ELEMENTS OF A COMMUNICATION SYSTEM
transmitter
communications
receiver
channel or medium
information
source
The information
source which
produces a
message or
sequence of
messages
to be communicated
to the receiving
terminal - such as
Converts electrical
signal suitable for
transmission over a
given medium.
Made up of
Oscillators and Amplifiers
Tuned circuits and Filters,
Modulators, Frequency
mixers and synthesizers
The channel is the
medium to convey the
signal from the
transmitter to the
receiver. During
The receiver performs
the inverse operation
of the transmitter to
reconstruct the
message from the
transmission, or at one of the signal.
terminals, the signal may be
affected by noise.
microphone
Mediums:
Electrical conductors.Cables and Wires
Optical Media
Fibre Optics and Photo
Diode
Free Space
Radio, visible or infrared
Other Types of Media
Sonar, earth power lines.
destination
Made up of
Amplifiers and Oscillators,
Mixers, tuned circuits and
filters, and a demodulator
or detector
TYPES OF ELECTRONIC COMMUNICATIONS
Full Duplex :
Half Duplex :
Simplex :
Here coms. is
Two way coms.
This is one way
only one party at a both ways at
communication. AM
the same time.
and FM broadcasting time transmits.
Citizens band (CB), Such as the
TV broadcasting,
Family Radio, and telephone or
Fax, IR Remote,
Cell Phone
amateur radio
Paging services,
GPRS, Telemetry.
Baseband Transmission
Can be sent directly and unmodified over the medium or modulate a
carrier for transmission over the medium. In telephone or intercom
systems, the voice is placed on the wires and transmitted. Computer
networks, the digital signals are applied directly to coaxial or twistedpair cables for transmission.
Broadband Transmission
A broadband transmission takes place when a carrier signal is
modulated, amplified, and sent to the antenna for transmission via
space.
The destination is the
entity (human or
otherwise) for which
the message is
intended.
voice signal to speaker
video signal LCD screen
binary data to monitor
Attenuation
Signal attenuation, or
degradation, is inevitable
no matter what the
medium of transmission.
Attenuation is proportional
to the square of the
distance between the
transmitter and receiver.
Digital Signals:
Change in steps or in
discrete increments.
Such as Binary which
is ON or OFF
Analog Signals:
Smooth and
continuous varying
voltage or current.
Such a sine wave
used in audio and
video signals.
Modulation
1)Amplitude Modulation (AM) Amplitude is varied
2)Frequency Modulation (FM) Frequency is varied
3)Phase modulation (PM), Phase angle of the sine wave
is varied.
4)Frequency-shift keying (FSK) data is converted to
frequency-varying tones from a digital modulate signal
Multiplexing
Multiplexing is the process of allowing two or more signals
to share the same medium or channel.
The three basic types of multiplexing are:
1)Frequency division
2)Time division
3)Code division
Frequency
Number of cycles of a repetitive wave that occur in a given
period of time. A cycle consists of two voltage polarity
reversals, current reversals, or electromagnetic field
oscillations.
Frequency is measured in cycles per second (cps) - hertz
(Hz).
Wavelength (λ)
Distance occupied by one cycle of a wave and is usually
expressed in meters. Its also the distance traveled by an
electromagnetic wave during the time of one cycle.
Bandwidth
The portion of the electromagnetic spectrum occupied by
a signal.
Channel bandwidth refers to the range of frequencies
required to transmit the desired information.
Frequency Ranges from 30 Hz to 300 GHz
Wavelength (λ) = speed of light ÷ frequency
Speed of light = 3 × 108meters/second
Therefore : λ = 3 × 108/ f
1
2
1.2.1 Simplex
1.2.2 One way communication
from Tx to RX .
3
4
8
λ = 3×10 /150 x 10 = 2m
λ = 2.1×108/150 x 10 = 1.4m
Wavelength (λ)
= speed of light ÷ frequency
8
Speed of light = 3 × 10 m/s
5
Baseband information
6
Half Duplex
CB and Amateur Radio
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3
8
9
λ = 3 ×10 /1.5 x10 = 200km
λ = 3 ×10 /1.5 x10 = 0.013m
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Multiplexing
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De-Multiplexing
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1. Fibre Optics
2. Laser/Photo Diode
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Simplex such as
FM Radio broadcasting
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Simplex and Analogue
Half Duplex and Digital
AMPLITUDE MODULATION AND SINGLE-SIDEBAND MODULATION
In amplitude modulation (AM) the information signal varies the amplitude of the carrier
sine wave.
Amplitude modulation is widely used in radio. AM broadcast stations are, of course,
amplitude modulated. So are citizens band radios, aircraft radios, and the video
modulation of a TV broadcast transmitter.
Vm
Modulator
v
2
Vc
Modulation Index and % Modulation
(m) is relationship between the amplitude
of the modulating signal and the
amplitude of the carrier signal.
m = Vm/Vc
Also known as the modulating factor or
coefficient, or the degree of modulation.
Multiplying the modulation index by 100
gives the percentage of modulation.
Information signal
Carrier
Envelope is
identical to the
shape of the
information
signal
= (Vc + Vmsin2pfmt)sin2pfct
= Vcsin2pfct + Vmsin2pfmt(sin2pfct )
Vm
m = VC
m =
Vm = Vmax - Vmin
2
Vc = Vmax + Vmin
2
Vmax - Vmin
Vmax + Vmin
Over Modulation and Distortion
The modulation index should be a number between 0&1. If the amplitude of the modulating
voltage is higher than the carrier voltage, m will be greater than 1, causing distortion.
Sidebands and the Frequency Domain
Side frequencies, or sidebands are generated as part of the
modulation process and occur in the frequency spectrum
directly above and below the carrier frequency.
Single-frequency sine-wave modulation generates two
sidebands. Complex wave (e.g. voice or video) modulation
generates a range of sidebands.
fUSB = fc + fm
fLSB = fc − fm
Bandwidth is the difference
between the upper and lower
sideband frequencies.
BW = fUSB − fLSB
Example: AM broadcast station is
allowed to transmit modulating
frequencies ≤ 5 kHz. Transmitting on
a frequency of 980 kHz, what are
sideband frequencies and total
bandwidth?
fUSB = 980 + 5 = 985 kHz
fLSB = 980 – 5 = 975 kHz
BW = fUSB – fLSB
= 985 – 975 = 10 kHz
BW = 2 (5 kHz) = 10 kHz
AM Power AM signal is amplified by a power amplifier. The AM signal is a composite of the carrier and sideband signal
voltages. Each signal produces power in the antenna. Total transmitted power PT = Pc + PUSB +PLSB.
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PT = (IT) R Where IT is measured RF current and R is antenna impedance
The greater the percentage of modulation, the higher the sideband power and the higher the total power transmitted.
Power in each sideband : PSB = PLSB = PUSB = (Pc m2)/4 Maximum power when the carrier is 100% modulated.
An AM transmitter has a carrier power of 30 W. The percentage of modulation is 85 percent. Calculate (a) the
total power and (b) the power in one sideband.
Peak envelope power (PEP) In SSB, the transmitter output is expressed in terms of
peak envelope power (PEP), the maximum power produced on voice amplitude
peaks. PEP is computed by the equation P = V^2/R. VRMS = Vp-p x 0.707
SSB Single-Sideband Modulation In amplitude modulation, two-thirds of the transmitted power is in the carrier, which
conveys no information. Signal information is contained within the sidebands. A single-sideband suppressed carrier
(SSSC) signal is generated by suppressing the carrier and one sideband.
SSB signals offer four major benefits:
1) Spectrum space is conserved and allows more signals to
be transmitted in the same frequency range.
2) All power is channeled into a single sideband. This produces
a stronger signal that will carry farther and will be more reliably
received at greater distances.
3) Occupied bandwidth space is narrower and noise in the
signal is reduced.
4) There is less selective fading over long distances.
Disadvantages of SSB and DSB
1) Single and double-sideband are not widely used
because the signals are difficult to recover (i.e.
demodulate) at the receiver. A low power, pilot carrier
is sometimes transmitted along with sidebands in
order to more easily recover the signal at the
receiver.
Applications of DSB and SSB
A vestigial sideband signal (VSB) is produced by partially suppressing the lower sideband. This kind of signal is used in
TV transmission.
v
1
2
= (Vc + Vmsin2pfmt)sin2pfct
= Vcsin2pfct + Vmsin2pfmt(sin2pfct )
2.1.1 m = Vm/Vc = 2/1 = 2
2.1.2 1V
2.1.3 40kHz
2.1.4 2kHz
2
fUSB = FC + FM = 884kHz
fLSB = FC - FM = 776kHz
BW = fUSB − fLSB = 8kHz
3
The modulation index (m) describes the relationship between the amplitude of the modulating signal and the
amplitude of the carrier signal. m = Vm/Vc AKA: modulating factor or coefficient, or the degree of modulation.
