Module 3 PDF

BASIC ELECTRONICS M#3

Prepared by : BINOSHI SAMUVEL Page 1 of 45 S1S2 2008 (AJCE Koovappally, Kanjirappally)

MODULE #3 Basic communication engineering



FREQUENCY BANDS

The number of pulses , or cycles per second is called frequency. A large range of frequency is called spectrum.

The call number for AM & FM radio station are actually the station allocated range of broadcast frequency (bandwidth) . An AM station with a call number of 89 has an assigned frequency of 890khz . An FM station with a call number of 99.8 MHz . The spectrum of usable frequency of broadcast radio is subdivided in this fashion so that two station don’t attempt to send signals on the same bandwidth

Television stations use a channel identification number that corresponding to their assigned frequency range. Common TV channel numbers are two though 12 channels 2 , 3 and 4 use the frequency range of 54 to 72 mhz, which falls in the very high frequency range (VHF) . Channels 5 & 6 use frequency



ranging from 76 to 88 mhz. The frequency range of 174 to 216 is used for channels 7 to 13.

Bandwidth is allocated in 6 mhz increment required by all television

signals.

The VHF range does not provide adequate frequency allocations,so that UHF or ultra high frequency range is also used.Channels 14 to 83 make use of frequencies of 470 through 890 MHz.

Another method of identifying a part of the frequency spectrum is by signal wave length we offers hear the term microwave, which identifies a part of the frequency spectrum by its wavelength rather than by its frequency.

Broadband is used to describe transmission equipment or a form of medium that uses a wide range of electromagnetic frequencies. Broadband is typically any channel that can support bandwidth greater than a common voice. grade telecommunication channel(4000Hz), also called wide band.

Name Symbol Frequency Wavelength Applications

Extremely low frequency

ELF 3–30 Hz 10,000–100,000 km

Directly audible when converted to sound, communication with submarines



Super low frequency SLF 30–300 Hz 1,000–

10,000 km

Directly audible when converted to sound, AC power grids (50–60 Hz)

Ultra low frequency ULF 300–3000 Hz 100–1,000 km

Directly audible when converted to sound, communication with mines

Very low frequency VLF 3–30 kHz 10–100 km

Directly audible when converted to sound (below ca. 20 kHz; or ultrasound otherwise)

Low frequency LF 30–300 kHz 1–10 km AM broadcasting, navigational

beacons, lowFER

Medium frequency MF 300–

3000 kHz 100–1000 m Navigational beacons, AM broadcasting, maritime and aviation communication

High frequency HF 3–30 MHz 10–100 m

Shortwave, amateur radio, citizens' band radio, skywave propagation

Very high frequency VHF 30–300 MHz 1–10 m

FM broadcasting, amateur radio, broadcast television, aviation, GPR, MRI

Ultra high frequency UHF 300–

3000 MHz 10–100 cm

Broadcast television, amateur radio, mobile telephones, cordless telephones, wireless networking, remote keyless entry for automobiles, microwave ovens, GPR

Super high frequency SHF 3–30 GHz 1–10 cm

Wireless networking, satellite links, microwave links, satellite television, door openers



Extremely high frequency

EHF 30–300 GHz 1–10 mm

MODULATION

Any wave has 3 significant characteristics viz

• Amplitude • Frequency • Phase

Modulation is a process of impressing information to be transmitted on a high frequency wave called carrier wave, by changing its one of the characteristics.

Modulation may also be defined as the process of altering some characteristics of carrier wave in accordance with the instantaneous value of some other wave calling modulating wave.

Need for modulation:- 1) Short operating range :- Audio signals having small frequency cannot

be transmitted over long distances directly in to the space . However modulated signal can be transmitted over long distance .

2) Poor radiation efficiency :- At audio frequencies radiation is not practicable as efficiency of radiation is poor .

3) Mutual interferences :- If low frequency signals are transmitted directly from different sources all of them would be mixed up . by modulation message of different frequency levels can be transmitted.

4) Huge antenna requirement :- modulation enables a low frequency signal transmission over long distances through space with the help of a high frequency carrier wave. These carrier wave needs reasonably sized antenna can be reduced .



AMPLITUDE MODULATION

DEFINITION: The process of varying

amplitude of the high frequency or carrier wave in accordance with the message signal ,keeping the frequency and phase of carrier wave unchanged is known as amplitude modulation .

ANALISIS AND FREQUENCY SPRECTRAM OF AM CARRIER WAVE: Let carrier wave and modulated voltage wave

Vc=Vcsin ct

Vm=Vmsin mt

The amplitude of carrier wave varies at a modulating signal frequency FM. The amplitude of modulated wave is given as,

A = Vc+Vm

= Vc+ Vmsin mt

= Vc[1 sin mt]

= Vc[1 sin mt]

Where - Modulation index – ratio of peak values of modulating and carrier signal .

The instantaneous values AM wave is given by the following equations ,

V = Asin ct = VC (1+ sin mt) sin ct

= Vc sin ct + mVc (sin mt sin ct)

= Vcsin ct + cos ( c- m)t - cos ( c+ m)

The inspection of equation reveals that the AM wave is equivalent to summation of three sinusoid . One having amplitude Vc and frequency ,



second having amplitude and frequency and third term having

amplitude and frequency . In practical radio transmission ,wc > wm .

