radio communication jto lice study material sample

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LICE-JTO Radio Communication Systems

2016ENGINEERS INSTITUTE OF INDIA .All Rights Reserved LICE / JTO / SSC : Classroom , POSTAL, All India TEST Series

28-B/7, Jia Sarai, Near IIT, HauzKhas, New Delhi-110016. Ph. 011-26514888. www.engineersinstitute.com

1

LICE -JTO

Limited Internal Competi t ive Examinat ion

STUDY MATERIAL

Radio Communication Systems

Principles of Radio Communication, A.M., F.M. Radio, Phase Modulation. Signal

conditioning and Transmission Study of special chips, output interfacing, output

instruments-indicators, recorders, data acquisition systems data loggers, servo

mechanism, electronic process control instrumentation. Wave propagation,

Microwave devices & components, microwave measurements, antenna

fundamental & their characteristic. Audio Engineering, sound transducers, sound

recording & reproduction, sound transmission, radio transmission, radio

reception.


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CONTENTS

Basics of Radio Communication 3- 6

Modulation 7-17

AM /FM Radio System 17-20

Microwave Engineering 20-63

ANTENNAS 63-75

Microwave measurements 76-86

Data acquisition system and data loggers 87-89

Audio Engineering 90-94Sample Questions 94-107

IMPORTANT TERMS - MEMORY 108-114

SYLLABUS: Radio Communication Systems

Principles of Radio Communication, A.M., F.M. Radio, Phase Modulation. Signal conditioning and

Transmission Study of special chips, output interfacing, output instruments-indicators, recorders,

data acquisition systems data loggers, servo mechanism, electronic process control instrumentation.

Wave propagation, Microwave devices & components, microwave measurements, antenna

fundamental & their characteristic. Audio Engineering, sound transducers, sound recording &

reproduction, sound transmission, radio transmission, radio reception.


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3

Basics of Radio Communication

Radio or radio communication means any transmission, emission, or reception of signs, signals,

writing, images, sounds or intelligence of any nature by means of electromagnetic waves of

frequencies lower than three thousand gigacycles per second (3000 GHz) propagated in space

without artificial guide.

Examples of radio communication systems:

Radio broadcasting.

TV broadcasting.

Satellite communication.

Mobile Cellular Telephony.

Wireless LAN.

Multimedia communication & Mobile Internet

Classification of radio spectrum

Application

TimeandFrequencyNormal,Navigation,

UnderwaterCommunication,Rem

otesensing

underground,Maritimetelegraphy

Longdistancecommunication(fixedandmarine),

Broadcasting,Navigation,Radiobeacons

AM

broadcasting,navigation,radiobeacons,and

distressfrequencies.

Fixedpointtopointcommunicatio

n,Mobile

maritimeaeronautical,landservic

es,military

communication,amateurradioan

dbroadcasting

Broadcasting,TV,FM,Mobileserv

icesfor

maritime,aeronauticalandland,W

ireless

microphones,Meteorburstcomm

unication

BroadcastingTV,satellites,Person

altelephone

systems,radarsystems,fixedand

mobilesatellite

services

Fixedservices,Fixedsatelliteservices,Mobile

services,Remotesensing

Frequencyassignmentsup60GHz

Frequency

300-

3000

Hz

3-30

kHz

30-300

kHz

300-

3000

KHz

3-30

MHz

30-300

MHz

300-

3000

MHz

3-30

GHz

30-300

GHz

Wavelength

1000

-100km

100

-10km

10

-1 km

1000

-100 m

100

-10 m

10

-1 m

100

-10 cm

10

-1 cm

10

-1 mm

Term ELF VLF LF MF HF VHF UHF SHF EHF

Radio Communication has three main problems:

The path loss


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4

Noise

Sharing the radio spectrum

In radio communication systems, the transmitted signal is very weak when it reaches the

receiver, particularly when it has traveled over a long distance.

The signal has also picked up noise of various kinds.

Receivers must provide the sensitivity and selectivity that permit full recovery of the original

signal.

The radio receiver best suited to this task is known as the super heterodyne receiver.

Superheterodyne receivers convert all incoming signals to a lower frequency, known as the

intermediate frequency (IF), at which a single set of amplifiers is used to provide a fixed

level of sensitivity and selectivity.

Gain and selectivity are obtained in the IF amplifiers.

The key circuit is the mixer, which acts like a simple amplitude modulator to produce sum

and difference frequencies.

The incoming signal is mixed with a local oscillator signal.

Block diagram of a superheterodyne receiver.

RF Amplifier

The antenna picks up the weak radio signal and feeds it to the RF amplifier, alsocalled a low-noise amplifier (LNA).