4
2.2.2
8-2
VMAX - VMIN
m = VMAX + VMIN = 8+ 2
6.7kHz
.3kHz
3.3kHz
3.3kHz
6
0.6
2.2.3
250 Hz
250 Hz
=
VC
Vm
9
VMAX - VMIN
VMAX + VMIN
m
=
V
C
=
15
= 0.6
V
m
=
2
x
3V/div
=
2
x 3V/div
8-2
8+2
=
2 x 3 = 9V fUSB = 2800 + 3.3 = 2803.3 kHz
=
2 x3
= 15V
fLSB = 2800 - 3.3 = 2796.7 kHz
BW = fUSB - fLSB
= 2803.3 - 2796.7 = 6.6kHz
7
Mixers are used to shift signals from one frequency range to another, a process known as heterodyning, for
convenience in transmission or further signal processing. It acts as a simple amplitude modulator to produce sum
and difference frequencies. Output = fRF , fLO , fLO + fRF, fLO - fRF
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9
Distortion
2.3.1 Input fRF = 1100kHz, 1098kHz and 1102kHz (fc, fc - fm, fc + fm)
Output = fLO , fLO + fRF, fLO - fRF
or fRF - fLO (if fRF > fLO)
= 1555kHz, 2655kHz, 2653kHz and 2657kHz
and 455khz, 457kHz, 453kHz,
2.3.1
455khz, 457kHz, 453kHz
The higher frequency carrier wave
has the lower frequency information
signal modulated onto it. This is
done by varying the carriers
Amplitude(AM) or Frequency(FM)
according the information signal. It
then conveys (transmits ) this
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information through space. as an
electromagnetic wave. At the
11
It modifies the carrier by impressing the message onto the carrier. The carrier can be receiver the carrier is demodulated
from its information.
modified in terms of amplitude, frequency and phase.
12
400Hz
1020Hz
400Hz
The purpose of the carrier is to
transmit information through space as
an electromagnetic
14
wave.
Modulation takes speech information or data information called an input signal, and impose it on top of a higher frequency signal
called the carrier wave. Modulation changes the shape of a carrier wave to encode the speech or data information on it so it can
be propagated over a distance and then received to be read or decoded.
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15
Transmitter functions:
Receiver functions:
(1) to generate an RF carrier wave via an
(1) uses and antenna to capture a selected radio
oscillator circuit at a selected frequency
wave
(2) to amplify the carrier wave
(2) amplifies the captured RF signal
(3) to modulate the carrier wave with a
(3) tunes or selects the required frequence by
message(data or audio)
oscillating the same frequence
(4) to amplify the modulated signal
(4) demodulates the message from the carrier wave
(5) to couple the modulated signal to an antenna (5) amplifies the audio or data signal
(6) to radiate the signal into the atmosphere
(6) presents the signal in a readable form such as a
speaker
fUSB
= 2000 + 30 = 2030 kHz
fUSB range = 2000.02 kHz to 2030kHz
16
1970kHz
030kHz
000kHz
30kHz
2.2.3
30kHz
20Hz
fLSB
= 2000 - 30 = 1970 kHz
fUSB range = 1970kHz to 1999.98kHz
20Hz
0
0
AMPLITUDE MODULATION CIRCUITS
Amplitude Modulated Signal are produced by:
1) By multiplying the carrier by a gain or attenuation factor that varies with the
modulating signal.
2) By linearly mixing or algebraically adding the carrier and modulating signals and
then applying the composite signal to a nonlinear device or circuit.
Amplitude modulation voltage is produced by a circuit that can
multiply the carrier by the modulating signal and then add the carrier.
Low-level modulators Generate AM with small signals and must be amplified before transmission.
Linear Amplifiers
Antenna
{
With a low level AM Transmitter a high frequency Carrier signal is
generated by the Oscillator. The small audio signal generated by the Carrier
Oscillator
Amplitude
Microphone is amplified before its modulated in the Modulator onto
Modulator
the Carrier signal.. This modulated signal is then amplified several
times before its finally amplified by the Power Amp so it has enough
Audio Amp
power to be radiated through space by the Antenna.
Diode Modulator
Microphone
Consists of a resistive mixing network, a diode rectifier,
and an LC tuned circuit. The carrier is applied to one input
resistor and the modulating signal to another input
resistor. This causes the two signals to be linearly mixed .
A diode passes half cycles when forward biased. The coil
and capacitor repeatedly exchange energy, causing an
oscillation or ringing at the resonant frequency.
Transistor Modulator
Consists of a resistive mixing network, a transistor, and an LC tuned circuit.
The emitter-base junction of the transistor serves as a diode and nonlinear
device. Modulation and amplification occur as base current controls a larger
collector current. The LC tuned circuit oscillates (rings) to generate the
missing half cycle.
PIN Diode Modulator
Variable attenuator circuits using
PIN diodes produce AM at VHF,
UHF, and microwave frequencies.
PIN diodes are special type silicon
junction diodes designed for use at
frequencies above 100 MHz. When
PIN diodes are forward-biased, they
operate as variable resistors.
Attenuation caused by PIN diode
circuits varies with the amplitude of
the modulating signal.
Differential Amplifier
Differential amplifier modulators make excellent amplitude modulators
because they have a high gain, good linearity and can be 100 percent
modulated. The output voltage can be taken between two collectors,
producing a balanced, or differential, output. The output can also be taken
from the output of either collector to ground, producing a single-ended output.
The modulating signal is applied to the base of a constant-current sourc
transistor. The modulating signal varies the emitter current and therefore the
gain of the circuit. The result is AM in the output.
Final
Power Amp
High-level modulators They produce AM at high power levels, usually in the final amplifier stage of a transmitter.
Collector Modulator The collector modulator is a linear power amplifier that takes the low-level modulating signals and
amplifies them to a high-power level. A modulating output signal is coupled through a modulation transformer to a class
C amplifier. The secondary winding of the modulation transformer is connected in series with the collector supply
voltage of the class C amplifier.
High-Level AM: Series Modulator
It improves frequency response, but
very inefficient. It replaces the
modulation transformer (expensive)
with an emitter follower. The
modulating signal is applied to the
emitter follower. The emitter follower
is in series with the collector supply
voltage. The collector voltage
changes with variations in the
amplified audio modulating signal.
Calculations:
An AM transmitter has an efficiency of 75 %. This transmitter uses high level modulation of the final RF power amplifier,
which has a dc supply voltage of 45 V with a total current of 4 A. Note that for 100 percent modulation, AF modulating
power Pm is one-half the input power. Calculate: (a) The RF input power to the final stage. (b) The AF power required
or 100 percent modulation. (c) The carrier output power. (d) The power in one sideband for 65 percent modulation.
(e) The maximum and minimum dc supply voltage swing with 100 % modulation.
(e) Minimum swing = 0 V
Maximum swing =
2 (VCC ) = 2 x 45 = 90 V
Amplitude Demodulators
Demodulators, or detectors, are circuits that accept
modulated signals and recover the original modulating
information.
Balanced Modulator
A balanced modulator is a circuit that generates a DSB signal, suppressing the carrier and leaving only the sum and
difference frequencies at the output. The output of a balanced modulator can be further processed by filters or phaseshifting circuitry to eliminate one of the sidebands, resulting in a SSB signal. Types of balanced modulators include
lattice, 1496/1596 IC, and the analog multiplier.
Lattice Modulator (diode ring)
Consists of an input transformer, an output transformer and four diodes connected
in a bridge circuit. The carrier signal is applied to the center taps of the input and
output transformers. The modulating signal is applied to the input transformer.
The output appears across the output transformer. This is an expensive modulator
as the receiver requires the re-insertion of the carrier at the correct frequency and
phase complicating the receiver’s design.
IC Balanced Modulators: Analog Multiplier 1496/1596 IC
IIts a versatile circuit available for communication applications. It can work at carrier frequencies up to 100 MHz.
METHODS TO GENERATE SSB
The Filter Method (Simplest)
The modulating signal is applied to the audio amplifier. The
amplifier's output is fed to one input of a balanced modulator. A
crystal oscillator provides the carrier signal which is also applied
to the balanced modulator. The output of the balanced modulator
is a double-sideband (DSB) signal. An SSB signal is produced
by passing the DSB signal through a highly selective bandpass
filter. With the filter method, it is necessary to select either the
upper or the lower sideband.
Phasing Method
uses a phase-shift technique that causes one of the sidebands to be
canceled out. The phasing method uses two balanced modulators
which eliminate the carrier. The carrier oscillator is applied to the
upper balanced modulator along with the modulating signal. The
carrier and modulating signals are both shifted in phase by 90° and
applied to another balanced modulator. Phase-shifting causes one
sideband to be canceled out when the two modulator outputs are
added together.
DSB and SSB Demodulation
To recover the intelligence in a DSB or SSB signal, the
carrier that was suppressed at the receiver must be
reinserted. A product detector is a balanced modulator used
in a receiver to recover the modulating signal. Any balanced
modulator can be used as a product detector to demodulate
SSB signals.
CRYSTALS AND CRYSTAL FILTERS
Crystal filters are the most commonly used filters in SSB
transmitters.
They are low in cost and relatively simple to design.
They have a high quality factor (Q) and thus provide
extremely good selectivity.
Crystal filters are made from the same type of quartz
crystals normally used in crystal oscillators.
When a voltage is applied across a crystal, it will vibrate
at a specific resonant frequency.
R
C1
Co
L
1
1) By multiplying the carrier by a gain or attenuation factor that varies with the modulating signal.
2) By linearly mixing or algebraically adding the carrier and modulating signals and then applying the composite
signal to a nonlinear device or circuit.
2
{
Linear Amplifiers
Carrier
Oscillator
Amplitude
Modulator
Audio Amp
Microphone
3
Final
Power Amp
1) A high frequency Carrier signal is generated
by the Oscillator.
Antenna
2) The small audio signal generated by the
Microphone is amplified before its modulated
in the Modulator onto the Carrier signal..
3) This modulated signal is then amplified
several times
\4) Then its amplified by the Power Amp so it
has enough power to be radiated through
space by the Antenna.
A balanced modulator is a circuit that generates a DSB signal, suppressing the carrier and leaving only the sum and
difference frequencies at the output. The output of a balanced modulator can be further processed by filters or
phase-shifting circuitry to eliminate one of the sidebands, resulting in a SSB signal.
4
Amplitude modulation voltage is produced by a circuit that can multiply the
carrier by the modulating signal and then add the carrier.