Hence the frequency of second and third term on right hand side of equation is generally close to carrier frequency. This frequencies can be represented graphically on a frequency spectrum spot. FREQUENCY SPECTRUM:

• The lower frequency component is called the lower side

frequency. • The upper frequency component is called the upper side

frequency. During the process of amplitude

modulation :- i. The original carrier frequency is not offered. But two new frequencies

(USB) and (LSB) , known as side band frequencies are

produced. ii. Carrier voltage component does not transmit any information because

the signal frequency is contained in side bands.

iii. In amplitude modulated wave , band width is from to

ie; or twice the signal frequency.

MODULATION INDEX (m): The extend by which the amplitude of carrier wave is varied by modulating signal is called the degree of amplitude modulation or modulation index (m) .Thus the ratio of change in amplitude of carrier wave to amplitude of normal carrier wave is called modulation index (m) m=



LIMITATIONS:- 1. Low efficiency:- In AM , the useful power that lies in side hands, quite

small, so the efficiency of AM system is low. 2. Limited operating range:- transmitters employing the AM have small

operating range. This is due to low efficiency. Hence information cannot be transmitted over long distances.

3. Noisy reception:- in case of AM , the reception is generally noisy. This is because a radio receiver cannot distinguish amplitude variation that represent noise and those contained desired signal.

4. Poor audio quality:- In order to obtain high facility reception, all audio frequency up to 15KHz must be reproduced and this necessities the band width of 30KHz while AM broadcasting stations are assigned band width of only 10KHz to minimize the inference from adjacent broadcasting station.

FREQUENCY MODULATION The frequency modulation is produced by varying the frequency of the carrier waves keeping the amplitude of it constant.

The deviation of the frequency of the carrier is proportional to the amplitude of the modulating signal. The rate at which carrier frequency shift from its normal value to a changed value is determined by the frequency of the modulating signal and this shift in carrier frequency from its normal value compared to the amplitude of the modulating signal is called deviation ratio.



EXPRESSION FOR FM WAVE Let the carrier voltage be

Ec = Ecsin( ct + ………….(1)

Let the modulating signal be

Em = Emcos ( mt)……………(2)

Let = wt + = instantaneous phase angle of carrier signal ie;

Ec = Ec sin ……………….(3)

c = angular frequency = …………….(4)

After frequency modulation, the frequency

= c + Kf em

Ie; = c + KF x Emcos ( mt)

Where K is the constant of proportionality.

On integrating eqn (4)

=

= c + Kf x Em cos ( mt)dt

= c + Kf x Em/ m x sin ( mt) + 1

Where 1 is constant of proportionality,

The instantaneous frequency , f = = fc + Kf x Em /2 cos ( mt)

fmax =fc + Kf Em/2

fmin = fc+ Kf Em/2

Frequency deviation = fmax – fc = fc – fmin = Kf Em/

The modulation index Mf = -

The expression for frequency modulated carrier is given by Ec = Ec sin[ ct + f sin( mt)]



In frequency modulated signal, the information is contained in the side bands. Since the information contained in the side bands. Since the side bands are seperated from each other by the frequency of modulating signal (fm)

Bandwidth = 2n fm

Where n is the number of significant sideband pairs in FM transmission the message is in the form of frequency variation of carrier wave therefore the noise gets amplitude modulated, does not harm the message signals. As a result in FM broadcast , the reception is of much better quality. FM transmission is highly efficiency as compared to AM . Due to a large number of sidebands it can be used for strew sound transmission.

PHASE MODULATION

a Phase modulation is not used in practical analog transmission system If the phase, in the equation Vc =Ec sin( ct + ) is varied so that the

magnitude of the phase change is proportional to the instantaneous amplitude of modulating voltage, the resulting wave is phase modulated.

Vc = E.sin ( ct + .sin mt)

-maximum value of the phase change introduced by the

particular modulating signal and is proportional to maximum amplitude of the modulating signal.

Vc = E.sin ( ct + p.sin mt)

Mp= = modulating index for phase modulation



The modulation index of a phase modulated wave is the phase shift in radians produced by the modulation.

In phase modulation, instantaneous phase of the output wave is varied in accordance with the magnitude of the modulating signal.

The instantaneous rate at which the phase variations occur depends on the instantaneous frequency of the modulating signal.

Phase modulation is accompanied by frequency modulation. In phase modulation, the frequency of the modulated wave differs

from the carries frequency only, while the phase is actually varying , ie ; only when the magnitude of the modulating voltage varies. Both are proportional to the change of modulating voltage .

In PM , the modulation index is independent of modulation frequency

PULSE MODULATION The carrier wave is not a continuous signal instead it is a train of

pulses . Two types of pulse modulation:

1. Pulse amplitude modulation 2. Pulse time modulation

These two come under the category of analog pulse modulation. Another type of pulse modulation is pulse code modulation . Pulse time modulation is sub divided in to two as ,

1. Pulse width modulation 2. Pulse position modulation

PULSE AMPLITUDE MODULATION The amplitude of a pulse train is changed in accordance with the

instantaneous amplitude of the message signal keeping the duration of the pulse unaltered.

PULSE WIDTH MODULATION The width of the pulse train is varied in accordance with the

instantaneous message signal. Here amplitude of pulse train remains unaltered .

PULSE POSITION MODULATION The amplitude and width of the pulse remains unaltered but the

position of the pulses in given time slot will vary in accordance with the amplitude of modulating signal.



PULSE CODE MODULATION DIGITAL MODULATION METHOD

Analog signal is converted in to pulses by process of sampling. Then each sample is quantized. The quantized samples are then represented as a binary numbers. Then this process is termed as encoding. The encoded output is then transmitted .

AM AUDIO TRANSMITTER

Generally, all communication systems consists of

(i) Source of signal or information to be transmitted.

