NOISE

IN
COMMUNICATION
SYSTEMS
Prepared by:
Engr. Analene
Montesines-Nagayo
NOISE IN COMMUNICATION
SYSTEMS
NOISE
- Any undesirable voltages or currents that ends up appearing in the receiver
output.
- Any unwanted form of disturbance of signal or energy tending to interfere
with the proper and easy reception and reproduction of desired signal.
- Random energy that corrupts and distort the desired signal.
- Noise is strong when the signal is weak.
- Example of noise in communication system:
Noise heard when tuning AM or FM receiver.
Hiss or static heard in the speaker.
Noise shows up in TV pictures as snow, known as confetti.
Noise that occurs in the transmission of digital data manifest itself as
bit error.
Signal loss
NOISE IN COMMUNICATION
SYSTEMS
Noise can affect the communication system performance in three
areas:
Noise can cause listener to misunderstand the original signal or
be unable to understand it at all.
Noise can cause the receiver to malfunction. Noise can cause
the receivers circuitry to function incorrectly, erratically or
improperly.
Noise can also result in a less efficient system.
Noise can be divided into two general categories:
A. UNCORRELATED NOISE
noise present regardless of whether there is a signal present
or not.
B. CORRELATED NOISE
noise present as a direct result of a signal.
mutually related to the signal and cannot be present in circuit
unless there is an input signal and is produced by non-linear
amplification.
No signal, no noise.
NOISE IN COMMUNICATION
SYSTEMS
NOISE IN COMMUNICATION
SYSTEMS
TYPES OF UNCORRELATED NOISE
1. EXTERNAL NOISE
Noise present in the received radio signal that has been
introduced in the communication medium.
Noise generated outside the device or outside the receiver.
2. INTERNAL NOISE
Noise produced by active and passive devices in the
receiver.
Noise generated within the device or within the receiver.
NOISE IN COMMUNICATION
SYSTEMS
TYPES OF EXTERNAL NOISE
1. MAN-MADE / INDUSTRIAL NOISE
produced by spark-producing mechanism such as commutators in
electric motors, automobile ignition system, power switching
equipment, fluorescent and gas filled lamps.
produced by any equipment that causes high voltages or currents to
be abruptly switched. This noise is called impulse noise.
Noise in DC power supply such as AC ripple.
Noise in AC power line such as surges of currents and voltage.
Noise introduced by nearby communication system.
In general, Man-made/Industrial noise are form of electromagnetic
interference that can be traces to non-natural causes.
Industrial noise frequency is between 15 to 160MHz, and it can
extend to 500MHz.
2. ATMOSPHERIC NOISE
- Noise caused by naturally occurring disturbances in the
Earths atmosphere, such as static caused by lightning and
thunderstorm.
- It has a great impact on signals at frequencies less than
30MHz.
Form of interference caused by rain, hail or snow is called
precipitation static.
NOISE IN COMMUNICATION
SYSTEMS
3. EXTRATERRESTIAL / SPACE NOISE
Noise coming from outer space due to sun, stars, distant planet
and other celestial bodies.
Noise caused by the sun is called solar noise. It is cyclical and
reaches a very annoying peak every 11 years.
Noise caused by stars, distant planet and other celestial objects
is called cosmic noise.
It has great impact on signals with frequencies beginning at
approximately 8 MHz and extends out to 1.43 GHz and beyond.
NOISE IN COMMUNICATION
SYSTEMS
NOISE IN COMMUNICATION
SYSTEMS
TYPES OF INTERNAL NOISE
1. THERMAL AGITATION/ WHITE/ JOHNSON NOISE
Noise generated in any power dissipating component in the
circuit such as resistor.
Thermal agitation refers to the rapid and random motion of
electrons in an electronic component caused by heat.
As temperature increases, increasing number of electrons
moves through the component, causing small noise voltage
to be produced across the component.
NOISE IN COMMUNICATION
SYSTEMS
TYPES OF INTERNAL NOISE
2. SHOT NOISE
Shot noise is produced by random movement of electrons
or holes across a PN junction due to discontinuities.
Noise due to the random arrival of charge carriers
(electrons or holes) at the output element of an active
electronic device such as diode, transistor and vacuum
tube.
NOISE IN COMMUNICATION
SYSTEMS
TYPES OF INTERNAL NOISE
3. TRANSIT TIME NOISE
High frequency noise caused by transistor and vacuum
tubes due to delayed movement of current carriers
(electron or holes) from input to output terminal of the
device (such as from emitter to collector of a transistor).
- Shows up as a kind of random noise within the device
and is directly proportional to the frequency of operation.
NOISE IN COMMUNICATION
SYSTEMS
4. EXCESS/ PINK/FLICKER NOISE
Noise caused by transistor and vacuum tubes due to crystal
surface defects.
Low frequency noise (below 1KHz.) from carrier density
fluctuations. It is proportional to emitter current and
junction temperature, inversely proportional to frequency.
5. MIXER NOISE
Noise caused by low transconductance of mixer compared
to amplifiers and inadequate image frequency rejection.
TYPES OF INTERNAL NOISE
NOISE IN COMMUNICATION
SYSTEMS
6. GENERATION-RECOMBINATION NOISE
Noise caused by random fluctuation of semiconductor
conductivity giving rise to noise current when DC signal
flows through the semiconductor.
7. CROSSTALK
Interference signal from one channel to another.
TYPES OF INTERNAL NOISE
NOISE IN COMMUNICATION
SYSTEMS
TYPES OF CORRELATED NOISE
1. HARMONIC DISTORTION
Unwanted multiples of a single frequency sine wave that
are created when sine wave is amplified in a non-linear
device, such as large signal amplifier.
2. INTERMODULATION INTERFERENCE
Unwanted cross-product (sum & difference) frequencies
created when two or more signals are amplified in non-
linear device.
NOISE IN COMMUNICATION
SYSTEMS
NOISE CALCULATIONS
NOISE POWER, Pn in Watts (W)
P
N
= K T B
3
10 1
log 10 ) (