RF amplifiers provide some initial gain and selectivity and are sometimes called pre-

selectors.

Tuned circuits help select the frequency range in which the signal resides.

RF amplifiers minimize oscillator radiation.

Bipolar and FETs can be used as RF amplifiers.

Mixers and Local Oscillators

The output of the RF amplifier is applied to the input of the mixer.


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The mixer also receives an input from a local oscillator or frequency synthesizer.

The mixer output is the input signal, the local oscillator signal, and the sum and

difference frequencies of these signals.

A tuned circuit at the output of the mixer selects the difference frequency, or

intermediate frequency (IF).

The local oscillator is made tunable so that its frequency can be adjusted over a

relatively wide range.

IF Amplifiers

The output of the mixer is an IF signal containing the same modulation that

appeared on the input RF signal.

The signal is amplified by one or more IF amplifier stages, and most of the gain is

obtained in these stages.

Selective tuned circuits provide fixed selectivity.

Since the intermediate frequency is usually lower than the input frequency, IF

amplifiers are easier to design and good selectivity is easier to obtain.

Demodulators

The highly amplified IF signal is finally applied to the demodulator, which recovers

the original modulating information.

The demodulator may be a diode detector (for AM), a quadrature detector (for FM),

or a product detector (for SSB).

The output of the demodulator is then usually fed to an audio amplifier.

Automatic Gain Control

The output of a demodulator is usually the original modulating signal, the amplitude

of which is directly proportional to the amplitude of the received signal.

The recovered signal, which is usually ac, is rectified and filtered into a dc voltage by

a circuit known as the automatic gain control (AGC) circuit.

This dc voltage is fed back to the IF amplifiers, and sometimes the RF amplifier, to

control receiver gain.

AGC circuits help maintain a constant output level over a wide range of RF input

signal levels.

Automatic Gain Control

The amplitude of the RF signal at the antenna of a receiver can range from a fraction

of a microvolt to thousands of microvolts; this wide signal range is known as the

dynamic range.

Typically, receivers are designed with very high gain so that weak signals can be

reliably received.


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However, applying a very high-amplitude signal to a receiver causes the circuits to

be overdriven, producing distortion and reducing intelligibility.

With AGC, the overall gain of the receiver is automatically adjusted depending on

the input signal level.

Frequency conversion

Frequency conversion is the process of translating a modulated signal to a higher or lower

frequency while retaining all the originally transmitted information.

In radio receivers, high-frequency signals are converted to a lower, intermediate frequency.

This is called down conversion.

In satellite communications, the original signal is generated at a lower frequency and then

converted to a higher frequency. This is called up conversion.

Mixing Principles

Frequency conversion is a form of amplitude modulation carried out by a mixer circuit or

converter.

The function performed by the mixer is called heterodyning.

Mixers accept two inputs: The signal to be translated to another frequency is applied to one

input, and the sine wave from a local oscillator is applied to the other input.

Like an amplitude modulator, a mixer essentially performs a mathematical multiplication of

its two input signals.

The oscillator is the carrier, and the signal to be translated is the modulating signal.

The output contains not only the carrier signal but also sidebands formed when the localoscillator and input signal are mixed.

Advantages and disadvantages of wireless communication (Important for LICE)

Advantages:

mobility

a wireless communication network is a solution in areas where cables are impossible

to install (e.g. hazardous areas, long distances etc.)

easier to maintain

Disadvantages:

has security vulnerabilities

high costs for setting the infrastructure

unlike wired comm., wireless comm. is influenced by physical obstructions, climatic

conditions, interference from other wireless devices

Frequency Carries/Channels

The information from sender to receiver is carrier over a well-defined frequency band.


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This is called a channel

Each channel has a fixed frequency bandwidth (in KHz) and Capacity (bit-rate)

Different frequency bands (channels) can be used to transmit information in parallel and

independently.

Example

Assume a spectrum of 90KHz is allocated over a base frequency b for communication

between stations A and B

Assume each channel occupies 30KHz.