(ii) Input signal conditioner or input processor and amplifier.

(iii)RF carrier generator.

(iv) Transmitter.

(v) A channel through which the carrier travels.

(vi) A receiver to receive, reprocess and retrieve the original data.

In AM transmission, modulation of the carrier can be effected on two

ways.



Both carrier and modulating signals are amplified to the required

power level using amplifiers and modulation process is done at the high

power level. This is called “high level modulation’.

(i) In the second case modulation process is done at low power levels and

the modulated carrier is amplified to the required power level and then

transmitted. This is called ‘low level modulation’.

In Am transmission techniques, the modulation techniques can be

done in two ways.

(i) High-level modulation

(ii) Low level modulation

In high level modulation

In high level modulation both the carrier signal and modulating signal

are amplified to a high power level modulation is done after the amplification

process and then transmitted. But in low level modulation, the modulated

signal in amplified to the required level and transmitted.

AM TRANSMITTER – BLOCK SCHEMATIC The block diagram a shown above represents a high level AM

modulation circuit. Basically these are classified into three sections.

(i) Audio section

(ii) RF section

(iii)Modulation section



(i) Audio section Audio section consists of AF source. This may be speech or sound

signal or electrical signal from a magnetic tape or other instrument. A

microphone may be used to convert the sound waves into electrical signal,

called audio frequency signal. For AM broadcast the AF input is limited to

a frequency of 50Hz. The input AF signal is initially processed and is pre-

amplified in the signal conditioning systems.

Audio Amplifier A series of audio amplifiers are used to amplify the amplitude of the

signal to be transmitted. Both AF voltage and power amplifiers are used.

This amplified output is the modulating signal.

Radio communication is an application of electronics. It is two types

Radiotelegraphy

Radiotelephony

Radiotelegraph – message transmitter through telegraphic code



Radiotelephony – message signal (music / speech) broadcast with the help

of radio wave modulated by the message.

Radio communication is also known as wireless communication since

wires are not necessary in transmission of message, which is done by radio

waves.

Key elements of radio communication;

1. A telegraphic key or microphone that controls the radio waves in

accordance with information.

2. Transmitter to produce radio waves.

3. Transmitting antenna that radio waves in specified directions.

4. Receiving antenna to receive a part of the radiated message.

5. Receiver that selects amplifier and detects the desired signal.

6. Headphone/ loudspeaker that converts detected electrical signal into

sound waves, reproducing the information.

CRYSTAL OSCILLATOR

The resonant frequencies of some naturally available crystals like

quartz are relatively constant. So far a high frequency stability, a crystal is

emblayed as the frequency determining element in an oscillator. Such

oscillators are referred to as crystal oscillators. Crystal oscillators use

conveniently cut quartz crystals. When complete, the cross-section of such a

crystal is hexagonal.

The property inter connecting the mechanical and the electrical

behaviors of a crystal is referred to as the piezo-electric effect. The

mechanical stress is applied along the crystal faces and the electric potential

appears along the edges.



When a proper alternating potential is impressed on a peizo-electric

crystal, the latter vibrates mechanically. If the frequency of the applied

alternating voltage and the natural frequency of the crystal are equal, the

amplitude of the mechanical oscillations attains a maximum value.

Crystal oscillators are generally used in the frequency range from

about 15KHz to 10MHz. For lower frequencies, the size of the quartz is

inconveniently large. At higher frequencies, the thickness of the crystal is so

small that it becomes fragile.

Circuit of a crystal Oscillator The high values quality factor Q and the stability of the quartz

characteristics with respect time and temperature account for the

remarkably high frequency stability of the crystal oscillator in the range

from a few KHz to several MHz. These are the distinct advantages of the

crystal oscillator.

BUFFER AMPLIFIER

It is nothing but an amplifier and it provides impedance matching

between the crystal oscillator and frequency translator (Multiplier).

RF AMPLIFIERS

RF signals constantly need to be made bigger as they move from place

to place. An amplifier is a filling station for RF signals. The RF signals

entering as a small signal into the amplifier leaves as a bigger one. RF

amplifiers fall into these main categories: low noise, high power and others.

A low noise amplifier is the very first amplifier a signal encounters

after it comes through the antenna in a receiver. A measure of LNA’s

quietness is called noise figure (NF). High power amplifiers are the last

amplifier a signal goes through before it flies out the antenna in a

transmitter. HPA boost the RF signal as big as possible just before it’s shout

out of the antenna.



SPECIAL AMPLIFIERS

Limiting amplifiers:- It limits the output power. This type of amplifier is

used in places where the component which follows will be damaged if its

input power is too high.

Balanced Amplifiers:- It’s a different amplifier design in which there are two

amplifiers in parallel.

Variable Gain Amplifiers:- Most amplifiers have fixed gain. Variable gain

amplifiers have an external control which allows the user to vary the gain

over some predefined range.

Fundamental Properties

Gain:-

Gain is a measure of how much bigger the output signal is than the input

signal.

Noise Figure:-

Low noise amplifiers listen for very small RF signals so they must be very

quiet. A measure of an LNA’s quietness is called noise figure.

Output Power:

A high power amplifier amplifies the RF signal to the needed amplitude just

before releasing from the antenna. A fundamental property of an HPA is

output power, measured in Watts. Higher the power, better the signal.

Linearity:-

One of the implications of digital wireless communications is that when a

digital signal rides on the top of an RF carrier, any amplifier, which the

signal goes through, must be linear. Linearity is a measure of how much the

amplifiers distort the shape of the signal.