=
x
P
dBm P
N
N
where :
K = Boltzmans constant , 1.38 x 10
23
J/K
T = absolute temperature in K
B = frequency bandwidth in Hz
NOISE IN COMMUNICATION
SYSTEMS
NOISE POWER DENSITY / POWER SPECTRUM
DENSITY, N
o
units: in Watts/Hertz (W/Hz)
average noise power / bandwidth
a figure that determines the amount of noise contained in a
specified bandwidth
3
10 1
log 10 ) (

=
= =
x
No
dBm No
KT
B
P
No
N
NOISE IN COMMUNICATION
SYSTEMS
NOISE VOLTAGE, V
N
in V
rms
4KTBR V
N
=
where :
K = Boltzmans constant , 1.38 x 10
23
J/K
T = absolute temperature in K
B = frequency bandwidth in Hz
R = equivalent noise resistance in O
NOISE IN COMMUNICATION
SYSTEMS
RMS NOISE VOLTAGE DUE TO SEVERAL SOURCES
3 2 1
2
3
2
2
2
1 NT
V V V V
N N N NT
N N N
R R R R + + =
+ + =
For series combination,
For parallel combination,
3 2 1
2
3
2
2
2
1 NT
1 1 1
1
V V V V
N N N
NT
N N N
R R R
R
+ +
=
+ + =
NOISE IN COMMUNICATION
SYSTEMS
SHOT NOISE CURRENT, i
N
in Amperes (A) rms
IB q i
N
2 =
where :
q = electron charge =1.6 x 10
19
C
B = frequency bandwidth in Hz
I= DC bias current of the device in Amperes
A form of internal noise which is due to the random
variations in current flow in active devices such as
diode,transistor and vacuum tubes.
NOISE IN COMMUNICATION
SYSTEMS
SIGNALTO NOISE RATIO, SNR
shows how much stronger or weaker the desired signal
power is compared to the unwanted noise.
indicates how easy or difficult it will be to extract the
desired information from corrupting noise.
SNR as power ratio:
Pn
Ps
log 10 dB in SNR
(Pn) power noise
(Ps) power signal
SNR
=
=
SNR as voltage ratio:

2
(Vn) voltage noise
(Vs) voltage signal
SNR
|
|
.
|

\
|
=
Vn
Vs
log 20 dB in SNR =
Pn
Ps
log 10 dB in SNR
(Pn) power noise
(Ps) power signal
SNR
=
=
NOISE IN COMMUNICATION
SYSTEMS
NOISE FIGURE (NF) / NOISE FACTOR (F)
shows exactly how much of noise in the amplified signal is due
to the original signal and its noise, compared to the noise added
by the amplifier itself.
shows how much noise is added to the received signal by the
noise of the circuitry itself.
) ( ) ( ) (
log 10 log 10 (dB) NF
F
o
i
o
i
dB SNR dB SNR dB NF
F
SNR
SNR
SNR
SNR
o i
=
= =
=
NOISE IN COMMUNICATION
SYSTEMS
NOISE FIGURE OF IDEAL NOISELESS AMPLIFIER
F = 1 and NF (dB) = 0 dB
NOISE FIGURE OF AMPLIFIER WITH INTERNALLY
GENERATED NOISE
F > 1
NOISE IN COMMUNICATION
SYSTEMS
REACTANCE NOISE EFFECTS
where :
B3dB = half power bandwidth, Hz
B
eq
= effective bandwidth, Hz
dB eq
B B
3
2
H
=
The significant effect of reactive circuits on noise is their
limititaion on frequency response
The equivalent bandwidth to be used in noise calculations with
reactive circuits is
NOISE IN COMMUNICATION
SYSTEMS
EQUIVALENT NOISE TEMPERATURE, NT or Teq in K
NT = Teq = T
o
(F 1)
where :
F = noise factor
T
o
= reference absolute temperature, 290 K
NOISE IN COMMUNICATION
SYSTEMS
NOISE DUE TOAMPLIFIERS IN CASCADE
Friss formula
Over-all noise factor of n stages
1 2 1 2 1
3
1
2
1 eq
..
1
...
1 1
F F