6
3.3.1
Pi = VCC × I
= 48 × 0.6 = 28.8W
3.3.2
Pm = Pi /2
= 28.8/2 = 14.4W
7
Power Amplifier
8
Square law response curve
3.3.1
Pi = VCC × I = 40 × 4 = 160W
9
3.3.2
Pm = Pi /2 = 160/2 = 80W
3.3.3
% efficiency = Pout/Pin
Pout = % x Pin
= 0.85 x 160
=136W
3.3.4
2
2
Ps = Pc(m) = 160(0.75)
2
2
= 45W
3.3.5 Minimum swing = 0 V
Maximum swing = 2 x VCC
= 2 x 40 = 80 V
10
AGC circuits help maintain a constant output voltage level over a wide range of RF input signal levels; they also
help the receiver to function over a wide range so that strong signals do not produce performance-degrading
distortion.
11
1) It is impractical to convert the information signal directly to electromagnetic radiation.
2) So that smaller practical antennas can be used at higher frequencies.
Constant amplitude, frequency
12
and phase
13
FREQUENCY MODULATION AND CIRCUITS
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Basic Principles of Frequency Modulation
A frequency modulated signal is produced by varying the frequency of a
constant-frequency carrier in accordance with a modulating signal.
A FM signal is identified by the following properties:
The information signal varies the frequency of the carrier.
The amount of frequency change from the carrier centre frequency is
called the frequency (fd) deviation.
The deviation is proportional to the amplitude of the modulating signal.
During FM, the carrier amplitude remains constant.
Basic Principles of Phase Modulation
A phase-modulated signal is produced by varying the amount of phase
shift of a constant-frequency carrier in accordance with a modulating
signal.
FM and PM signals. The carrier is drawn as a
A PM signal is identified by the following properties:
triangular wave for simplicity, but in practice it
is a sine wave. (a) Carrier. (b) Modulating
The phase shift of the carrier is varied by the amplitude of the modulating
signal. (c) FM signal. (d) PM signal
signal.
The maximum frequency deviation occurs where the rate of change of the
modulating signal amplitude is greatest, which is at its zero-crossing
points.
A low-pass fi lter on the modulating compensates for increased frequency
deviation at the higher modulating frequencies.
PM to FM conversion
A phase modulated (PM) signal is made compatible with a
frequency modulated (FM) signal by compensating frequency
variations produced in the modulating signal. This is achieved
by passing the information signal through a low-pass filter.
The low-pass filter causes the higher modulating frequencies to be attenuated. This filter is also
known as a frequency-correcting network or 1/f filter. The result is an output that is the same as
an FM signal. The FM produced by a phase modulator is called indirect FM.
SIDEBANDS AND THE MODULATION INDEX
Modulation Index and Deviation Ratio
The ratio of the frequency deviation to the modulating frequency is known as the modulation
index (mf).
In standard FM broadcasting, the maximum permitted frequency deviation is 75kHz, and the
maximum permitted modulating frequency is 15 kHz. This produces a modulation index of 5.
For amateur FM transmissions, the deviation ratio is restricted by regulation to 5 kHz. The
highest audio used for voice communications is 3 kHz.
Sidebands
Any modulation process produces sidebands.
When a constant-frequency sine wave modulates a carrier, two side frequencies are produced.
Side frequencies are the sum and difference of the carrier and modulating frequency.
The bandwidth of an FM signal is usually much wider than that of an AM signal with the same modulating signal.
Bessel Functions
Is a standard table showing the number of significant sidebands for a specific
Modulated Frequency (fm). The carrier amplitude, number of sidebands and
their amplitudes are listed.
Significant sidebands are those that have an amplitude of greater than 1%
(.01) in the Bessel table. The symbol ! means factorial. This tells you to
multiply all integers from 1 through the number to which the symbol is
attached. (e.g. 5! Means 1 × 2 × 3 × 4 × 5 = 120).Narrowband FM (NBFM) is
any FM system in which the modulation index is less than π/2 = 1.57, or mf <
π /2
FM Signal Bandwidth
The higher the modulation index in FM, the greater the number of significant sidebands and the wider the
bandwidth of the signal.
Ÿ When spectrum conservation is necessary, the bandwidth of an FM signal can be restricted by putting an upper
limit on the modulation index.
mf = 6/3 = 2
Carson’s: BT
Example:
BW = 2fmN (check Bessel F.)
If the highest modulating frequency is 3 kHz and the
maximum deviation is 6 kHz, what is the modulation index?
Where N is the number of
significant* sidebands
BW = 2(3kHz)(4) = 24Khz
What is the Bandwidth?
Ÿ
FREQUENCY MODULATION VERSUS AMPLITUDE MODULATION
FM offers better noise immunity as it rejects interfering signals because of the capture effect and it provides
better transmitter efficiency. The disadvantage of FM lies in the fact that it occupies a wider frequency spectrum
that an AM signal.
Noise Immunity
One of the benefits of FM over AM is its superior noise immunity. Noise is any interference that disturbs the
legible transmission of a signal. Noise could be in the form of narrow spikes with very broad frequency content.
These spikes add to a signal and interfere with it. Noise is essentially amplitude variations. An FM signal has
constant carrier amplitude. Because of this, FM receivers
contain limiter circuits that are used to limit the amplitude of
the received signal. Any amplitude variations are effectively
clipped off without disturbing the information content of the
information signal.
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Capture effect
Interfering signals which are at the same frequency with the
twice the amplitude of the other, the stronger signal will “capture”
FM signals are effectively rejected. If one signal is more than
the channel and will totally eliminate the weaker, interfering signal. This is known as the capture effect. In AM, the
two signals are likely to interfere with each other, making the information signal unintelligible.
Preemphasis
Noise can interfere with an FM signal and particularly with the highfrequency components of the modulating signal.
Noise is primarily sharp spikes of energy and contains a lot of harmonics
and other high-frequency components.
To overcome high-frequency noise, a technique known as preemphasis
is used.
A simple high-pass filter can serve as a transmitter’s pre-emphasis
circuit.
Pre-emphasis provides more amplification of only high-frequency
components.
Deemphasis
A simple low-pass filter can operate as a deemphasis circuit in a
receiver.
A deemphasis circuit returns the frequency response to its normal flat
level.
The combined effect of preemphasis and deemphasis is to increase the
signal-to-noise ratio for the high-frequency components during
transmission so that they will be stronger and not masked by noise.
Advantages of FM
FM typically offers some significant benefits over AM.
FM has superior immunity to noise, made possible by clipper limiter circuits in the receiver.
In FM, interfering signals on the same frequency are rejected. This is known as the capture effect.
FM signals have a constant amplitude and there is no need to use linear amplifiers to increase power levels. This
increases transmitter efficiency.
Disadvantages of FM
FM uses considerably more frequency spectrum space with a wider bandwitdh than AM.
FM has used more complex circuitry for modulation and demodulation.
In the past, the circuits used for frequency modulation and demodulation involved were complex. With the
proliferation of ICs, complex circuitry used in FM has all but disappeared. ICs are inexpensive and easy to use.
FM and PM have become the most widely used modulation method in electronic communication today.
Transmission efficiency
FM signals are always generated at a lower level and then amplified by a series of class C amplifiers to increase
their power. The result of this is greater use of available power because class C amplifiers are far more efficient.
AM signals are generated at a lower level and are amplified by linear amplifiers that operate as class A or class B.
Class A and class B amplifiers are less efficient.
Sideband Power Calculations
If Vc = 2000V,
m = J0 J1 J2 J3 J4
2.0 = 0.22 0.58 0.35 0.13 0.03
VcJ0
VcJ1
VcJ2
VcJ3
VcJ4
= 0.22 x 2000V = 440V = V0
= 0.58 x 2000V = 1160V= V1
= 0.35 x 2000V = 700V = V2
= 0.13 x 2000V = 260V = V3
= 0.03 x 2000V = 60V = V4
1
1)The information signal varies the frequency of the carrier.
2)The amount of frequency change from the carrier centre frequency is called the frequency (fd) deviation.
3)The deviation is proportional to the amplitude of the modulating signal.
4)During FM, the carrier amplitude remains constant.
2
4.2.1 mf = 5kHz/2kHz = 2.5kHz There are 5 significant
4.2.1 BW = 2fmN (check Bessel F.)
sidebands with amplitude
BW = 2(2kHz)(4) = 20 kHz more than 0.01
3
1) FM has superior immunity to noise, made possible by clipper limiter circuits in the receiver.
2) In FM, interfering signals on the same frequency are rejected. This is known as the capture effect.
3) FM signals have a constant amplitude and there is no need to use linear amplifiers to increase power levels. This
0.58
4.12
0.58
increases transmitter efficiency.
0.35
4
0.24
4.11
0.13
0.13
0.24
0.03
0.03
0.03
0.35
0.22
0.94
0.03
-f3
-f2
-f1
fc
f1
f2
f3
L = 40uH
C = 330 pF + 50pF = 380pF
1
f0 =
2π LC
1
-6
-12
= 2π 40x10 x 380 x 10 = 1.29MHz
L = 40uH
C = 330 pF + 25pF = 355pF
1
f0 =
2π -6LC
-12
= 2π 40x10 x 355 x 10 = 1.34MHz
1)The phase shift of the carrier is varied by the amplitude of the modulating signal.
2)The maximum frequency deviation occurs where the rate of change of the modulating signal amplitude is
greatest, which is at its zero-crossing points.
3)A low-pass filter on the modulating compensates for increased frequency deviation at the higher modulating
frequencies.
