This document discusses different types of noise that can affect telecommunication systems, including external noise from natural and man-made sources, and internal noise generated within system components. It defines various internal noise sources such as thermal noise, shot noise, partition noise, and flicker noise. Thermal noise is explained using Johnson noise equation, and how it can be minimized by reducing bandwidth, temperature, or resistance. Noise factor and noise figure are introduced to measure noise performance, with the Friis formula provided to calculate the overall noise factor of networks in cascade. Effective noise temperature is also defined to represent noise effects in a network.

1. Noise
TELECOMMUNUCATION
NOISE
Presented by
Mr M Nkoloma
To
D3E Students

2. Noise
NOISE CATEGORIES
– External
• Natural noise
• Man-made noise
– Internal
• Thermal noise
• Shot noise
• Partition noise
• Flicker noise

3. Noise
External noise
• Noise originating from outside the
communication system.
– Natural noise
• Noise that arise due to natural phenomenon
– eg Lighting, thunderstorm, solar flares and other general
atmospheric disturbances (eg cosmic noise)
– Man-made noise
• Noises that arise due to pick up of undesired signals from
other sources.
– eg faulty electrical contacts, electrical appliances, ignition
radiation, fluorescent lighting.
» Removing the noise source or shielding the
communication system minimise their effects.

4. Noise
Internal noise
• Noise generated within the components
used in the communication system.
• Difficult to eliminate as they come from
various components.
– Eg:
• Thermal noise or Johnson noise
• Shot noise
• Partition noise
• Flicker noise

5. Noise
Internal noise cont.
• Internal noise obey certain physics laws.
• An understanding of these laws enables
systems to be designed in such a way as
to minimise their effects.

6. Noise
Thermal noise
or Johnson noise
• Arises from the random motion of free
electrons in a conductor as a result of
receiving thermal energy
• Relationship exist between
instantaneous thermal noise voltage and
temperature:
KTBR
Vn 4
2


7. Noise
KTBR
Vn 4
2

• Vn is the thermal noise voltage.
• K is the Boltzmann’s constant.
• T is the absolute temperature of the
conductor.
• R is the resistance of the conductor.
• B is the bandwidth over which noise is
measured.
K
J /
10
38
.
1 23



8. Noise
• This noise can be minimised by:
– Reducing the bandwidth of the system.
– Keeping the temperature of the
communication system low.
– Designing the communication system
circuits to have a low resistance.
KTBR
Vn 4
2


9. Noise
Noise equivalent circuit
• Consist of a noise voltage generator in
series with a noiseless resistance, or a
noise current generator in parallel with
noiseless conductance.

10. Noise
Maximum noise power
• Noisy resistance and its equivalent circuit
• Maximum noise power is delivered to the load
when load resistance = R
• Hence the maximum noise delivered to the
load is given as
 2
2
2
L
n
n
R
R
V
I


 2
2
2
L
L
n
L
n
N
R
R
R
V
R
I
P



KTB
R
V
P n
N 

4
2
max

11. TELECOMMUNICATION
NOISE
Presented
by
Mr M Nkoloma
to
D3E Students

12. Signal to Noise ratio
• Telecommunication systems are characterized by the
fact that received signals are always accompanied by
noise.
• Effectiveness is measured in terms of the ratio of
signal power to noise power, SNR.
• SNR measures:
– Performance of the communication system.
– Purity of the signal.

13. Signal to Noise Ratio
• Signal power is given as:-
• Noise power is given as:-
• Signal to noise ratio (SNR) is given as
– This is usually expressed in decibels (dB)
• Minimum accepted SNR depends on application
– Telephone circuits = 16db
– High quality audio transmission = 60db
– Tv transmission = 47db
R
V
P s
S
4
2

R
V
P N
N
4
2

2
2
2










N
s
N
s
V
V
V
V
SNR









N
s
dB
V
V
SNR log
20

14. • Measure of the degradation of the signal to noise ratio.
• Mathematically noise factor is defined as:
• Where
– G is the gain of the network
– Nai is the noise introduced by the network referred to the input
of the network.
– Si the input signal power and So the output signal power.
Noise factor & noise figure
O
i
SNR
SNR
F 
 














ai
i
i
i
i
o
o
i
i
N
N
G
GS
N
S
N
S
N
S

15. • Therefore noise factor can also be reduced and
defined as:
• Noiseless network has unity noise factor.
• Noise figure
– Noise factor expressed in dB
Noise factor & noise figure
dB
SNR
SNR
F
o
i
dB 







 log
10
i
ai
i
N
N
N 

 













ai
i
i
i
i
o
o
i
i
N
N
G
GS
N
S
N
S
N
S
F

16. Noise introduced by a network
• Assuming that only thermal noise is available.
– Noise introduced by a network can be defined as
i
ai
i
N
N
N
F


free
noise
was
network
the
if
noise
output
Total
power
noise
output
Total
F 
  i
ai N
F
N 1


 KTB
F
Nai 1



17. Problem
• For a system with an input noise of 8µV,
an output noise of 500µV and power gain
of 24.771dB. Calculate the noise factor.

