electronic circuit for communication system

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Md. Johurul Islam, Ph. D. ( Jaist, Japan )Associate Professor/APEE/RU

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Electronic Circuit For Communication System

OUTLINE

Modulation and Demodulation

Radio Communications

Radio Receivers Television

Radar

Book: Electronic communication systemsGeorge Kennedy (3rded)Hand book of electronics Gupta and Kumar.

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Concept of communications

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Communication without wires

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Radio Comm. Building Blocks

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Block Diagram of Communication Systems

Information

Source

Transmitter Channel

Noise

Source

Receiver Destination

Encoding

Modulation

(Distortion)

Decoding

Demodulation

(Distortion)

Information: Itself is that which is conveyed: Voice, Music, Pictures, Data etc.

Transmitter: The information modulates the high frequency carrier and release

into the air for propagation.

Channel: It will be Radio channel, Optical Fiber, or Wire.

Noise: Unwanted signal added into the information.

Receiver: It is used to recover the original information from the modulated

signal. 6

Modulator

Demodulator

Transmission

Channel

Input

transducer

Transmitter

Receiver

Output

transducer

Carrier

EM waves (modulated

signal)

Baseband signal

(electrical signal)

Baseband signal

(electrical signal)


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Block diagram of typical radio transmitter

Generate high frequency carrierLow frequency information

Crystal

oscillator

RF buffer

amplifier

Modulation

voltage

amplifiers

Modulation

processing

RF output

power

amplifiers

Modulating

Signal

RF voltage

and power

amplifiers

Modulation

power

amplifiers


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Block diagram of typical radio Receiver

Faradays Electromagnetic Induction

Tuning carrier wave and amplify it

Mix carrier with L.O. wave to produce

Intermediate frequency (IF)

High frequency wave generator

Separate original information from IF wave


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Types of Modulation


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Analog Modulation

Amplitude Modulation (AM)

Amplitude modulation is the process ofvarying the amplitude ofa carrier wave in proportion to the amplitude of a basebandsignal. The frequency of the carrier remains constant.

Frequency Modulation (FM)

Frequency modulation is the process ofvarying the frequency ofa carrier wave in proportion to the amplitude of a basebandsignal. The amplitude of the carrier remains constant.

Phase Modulation (PM)

In phase modulation, the phase deviation is proportional to theamplitude of the modulating signal and therefore independent ofits frequency.

Carrier Signal

ModulatingSignal

Amplitude

Modulated

Signal

FM


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Need for Modulation

For effective radiation energy from antenna, the antenna height must be

comparable with the wavelength of the carrier frequency.

Modulation: In the process of modulation, some characteristic of a high

frequency sine wave ( carrier) is varied in accordance with theinstantaneous value (amplitude) of the information (modulating signal) .

Similarly: /2=75 m at frequency of

1MHz (possible in practical)


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Classification of electrical noiseNoise: It may be defined as any unwanted form of electrical/electromagnetic energy

which tends to interfere with proper reception and reproduction of wanted signal.

1. External Noise: Atmospheric noise---discharge of thunderstorms, natural electrical discharge etc.

Extraterrestrial noise (space noise) ---- Solar and Cosmic noises.

Man made noise (Industrial noise) --- automobile, aircraft, electrical motors,

leakage from high voltage line, fluorescence light etc.

2. Internal noise: Thermal /white/Johnson noise---The random noise generated in a

resistor/resistive components. Noise voltage

Shot noise (another form of thermal noise)--- results from shot effect present in

all amplifying devices. It is caused by random variation in the arrival of electrons

(holes) at the output electrodes. Noise current

Transit noise: In VHF range the transit time become comparable with period ofthe signal to be amplified. As a result the input admittance of the transistor

increases and thermal noise is generated.

Miscellaneous noise: (i) Flicker noise due to VLF signal, (ii) Transistor thermal

noise due to heat,(iii) Partition noise due to current division at collector/base.

RkTBVn 4

BqII pn 2


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Thermal noise

It is seen that the noise power generated in a resistor is directly proportional to its

absolute temperature and the bandwidth over which the noise is to be measured.


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Noise due to more than one sources

Additi f i d t l lifi i d


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Addition of noise due to several amplifiers in cascade:

The rms noise voltage across R3 33 4 fRkTVn

3

R

The same noise voltage would be present at the output if there were no R3 there,

instead was present at the input of stage 2. corresponding voltage would be

3

2

3

2

3

34

4RfkT

A

fRkT

A

VV nn

Where is the resistance which if placed at the input of the second stage would

produce the same noise voltage at the output as does R33

R

2

2

3

3 A

R

R

Equivalent noise resistance at the input of second stage is

2

2

3

232

A

RRRRReq

Similarly a resistance may be placed at the input of first stage to replace2R eqR

Scan Figure


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2

2

2

1

3

2

1

2

2

1

2

232

2

1

2

/

AA

R

A

R

A

ARR

A

RR

eq

Therefore the actual noise resistance present at the input of the

first stage is

2

2

2

1

3

2

1

2

121

AA

R

A

RRRRReq

These can be apply for n stage amplifier.

Scan Problems.


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Signal to noise ratio (S/N):

Why equivalent noise resistance is so important?

Comparison of two kinds of equipment in evaluating their performance

Comparison of noise and signal at the same point to ensure that the

noise is not excessive.

If equivalent noise resistance is difficult to obtain then S/N ratio is very

often used and defined as the ratio of signal power to noise power at the

same point. Thus 2

2

2

/

/

n

s

n

s

n

s

V

V

RV

RV

X

X

N

S

Noise Figure:

For comparison of receivers or amplifiers working at different impedance levels

the use of equivalent noise resistance is misleading, and another quantity knownas noise figure or noise factor is defined as follows

NSOutput

NSInputF

/

/ For ideal receiver F = 1

Practical receiver F > 1


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electronic circuit for communication system

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