FREQUENCY TRANSLATOR

Usually the crystal oscillator frequency may not be same as the required

carrier frequency assigned for that particular station. It will be s a sub

multiple of the carrier. Hence a frequency multiplier (harmonic generator) is

used to multiply the crystal oscillator frequency the required carrier.

Antenna

An antenna is device designed to accommodate RF signal usually in the

form of standing waves or it is a structure that transform the energy

contained in a guided wave to that of free space or on the receiving end, free

space to guided wave.

The modulated carrier is fed to the transmitting antenna for

transmission into free space. Antenna matching network provides the

required impedance matching between the antenna and output of the

moderator. This is essential for getting maximum power transfer and high

efficiency. The height of the antenna shall be at-least one fourth of the

wavelength of the carrier.

Modulator

The amplifier AF signal from the carrier section and high power. RF

carrier from the carrier section and fed to the modulator. High-level

modulation takes place. Usually this may be “Collector modulation”. In the

modulator, the modulating signal modulates the RF carrier to the required

depth of modulation. Which will be at predetermined value.

AM RECEIVERS

A radio transmitter transmits or radiates a modulated carrier wave. This

modulated carrier is picked up by the antenna of the radio receiver. This

signal so received is very weak. Hence generally this signal is first amplified

in an RF amplifier stage of the radio receiver. Further since the signal



accompanied by lots of unwanted signals at adjacent frequencies, it must be

seated and the noise be rejected. Finally the RF carrier must be

demodulated to get back the original modulating signal. Also since the

detected signal is usually weak, it has got to be amplified is one or more

stages of audio amplifiers.

Thus the following are the main functions of a radio receiver:

I. Interrupt the electro magnetic wave in the receiving antenna to

produce the desired RF modulated carrier.

II. Select the desired signal and reject the unwanted signal.

III. Amplify the RF signal.

IV. Detect the RF carrier to get back the original modulation frequency

voltage.

V. Amplify the modulation frequency voltage.



VI.

(I) AM Broadcast Receiver

These are meant for listening to broadcast of speech or music radiated

from amplitude modulation broadcast transmitter operating on long

wave, medium wave or short wave bands.

(II) FM Broadcast Receiver

These are used for receiving broadcast programmes from FM

broadcast transmitters operating in VHF or in UHF bands.

(III) T.V Receiver

These receivers are used for receiving television broadcast in VHF ot in

UHF bands.

(IV) Communication Receivers These are super heterodyne receivers

used for reception of code and short wave telephone signals and include

circuit refinements such as IF beating oscillator for code reception, noise

limiters or noise



suppressors, band spread for fine tuning, crystal filter for high and

adjustable selectivity, sensitivity control, volume expander, inter channel

noise suppressor, tuning indicator etc. These communication receivers are ,

there fore more costly and complicated and their operation involves some

technical knowledge and skill as possessed by radio operators.

(V) Code receivers

These are in general, simple super heterodyne receivers with the

addition of IF beating oscillator to produce audio beat note with IF signal.

Other code receivers are meant for receiving code signals, ie. radio

telegraph signals and consist of an oscillating detector with amplifier

stages.

(VI) Radar receivers

These are receivers used for receiving Radar (Radio Detection and

Ranging signals).

Salient features of Broadcast Receivers

(a) Simplicity of Operation:- These receivers are required to be handled by

listeners who have little technical knowledge and hence simplicity of

operation is essential.

(b) Good Facility:- Since these broadcast receivers are primarily designed

for entertainment purpose. There should have good electrical fidelity

ie: a reasonably large and uniform frequency response over almost the

entire audio frequency bond.

(c) Good Selectivity:- By selectivity of a radio receiver is meant its ability

to discriminate the desired signal from other frequencies, notably the

side band of adjacent channels in the frequency spectrum.

(d) Average Sensitivity:- Broadcast receivers should have reasonably high

sensitivity. So that it may have good response to the desired signal of

medium and long strengths but should not have excessively high



(e) sensitivity otherwise it will pick up all the undesired electrical

disturbances produced in incinity

(f) Adaptability to different types of Aerials:- A broadcast receivers should

be designed to operate satisfactory with any type of Aerial.

Basic Functions AM Receivers

A radio receiver in its most elementary form performs the following

four essential functions:

(i) Reception: This consists in receiving or picking up energy from the

various electro magnetic waves radiated by the radio transmitter. This

function is performed by the receiving antenna. When EM wave

strikes the antenna, a voltage of the wave frequency is induced in the

antenna.

(ii) Selection: This consists in selecting or responding to desired radio

wave with the exclusion of all others. Thus at any instant of time, a

large number of EM waveform of different radio stations at different

frequencies are intercepted by the antenna and each of these induces

a voltage in the antenna. The selector circuit or tuner in the form of a

parallel tuned circuit responds to the desired signal only and rejects

all other signals.

(iii) Detection or Demodulation: The desired signal in the form of

a modulated carrier voltage is detected in a detector circuit to recover

the original modulating voltage.

(iv) Reproduction: This consists in feeding the detected signal to a

loudspeaker or headphones to reproduce the sound wave giving the

original programme. Based on the technique of operation, radio receivers may be put into two

categories.

(i) Straight receivers: The receivers which operate in straight forward

manner without frequency conversion.



Super heterodyne receivers: The receivers in which incoming RF

signal is converted to standard Intermediate Frequency (IF) before

detection takes place. These receivers were extensively used earlier

but are not used in these days.