+ +

+ =
n
n
G G G
F
G G
F
G
F
Over-all noise temperature of n stages
1 2 1 2 1
3
1
2
1 eq
..
... T T

+ + + + =
n
n
G G G
T
G G
T
G
T
where: G is the power gain
NOISE IN COMMUNICATION
SYSTEMS
NOISE DUE TOAMPLIFIERS IN CASCADE
Friss formula
Over-all noise resistance
2
1
2
2
2
1
2
2
2
1
3
2
1
2
1 eq
...
... R R

+ + + + =
n
n
A A A
R
A A
R
A
R
where: A is the voltage gain
Example #1. Areceiver has a 50O input resistance, a bandwidth of
2 MHz, and a temperature of 30
o
C.
1.1 The noise power is ___________W.
(a) 8.26x10
-16
(b) 8.36x10
-15
(c) 2.76x10
-17
(d) 1.29x10
-6
1.2 The thermal noise voltage is ______ Vrms.
(a) 3.35x10
-14
(b) 1.67x10
-12
(c) 4.18x10
-11
(d) 1.29x10
-6
1.3 The noise density in dBm is ______.
(a)203.79 (b) -173.79 (c) 347.57 (d) 407.57
Example #2. What is the noise temperature when the noise power is
12 x 10
-18
Wand the bandwidth is 3KHz?
(a) 289.86 K (b) 16.86 K (c) 273 K (d) 0K
NOISE IN COMMUNICATION
SYSTEMS
Example #3. What is the noise resistance when the noise voltage is
1.15mV with a bandwidth of 800KHz and a temperature of 29
o
C?
(a) 898M O (b) 99.17M O (c) 86.23MO (d) 1.03K O
Example #4. Anoiseless amplifier having a gain of 60, a bandwidth
of 20MHz and an operating temperature of 40
o
C amplify the noise
voltage generated by a resistor. The output noise voltage is
measured as 1mVrms.
4.1 The rms input noise voltage is ______.
(a) 60mV (b) 1 V (c) 23.57 V (d) 16.67V
4.2 The noise resistance is _______.
(a) 10.42G O (b) 2.89MO (c) 804.19 O (d) 1.61K O
NOISE IN COMMUNICATION
SYSTEMS
Example #5. What is the SNR in dB for signal power of 50Wand a
noise power of 0.1 W?
(a) 50 dB (b) 26.99 dB (c) 26.99 dB (d) 53.98 dB
Example #6. What is the noise figure in dB when the input signal
and noise values are 1W and 0.01W, and the output signal and
noise values are 10Wand 0.1W?
(a) 0 dB (b) 100 dB (c) 1 dB (d) 10 dB
Example # 7. Two resistor, 20KO and 50KO, are at ambient
temperature. Calculate for a bandwidth equal to 100KHz, the
thermal noise voltage for the two resistors connected in parallel.
(ECE Board, November 1997)
(a) 4.28mV (b) 47.8 V (c) 4.78 V (d) 0.48V
NOISE IN COMMUNICATION
SYSTEMS
Example # 8. What is the signal to noise ratio at the output of the
amplifier in dB when the input signal and noise values are 0.1 x 10

3
V and 0.01 x 10
-6
V respectively? The amplifiers voltage gain
and internal noise voltage are 1000 and 1 x 10
-5
V respectively.
(a) 80 dB (b) 74 dB (c) 34 dB (d) 40 dB
Example # 9. What is the equivalent noise temperature for a totally
noiseless amplifier?
(a) 290 K (b) 17
o
C (c) 30
o
C (d) 0K
Example # 10. Determine the shot noise current for a diode with a
forward bias current of 1mAover a 100KHz bandwidth.
(a) 3.2x10
-17
A (b) 5.66nA (c) 4nA (d) 8nA
NOISE IN COMMUNICATION
SYSTEMS
REFERENCES
- Electronic Communication Systems Through
Advanced by W. Tomasi
- Communication Electronics by L. Frenzel
- Lecture Notes in Principles of Communication
by A.H. Ballado and M.M. Sejera
- Electronic Communication Systems by
G. Kennedy