-f2
6
7
8
-f1
fc
f1
f2
fd
m(t)=Vccos(2πfct + fm sin(2πfmt))
9
fd = mf x
3x2π
fd
3MHz
mf = fm
1250Hz
fm = 5 x 1250 = 6250Hz
2
Vrms (12x0.707)2
P=R =
10
= 7.2W
=5
10
The ratio of the frequency deviation to the modulating frequency is known as the modulation index (mf).
11
4.2.5 BW = 2fmN (check Bessel F.)
BW = 2(5kHz)(4) = 40 kHz
fd = mf x fm = 2 x 5kHz = 10kHz
BT = 2(fd+fm) = 25kHz
100MHz V 2
20002
P = 2R = 2x50 = 40kW
2
5kHz (0.5 x 2p = p
4
thus 0.5 x 10 )
2
2V 2(1160)2
PJ1 = 2R = 2x50 = 26.9kW
2V 2(60)2
PJ4 = 2R = 2x50 = 70W
12
In radio, a guard band is an unused part of the radio spectrum between radio bands, for the purpose of preventing
interference. It is a narrow frequency range used to separate two wider frequency ranges to ensure that both can
transmit simultaneously without interfering with each other. It is used in frequency-division multiplexing.
13
88 to 108 MHz = 20MHz with Bandwidth of 200kHz leaving space for 100 channels
RADIO TRANSMITTERS
OPERATION OF CW, AM, FM OR SSB TRANSMITTER
A transmitter is an electronic unit that processes the information signal and converts
it into an RF signal capable of being transmitted over long distances. Basically, a
transmitter performs the following functions:
(1) It must generate a carrier signal of the correct frequency.
(2) It must provide some form of modulation that causes the information signal to modify the carrier signal.
(3) It must provide sufficient power amplification to ensure that the signal level is high enough to carry over the
desired distance.
(4) It must provide circuits that match the impedance of the power amplifier to that of the antenna for maximum
transfer of power.
Single-Sideband (SSB) Transmitter
• Oscillator generates the
carrier.
• Carrier is fed to buffer
amplifier.
• Signal is applied to
balanced modulator.
• DSB signal fed to
sideband filter to select
upper or lower sideband.
• SSB signal sent to mixer
circuit.
• Final carrier frequency fed
to linear driver and power
amplifiers.
Power Amplifiers
The class of an amplifier indicates how it is biased.
Ÿ Class A amplifiers are biased so that they conduct continuously. The output is an amplified linear reproduction of
the input.
Ÿ Class B amplifiers are biased at cutoff so that no collector current flows with zero input. Only one-half of the sine
wave is amplified.
Ÿ Class AB linear amplifiers are biased near cutoff with some continuous current flow. They are used primarily in
push-pull amplifiers and provide better linearity than Class B amplifiers, but with less efficiency.
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Switching Amplifiers act like on/off or digital switches.
They effectively generate a square-wave output.
Harmonics generated are filtered out by using high-Q tuned circuits.
The on/off switching action is highly efficient.
Switching amplifiers are designated class D, E, F, and S.
Benefits of Class D and Class E power amplifiers
Reduced power waste as heat dissipation and hence:
Reduction in cost, size and weight of the amplifier due to smaller (or no) heat sinks, and compact
circuitry.
Very high power conversion efficiency.
Class C amplifiers
These amplifiers are used for power amplification in the form of drivers, frequency multipliers, and final amplifiers.
Biasing takes place on the positive cycle of the input signal.
The RC circuit allows for the transistor to conduct in short pulse during the negative cycles. Less than less than
180° of the input is conducted
The primary purpose of a tuned circuit is to form the complete AC sine-wave output.
The tuned circuit oscillates, at its resonant
frequency whenever it receives a DC pulse.
The pulse charges a capacitor, which then
discharges into an inductor.
The exchange of energy between the inductor
and the capacitor is called the flywheel effect
and produces a damped sine wave at the
resonant frequency.
Self-oscillation exists when some of the
output voltage finds its way back to the input
of the amplifier with the correct amplitude and
phase, and the amplifier oscillates.
This type amplifier makes a good frequency
multiplier as harmonics are generated in the
process.
OPERATION OF A FREQUENCY MULTIPLIER
The frequency multiplier is a special form of class C
amplifier.
Ÿ Any class C amplifier is capable of performing
frequency multiplication if the tuned circuit in the
collector resonates at the some integer multiple of the
input frequency.
Ÿ
Impedance Matching Networks
An impedance-matching network provides a match
between Zi and ZL .
The basic function of a matching network:
To provide for an optimum transfer of power through
impedance-matching technique.
Ÿ To provide filtering and selectivity.
Ÿ
LC NETWORKS
One of the simplest forms of impedance-matching networks is the L
network. It consists of an inductor and a capacitor connected in various Lshaped configurations.
Ÿ They are used as low- and high-pass networks.
Ÿ Low-pass networks are preferred because harmonic frequencies are
filtered out.
Ÿ The L-matching network is designed so that the load impedance is matched
to the source impedance.
Four L-type impedance-matching networks
(a) ZL< Zi
(b) ZL > Zi
(c) ZL < Zi
L-Type impedance-matching networks for RL < Ri and RL > Ri
RL < Ri
RL > Ri
Pi and t networks
BENEFITS OF SPEECH PROCESSING
Speech-processing circuits in a transmitter prevent over-modulation, prevent
excessive signal bandwidth and increase the average transmitted power in AM and
FM systems. It is performed digitally by converting the audio signal to digital form,
manipulating it in a digital signal processor and reconverting it to analog.
(d) ZL > Zi
1
2
1) To provide for an optimum transfer of power through impedance-matching technique.
2) To provide filtering and selectivity.
RL > Ri
3
5.3.2
4
@120MHz
(10)
RL < Ri
6
The carrier, which may initially be a sine wave, is applied
to a shaping circuit that effectively converts it to a square
wave. The carrier is usually frequency-modulated. The
square wave carrier signal is then applied to the base of
the class E bipolar power amplifier. The Q1 switches off
and on at the carrier rate. The signal at the collector is a
square wave that is applied to a low-pass filter and tuned
impedance-matching circuit made up of C1, C2, and L1.
The odd harmonics are filtered out, leaving a fundamental
sine wave that is applied to the antenna. A high level of
efficience is achieved with this arrangement.
7
Class A amplifiers are biased so that they conduct
continuously. The RF input from a 50Ω
source is connected to the base of T1 via an impedancematching circuit made up of C1,
C2, and L1. The output is matched to a 50Ω load by the
impedance-matching network
made up of L2, L3, C3, and C4.The bias is set so that the
input varies the collector (or
drain) current over a linear region of the transistor’s
characteristics. Thus its output is an
amplified linear reproduction of the input. Class A
amplifiers have a maximum efficiency of
50 percent.
Class B amplifiers are biased at cutoff so that no collector
current flows with zero
input. The transistor conducts on only one-half, or 180°,
of the sine wave input. This means
that only one-half of the sine wave is amplified. Here two
class B amplifiers are connected
in a push-pull arrangement so that both the positive and
negative alternations of the input
are amplified. The RF driving signal is applied to Q1 and
Q2 through input transformer
T1. It provides impedance-matching and base drive
signals to Q1 and Q2 that are
180° out of phase. An output transformer T2 couples the
power to the antenna or load.
Bias is provided by R1 and D1.
RADIO RECEIVERS
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Sensitivity and Selectivity
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.
A communication receiver must be able to identify and select a desired signal from the thousands of others
present in the frequency spectrum (selectivity) and to provide sufficient amplification to recover the modulating
signal (sensitivity).
A receiver with good selectivity will isolate the desired signal and greatly attenuate other signals.
A receiver with good sensitivity involves high circuit gain
Receivers must provide the sensitivity and selectivity that permit full recovery of the original signal.
Noise and types of noise
(a) External noise: noise generated from external sources, such as industrial, atmospheric,
terrestrial or space sources.
(b) Internal noise: noise generated within a communications receiver, such as shot
noise, transit time noise and white noise or Johnson noise or thermal noise.
1) Thermal Noise.
Components are conductors which offer resistance to current flow and in turn produce heat. This heat increases
the atomic motion in the conductor .This movement fluctuates the component’s resistance producing the
thermally random voltage we call noise.
2) Semiconductor Noise.
Electronic components such as diodes and transistors are major contributors of noise such as shot noise, flicker
noise and transit-time noise. Shot noise in electronic devices results from fluctuations of the electric current
when electrons traverse a gap such as the PN junction. Flicker noise, also occurs in resistors and conductors.
This disturbance is the result of minute random variations of resistance in the semiconductor material. Transit
time refers to how long it takes for a current carrier such as a hole or electron to move from the input to the
output. At high frequencies these transitions generate transit-time noise.
3) Inter-modulation Distortion.
Inter-modulation distortion results from the generation of new signals and harmonics caused by circuit nonlinearities.
thermal noise
vn = rms noise voltage
k = Boltzman's constant 1.38 Χ 10 J/K
T = temperature, K (°C+ 273)
B = bandwidth, Hz
R = resistance, Ω
Signal-to-Noise Ratio
The signal-to-noise (S/N) ratio, also designated SNR, indicates the relative strengths of the signal and the noise
in a communication system. The stronger the signal and the weaker the noise, the higher the S/N ratio. If the
signal is weak and the noise is strong, the S/N ratio will be low and reception will be unreliable. Communication
equipment is designed to produce the highest feasible S/N ratio. Signals can be expressed in terms of voltage or
power. The S/N ratio is computed by using either voltage or power values:
-23
Vs = signal voltage, Vn = noise voltage, Ps = signal power, Pn = noise power
Assume, e.g., that the signal voltage is 1.2 μV and the noise is 0.3 μV. The S/N ratio is 1.2/0.3 = 4. Most S/N
ratios are expressed in terms of power rather than voltage. For example, if the signal power is 5 W and the power
is 1.25 W, the S/N ratio is 5/1.25 = 4
The preceding S/N values can be converted to decibels as follows:
For voltage: dB = 20 log S/N = 20 log 4 = 20(0.602) = 12 dB
For power: dB = 10 log S/N = 10 log 4 = 10(0.602) = 6 dB
However, it is expressed, if the S/N ratio is less than 1, the dB value will be negative nd the noise will be stronger than
Quality factor and Bandwidth
The quality factor (Q) is the ratio of inductive reactance (XL) to resistance (R)
The quality factor (Q) of a tuned circuit is in fact a measure of how selective its
passband is compared to its centre frequency. where fr is the resonant frequency.
