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LICE-JTO
Radio Communication Systems
1
LICE-JTO
Limited Internal Competitive Examination
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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LICE-JTO
Radio Communication Systems
2
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 90-94
Sample 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.
© 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
LICE-JTO
3
Radio Communication Systems
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
-10
km
Term
ELF
VLF
•
30-300
kHz
3003000
KHz
3-30
MHz
30-300
MHz
3003000
MHz
3-30
GHz
30-300
GHz
10
1000
100
10
100
10
10
-1 km
-100 m
-10 m
-1 m
-10 cm
-1 cm
-1 mm
LF
MF
HF
VHF
UHF
SHF
EHF
Radio Communication has three main problems:
–
Frequency assignments up 60 GHz
-100
km
Fixed services, Fixed satellite services, Mobile
services, Remote sensing
Wavelength
Broadcasting TV, satellites, Personal telephone
systems, radar systems, fixed and mobile satellite
services
100
Broadcasting, TV, FM, Mobile services for
maritime, aeronautical and land, Wireless
microphones, Meteor burst communication
1000
Fixed point to point communication, Mobile
maritime aeronautical, land services, military
communication, amateur radio and broadcasting
3-30
kHz
AM broadcasting, navigation, radio beacons, and
distress frequencies.
3003000
Hz
Frequency
Long distance communication (fixed and marine),
Broadcasting, Navigation, Radio beacons
Time and Frequency Normal, Navigation,
Underwater Communication, Remote sensing
underground, Maritime telegraphy
Application
The path loss
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LICE-JTO
Radio Communication Systems
–
Noise
–
Sharing the radio spectrum
4
•
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, also
called a low-noise amplifier (LNA).

RF amplifiers provide some initial gain and selectivity and are sometimes called preselectors.

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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LICE-JTO
Radio Communication Systems

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.
5
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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LICE-JTO
Radio Communication Systems

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.
6
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 local
oscillator 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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LICE-JTO
Radio Communication Systems
 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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LICE-JTO
Radio Communication Systems
Modulation
 Modulation = adding information (e.g. voice) to a carrier electromagnetic (radio) signal
Digital modulation techniques
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LICE-JTO
Radio Communication Systems
Frequency Modulation (FM) / Amplitude Modulation (AM)
Radio frequency interference
Radio signal attenuation (path loss)
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LICE-JTO
Radio Communication Systems
10
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 a
communication 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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Radio Communication Systems
= 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:
m=
•
Em
Ec
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 m  m12  m22    
Bandwidth
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Radio Communication Systems
12
 Signal bandwidth is an important characteristic of any modulation scheme
 In general, a narrow bandwidth is desirable
 Bandwidth is calculated by:
B  2Fm
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
 m2 

Pt  Pc1 
2 

Quadrature AM and AM Stereo
 Two carriers generated at the same frequency but 90º out of phase with each other allow
transmission 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:
PEP 
Vp
2
2 RL
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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Radio Communication Systems
13
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 symbol fc. This sets the dynamic range (i.e. voltage range) of
the 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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Radio Communication Systems
14
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:

f c
fi
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 the
bandwidth is much larger than AM.
In national radio broadcasts using FM, the frequency deviation of the carrier fc , is chosen to be
75kHz, 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:
BandwidthFM
Radio
 2(f c  f i (max) )
 2(75  15)
 180kHz
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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Radio Communication Systems
15
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 acquisition
andautomated 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 Communication Systems
16
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 dotand-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 method
of 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 Systems
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