TRF Receivers

It is a straight receiver in which the incoming signal is first amplified

in one or more tuned RF amplifier stage. This increases the magnitude of

the signal and hence improves the sensitivity of the receiver. The amplifier

signal is then fed to the detector to re-obtain the original modulation

frequency signal. The modulation frequency is further amplified in one or

more stages of audio frequency amplifiers before being fed to the

loudspeaker.

Principle of Super Heterodyne Receiver

Heterodyne reception stands for the radio reception after converting

the modulated carrier voltage into similarly modulated voltage at a different

carrier frequency. Thus the heterodyning process involves a simple change

or translation of carrier frequency. This change in carrier frequency is

achieved by heterodyning or mixing the modulated carrier voltage with a

locally generated high frequency voltage in a non-linear device to obtain at

the output a similarly modulated carrier voltage at the difference carrier

frequency called intermediate frequency.

Super heterodyne reception is a form of heterodyne reception in

which frequency conversion takes place one or more times before the

modulated carrier voltage is fed to the detector to recover the original

modulation frequency voltage. In practice, however the name super

heterodyne is applied to receivers in which only one frequency conversion

takes place before detection.

Because of the various merits of super heterodyne receivers over TRF

receivers, these super heterodyne are most properly use in almost all radio



receiver applications such AM broadcast receivers, AM communication

receivers, FM receivers, SSB receivers, TV receiver, RADAR receiver etc.

Constituent Stages of a Super heterodyne Receiver

Antenna or Aerial

It intercepts the EM waves, voltages induced in the antenna are communicated to the receiver input circuit by means of a feeder wire or lead in wire. A parallel tuned circuit at the input of the receiver responds only to voltage at the desired carrier frequency and rejects voltages at all other frequencies. The voltage so picked up is fed to the input of the RF amplifier stage.

RF Amplifier

This stage is generally a tuned voltage amplifier tuned to the desired

carrier frequency. The chief functions of RF amplifier stages are:

(i) To amplify the input signal voltage to a suitably high level before

feeding it to the frequency mixer which contributes large noise. Thu

signal / noise ratio is improved.

(ii) To provide discrimination or selectivity against image frequency signal

and intermediate frequency signal.

Frequency converter stage

This consists of a local oscillator and frequency mixer. To the frequency mixer are fed both the local oscillator voltage as well as signal voltage. The mixer, being a non-linear device produces at its output the various inter modulation terms.

The difference frequency voltage is picked-up by the tuned circuit in the output circuit of the mixer. This difference frequency is called the intermediate frequency the value of which is constant for a receiver. Sometimes two separate transistors are used as local oscillator and frequency mixer but more often only one transistor functions both local oscillator and frequency mixer. Thus with the help of frequency converter stage. RF signal of any carrier frequency is converted into similarly modulated fixed frequency IF signal.



IF Amplifier Stage It consists of two or more stages of fixed frequency voltage amplifier

having a 3-dB bandwidth of 10KHz for AM broadcast. This IF amplifier

provides most of the receiver application and selectivity.

Second Detector Output of the last IF amplifier stage is fed to this second detector, which is generally a linear diode detector. Output of this is the original modulation frequency voltage.

Audio Frequency Amplifier Audio frequency output from second detector is fed to the AF amplifier which provides additional amplification. Usually one stage of audio voltage amplifier is used followed by one or more stages of audio power amplifier.

Loudspeaker Amplified audio output voltage of audio power amplifier is fed to loudspeaker through impedance matching transformers. The loudspeaker reproduces the original programme.



Basic Principles of T.V Television means vision at a distance. It have good TV system, the television

system has to reproduce faithfully the following.

Shape of the object

Relative brightness of the subject

Motion

Sound

Colour

Perspective

In TV system, the picture or image of the object is converted in to

electrical signals using a transducers by a video camera. Video signal

obtained has a frequency rang of about 5 Mhz is amplitude modulated with

a carrier wave.

The round of the object is recorded by a microphone. This amplitude

to the required levels using AF amplifier. Their audio signal is frequency

modulated by a separate carrier wave.

A TV ration radiate two separate RF carriers by a single antenna. One

carriers is frequency modulated by audio signal and the other is amplitude

modulated by video signal. The audio carriers frequency is 5.5 MHz higher

than the video carrier frequency. Both there carriers are transmitted

simultaneously through TV transmitters. For colour TV systems, the total

bandwidth required for a particular ration about 7 MHz.



After incorporating special signals like synchronizing pulses, blanking

pulses, etc …. For initiating various operations of the receiver, the composite

video signal is obtained.

(In exam explain

about each block with its figure on the right side)

At the receiver on antenna is used to receive the TV signals. The

composite video signal received is selected using a tuner. The tuned stage is

a mixer which mix the received signal with the local oscillator. And produce

two intermediate frequencies viz Picture IF and sound IF)

In various triggering pulses like horizosntal and vertical synchronizing

pulse are separated for triggering various circuits in the receiver. There are

for deflecting the election beam in picture tube in horizontal as well as

vertical direction.

Detectors separate video and audio signals. Video signals obtained is

amplified and applied to TV Picture tube for reproduction. Audio signal

obtained is applied to loud separate after proper amplification



Monochrome Picture tube

The picture tube is a

special kind of cathode ray tube. In the cathode ray tube (CRT), the

electrical or the video signals are translated in to a visible picture, or image

on a phosphorescent screen.

A monochrome Picture tube consists of

i. Electron gun

ii. Focusing System

iii. Deflection system

iv. Display system – screen etc…

A amplified arrangement of a monochrome picture tube is given below:

(i) Electron Gun It units and control electron beam which are

liberated from the cathode when heated.