B is the bandwidth
The bandwidth (B) of a tuned circuit is a measure of the circuit’s selectivity. The
bandwidth of a tuned circuit is the difference between the upper and lower cutoff
frequencies that are located at the 3-dB (that is 0.707) points on the selectivity curve.
The bandwidth is determined by the resonant frequency ( fr ) and the quality factor
Shape factor
A measure of the steepness of the skirts or the skirt selectivity of a receiver is the
shape factor (SF). The shape factor is the ratio of the 60-dB-down bandwidth to the
6-dB-down bandwidth of a tuned circuit or filter. If the bandwidth at the 60-dB-down
is f4 - f3 and the bandwidth at the 6-dB-down is f2 - f1 , the shape factor is therefore
expressed as follows:
the signal.
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
SUPERHETERODYNE 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.
1. Antenna: picks up the weak signal and feeds
it to the RF amplifier
2. RF amplifier: provides some initial gain and
selectivity and also minimizes oscillator
radiation.
3. Mixer: converts RF into IF
4. Local oscillator: generates frequency that
gets mixed with the
incoming RF frequencies in the mixer stage.
5. The output of the mixer is an IF signal containing the same modulation as input RF signal but lower frequency selected and amplified
several times in IF stage.
6. The highly amplified IF signal is finally applied to the demodulator, which recovers the original modulating information.
7.The output of a demodulator is usually the original modulating signal with amplitude same as received signal. This ac signal, 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. This to maintain a constant output level.
8. Audio amplifier: amplifies the IF signal
9. Speaker: converts electrical signals into audio
FREQUENCY CONVERSION
Mixers accept two signals, the incoming signal ( fRF ) and
the local oscillator signal (fLO ).
Ÿ The output from the mixer will therefore consist of the
following signals: fLO ,fLO + fRF, fLO - fRF , or fRF - fLO
Ÿ The sum and difference signals are given by the following
expressions:
fIF = fLO + fRF , fIF = fLO - fRF or fIF = fRF - fLO
Ÿ
INTERMEDIATE FREQUENCY (IF) AND IMAGE FREQUENCIES
Image frequency is an RF signal that is spaced from the desired incoming signal by a frequency that is two times
the IF above or below the incoming frequency.
where fimage = image frequency fRF= desired signal frequency fIF = intermediate frequency
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
DUAL-CONVERSION
RECEIVERS
Another way to obtain
selectivity while
eliminating the image
FLO2 = FIF1 +FIF2
problem is to use a dual- FLO1 = FRF +FIF1
conversion
superheterodyne
receiver.
A typical receiver uses two mixers and local oscillators, so it has two IFs.
The first mixer converts the incoming signal to a high intermediate frequency to eliminate the images.
The second mixer converts that IF down to a much lower frequency, where good selectivity is easier to obtain.
AGC CIRCUITS
Automatic gain control is a feedback system that automatically adjusts the gain of the receiver based on the
amplitude of the received signal. Very low level signals cause the gain of the receiver to be high. Large input
signals cause the gain of the receiver to be reduced.
Here are some of the problems that are encountered in a receiver without AGC:
Tuning the receiver would be difficult. The volume control would have to be turned way up to receive weak
stations.
The received signal from any station is constantly changing as a result of changing weather and ionospheric
conditions.
To receive a signal under mobile conditions would be difficult. For instance, a standard broadcast AM car radio
would be almost unusable without a good AGC to compensate for the signal variation in different locations.
Squelch circuit
A squelch circuit provides a means of keeping the audio amplifier tuned off during
the time that noise is received in the background. When a signal appears at the input,
the audio amplifier is enabled. A practical example is in AM systems such as CB
radios where the noise level is relatively high and very annoying.
1
The frequency mixer does the actual heterodyning; it changes the incoming radio frequency signal to a higher or
lower, fixed, intermediate frequency (IF). The Mixer accept two signals, the incoming signal ( fRF ) and the local
oscillator signal (fLO ). From this it creates an intermediate frequency (IF) which is the sum and difference of the
oscillator frequency and original incoming signal. fIF = fLO + fRF , fIF = fLO - fRF or fIF = fRF - fLO
2
6.2.1 Input fRF = 1100kHz, 1098kHz and 1102kHz (fc, fc - fm, fc + fm)
Output = fLO , fLO + fRF, fLO - fRF
or fRF - fLO (if fRF > fLO)
= 1555kHz, 2655kHz, 2653kHz and 2657kHz
and 455khz, 457kHz, 453kHz,
3
6.2.2 455khz, 457kHz, 453kHz
Input fRF = 1000kHz, 1001kHz
and 999kHz (fc, fc - fm, fc + fm)
Output = fLO , fLO + fRF, fLO - fRF
or fRF - fLO (if fRF > fLO)
= 1455 kHz,
454 kHz, 455 kHz, 456 kHz;
2454 kHz, 2455 kHz, 2456 kHz
455khz, 453kHz, 454kHz
4
5
V1 = 4kTB1R
4.2.1
V2 = 4kTB2R
All stay constant accept B
V1
B1
V2 = B2
B2
V2 = V1 x B1
2
( (
4.2.2 V1
V1
G1 = 4kTB1R
G1 = 4kTB1R
=
4kTB1
= 1 x 10 x
-3
2
(V1/G1)
R
-3
3
5 x 10
20 x 10 3 = 5mV
2
(1x 10 /60)
=
4(1.38x10-23 )(80+273)(20x103 ) = 712.8kΩ
6
NB! 10logP = 20logV
Vn = 4kTBR
2
(Vn) = 4kTBR
-6 2
2
(Vn)
(0.11x10 )
3
T = 4kBR = 4(1.38x10-23 )(10x10 )(75) = 292
T = 292 - 273 = 19ºC
7
Automatic gain control is a feedback system that automatically adjusts the gain of the receiver based on the
amplitude of the received signal. Very low level signals cause the gain of the receiver to be high. Large input signals
cause the gain of the receiver to be reduced.
8
9
A communication receiver must be able to identify and select a desired signal from the thousands of others present
in the frequency spectrum (selectivity) including noise.
The receiver selectivity performance determines the level of interference that may be experienced, and therefore it
is very important that the selectivity enables sufficient rejection of signals on other frequencies to be achieved to
enable interference free operation.
10
Components are conductors which offer resistance to current flow and in turn produce heat. This heat increases the
atomic motion in the conductor .This movement fluctuates the component’s resistance producing the thermally
random voltage we call noise.
11
6.5.1 Si = 120dB
No = 8dB
=
=
120/10
10
8/10
10
12
=
=
10
6.31
12
SNR(dB) = 10log(10 /6.31) = 112dB
9
12
6.5.2 SNR ratio: 10 /6.31 = 158.5x10 W
12
6.1.1 RF amplifier: receives the minute signal from the antenna and provides some initial gain and selectivity and
also minimizes oscillator radiation.
6.1.2 The mixer receives an input from the RF amplifier and the 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
6.1.3 While the mixer is the first detector stage of the receiver the second detector is the IF amplifier.
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. 20Mhz
13
6.2.1 fIF1 = fLO1 - fRF = 30 - 20 = 10MHz
6.2.2 fLO2 = fIF1 + fIF2 = 1 0 + 1 = 11MHz
14
Industrial, extraterrestrial and
atmospheric
15
External noises can only be avoided by changing the location of the receiver or changing the frequency to a less
affected one. A receiver with a good selectivity will isolate the desired signal in the RF spectrum and eliminate or at
least greatly attenuate all other signals.
16
The sun
.
17
1) Thermal Noise. Components are conductors which offer resistance to current flow and in turn produce heat. his
heat increases the atomic motion in the conductor .This movement fluctuates the component’s resistance producing
the thermally random voltage we call noise.
2) Semiconductor Noise. Electronic components such as diodes and transistors are major contributors of noise
such as shot noise, flicker noise and transit-time noise. Shot noise in electronic devices results from fluctuations
of the electric current when electrons traverse a gap such as the PN junction. Flicker noise, also occurs in resistors
and conductors.
The two major sources of noise in transistors
3) Inter-modulation Distortion.
when electrons crossing junctions at high
Inter-modulation distortion results from the generation of new
speeds are shot noise due to current flow of
signals and harmonics caused by circuit non-linearities.
carriers in the base and thermal noise due to
device resistance.
18
19 The signal-to-noise (S/N) ratio, indicates the relative strengths of the signal and the noise in a communication
system. The stronger the signal and the weaker the noise, the higher the S/N ratio. If the signal is weak and the
noise is strong, the S/N ratio will be low and reception will be unreliable. Communication equipment is designed to
produce the highest feasible S/N ratio.
ANTENNAS, TRANSMISSION LINES AND RADIO WAVE PROPAGATION
Electromagnetic waves are signals that oscillate, that is,
the amplitudes of the electric and magnetic fields vary at a
specific rate. The oscillations may occur at a very low
frequency or at an extremely high frequency. This entire
range of frequencies is known as the electromagnetic
spectrum
TRANSMISSION LINES
A transmission line is a two-wire cable that connects the transmitter to the antenna or the antenna to the receiver.
