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CHAPTER 5

- Noise in Communication Systems

Noise in Communication Systems

- Outline
- Introduction
- Thermal Noise
- Shot Noise
- Signal - to Noise Ratio
- Noise Factor Noise Figure
- Noise Temperature
- Cascaded Networks

By the end of this chapter you should be able to

- Define noise and describe the prominent sources

of electrical noise - Explain and calculate the most common types of

noise in communication system

Introduction

Noise is the static you hear in the speaker when

you tune any AM or FM receiver to any position

between stations. It is also the snow or

confetti that is visible on a TV screen.

Introduction

- Noise is a general term which is used to describe

an unwanted signal which affects a wanted signal.

- Noise is a random signal that exists in a

communication system. - Random signal cannot be represented with a simple

equation.

Sources of noise

Noise

Internal Noise

External Noise

- Man-made noise and natural resources
- External noise comes from sources over which we

have little or no control - Industrial sources such as motors, generators,

and manufactured equipment - Atmospheric sources / static electricity such as

speaker when there is no signal present

- Due to random movement of electrons in electronic

circuit. - Electronic components in a receiver such as

resistors, diodes, and transistors are major

sources of internal noise - Thermal noise/Johnson noise
- Shot noise

Introduction (Contd)

- The noise level in a system is proportional to
- temperature and bandwidth,
- the amount of current flowing in a component,
- the gain of the circuit,
- the resistance of the circuit.

Noise Effect

- The effects of noise are as follow
- Degrade system performance for both analog and

digital systems. - The receiver cannot understand the original

signal. - The receiver cannot function as it should be.
- Reduce the efficiency of communication system.

Types of Noise

- The are several types of noise, among them are
- 1. Thermal Noise/White Noise
- 3. Shot Noise
- 4. Noise Temperature
- 5. Quantization Noise

Thermal Noise (Johnson Noise /white noise)

Thermal Noise (Johnson Noise /white noise)

Thermal noise is the result of the random motion

of charged particles (usually electrons) in a

conducting medium such as a resistor.

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).

Thermal Noise (Johnson Noise /white noise)

Movement of the electrons will forms kinetic

energy in the conductor related to the

temperature of the conductor.

When the temperature increases the movement of

free electrons will increases and causes current

flows through the conductor.

Thermal Noise (Johnson Noise) (Contd)

Thermal noise is often referred to as white

noise because it has a uniform spectral density

across the EM frequency spectrum. (analogous to

the colour white which consists of all the colour

spectrum)

Thermal Noise (Johnson Noise) (Contd)

- Experimental results (by Johnson) and theoretical

studies (by Nyquist) give the mean square noise

voltage as

Where k Boltzmanns constant 1.38 x 10-23

Joules per K T absolute temperature (Kelvin)

B bandwidth noise measured in (Hz) R

resistance (ohms)

Thermal Noise (Johnson Noise) (Contd)

In 1928, J. B. Johnson have proven that noise

power generated is proportional to the

temperature and the BW.

- In dB, it is defined as
- PdBm 10log(KTB/0.001)

Noise power can be modeled using voltage

equivalent circuit (Thevenin equivalent circuit)

or current equivalent circuit (Norton equivalent

circuit)

Analysis of Noise In Communication Systems

Thermal noise may be represented by an equivalent

circuit as shown below

(mean square value , power) then VRMS

i.e. Vn is the RMS noise voltage.

Resistors in Series

Resistors in Series

Assume that R1 at temperature T1 and R2 at

temperature T2, then

The resistor in series at same temperature behave

as a single resistor

Analysis of Noise In Communication Systems

Resistance in Parallel

Analysis of Noise In Communication Systems

Resistance in Parallel

Example 1

Given a 50 kO resistor at a temperature of

290 K, 3 kHz bandwidth. Find Vrms value of noise

NOTE Temperature unit conversion

Example 2

- One operational amplifier with a frequency range

of (18-20) MHz has input resistance 10 k?.

Calculate noise voltage at the input if the

amplifier operate at ambient temperature of 270C.

Remember to convert the temperature to Kelvin

EXAMPLE 3

- A receiver has a noise power bandwidth of 10 kHz.

A resistor that matches the receiver input

impedance is connected across its antenna

terminals. Determine the Noise Power if the

resistor has temperature of 27 oC.

SOLUTION 1. Use Noise Power formula 2. P

(1.38 x 10-23 J/K)(273o 27oK)(10000 Hz)

4.14 x 10-17 W. 3. in dB, P(dB)

10log(4.14 x 10-17 W) / 0.001 -133.8

P KTB

Shot Noise

- Shot noise is a type of electronic noise that

occurs when there are finite number of particles

that carry energy, such as electrons in an

electronic circuit or photons in an optical

device - Shot noise was originally used to describe noise

due to random fluctuations in electron emission

from cathodes in vacuum tubes (called shot noise

by analogy with lead shot). - Shot noise also occurs in semiconductors due to

the release of charge carriers. - Shot noise is found to have a uniform spectral

density as for thermal noise (White noise)

How to determine noise level in communication

system?

