
➢ Often called Black – Body Noise
➢ Distributed evenly throughout the sky.
➢ Intensity is relatively small.
Man-Made Noise
➢ Produced by human.
➢ Predominant sources are spark-producing mechanisms such as
❖commutators in electric motors
❖automobile ignition systems
❖ac power generating and switching equipment
❖Fluorescent lights
❖leakage from high voltage lines
➢ Man - made noise is impulsive in nature and contains a wide range of
frequencies that are propagated through space in the form of radio
waves.
➢ This noise ranges between 1 to 600 MHz and is most prominent.
➢ This noise is most intense in industrial and densely populated areas.
Internal Noise
➢ Noise created by any of the active or passive devices found in
receivers due to continuous functioning.
➢ Such noise is generally random, impossible to treat on
individual voltage basis, but easy to observe and describe
statistically.
➢ Because the noise is randomly distributed over the entire radio
spectrum, it is proportional to bandwidth over which it is
measured.
➢ This noise is quantifiable. A proper receiver design may lower
the effect of this internal noise.
Internal Noise
Internal noise can be further classified as:
1. Thermal Noise or Johnson Noise
2. Shot Noise
3. Low frequency or flicker Noise
4. Burst Noise or Popcorn Noise
Thermal Noise
➢ Thermal noise is associated with the rapid and random
movement of electrons within a conductor due to thermal
agitation.
➢ The mean square noise voltage is given as
ഥ 𝟐 = 𝟒𝒌𝑻𝑩𝑹 (𝒗𝒐𝒍𝒕𝟐 )
𝑽
Where
k = Boltzmann’s constant = 1.38 x 10-23 Joules per K
T = absolute temperature in Kelvin (T = oC + 273)
B = bandwidth noise measured in (Hz)
R = resistance (ohms)
Thermal Noise
➢ This type of noise is generated by all resistances (e.g. a resistor,
semiconductor, the resistance of a resonant circuit, i.e. the real part
of the impedance, cable etc).
➢ Also called as Johnson noise, Brownian noise, White noise – because
the random movement is at all frequencies and has a uniform
‘spectral density’.
Thermal Noise
➢ Thermal noise power is proportional to the product of bandwidth
and temperature.
N = KTB
➢ Thermal noise power is given by
𝑁𝑑𝐵𝑚 = 10 𝑙𝑜𝑔
𝐾𝑇𝐵
0.001
= 10 𝑙𝑜𝑔
𝐾𝑇
+
0.001
10log B
➢ For I Hz Bandwidth at room temperature (17oC or 290K)
𝑁𝑑𝐵𝑚 = 10
1.38−23 𝑥 290
𝑙𝑜𝑔
+
0.001
10log 1 = −174dBm
➢ Thus, at room temperature, 𝑁𝑑𝐵𝑚 = −174dBm + 10log B
Thermal Noise
Thermal Noise
Equivalent Voltage Source
Thermal Noise
Problem:
An amplifier operating over the frequency range from 18
to 20 MHz has 10K ohms. What is the rms noise voltage at
the input to this amplifier if the ambient temperature is
27 degree Celsius.
Ans: 18.2 microvolts
K=1.38x10-23
Analysis of Thermal Noise In
Communication Systems
➢ This thermal noise may be represented by an equivalent circuit
as shown below
Vn is the RMS noise voltage
VRMS =2 𝑘𝑇𝐵𝑅 = 𝑉𝑛
Analysis of Noise In Communication Systems
Resistors in Series
Assume that R1 at temperature T1 and R2 at temperature T2, then
____
2
n
___
V =V
____
2
n1
____
V
2
n1
___
+V
2
n2
= 4 k T1 B R1
Vn 2 = 4 k T2 B R2
2
____
2
n
 V
____
2
n
V
= 4 k B (T1 R1 + T2 R2 )
= 4 kT B ( R1 + R2 )
The resistor in series at same temperature behave as a single resistor
Analysis of Noise In Communication Systems
Resistors in Parallel
Assume that R1 at temperature T1 and R2 at temperature T2, then
R2
Vo1 = Vn1
R1 + R2
____
2
n
V
____
2
n
V
___
=V
2
o1
Vo 2 =Vn 2
___
+V
R1
R1 + R2
2
o2
 R1 R2
4kB
2
2
R
T
R
+
R
T
R