The purpose of the transmission line is to carry the RF energy for the desired distance. The two most common
types of transmission lines are
(a) Balanced line - Neither wire is connected to ground.
(b) Unbalanced line - One conductor is connected to ground
The two primary requirements of a transmission line are:
1) The line should introduce minimum attenuation to the signal.
2) The line should not radiate any of the signal as radio energy.
Wavelength
Wavelength is the distance that an ac wave travels in the time required for one cycle of that signal.
Mathematically, wavelength (λ) is expressed as
where c = velocity of electromagnetic wave m s
f = frequency Hz
The velocity of electromagnetic wave (c) in free space is equal to 3×10^8 m/s .
Velocity factor
The speed of the signal in the transmission line is slower than the speed of a radio signal in free space. The
difference of these speeds leads to the development of a velocity factor (VF), which is the
ratio of the transmission speed (VL) in the transmission line and the transmission speed (VT) .
The velocity factor of a coaxial cable is typically 0.6 to 0.8, that is, between 60% and 80%. In
order to maintain consistency and for ease of assessment, use a factor of 0.70 in your calculations
Characteristic impedance
The RF generator connected to the transmission line sees
impedance that is a function of the inductance and
capacitance in the circuit. This impedance is known as the
characteristic impedance (Z0 ).
Characteristic impedance is also
referred to as surge impedance .
Standing waves
The magnitude of the standing waves on a transmission line is determined by the
ratio of the maximum current (Imax )to the minimum current (Imin) along the line, or the
ratio of the maximum voltage (Vmax) to the minimum voltage (Vmin) . These ratios are
referred to as standing wave ratios.
When a line is properly terminated,
(1) There would be no standing waves.
(2) The voltage and current are constant along the line, ie the maximums and minimums are the same.
(3) The SWR is 1 (ideal).
ALSO If the impedance of the transmission line and the actual impedance of the load are
known, SWR can be calculated. SWR is the ratio of the load impedance (ZL) to the
characteristic impedance (Z0)vice versa.
If the incident voltage (Vi) and the reflected voltage (Vr)
are known, reflection coefficient (Γ) is calculated as follows:
Similarly, if the maximum voltage (Vmax) and the minimum voltage (Vmin) are known,
reflection coefficient (ﾃ) is calculated as follows:
To obtain the SWR from the refection coefficient, use the following equation:
The reflection coefficient can also be derived from the line and load impedances as
follows:
Line of sight communications by direct or space waves Transmitting distance with direct waves is a function of the
height of the transmitting and receiving antennas. The expression to compute the distance between a transmitting
antenna and the horizon is d = 2ht
where ht is the height of the transmitting antenna, in feet d is the distance
from the transmitter to the horizon, in miles Optical Range: The distance between transmitting and receiving
antennas could be given by the expression D = 2ht + 2hr In the above expression, hr is the height of the
receiving antenna in feet. In MKS units, the above expression could be modified to D = 4.12 ( ht + hr ) where h is
the height of the transmitting antenna, in meters h is the height of the receiving antenna, in meters d is the distance
from the transmitter to the horizon, in kilometers.
ANTENNA FUNDAMENTALS
What is an antenna?
In simple terms one would define an antenna as an
electromechanical device used to transmit or receive radio waves.
What are radio waves?
Radio waves, also called electromagnetic waves, are made up of
both electric and magnetic fields. An electric field is generated when
voltage is applied to an antenna. The voltage results in current
flowing through the antenna, producing a magnetic field. The electric
and magnetic fields are orthogonal or mutually perpendicular (ie at
right angles) to each other.
Polarization
The orientation of magnetic and electric fields with respect to earth is
referred to as polarization. Parallel (horizontal)to the earth- horizontally polarized.
If an electromagnetic wave is perpendicular to the earth, - vertically polarized.
The near field describes the region directly around the antenna where the electric and magnetic fields are
distinct. The far field is approximately 10 wavelengths from the antenna. It is the radio wave with the composite
electric and magnetic fields.
Dipole antenna Hertz antenna
It was earlier discovered that if a parallel-wire transmission line is left open,
the electric and magnetic fields escape from the line and radiate into space.
The radiation from a transmission line can be improved by bending the
transmission line conductors at a right angle to the transmission.
Optimum radiation occurs if the segment of transmission wire is one
quarter wave
length long at the operating frequency. This makes an
antenna that is one-half wave
length long.
Half-wave dipole antenna
An antenna is a frequency-sensitive device. To get the dipole to resonate at the frequency of operation, the
physical length must be shorter than the one-half wavelength computed by λ = 492/f. Actual length is related to
the ratio of length to diameter, conductor shape, Q, the dielectric (when the material is other than air), and a
condition known as end effect. The ratio of Electric to magnetic fields is a constant in free space and is called the
wave impedance and is 377 Ω. The fields are radiated at the speed of light (c), of 3x10^8 m/s .
Antenna Quality factor (Q) and Bandwidth
The bandwidth of an antenna is determined by the frequency of operation and the Q of the antenna according to
the relationship BW = fr/Q. The higher the Q, the narrower the bandwidth. For an antenna, low Q and wider
bandwidth are desirable so that the antenna can operate over a wider range of frequencies with reasonable
SWR. In general, any SWR below 2:1 is considered good in practical antenna work.
Antenna terminology
Antenna gain: The ratio of the focused power transmitted (Pt) to the input power (Pi)
of the antenna. When expressed in decibels of power gain, it is
Effective radiated power: The total amount of power radiated by an antenna. The ERP
is calculated by multiplying the transmitter power(Pt) fed to the antenna by the power gain
(Ap)of the antenna
a) Folded Dipole - VHF FM Broadcasting
b) Loop antenna - Direction Finding
c) Log-periodic antenna- HF
Communications with directivity and gain
d) Yagi–Uda antenna - High gain
directional used to receive TV signals
Antenna impedance matching
The maximum amount of power should be transferred from the transmitter to the antenna. For this to happen, the
characteristic impedance of the transmission line must match the output impedance of the transmitter and the
impedance of the antenna itself. In other words, the SWR should be 1.
In reality, to have a perfect match between an antenna, transmission line and the transmitter is not possible.
However, the following techniques can be used to match impedances:
1 antenna tuner or antenna matching
2 using a matching stub or Q section (a quarter-wave transformer)
3 using a balanced-unbalanced (balun) coax
The following relationship is used to match the impedances using a quarter-wave
transmission line:
where ZQ is the characteristic impedance of quarter -wave
matching stub or Q section. Z0 is the characteristic impedance of
transmission line or transmitter at the input of a Q section ZL is the
impedance of load, usually antenna feed point impedance
1
7.1.1 a theoretical point source that radiates electromagnetic energy in all directions. In reality, no practical antennas
radiate isotropically; instead, the radiation is concentrated into a specific pattern.
7.1.2 Beamwidth: The measure of antenna’s directivity or the angle of the radiation pattern over which a transmitter’s
energy is directed or received or the angular separation between two half-power points (3 dB points).
2
To calculate the gain with respect to an isotropic radiator (dBi), add 2.15 dB
to the gain over the dipole. dBi = dBd + 2.15 = 6 + 2.15 = 8.25 dBi
3
7.3.1d =4.12 ( ht ) = 15.96km
.
7.3.2 D = 4.12( ht + hr )
= 4.12( 15 + 1.5)
= 21 km
7.4.1 BW = 0.05 x 24
= 1.2Mhz
7.4.2 Fm = BW/2 =0.6kHz
24Mhz ± 0.6kHz
7.4.3 22.8Mz to 25.2Mhz
4
For example, if the bandwidth of a 24-MHz dipole antenna is given as 4 percent, the bandwidth can be calculated
as 0.04 x 24 = 0.96 MHz (960 kHz). The operating range of this antenna, then, is the 960-kHz bandwidth centered
on 24 MHz. This gives upper and lower frequency limits of 24 MHz ± 480 kHz or one-half the bandwidth. The
operating range is 23.52 to 24.48 MHz, where the antenna is still close to resonance. The field strength will
decrease the further away
from antenna
E =
√ 30 Pt
d
Z0 = √ μ0/ 0
-12
= √1.26 x 10-6/ 8.85 x 10
= 377Ω
5
6
7
1) Ground Wave < 1MHz uses ground to propagate 2) Sky Wave 3-30Mhz
3) Line of Sight > 30Mhz uses line of sight propagation
Uses ionosphere to bounce wave
8
The orientation of magnetic and
electric fields with respect to earth
is referred to as polarization.
9
Dipole or Hertz antenna.
1/2λ wave length
10(1) There would be no standing waves.
(2) The voltage and current are constant along the line, ie the maximums and minimums are the same.
(3) The SWR is 1 (ideal).
Loss can be expressed in decibels
(dB) in terms of power as in the
following expression where Po is the
output power Pi is the input power
Table 7.1 provides the attenuation of 7.8 dB/30.48 m = attenuation per meter = 0.256 dB/m.
A cable of 20 m has an attenuation of 0.256 × 20 = 5.12 dB
11
12
Non Termination is undesired and results in standing waves along the transmission
line which magnifies transmission line losses (significant at higher frequencies and
for longer cables). The SWR is a measure of the depth of those standing waves and
is, therefore, a measure of the matching of the load to the transmission line
7.3.1
13
8
8
7.3.2 C = 3x10 x0.71 = 2.13x10
6
8
= 2.13x10 /27x10 =7.9m
7.3.3
Zout = 50Ω
Y
14
The orientation of magnetic and electric fields with respect to earth is referred to
as polarization. Parallel (horizontal)to the earth- horizontally polarized. If an
electromagnetic wave is perpendicular to the earth, - vertically polarized.