18. Noise factor for networks in
cascade
• Consider two networks in cascade
• Noise available at the output
• Noise power available at the output if the
network was noise free
2
1
2
02 a
o N
N
G
N 
   2
1
1
2
02 a
a
i N
N
N
G
G
N 


i
N
G
G
N 2
1
02 

19. Noise factor for networks in
cascade
• Noise factor is given as
• Hence the overall noise factor F is given as
• But and
• Thus hence
free
noise
was
network
the
if
noise
output
Total
power
noise
output
Total
F 
 
i
a
a
i
N
G
G
N
N
N
G
G
F
2
1
2
1
1
2 


 
i
a
i
a
i
N
G
G
N
N
G
N
N
G
F
2
1
2
1
1
1



 
i
a
i
N
G
N
N
G
F
1
1
1
1


 
i
a
i
a
i
N
G
N
N
G
N
N
G
F
2
2
2
1
2
2 1



i
a
N
G
N
F
2
2
2 1

1
2
1
1
G
F
F
F




20. Noise factor for networks in
cascade
• In general, if n networks are connected in cascade
the overall noise factor can be obtained as follows;
• This is called the FRIIS’s formula
• Observe that the major contribution to the overall
noise factor is produced by the first stage network.
– Extremely important to ensure that the first network in any
cascade system has a low noise figure.
1
3
2
1
2
1
3
1
2
1
.....
1
.....
1
1









n
n
G
G
G
G
F
G
G
F
G
F
F
F

21. Problems
• A preamplifier with power gain to be found
and a noise figure of 2.5 dB is cascade with a
mixer with a gain of 2 dB and a noise figure
of 8 dB. Find the preamplifier gain such that
the overall noise figure of the cascade is at
most 4 dB.
– Answer: 8.6 dB
• Given that three amplifiers are connected in
cascade and have the following data: F1 = 7,
F2 = 5, F3 = 4, A1 = 100, A2 = 30 and A3 = 20.
Calculate the overall noise figure in dB.

22. • F= 4dB
F=F1 + (f2-1)/G1
• Change to non-dB’s
F1=1.778 (ant-log)
F= 2.511
F2= 6.3096
2.511= 1.778 + ( 6.3096-1)/G1
G1= 7.244
G1= 8.5998 (10 log 7.244)
Noise

23. EFFECTIVE NOISE TEMPERATURE
• This is a fictitious temperature, Te used
to represent the effects of noise in a
network.
• Noise introduced by a network
– Nai = KTeB
– Nai = K(F-1)ToB
• Where Te is the effective noise temperature
To is the reference temperature

24. EFFECTIVE NOISE TEMPERATURE - Passive
network
• Passive network
– Realized when network gain G<1
– Characterized by insertion loss, L.
• L is the reciprocal of power gain, 1/G
– L = F when a network is matched both at the input and output
• Therefore Te = (F-1)To
• OR Te = (L-1)To for a passive matched network
– L is attenuation loss factor or insertion loss
• F = 1 + Te/To
power
Output
power
Input
L 

25. EFFECTIVE NOISE TEMPERATURE
• Overall noise factor, F is given as
• From the relationship: F = 1 + Te/To
• Hence the overall effective noise temperature is
given as
• Therefore the overall effective noise
temperature is given as
1
3
2
1
2
1
3
1
2
1
.....
1
.....
1
1









n
n
G
G
G
G
F
G
G
F
G
F
F
F
o
n
en
o
e
o
e
o
e
o
e
T
G
G
G
G
T
T
G
G
T
T
G
T
T
T
T
T
1
3
2
1
2
1
3
1
2
1
.....
.....
1
1








1
3
2
1
2
1
3
1
2
1
.....
.....






n
en
e
e
e
e
G
G
G
G
T
G
G
T
G
T
T
T

26. Problem
• Consider a receiver system consisting of an antenna with lead-in
cable having a loss factor of L = 1.5dB = F, an RF preamplifier
with a noise figure of 7dB and a gain of 20dB, followed by a
mixer with a noise figure of 10dB and a conversion gain of 8dB,
and finally an integrated-circuit IF amplifier with a noise figure
6dB and a gain of 60dB.
– Find the overall noise figure and noise temperature of the
system. 8.57dB, 1796K
– Find the overall noise figure and noise temperature of the
system with the preamplifier and cable interchanged.
7.12dB, 1203K

27. • L1= 1.5dB=F1
• F2= 7dB, G2= 20dB
• F3= 10dB, G3= 8dB
• F4= 6dB, G4= 60dB
(Hint= change the dBz into Non-dBz;
using ant-logs)
Noise