= (R + R )2 2 1 1 1 2 2  R R
 1 2
1
2

_____
2
n
V
=
_____
2
n
V

4kB R1 R2 (T1 R1+ T2 R2 )
(R1 + R2 )2
 RR
= 4kTB  1 2
 R1 + R2






Analysis of Noise In Communication Systems
REACTANCE
➢ Reactances do not generate thermal noise.
➢ This follows from the fact that reactances
cannot dissipate power.
➢ Consider an inductive or capacitive reactance
connected in parallel with a resistor R.
➢ In thermal equilibrium, equal amounts of
power must be exchanged; i.e, P1 = P2 .
➢ But since the reactance cannot dissipate
power, the power P2 must be zero, and hence
P1 must also be zero.
Shot Noise
➢Caused by the random arrival of carriers at the output element
of an electronic device such as diode, FET, BJT.
➢It is randomly varying and is superimposed onto any signal
present.
➢When amplified shot noise sounds like Metal pellets falling on a
tin roof.
➢Also called transistor noise. It is additive with thermal noise.
Shot Noise
➢ The noise is uniformly distributed over frequency spectrum.
➢ The noise energy increases with the current in device, so
collector currents are maintained at a few hundred mA for
low noise application
➢ Shot noise is well known to occur in solid-state devices
➢ Tunnel junctions
➢ Schottky barrier diodes
➢ p-n junctions
Shot Noise
Nt =Number of electrons
Poisson process
➢ An electric current is the flow of discrete electric charges. The finiteness
of the charge quantum result in statistical fluctuation of the current.
➢ Unlike thermal noise, this noise is dependent upon the current flowing
and has no relationship to the temperature at which the system is
operating.
Shot Noise
➢ The mean square noise component is proportion to the DC flowing,
and for most devices the mean-square, shot-noise is given by:
𝑰𝟐𝒏 = 𝟐 𝑰𝑫𝑪 + 𝟐𝑰𝒐 𝒒𝒆 𝑩𝒏 (𝒂𝒎𝒑𝒆𝒓𝒆)𝟐 = 𝑰𝟐𝒏 = 𝟐𝒒𝒆 𝑰𝒅𝒄 𝑩𝒏 (𝒂𝒎𝒑𝒆𝒓𝒆)𝟐
where
Idc is the direct current in ampere’s
Iois the reverse saturation current (amps)
qe is the electronic charge = 1.6 x 10-19 coulombs
Bn is the equivalent noise bandwidth in Hertz
Shot Noise
Transit Time Noise
Any modification to a stream of carriers as they pass
from the input to the output of a device (From emitter
to collector of a transistor) produces an irregular,
random variation categorized as transit – time noise.
Miscellaneous Noise
Flicker, 1/f Noise
➢ Flicker noise is a form of electronic noise that dominates at low
frequencies or low frequency offsets from oscillators.
➢ Spectral density increases with decrease in frequency. Hence it is
sometimes referred to as 1/f noise
➢ Flicker noise becomes significant at frequency lower than about 100 Hz.
➢ Flicker noise can be reduced significantly by using wire-wound or
metallic film resistors rather than the more common carbon
composition type.
Miscellaneous Noise
Flicker, 1/f Noise
➢In semiconductors, flicker noise arises from fluctuations in the
carrier densities (holes and electrons), which in turn give rise
to fluctuations in the conductivity of the material.
➢The noise voltage will be developed whenever direct current
flows through the semiconductor.
➢The mean square voltage will be proportional to the square of
the direct current.
Avalanche Noise
➢Avalanche noise is a form of noise that is created when
avalanche breakdown occurs. It can be used for noise
generators.
➢Avalanche noise is a form of noise that does not occur in
most circuit.
➢But can be experienced with PN junctions that are operated
at the point of avalanche breakdown or close to it.
Burst Noise: popcorn noise
➢ Burst noise or "popcorn noise" is experienced in a variety of RF and
other electronic circuits.
➢ Burst noise, or as it is sometimes called, popcorn noise, or random
telegraph signal, RTS, consists of sudden step-like transitions
between two or more levels.
➢ Burst noise, or popcorn noise was an issue when the first
operational amplifiers were introduced. It made a noise like cooking
popcorn if sent to a loudspeaker - hence the name.
Correlated Noise
➢ Nonlinear
distortions
produced
by
nonlinear
amplification.
➢ Also produced when signals pass through nonlinear
devices like diodes.
➢ Nonlinear Distortions includes
❖Harmonic Distortion
❖Inter-modulation Distortion
Harmonic Distortion
➢Harmonics are the integral multiples of the original input
signal.
➢Original signal is the first harmonic and is called
fundamental frequency.
➢Total Harmonic distortion(THD) is given by V(higher) =
V(higher) is Quadratic sum of RMS voltages of the harmonics =
V(fundamental) is the RMS voltages of the fundamental frequency.
Harmonic Distortion
Inter modulation distortion
➢ Generation
of
unwanted
sum
and
difference
frequencies when two or more signals are amplified
in a nonlinear device.
➢ In communication circuits it is more often desirable
to mix two or more signals and produce sum and
difference frequencies.
➢ The sum and difference frequencies are called cross
products.
𝐶𝑟𝑜𝑠𝑠 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 = 𝑚𝑓1 ± 𝑛𝑓2
Inter modulation distortion
Noise Classification
➢
➢
Signal to Noise Ratio
The signal to noise ratio is given by
S Signal Power
=
N Noise Power
The signal to noise in dB is expressed by
S 
 