15
16
17
VHF Transmissions
TV Reception
Direction Finding
7.2.1 The electric and magnetic fields are
orthogonal or mutually perpendicular (ie
at right angles) to each other
7.2.2
18
For antenna applications, low Q, and hence wider bandwidth, is
desirable so that the antenna can operate over a wider range of
frequencies with reasonable SWR. A rule of thumb is that any SWR
below 2:1 is considered good in practical antenna work.
19
Radio waves, also called electromagnetic waves, are made up of both electric and magnetic fields. An electric field
is generated when voltage is applied to an antenna. The voltage results in current flowing through the antenna,
pproducing a magnetic field. The electric and magnetic fields are at right angles to each other
20
SWR = 75/93 = 0.81
r = (93-75)/(93+75) = 0.107
21
m
m
Power Desity:
Pt
Pd = 4pd²
10
= 4p(36000)²
-10
= 6.14 x 10 W
22
23
24
When a wave deflects while going through an object changing angle
Diffraction refers to the "bending of waves around an edge" of an object
When a wave bounce of a surface substance
25
Surface and Space waves, Horizontally
26
PAc = Pd x Ac -10
6
= 6.14 x 10 x 2.5 x10
= 1.54mW
5.3.1
150
150
5.3.3 Prefl = r² x Pout
= 0.5² x 200W = 50W
5
5.3.2
Power Absorbed PLoad:
PLoad = Pout - Prefl
= 200W - 50W = 150W
5
5
3
DATA TRANSMISSION
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
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Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
Ÿ
DIGITAL COMMUNICATIONS CONCEPTS
Digital signal
Digital signals are binary pulses that have two distinct states, each represented by a voltage level. The pulses
switch rapidly between these two levels. One level is referred to as a binary 0 or low, and the other as a binary 1
or high. Please note that any other voltages can be used to represent binary pulses.
American Standard Code for Information Interchange(ASCII)
The most widely used data communication code is the 7-bit binary code known as the
ASCII code can represent 128 numbers, letters, punctuation marks, and other
symbols. 1001010
ASCII code combinations are available to represent both uppercase and lowercase
letters of the alphabet.
Several ASCII codes have two- and three-letter designations which initiate operations
or provide responses for inquiries.
ASCII letter “K” =01001011
Hexadecimal Values.
Binary codes are often expressed by using their hexadecimal, rather than
decimal, values. To convert a binary code to its hexadecimal equivalent, first
divide the code into 4-bit groups, starting at the least significant bit on the
right and working to the left. (Assume a leading 0 on each of the codes.)
1. The ASCII code for the number 4 is 0110100. Add a leading 0 to make 8
bits and then divide into 4-bit groups: 00110100 = 0011 0100 = hex 34. 2.
The letter w in ASCII is 1110111. Add a leading 0 to get 01110111; 01110111 =
0111 0111 = hex 77.
Serial Transmission
Data can be transmitted in two ways: Parallel and Serial
Data transfers in long-distance communication systems are made serially.
In a serial transmission, each bit of a word is transmitted one after another.
Parallel data transmission is not practical for long-distance communication.
Expressing the Serial Data Rate
The speed of data transfer is usually indicated as number of bits per second (bps or b/s).Data rate (bits/sec)=1/Tb
Another term used to express the data speed in digital communication systems is baud rate.
Baud rate is the number of signaling elements or symbols that occur in a given unit of time.
A signaling element is simply some change in the binary signal transmitted. ASCII letter “M” = 01001101
Asynchronous Transmission
In asynchronous transmission each data word is accompanied
by start and stop bits that indicate the beginning and ending of
the word.
When no information is being transmitted, the communication
line is usually high, or binary 1.
In data communication terminology, this high level is referred to as a mark.
To signal the beginning of a word, a start bit, a binary 0 or space is transmitted.
Most low-speed digital transmission (the 1200- to 56,000-bps range) is asynchronous.
Asynchronous transmissions are extremely reliable.
The primary disadvantage of asynchronous communication is that the extra start and stop bits effectively slow
down data transmission.
Synchronous Transmission
The technique of transmitting each data word one after
another without start and stop bits, usually in multiword
blocks, is referred to as synchronous data transmission.
To maintain synchronization between transmitter and
receiver, a group of synchronization bits is placed at the
beginning and at the end of the block.
Each block of data can represent hundreds or even thousands of 1-byte characters.
The special synchronization codes at the beginning and end of a block represent a very small percentage of the
total number of bits being transmitted, especially in relation to the number of start and stop bits used in
asynchronous transmission.
Synchronous transmission is therefore much faster than asynchronous transmission because of the lower
(b)
(a)
overhead
Assume we wanted to transmit the decimal number 210. This can be
represented in binary by the 8-bit number 11010010.
(a) Sketch a digital signal to represent serial transmission of this number, if
each bit is 1 ms.
(b) If two bits are transmitted per 1 ms interval, sketch a digital signal to
(c)
represent the transmitted signal.
Hint: There are four possible combinations of 2 bits, and the discrete levels re
assigned as follows: 00 = 0 V, 01 = 1 V, 10 = 2 V and 11 = 3 V.
(c) Determine (i) The bit rate if each bit interval is 1 ms.
(ii) The baud rate if each bit interval is 1 ms.
(iii) The bit rate if two bits are transmitted per 1 ms bit interval.
(iv) The baud rate if two bits are transmitted per 1 ms bit interval.
Determine channel capacity of a 6 MHz
CHANNEL BANDWIDTH AND DATA RATE
channel with a SNR ratio of 25 dB.
Channel capacity (C) is twice the channel bandwidth (B). Channel capacity is then Solution: The 25 dB power should be
converted to a power ratio of
determine by the following expression: C = 2B where B = bandwidth [Hz]
The channel capacity can be modified by taking into consideration multiple-level
encoding schemes (or signalling elements), denoted as M, that permit more bits per
symbol or baud to be transmitted. The new expression becomes C = 2Blog2M
The above expression says that for a given bandwidth, the channel capacity in bits
per second can be higher if there are more than two levels or other symbols per time
interval. The relationship between channel capacity, bandwidth and noise is
summarized in what is known as the Shannon-Hartley theorem, illustrated by the
expression
where C = channel capacity b/s
B = bandwidth Hz
SNR = signal power to noise power ratio
MODEM: FUNCTION AND APPLICATION
A Modem is a device that converts binary signals to
analog signals capable of being
transmitted over telephone and cable TV lines and by
radio and then demodulate such analog signals,
reconstructing the equivalent binary output. Essentially, a Modem contains both a modulator and a demodulator.
There are four widely used modem types:
It enables transmission of digital data over cables and wireless links.
• Conventional analog dial-up modems.
TYPES OF MODEMS AND OPERATION
• Digital subscriber line (DSL) modems.
• Cable TV modems.
There are various modulation and demodulation techniques that are used in
• Wireless modems.
modern data communications Modems. These include frequency-shift keying
(FSK), phase shift keying (PSK) and quadrature amplitude modulation (QAM).
Modulation for Data Communication: Frequency-Shift Keying (FSK)
In FSK, two sine-wave frequencies are used to represent binary 0s and 1s.
A binary 0, usually called a space, has a frequency of 1070 Hz.
A binary 1, referred to as a mark, is 1270 Hz.
These two frequencies are alternately transmitted to create the serial binary data.
TYPES OF COMMUNICATIONS PROTOCOLS AND ERROR DETECTION AND ERRORCORRECTION SCHEMES
When high-speed binary data is transmitted over a communication link, whether it is a cable or radio,
errors will occur. These errors are changes in the bit pattern caused by interference, noise or
equipment malfunctions. Such errors will cause incorrect data to be received. To ensure reliable
communication, schemes have been developed to detect and correct bit errors. The number of bit
errors that occur for a given number of bits transmitted is referred to as the bit error rate (BER) The
main objective in error detection and correction is to maximize the probability of 100 percent accuracy.
Error detection and correction
Error detection just identifies that a bit (or bits) has been received in error. Error correction corrects
errors at a far-end receiver. Both require a certain amount of redundancy to carry out the respective
function. Redundancy, in this context, means those added bits or symbols that carry out no other
function than as an aid in the error-detection or error-correction process.
(a) Parity check
With the 7-bit ASCII code, a bit was added for parity, making it an 8-bit code. It is also referred to as
vertical redundancy checking (VRC). There are two types of parity check, even parity and odd parity.
Both systems are based on the number of marks or 1s in a 7-bit character. The eighth bit is appended
accordingly, either a 0 or a 1.
(b) Cyclic redundancy check
The cyclic redundancy (CRC) is a mathematical technique that is used in synchronous data
transmission. It is said to be able to catch 99.95 percent of transmission errors.
In CRC, each string of bits (binary data) is represented by a polynomial function, M(x). M(x)
is also known as a message function. A data block or frame is placed in memory (shift
registers). M(x) is divided by a special code called the generating function, G(x). The
process yields a quotient function, Q(x), and a remainder function, R(x). The remainder,
which is the CRC block check code (BCC), is appended to the end of the message. This is
called systematic code, where the BCC and the
message are transmitted as separate parts within the transmitted code. At the
receiver, the message and CRC are checked by characters passing through its
block check register, BCR. In the receiver, the frame is stored, and then divided
by the same generating polynomial G(x). The calculated remainder is compared
to the received remainder. If the values are the same, the message is error-free
(contains no errors). If they are not, there is at least one bit in error in the
message signal. Cyclic block codes are expressed as (n, k) cyclic codes,
where n = length of the transmitted code k = length of the message
Block check code (BCC) = n – k
(a) Append the eighth bit to
the following ASCII bit
sequence to implement
even parity: 1010010.