Intensity of the beam at any instant is controlled by the control grid.

There are two anodes

First Anode

Accelerate electrons towards the screen.

Applied potential is about 250-450v

Second Anode

Provide grater acceleration to the beam.

Applied potential is around 8-20 kv.

Anode is connected to the inside to the tube at a special waiting called

aquadag coating (conducting granules of graphite)

Focusing System Focusing of the beam is done by employing electrostatic focusing

techniques.

Deflection System Electromagnetic deflection is used for

deflecting the electron beam such that beam

traverses the screen from top left corner to

bottom right corner.

Two pairs of deflection coils are mounted

externally to picture tube called deflection “YOKE”

Yoke contains two separates coils called horizontal and vertical

winding which produce separate magnetic fields to produce a raster

Horizontal deflection coil helps to move the beam from left to right.



Vertical deflection coil makes movement of beam from top to bottom.

Display System Screen

At is composed of a fluorescent material made of a compound

containing Zn, Co, Cd, etc.

All light produced by screen material is emitter outward but a great

amount of light to reflected book into tube there by reducing

brightness.

To present this, tube is ‘aluminized’ (Screen is coated with a very fine

thin layer of aluminium)

So, screen behaves as a miners that reflect electrons out of the tube

towards the viricer )

Face plats

Rectangular face plates with a breadth to right ratio of 4:3 are

commonly used in picture tubes.

(A 54 cm picture tube means that the distance between the two

diagonal ratio of the face of the picture take of 54cm)

Working Electron gun produces electron beam which is accelerate towards

screen.

Before that it has to be focused and accelerated by electrostatic

methods.

Electrons are detected using electromagnetic schemes by deflection

“YOKE”



Electrons hit the inner rids of glass face plate coaked with

luminescent material and procedure light.

Received video signal derives electron gun and controls intensity of

beam. The resulting brightness variations of raster generators black

and white pictures.

Final anode held at 18Kv is employed in order to give the electrons

sufficient energy to produce Florence.

Final anode is connected to tube at aquadag.

Electrons that are accelerated under this high voltage attain high

velocities and more straight to the screen and are not collected by

higher positive potential because its circular structure provides a

symmetrical according field around all ride of the beam.

Due to high velocity of ě beam there will be secondary mission and

conducting coating near the screen collect there secondary electron.

Scanning To obtain composite video signal or picture signal, the picture or image is

scanned by the pick up tube or video camera.

The program scan at

the studio is projected on to a

screen inside the camera to

form an image. The scanning

system has the effect of

deciding the scene in to

narrow horizontal strips

called lines. Each line is

scanned from left to right by the scanning beam in the camera tube.



There are two scanning procedures taking place simultaneously: Moving the

beam horizontally from left to right at a fast rate. Moving the beam vertically

downwards at a slower rate.

The amount of light intensity encountered by this beam at any

particular instant determines the camera current and hence a cottage

proportional to this light intensively appears at the output of camera take.

This is the video signal.

Interlaced Scanning Generally to avoid flickering interlaced scanning is used. Here the are two

fields, odd field & even field. Even fields are placed in between the odd field.

In odd field the

electron beam

initially scans

the line a-b at

an angle as

shown. The

first line starts

from the left

end of the

picture to be

scanned. After reaching right and it returns to the left ends directly and

starts the line c-d. There is a small space ‘s’ between the line a-b& c-d equal

to the diameter of the scan beam. During the motion of the beam from left to

right, the picture information in concentrated in to electrical signal. The

electrical signal so obtained at the camera output will be at video frequency

312.5 lines are scanned in the first field.



(50/second in india)

While reaching the

bottom middle the odd

field has converted 312.5

lines of pictures

information in to

electrical signal now the

beam is shifted vertically

to the top middle and

starts scanning the

second field (even field).

The scan line of this field

lie in between the odd

field lines. After

completing the second

field it again goes to the

top end to start a new odd field.

The two fields are interlaced. Complete picture information is obtained only

with two fields taken together .

Two fields constitute a ‘frame’. To audio flickering effect, each field is

repeated 25 times ie, today 50 fields are taken per second.

In odd field 312.5 lines are scanned. In even fields, which are placed in

between the line of odd lines, another 312.5 lines are scanned there toe

fields constitute one full picture.

At the receiver the same antenna collects the audio and video RF

signals. A unit called linear stretch the required channel. The audio and RF

signal are amplified by the suitable stage and then they heterodyne with

local oscillator to generate the intermediate frequency IF.



This IF is then amplified and is fed to a video detector, which

demodulates the composite video signals and separates the audio If from

CVS. The audio IF is amplified and the demodulated and further amplified

so that it can drive a loud speaker. The demodulated CVS is fed to video.

Amplifier and Sign Separator Circuit, the video signals interact with an

electron beam to form a visible image on the face of the picture tube the sign

pulses and they are used to

respectively. The find accelerating anode voltage 18KV is generated

with the help of horizontal deflection pulse and a special transformer called

line output Transformer (LDT).



Mobile Communication System In mobile communication system the subscribers maybe fixed or mobile. The service provider must be able to locate the subscriber & thus assign a channel frequency for communication. If the called subscriber is in motion, the system has to track continuously & maintain communication link with the moving subscriber.

The Cells In the system, the whole area of service is divided into small regions called cells. The area of each cell will depend on the number of subscribers, nature of the terrain of the cell density of buildings etc..