There are 3 channels

Each channel is simplex (Transmission occurs in one way)

For full duplex communication:

Use two different channels (front and reverse channels)

Use time division in a channel

Radio waves generation

when a high-frequency alternating current (AC) passes through a copper conductor it

generates radio waves which are propagated into the air using an antenna

radio waves have frequencies between:

3 Hz 300 KHz - low frequency

300 KHz 30 MHz high frequency

30 MHz 300 MHz very high frequency

300 MHz 300 GHz ultra high frequency

radio waves are generated by an antenna and they propagate in all directions as a straight

line

radio waves travel at a velocity of 186.000 miles per second

radio waves become weaker as they travel a long distance


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Modulation

Modulation = adding information (e.g. voice) to a carrier electromagnetic (radio) signal

Digital modulation techniques


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Frequency Modulation (FM) / Amplitude Modulation (AM)

Radio frequency interference

Radio signal attenuation (path loss)


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Amplitude Modulation

Amplitude Modulation is the simplest and earliest form of transmitters

AM applications include broadcasting in medium- and high-frequency applications, CB radio,

and aircraft communications.

Transmit information bearing (message) or baseband signal (voice music) through acommunication channel

Baseband = is a range of frequency signal to be transmitted. eg: Audio (0 - 4 kHz), Video (0

- 6 MHz).

Communication channel:

Transmission without frequency shifting.

Transmission through twisted pair cable, coaxial cable and fiber optic cable.

Significant power for whole range of frequencies.

Not suitable for radio/microwave and satellite communication.

Carrier communication

Use technique of modulation to shift the frequency.

Change the carrier signal characteristics (amplitude, frequency and phase) following

the modulating signal amplitude.

Suitable for radio/microwave and satellite communication.

The instantaneous amplitude of a carrier wave is varied in accordance with the

instantaneous amplitude of the modulating signal. Main advantages of AM are small

bandwidth and simple transmitter and receiver designs. Amplitude modulation is

implemented by mixing the carrier wave in a nonlinear device with the modulating signal.

This produces upper and lower sidebands, which are the sum and difference frequencies of

the carrier wave and modulating signal.

The carrier signal is represented byc(t) = A cos(wct)

The modulating signal is represented bym(t) = B sin(wmt)

Then the final modulated signal is

[1 + m(t)] c(t)= A [1 + m(t)] cos(wct)= A [1 + B sin(wmt)] cos(wct)


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= A cos(wct) + A m/2 (cos((wc+wm)t)) + A m/2 (cos((wc-wm)t))

The information signal varies the instantaneous amplitude of the carrier

AM Characteristics

AM is a nonlinear process

Sum and difference frequencies are

created that carry the information.

Modulation Index - The ratio between the amplitudes between the amplitudes of the

modulating signal and carrier, expressed by the equation:

c

m

EEm =

When the modulation index is greater than 1, over modulation is present

Modulation Index for Multiple Modulating Frequencies

Two or more sine waves of different, uncorrelated frequencies modulating a single carrier is

calculated by the equation ++= 222

1 mmm

Bandwidth


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Signal bandwidth is an important characteristic of any modulation scheme

In general, a narrow bandwidth is desirable

Bandwidth is calculated by:

mFB 2=

Power Relationships

Power in a transmitter is important, but the most important power measurement is that of

the portion that transmits the information

AM carriers remain unchanged with modulation and therefore are wasteful

Power in an AM transmitter is calculated according to the formula at the right

+=

21

2mPP ct

Quadrature AM and AM Stereo

Two carriers generated at the same frequency but 90 out of phase with each other allowtransmission of two separate signals

This approach is known as Quadrature AM (QUAM or QAM)

Recovery of the two signals is accomplished by synchronous detection by two balanced

modulators

Suppressed-Carrier AM

Full-carrier AM is simple but not efficient

Removing the carrier before power amplification allows full transmitter power to be applied

to the sidebands

Removing the carrier from a fully modulated AM systems results in a double-sideband

suppressed-carrier transmission

Single-Sideband AM

The two sidebands of an AM signal are mirror images of one another

As a result, one of the sidebands is redundant

Using single-sideband suppressed-carrier transmission results in reduced bandwidth and

therefore twice as many signals may be transmitted in the same spectrum allotment

Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC

Power in Suppressed-Carrier Signals

Carrier power is useless as a measure of power in a DSBSC or SSBSC signal

Instead, the peak envelope power is used

The peak power envelope is simply the power at modulation peaks, calculated thus:

RL

VPEP

p

2

2

=

Frequency Modulation.

FM is widely used for a variety of radio communications applications. FM broadcasts on the VHF

bands still provide exceptionally high quality audio, and FM is also used for a variety of forms of two

way radio communications, and it is especially useful for mobile radio communications, being used

in taxis, and many other forms of vehicle.


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In Frequency Modulation (FM) the instantaneous value of the information signal controls the

frequency of the carrier wave. This is illustrated in the following diagrams.

Notice that as the amplitude of the information signal increases above 0 volts, the frequency of the

carrier increases, and as the amplitude of the information signal decreases below 0 volts, the

frequency of the carrier decreases.

