NOISE IN

COMMUNICATION SYSTEMS

Prepared by:

Engr. Analene

Montesines-Nagayo

NOISE IN COMMUNICATION

SYSTEMSNOISE

- 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 receiver’s 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

Earth’s 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)

PN = K T B

3101log10)(

x

PdBmP N

N

where :

K = Boltzman’s constant , 1.38 x 10–23 J/K

T = absolute temperature in K

B = frequency bandwidth in Hz

NOISE IN COMMUNICATION

SYSTEMSNOISE POWER DENSITY / POWER SPECTRUM

DENSITY, No

units: in Watts/Hertz (W/Hz)

average noise power / bandwidth

a figure that determines the amount of noise contained in a

specified bandwidth

3101log10)(

x

NodBmNo

KTB

PNo N

NOISE IN COMMUNICATION

SYSTEMS

NOISE VOLTAGE, VN in Vrms

4KTBR VN

where :

K = Boltzman’s constant , 1.38 x 10–23 J/K

T = absolute temperature in K

B = frequency bandwidth in Hz

R = equivalent noise resistance in W

NOISE IN COMMUNICATION

SYSTEMS

RMS NOISE VOLTAGE DUE TO SEVERAL SOURCES

321

2

3

2

2

2

1NT VVV V

NNNNT

NNN

RRRR

For series combination,

For parallel combination,

321

2

3

2

2

2

1NT

111

1

VVV V

NNN

NT

NNN

RRR

R

NOISE IN COMMUNICATION

SYSTEMS

SHOT NOISE CURRENT, iN in Amperes (A) rms

IBqiN 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

SIGNAL TO 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 inSNR

(Pn)power noise

(Ps)power signal SNR

SNR as voltage ratio:

2

(Vn) voltage noise

(Vs) voltage signal SNR

Vn

Vs log 20 dB inSNR

Pn

Ps log 10 dB inSNR

(Pn)power noise

(Ps)power signal SNR

NOISE IN COMMUNICATION

SYSTEMSNOISE 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.

)()()(

log10 log 10 (dB) NF

F

o

i

o

i

dBSNRdBSNRdBNF

FSNR

SNR

SNR

SNR

oi

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

Beq = effective bandwidth, Hz

dBeq BB 32

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 = To (F – 1)

where :

F = noise factor

To = reference absolute temperature, 290 K

NOISE IN COMMUNICATION

SYSTEMS

NOISE DUE TO AMPLIFIERS IN CASCADE

Friss formula

Over-all noise factor of n stages

12121

3

1

21eq

..

1...

11F F

n

n

GGG

F

GG

F

G

F

Over-all noise temperature of n stages

12121

3

1

21eq

..... T T

n

n

GGG

T

GG

T

G

T

where: G is the power gain

NOISE IN COMMUNICATION

SYSTEMS

NOISE DUE TO AMPLIFIERS IN CASCADE

Friss formula

Over-all noise resistance

2

1

2

2

2

1

2

2

2

1

3

2

1

21eq

...... R R

n

n

AAA

R

AA

R

A

R

where: A is the voltage gain

Example #1. A receiver has a 50W input resistance, a bandwidth of

2 MHz, and a temperature of 30oC.

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 W and 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 29oC?

(a) 898M W (b) 99.17M W (c) 86.23MW (d) 1.03K W

Example #4. A noiseless amplifier having a gain of 60, a bandwidth

of 20MHz and an operating temperature of 40oC 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 mV (c) 23.57 mV (d) 16.67mV

4.2 The noise resistance is _______.

(a) 10.42G W (b) 2.89MW (c) 804.19 W (d) 1.61K W

NOISE IN COMMUNICATION

SYSTEMS

Example #5. What is the SNR in dB for signal power of 50W and 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 10W and 0.1W?

(a) 0 dB (b) 100 dB (c) 1 dB (d) 10 dB

Example # 7. Two resistor, 20KW and 50KW, 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 mV (c) 4.78 mV (d) 0.48mV

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 amplifier’s 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) 17oC (c) 30oC (d) 0K

Example # 10. Determine the shot noise current for a diode with a

forward bias current of 1mA over a 100KHz bandwidth.

(a) 3.2x10-17A (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