Each cell contains an antenna and is controlled by a small office within the cell called cell office. All the subscribers inside a particular cell are monitored & tracked continuously by the base station system [BTS], & the data is passed on to the central office [MTSO].

MTSO is Mobile (Telephone) Telecommunication Switching office. MTSO controls the whole traffic in the region. There need be only one MTSO for a state like Kerala.



Working Of Mobile Telephony

(1) Call From Fixed Phone To Mobile Phone

Let the call from a fixed land line is originated in the normal manner. Getting the mobile subscriber number, the local exchange knows the called party is a mobile user. The call is routed to MTSO of that particular mobile service provider. The MTSO then verifies the details of the called subscriber regarding the genuineness. It then tracks the cell in which the called party is currently located. This data is available at MTSO which are obtained from the BTS of that particular cell. This data will be continuously updated. The called subscriber's number is transmitted to the particular BTS & is retransmitted from the antenna there. Then the ring tone is activated.

(2) Mobile To Mobile [With Same Service Provider]

The calling subscriber enter the number of the called subscriber & presses the send/called key on his hand set. This number is transmitted along with some data from the handset to all the directions. The nearest base stations receiving antenna within the cell capture this & sends this to MTSO. At MTSO after verifying the authenticity of both numbers the current location of the called number is traced. MTSO sends the called number to that cell. Transmitter at that cell transmits this to all directions, inside the cell. This is received by the subscriber handset.



(3) Hand Off

Sometimes both calling & called subscribers may be in motion. During conversation the parties may move from one cell to another. But we know that each cell has different frequencies for transmission. When mobile units move away from a particular cell antenna the strengths of the received signal reduces. The MTSO shifts the operation to a new channel having highest strength. This will naturally be the frequency of a cell nearest to the subscribers. The change overtakes smoothly without the knowledge of calling & called parties. This is called “Hand Off”.

Frequency Reuse A radio channel consists of a pair of frequencies, one for each direction of transmission, i.e. used for full duplex operation. A particular channel, say F1, used in one geographic zone to call a cell called C1 with a coverage radius R can be used in another cell with a same coverage radius at a distance D away.

Frequency reuse is the core concept of a cellular mobile radio system. If a service provider has a 24 channel frequencies, the whole region cannot be served. Frequency is reused.

MICROWAVE COMMUNICATION SYSTEM Initially microwave radio transmission was exclusively by long distance communication carriers, the military & publicity. The microwave signal requires an obstruction free line of sight communication between sending & receiving locations. The useful frequency range is between 3000 Mhz to 150 Mhz.

Main characters of microwave communication system:

1. Line of Sight (LOS) communication is limited by the earth's curvature.

2. Different techniques like frequency & phase modulation, spread spectrum, TDMA etc.. are used.

3. Good reliability & large transmission bandwidth.

4. Due to free space attenuation of signal, repeater stations are required at both 50Km intervals.

5. Both digital & analog communication systems are in use.



Simplified Microwave Communication System The input data to be transmitted in applied to the microwave transmitter after processing & modulation. It's transmitted through the microwave antenna in a particular direction. At a LOS distance & about 50Km there will be an

Intermediate receiving station called repeater. At the repeaters, the received signal is amplified and retransmitted in the desired direction using the microwave antenna. Depending upon the distance of destination there can be several repeater stations. At the end or receiving station, the signal is received, demodulated and processed and send to various destination points.

Applications Of Microwaves The advantage of the microwave spectrum is that it can accommodate many more channels than the T.V and radio bands. RADAR systems represent other major applications of microwaves. They are used to detect aircrafts guided missiles, observe and predict weather conditions, control of flight traffic etc. Heating property of microwave power is useful in a wide variety of commercial and industrial applications. Microwave oven is an example. In medical applications the possibility of exposing malignant cells to microwaves heat is being investigated as method for treating cancer. The analysis and interpretation of molecular resonance called microwave spectroscopy, is an important vehicle in the scientific effort to understand the fundamental nature of solids, liquids and gases.

The movement of microwaves between the transmitter and receiver is called propagation. Differences in frequency, transmitter power and antenna design results in varying degrees of signal loss. For example the microwave signal expands as it moves from the transmitter to the receiver. The receiver can see only a portion of the signal being radiated. This is called free space



attenuation and it is one reason why microwave transmission systems are limited to 30 miles. At greater distances the receiver would not get enough of the signal for and accurate transmission.

Microwave Transmitter The signal to be transmitted consists of telephone channels, data channels and video channels. These are to be multiplexed. For this FOM is used. Then it is modulated with a carrier frequency. This modulated signal is again frequency translated to that of transmission frequency in GHz range.

Repeater Stations A repeater station is characterized by two antennas in two directions. One for receiving and the other for transmission. The repeater will receive, amplify and retransmit the signals to the next repeater station on the link

Microwave Receiving Station At the receiving station, the signal is received through the directional receiving antenna. It is down converted to IF by adding local oscillator signal and demodulated to receive the base band signals several microwave amplifiers are used to amplify the signal to the desired level. The amplified signal is then demodulated to recover the original base band signal transmitted from the terminal transmitting station. The base band signal is then demultiplexed using frequency division technique to receive the individual telephone channel or TV channel signals.

Satellite Communication

Defintion:

A satellite communication is a microwave repeats in the sky that



consists of diverse combination of one or more of the following:

Receiver, transmitter, amplifier, regenerator, filter, onboard computer, antenna, waveguide and about any other electronic communication system ever developed.

Basic Concept Of Satellite Communication Link

The basic components of a satellite communication are.