N
dB
=S dBm − N dBm
for S and N measured in mW.
Noise Factor (F)
➢ Noise Figure represents the degradation
in signal/noise ratio as the signal passes
through a device.
➢ The amount of noise added by the
network is embodied in the Noise Factor
F, which is defined by
Noise factor F =
(S N )
(S N )
IN
OUT
➢ F equals to 1 for noiseless network. Generally F is always greater than 1.
Noise Figure (NF)
➢ The noise figure in the noise factor quoted in dB
Noise Figure F dB = 10 log10 F
NF ≥ 0 dB
NF = (Si/Ni)dB – (So/No)dB
➢ The noise figure / factor is the measure of how much a
network degrades the (S/N)IN, the lower the value of F, the
better the network.
(Si/Ni)dB = 40 dB
(So/No)dB = = 30 dB
Noise Figure = 10 dB
Noise Figure of Cascaded Amplifiers
➢ The total noise factor is accumulation of individual noise factors
➢ Friiss’ Formula to calculate total noise factor is
FT = F1 +
𝐹2 −1
𝐴1
+
𝐹3 −1
𝐴1𝐴2
➢ The total noise figure is then,
NFT = 10 log FT
𝐹4 −1
+
𝐴1𝐴2𝐴3
+ ………+
𝐹𝑛 −1
𝐴1𝐴2𝐴3…𝐴𝑛
Noise Figure of Cascaded Amplifiers
Example:
For three cascaded amplifier stages each with noise figures of 2dB and
power gain of 10dB. Calculate the total noise factor
Friiss’ Formula to calculate total noise factor is
FT = F1 +
FT = 2 +
2−1
10
+
𝐹2 −1
𝐴1
+
𝐹3 −1
𝐴1𝐴2
𝐹4 −1
+
𝐴1𝐴2𝐴3
2−1
= 2.11
100
N FT = 10 log10 FT = 10 log10 (2.11) = 3.24 dB
+ ………+
𝐹𝑛 −1
𝐴1𝐴2𝐴3…𝐴𝑛
Noise Temperature
➢ Comes from the random motion of electrons
N = KTB
𝑇=
Thermal Noise
𝑁
𝐾𝐵
➢ Convenient! Common basis for measuring
➢ random electrical noise from any source
➢ Relation with Noise Figure
Te = T o ( F – 1 )
F = 1+
Te
To
➢ Te : The effective noise temperature of device
➢ T0 : a reference temperature 290K (room temperature)
Noise Temperature