The frequency fi of the information signal controls the rate at which the carrier frequency increases

and decreases. As with AM, fi must be less than fc. The amplitude of the carrier remains constant

throughout this process.

When the information voltage reaches its maximum value then the change in frequency of the

carrier will have also reached its maximum deviation above the nominal value. Similarly when the

information reaches a minimum the carrier will be at its lowest frequency below the nominal carrier

frequency value. When the information signal is zero, then no deviation of the carrier will occur.

The maximum change in frequency that can occur to the carrier from its base value fc is called the

frequency deviation, and is given the symbolfc. This sets the dynamic range (i.e. voltage range) ofthe transmission.

The dynamic range is the ratio of the largest and smallest analogue information signals that can be

transmitted.

Modulation Index


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All FM transmissions are governed by a modulation index,, which controls the dynamic range of

the information being carried in the transmission. The modulation index,, is the ratio of the

frequency deviation, fc , to the maximum information frequency, fi , as shown below:

i

c

f

f=

Theoretically, an FM spectrum has an infinite number of sidebands, spaced at multiples of fi above

and below the carrier frequencyfc . However the size and significance of these sidebands is very

dependent on the modulation index,. (As a general rule, any sidebands below 1% of the carrier can

be ignored.)

Determination of Bandwidth for FM Radio

FM radio uses a modulation index,> 1, and this is called wideband FM. As its name suggests thebandwidth is much larger than AM.

In national radio broadcasts using FM, the frequency deviation of the carrierfc , is chosen to be75kHz, and the information baseband is the high fidelity range 20Hz to 15kHz.

Thus the modulation index, is 5 and such broadcast requires an FM signal bandwidth given by:

kHz

ffBandwidth icRadioFM

180

)1575(2

)(2 (max)

=+=+=

Points to remember.

An FM transmission is a constant power wave, regardless of the information signal or modulation

index,, because it is operated at a constant amplitude with symmetrical changes in frequency.

As increases, the relative amplitude of the carrier component decreases and may become much

smaller than the amplitudes of the individual sidebands. The effect of this is that a much greater

proportion of the transmitted power is in the sidebands (rather than in the carrier), which is more

efficient than AM.

Signal Conditioning


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PC-based Data Acquisition System

In the last few years, industrial PC I/O interface products have becomeincreasingly reliable,

accurate and affordable. PC-baseddata acquisition and control systems are widely used in

industrial andlaboratory applications like monitoring, control, data acquisitionandautomated testing.

Selecting and building a DA&C (Data Acquisition and Control) systemthat actually does what

you want it to do requires some knowledge ofelectrical and computer engineering.

Transducers and actuators

Signal conditioning

Data acquisition and control hardware

Computer systems software

A data acquisition system consists of many components that are integrated to:

Sense physical variables (use of transducers)

Condition the electrical signal to make it readable by an A/D board

Convert the signal into a digital format acceptable by a computer

Process, analyze, store, and display the acquired data with the help of software


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Radio Transmission and Reception

For the propagation and interception of radio waves, a transmitter and receiver are

employed.

A radio wave acts as a carrier of information-bearing signals; the information may

be encoded directly on the wave by periodically interrupting its transmission (as in dot-

and-dash telegraphy) or impressed on it by a process called modulation.

The actual information in a modulated signal is contained in its sidebands, or

frequencies added to the carrier wave, rather than in the carrier wave itself.

The two most common types of modulation used in radio are amplitude modulation

(AM) and frequency modulation (FM). Frequency modulation minimizes noise and

provides greater fidelity than amplitude modulation, which is the older methodof broadcasting. Both AM and FM are analog transmission systems, that is, they

process sounds into continuously varying patterns of electrical signals which resemble

sound waves.

Digital radio uses a transmission system in which the signals propagate as discrete

voltage pulses, that is, as patterns of numbers; before transmission, an analog audio

signal is converted into a digital signal, which may be transmitted in the AM or FM

frequency range. A digital radio broadcast offers compact-disc-quality reception and

reproduction on the FM band and FM-quality reception and reproduction on the AM

band.

In its most common form, radio is used for the transmission of sounds (voice and

music) and pictures (television). The sounds and images are converted into electrical

signals by a microphone (sounds) or video camera (images), amplified, and used to

modulate a carrier wave that has been generated by an oscillator circuit in a transmitter.

The modulated carrier is also amplified, and then applied to an antenna that converts

the electrical signals to electromagnetic waves for radiation into space. Such waves

radiate at the speed of light and are transmitted not only by line of sight but also by

deflection from the ionosphere.


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radio communication jto lice study material sample

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