A) Earth station

B) Satellite

Earth station:

Both transmit and receive the signals

Satellite:

It is classified as

Domestic

Regional

Global

They can also be classified in terms of the type of services allowed as

1. Fixed



2. Mobile

3. Maritime

4. Aeronautical

5. Experimental

6. Broadcasting

Orbit 1. Polar Circular Orbit

These are closer or the earth and passes over the poles

Average height – 800 to 10000 km above the earth.

Uses – observational, survey lens, navigational purposes

2. Inclined Elliptical Orbits

Used where communication is desired for regions of high altitude, the Russian “MOLNIYA” series of satellites are highly inclined orbits.

GEOSTATIONARY SATELLITE

Geostationary satellites should appear fixed in the sky; it requires a period of 24 hrs. Using the eq. For the distance between earth and the satellite r = ( gR2 /2πf2)13 and perio0d should be 24 hrs. The resulting distance is 35786km.

Advantages Senders and receivers can use fixed antenna positions, no adjusting is needed.

Geo's are ideal for TV and radio broadcasting.

Life time expectation is rather high at about 15 yrs.

Geo's don’t exhibit any droplet shift becoz the relative movement is zero.

Disadvantages Larger antennas are needed.

Limited transmission quality

transmit power needed is relatively high



transferring a geo into orbit is very expensive

Communication Subsystems SUBSYSTEM FUNCTIONS IMPORTANT

PARAMETERS 1) Repeaters 2) Antennas

Signal amplification Reception of signals, transmission of signals

Noise figure, Linearly output RF power. Coverage gain.

Merits Of Satellites Satellite relays are inherently wide area broadcast where as all the

terrestrial relays are point-to-point.

Satellite circuit can be installed quickly. Having installed the satellite in its proper position, earth station can be installed and communication established in a day or so.

Mobile communication can be easily established by satellite communication since it has a high degree of flexibility in interconnecting mobile vehicles.

Satellite communication is economical compared with terrestrial communication particularly where long distances are involved. Cost of satellite communication is independent of distance whereas cost of terrestrial network increases in proportion to the distance.

Compared to the fibre optical cable communication, satellite communication has the advantage that quality of transmitted signal and the locations of sending and receiving stations are independent of distance. Thus using satellite communication, quality remains the same when the distance between the transmitting stations increases from say 50 to 2000 km.

For thin traffic remote areas like NE regions in Himachal Pradesh, ladakh.etc. Satellite communication is most cost effective.

For search rescue and navigation, satellite communication is far superior and economical compared to other systems.



Drawbacks Of Satellite Communication With satellite having been installed, communication path length between terrestrial transmitter and the receiver is about 75000 km with velocity of electromagnetic waves of the transmission delay corresponding to 75000km amounts to 0.25 seconds. Thus there is this delay of 0.25 secs between transmission and reception. Hence between talks there is time gap of o.5 secs and this gap may become annoying.

Imperfect impedance may cause echo received back after delay of .5 secs. Echo suppression has to be used.

Time delay of .5 secs also reduces the efficiency of satellite in data transmission.

Repair of satellite is almost impossible. Once it has been launched, further the satellite is subjected to extreme environmental stresses.

Applications 1. Weather Forecasting

Several satellites deliver pictures of the earth using eg. Infrared or visible light without the help of satellite, the forecasting of hurricanes would be impossible.

2. Radio And TV Broadcast Satellites.

Hundreds of TV and radio programmes are available via satellite.

3. Military Satellites

One of the earliest applications.

4. Satellites For Navigation

Even though it was only used for military purposes in the beginning, the global positioning system is now a days well known and available for everyone.



SATELLITE LINK MODELS A satellite system consists of three basic sections. They are: 1) Up Link (Transmitter)

2) Transponder (repeater)

3) Down Link (Receiver)

Up Link A satellite uplink model is given above; the main component on up link is the earth station transmitter. The transmitter consist of an IF modulator and an IF-to-RF converter, a high amplifier(HPA) and finally a band pass filter. The IF modulator converts the input base band signals to either an FM, a PSK (phase shift keying) or a QAM (Quadrature Amplitude Modulation) modulated intermediate frequency. The UP converter converts the IF to an appropriate RF carrier frequency. The HPA



provides the necessary sensitivity and output power to propagate the signal to the satellite transponder.

Down Link An earth station receiver includes an input band pass filter, a low

noise amplifier and an RF-to-IF down converter. The BPF limits the input noise to LNA. The LNA is highly sensitive low noise device such as tunnel diode amplifier or a parametric amplifier. The RF-to-IF down converter is amine or band pass filter combinations which converts the received RF signals to the desired IF frequency. THE demodulator output gives the original base band signal.

Note: Satellites are always built with the intention that many users will share the band width allocated to the satellites, allowing many separate communications link to be established through the satellite transponders. The ability of the satellite to carry many signals at the same time is known as multiple accesses. Multiple accesses allow the communication capacity of the satellite to be shared among a large number of earth stations, and to accommodate the different mixes of communication traffic that are transmitted by the earth stations. The signals that earth station transmit to a satellite may differ widely entire character voice, video, data, facsimile-but they can be send through the same satellite using multiple accesses and multiplexing techniques. Multiplexing is the process of combining a number of signals into a single signal so that it can processed by a single amplifier or transmitted over a single radio channel.

Module 3 PDF

Documents

frequency spectrum

assigned frequency range

large range of frequency

low frequency vlf

low frequency lf

low elf frequency

low frequency ulf

high frequency range