(b) Suppose we use odd
parity and transmit the
same character as in (a).
What would be the new bit
sequence?
Solution:
(a) There are three 1s,
making it an odd number.
Thus a 1 is appended as
the eighth bit to make it an
even number. The new bit
sequence is 10100101.
(a) There are three 1s,
making it an odd number.
Thus a 0 is appended as
the eighth bit to make it an
odd number. The new bit
sequence is 10100100.
What does (7, 4) cyclic code mean? Hence,
determine BCC. Answer:
The bit length of the transmitted code is 7
bits (n = 7) and the message length is 4 bits
(k = 4). BCC = n – k = 7 – 4 = 3 The BCC is
used at the receiver to determine if the
transmitted message contains an error.
For a (7, 4) cyclic code and given a
message polynomial M(x) = (1 1 0 0) and
a generator polynomialG(x) = x3 + x +1 ,
determine the BCC
(a) Mathematically.
(b) Using the CRC code generator circuit.
Solution: The code message M(x) defines
the length of the message (4 bits). The
number of shift registers required to
generate the block check code is
determined from the highest order in the
generating polynomial G(x), which is x3 .
Three shift registers will thus be required.
Therefore three zeros will be needed to pad
the message (1 1 0 0) to get (1 1 0 0 0 0 0).
BCC = n – k = 7 – 4 = 3
The modified code word is now divided by
the G(x). This is called modulo-2 division.
The remainder 010 is attached to M(x) to
complete the transmit code. The
transmit code word is 1100010.
(c) Hamming Code
Hamming code is a set of error-correction codes that can be used to detect and correct the errors that can occur
when the data is moved or stored from the sender to the receiver.
Redundant bits
Redundant bits are extra binary bits that are generated and added to the information-carrying bits of data transfer
to ensure that no bits
were lost during the data transfer. The number of redundant bits can be calculated using the
n
following formula: 2 ≥ m+ n +1 where, n = redundant bit, m = data bit
Parity bits
A parity bit is a bit appended to a data of binary bits to ensure that the total number of 1’s in the data are even or
odd. Parity bits are used for error detection. There are two types of parity bits:
Even parity bit: For a given set of bits, count 1 . If count is odd, the parity bit value is set to 1, even = 0
Odd Parity bit: For a given set of bits, count 1 . If count is odd, the parity bit value is set to 0, even = 1
How to calculate: 10011010 (Even)
1) Draw table
1
2 3 4 5 6 7 8 9 10 11 12 13
P1 P2 D1 P4 D2 D3 D4 P8 D5 D6 D7 D8
position:
1)
0
1
0
0
1
1
0
1
0
Check Positions for
P1: 1,3,5,7,9,11.... = 10111 = Even = 0
2)
0
1
1
0
0
1
1
0
1
0
Check Positions for
P2: 2,3,6,7,10,11....= 10101 = Odd = 1
3)
0
1
1
1
0
0
1
1
0
1
0
Check Positions for
P4: 4,5,6,7,12,13....= 001
4)
0
1
1
1
0
0
1
1
0
1
0
Check Positions for
P8: 8-15,24-31 .... = 0101 = Even = 0
0
= Odd = 1
Fixing an error bit: lets say code becomes 10011110 (don’t count Parity bits)
Now redo 1) P1 = 10111 = Even = 0 Correct,
2) P2 = 10111 = Even = 0 Wrong
3) P4 = 1001 = Odd = 1 Correct
4) P8 = 1101 = Odd = 1 Wrong P2 and P8 is wrong 2 + 8 = 10 Bit position 10 is 1 should be 0
Communications Networks
A network is any interconnection of two or more stations that wish to communicate.
Each station in a communications network is a node. Figure 8.2 shows an example of a
simple network with four nodes. Each node is connected to the other by a link.
There are 4 types of Networks:
(a) WANs: Wide-Area Networks : cover a complete country, for example, a telephone
system.
(b) MANs: Metropolitan-Area Networks: medium-sized networks, for example, a local
cable television system.
(c) LANs: Local-Area Networks : interconnect multiple stations over a very small area, for
example, networks where offices are connected within the same building or different
Network topologies
floors of the building.
(d) PANs: Personal-Area Networks is a short-range wireless network that is set up
automatically
between two or more devices such as laptop computers
or cell phones.
Internet applications
Internet applications are too numerous to itemise. Some of the Internet
applications are World Wide Web, e-mail, fi le transfer, podcasting, e-commerce, searches,
voice over Internet protocol (VoIP), video over Internet protocol (VoIP), and chat.
How does the Internet Work?
The Internet works through a packet routing network in accordance with the Internet Protocol (IP), the Transport Control Protocol
(TCP) and other protocols. The Internet is made up of a massive network of specialized computers called routers. Each router's
job is to know how to move packets along from their source to their destination. A packet will have moved through multiple routers
during its journey. When a packet moves from one router to the next, it's called a hop.
What’s a protocol?
A protocol is a set of rules specifying how computers should communicate with each other over a network. The Internet Protocol
which specifies how computers should route information to other computers by attaching addresses onto the data it sends.
What’s a packet?
Data sent across the Internet is called a message. Before a message is sent, it is first split in many fragments called packets.
What’s a packet routing network?
It is a network that routes packets from a source computer to a destination computer. The Internet is made up of a massive
network of specialized computers called routers. Each router’s job is to know how to move packets along from their source to
their destination. A packet will have moved through multiple routers during its journey.
1
8.2.1
2
8.2.2
V (2)
3
(a) WANs: Wide-Area Networks : cover a complete country, for example, a telephone system.
(b) MANs: Metropolitan-Area Networks: medium-sized networks, for example, a local cable television system.
(c) LANs: Local-Area Networks : interconnect multiple stations over a very small area, for example, networks
where offices are connected within the same building or different floors of the building.
4
5
6
The bit length of the transmitted code is 7 bits (n = 7)and the message length is 4 bits (k = 4). BCC= n – k = 7– 4 = 3
The BCC is used at the receiver to determine if the transmitted message contains an error.
BCC= n – k = 7– 4 = 3
8.3.1Baud rate is the number of signaling elements or symbols that occur in a
given unit of time.
8.3.2 A signaling element is simply some change in the binary signal transmitted.
7
-6
Bit Rate = 1 / Tb = 1/ 70 x 10 = 14286 bps
8
Convert 26dB power to S/N ratio: S/N = 10
(26/10)
= 398.1
6
Now: C = B log 2(1 + S/N) = 15 x 10 log 2(1+ 398.1)
6
6
= 15 x 8.6 x 10 = 129 x 10 bps
Base 2 log 0f 399.1
log399.1
P =
log2
=
8.6
9
8.2.1
BW = f2 - f1
= 2.2725GHz - 2.2675GHz = 5MHz
Base 2 log 0f 19.2
log19.2
P =
log2
8.2.2
SNR (dB) = 10 log (Ps/Pn)
SIGNAL TO NOISE IN RATIO
12.6dB/10 =
log (SIGNAL TO NOISE IN RATIO)
(SIGNAL TO NOISE IN RATIO) = 10 12.6/10 = 18.2
=
4.26
6
8.2.3 C = B log 2(1+SNR) = 5 x 10 x log2(1+18.2) = 21 Mbps
signalling levels
0
21/2x5
C/2B
8.2.4 C = 2B log2M thus C/2B = log M and M = 2
= 2
= 4.28 ≈ 4
2
10
FALSE
The special synchronization codes at the beginning and end of a block represent a very small percentage of the
total number of bits being transmitted, especially in relation to the number of start and stop bits used in
asynchronous transmission. Synchronous transmission is therefore much faster than asynchronous transmission
because of the lower overhead.
11
Tb = 1/ Bit Rate = 1/ 14400 = 69 us.
12
8.1.1
1 2 3
1 1 0
4
1
5
0
6
0
7 8 9
0 1 1
ODD PARITY :
Parity Bits : For P1 = 1, 3, 5, 7, 9
:
For P2 = 2, 3, 6, 7
:
For P4 = 4, 5, 6, 7
:
For P8 = 8, 9,
:
ODD
?0001
?000
?000
?1
=
=
=
=
=
NEW CODE : 1 1 0 1 0 0
8.1.2 ODD PARITY : ODD = 1
1 2 3 4 5 6 7 8 9
1 0 0 0 0 1 1 0 1
ODD PARITY :
ODD =
Parity Bits : For P1 = 1, 3, 5, 7, 9
: ?0011 =
For P2 = 2, 3, 6, 7
: ?000 =
For P4 = 4, 5, 6, 7
: ?000 =
For P8 = 8, 9,
: ?1
=
NEW CODE :
1 0
0
0
0
0
0
0
1
1
0
0
VS
1
0
1
1
1
0
1 0
1 x
1
1
1 x
}
1
0 x
0 x
0
} P2 + P4
P1 + P8 = 9
0
VS
= 6
1
8.1.2
13
14
8.1.1 J = 10 = 1100 = 100 1100 ASCII
LSB
0 0 1
1
0
0
1
1
0
0
1
0
0
1
1
0
0
MSB
1 0 0 1 0
Time
15
The code message M(x) defines the
length of the message (4 bits). The
number of shift registers required to
generate the block check code is
determined from the highest order in
the generating polynomial G(x), which
is x3 . Three shift registers will thus
be required. Therefore three zeros
will be needed to pad the message (1
1 0 0) to get (1 1 0 0 0 0 0).
BCC = n – k = 7 – 4 = 3
The modified code word is now
divided by the G(x). This is called
modulo-2 division.
The remainder 010 is attached to M(x) to
complete the transmit code. The transmit
code word is 1100010.