- Noise effect can be determined by measuring
- - Signal to Noise Ratio, SNR for analog system
- - Noise Factor, F
- - Noise Temperature, Te .
- - probability of error or bit error rate,

BER for digital system - To determine the quality of received signal at

the receiver or an antenna, SNRi is used. - SNR o is always less than SNRi , due to the facts

that the existence of noise in the receiver

itself. In the receiver usually constitute a

process of filtering, demodulation and

amplification.

Noise Calculation

- SNR is a ratio of signal power, S to noise power,

N. - Noise Figure, F
- Noise factor, NF

Signal to Noise Ratio

The signal to noise ratio is given by

The signal to noise in dB is expressed by

for S and N measured in mW.

Signal to Noise Ratio

- Example
- 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

- Solution
- To express in dB

Signal to Noise Ratio

- Signal to noise power ratio can be expressed in

terms of voltages and resistances.

If the input and output resistances of the

amplifier, receiver or network being evaluated

are equal

- Where
- Rin input resistance (ohms)
- Rout output resistance (ohms)
- Vs signal voltage (volts)
- Vn noise voltage (volts)

Signal to Noise Ratio

- Example
- For an amplifier with an output signal voltage

of 4V, an output noise voltage of 0.005 V, and an

input and output resistance of 50 ohm, determine

the signal to noise power ratio. - Solution

Noise Factor- Noise Figure

Consider the network shown below,

Noise factor, F

lower the value of F, the better the network.

- F equals to 1 for noiseless and in general F gt

1.

Noise Factor- Noise Figure (Contd)

- Noise figure (NF) is the Noise factor converted

to dB

Noise Figure (NF) dB 10 log10 (F)

If every variable is a dB Noise figure

NF SNRin - SNRout

Noise Factor- Noise Figure (Contd)

- Example
- The signal to noise ratio at the input to a

communication receiver is 40 dB. If the receiver

has a noise figure of 12 dB, calculate the output

signal to noise ratio

- Solution

NF SNRin - SNRout

SNRout SNRin - NF 40 -12

28 dB

Noise Temperature

Equivalent noise temperature Te is not the

physical temperature of the amplifier, but rather

a theoretical construct that is an equivalent

temperature that produces that amount of noise

power

Noise temperature (Te) is expressed as

Where Te equivalent noise temperature

(Kelvin) T environmental temperature

(reference value of 290 K) F Noise factor

Te T(F-1)

Cascaded Network

A receiver systems usually consists of a number

of passive or active elements connected in

series. A typical receiver block diagram is shown

below, with example

In order to determine the (S/N) at the input, the

overall receiver noise figure or noise

temperature must be determined. In order to do

this all the noise must be referred to the same

point in the receiver, for example to A, the

feeder input or B, the input to the first

amplifier.

Cascaded Network

Total noise factor is the accumulation of the

individual noise factors. Friiss formula is used

to calculate the total noise factor of several

cascade amplifiers.

F1

F3

F2

Where Fn Noise factor (dB) Gn Power gain ,

amplifier n

System Noise Figure

Assume that a system comprises the elements shown

below,

Assume that these are now cascaded and connected

to an aerial at the input, with

from the aerial.

Now ,

Since

similarly

System Noise Figure (Contd)

The overall system Noise Factor is

The equation is called FRIIS Formula.

System Noise Temperature

Attenuator, Transmission Loss

- All transmission medium will attenuate power and

caused power loss gt Pout lt Pin. - Power loss or power attenuation is given by

Transmission Loss

Summary

- Thermal Noise
- Signal - to Noise
- Noise Factor
- Noise Figure

Noise Figure (NF) dB 10 log10 (F)

Summary

- Noise Temperature
- Cascaded Networks

Te T(F-1)

Example 1.1 Calculate signal power if its value

in dBm is 0 dBm.

dBm 10 log P2 / P1 10 log P2 / 1 mW 0 P2

1 mW

Example 1.2 Calculate signal power in dB if its

value is 1 mW.

dB 10 log P2 / P1 10 log P2 / 1 W 10 log 1

mW / 1 W - 30 dB

Example

- For three cascaded amplifier stages, each with

noise factor of 2 dB and power gains of 10 d,

determine the total noise figure.

Example Cascade Three amplifiers, ABC was

connected in series. Noise figure and power gain

of the amplifiers are given below Amplifier A

GA 20 dB FA 3 dB Amplifier B GB 10 dB FB

5 dB Amplifier C GC 5 dB FC 10 dB An

input signal of 50 dB higher than noise level was

fed at the input of the network.

Calculate (a) Total noise factor (b) SNR at the

output

Solution

Amplifier A GA 20 dB FA 3 dB Amplifier B

GB 10 dB FB 5 dB Amplifier C GC 5 dB FC

10 dB

(a) Angka hingar 10 log10 2.03 3.05 dB

(b) Di beri, SNRmasukan 50 dB

FdB SNR masukan (dB) SNR keluaran

(dB) SNR keluaran 50 dB 3.05 dB 46.95

dB

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