4. • There were some methods for communications
like communication via smoke , mors alphabet,
walkie talkie etc.
• All of them has some receivers, transmitters , and
more important than all of them , has some
signals with some frequencies.
5. • Let’s compare the communication via smoke
with walkie talkie.
• In the smoke communication , telling a situation to the
other people depends on the time interval between
smokes. Different intervals for different situations. This
means there are some frequencies for some situations and
if there are frequencies there is a signal . This signal is
called the smoke signal.
6. • In the mors alphabet, walkie talkie or the other
modern communication systems , there are
electrical signals with some frequencies.
• Shortly , the most important similarity is the
signals and frequencies.
7. • Walkie Talkies were first patented
in 1938 by Al Gross. They were first
used by the military only most likely
because they were so expensive and
very large and heavy for the average
person to buy and use.
8. • In fact, someone needed to carry part of the walkie talkie on their
back because it was so large. It is not so clear who should get
credit for the invention of the device.
• Around the same time Al Gross was working on his model,
Donald Hings was working on his version of the walkie talkie
which he called a “packset”. Hings invention was used during
War World Two starting in 1942 and was very important in the
war effort.
9. • After World War Two the use of the hand held radio
spread throughout different public sectors. With more
compact designs police and fireman began to rely more on
the devices for communication. Later walkie talkie use
moved from public to private sectors and to everyday use
for the average person or even toys for children.
10.
11. • The walkie talkie communication system consists of two
receiver and two transmitter circuits. One receiver and one
transmitter circuits are for each walkie talkies. The general
structure of this system :
13. • A transducer is a device that converts a signal
in one form of energy to another form of energy.
Microphone and the loudspeaker are the
transducers.
• After the sound is converted to the electrical
signal, amplifier amplifies it in the transmitter
circuit. Modulation is the carrying the message
signal with the carrier signal . Modulator makes
this duty.
• In the receiver circuit , demodulator seperates the
message signal from modulated signal , and
amplifier increases the message signal to be
heard easily. Loudspeaker converts the electrical
signal to the sound.
• Transducer and the receiver must be in the same
frequency interval to communicate each other.
14. Types of Communication
A B
Simplex – A can talk to B
Radio, T.V. broadcasting, CD/DVD ROM
Simplest type, requires one transmitter and one receiver
Simplex
Duplex – A and B both can talk to each other simultaneously
Telephone, Telegraph
Complex, requires two transmitter and two receiver at both ends
Needs two different channels for simultaneous transmission
A B
Duplex
15. • A transceiver is a small unit that combines a transmitter and a
receiver
• A small hand-held unit of transceiver is popularly called a
walkie-talkie
• The usual controls on the small transceivers unit are off-on
switch with volume control, push-to-talk button, squelch control
(eliminates background noise) and jack for earphones
A B
Half-Duplex
Half-Duplex – A and B can both talk to each other but not simultaneously
Fax, CD/DVD RW
Needs one single channel for transmission
Compromise between two, don’t require separate transmitter and receiver
Same antenna and circuitry may be used for both transmission and reception
16.
17. WHAT IS THE MODULATION ?
• Modulation is the process of superimposing a
signal (message signal ) on a high frequency
signal (carrier signal ) to transmit easily.
s(t)
The wave you
want to transmit
c(t)
A wave that can be
transmitted
+
Signal Carrier r(t)
The actual wave
that is transmitted
=
Modulated signal
t t t
18. DEMODULATION
• Demodulation is the opposite of the modulation.
Demodulation is used in receiver. When the
modulated signal is come into the receiver , the
demodulator seperate the message signal from the
modulated signal.
r(t)
The received signal at
demodulator
Modulated signal s(t)
Output of the
demodulator
Original Signal
t t
Demodulation
20. AM vs FM
• In amplitude modulation (AM) , the message signal m(t)
is impressed on the amplitude of the carrier signal c(t).
Modulation is done with changing the amplitude of carrier
signal.
• In frequency modulation (FM) , the frequency of the
carrier is changed according to the variations in the
message signal.
21. • QUESTION IS WHAT TYPE OF
MODULATION SHOULD BE USED TO
TRANSMIT THE SIGNAL THROUGH THE
LONG DISTANCES ???
• IN ORDER TO ANSWER THIS
QUESTION , THE ADVANTAGES FM
AND AM MUST BE EXAMINED
22. ADVANTAGES OF FM
• Resilient to the noise : AM waves do not have constant
envelopes and therefore more affected by static or noise
than FM. Unwanted electromagnetic waves do not cause the
frequency of FM carrier wave to change.
• Resilient to signal strength variations
• Does not require linear amplifiers in the transmitter
• Enables greater efficiency than many other modes
23. ADVANTAGES OF AM
It is simple to implement
it can be demodulated using a circuit consisting of very few
components
AM receivers are very cheap as no specialized components
are needed.
.
24. SHORTLY ;
• Frequency modulated signal has wider coverage
than AM radio
• Frequency modulated signal has better sound
quality than AM radio
That’s why FM is more preferred than the
AM in walkie talkies.
25. Carrier Frequency bands
Name Freq.
Range
Wave
length
Application Propagation
ELF 300Hz
to 3kHz
100 km
to 1000km
Navigation, long distance
communication with
ships
Wave tube between earth surface and
the ionosphere
VLF 3kHz
to 30kHz
10km
to 100km
Navigation, long distance
communication
Ground propagation, stable
LF 30kHz
to
300kHz
1km
to 10km
Navigation, long distance
communication with
ships
Ground propagation, stable
MF 300kHz
to 3MHz
100m
to 1km
AM broadcasting, radio
navigation
Ground-wave, sky-wave propagation.
Fading
HF 3MHz
to
30MHz
10m
to 100m
Radio broadcasting,
fixed point-to-point
(around the world)
Large perturbation, reflection in
ionosphere
VHF 30MHz
to
300MHz
1m
to 10m
Radio and TV
broadcasting, mobile
services
Diffraction
UHF 300MHz
to 3GHz
10cm
to 100cm
Cellular telephony (GSM,
NMT, AMPS), digital TV,
fixed point-to-point,
satellite, radar
Shadowing by mountains and buildings
SHF 3GHz
To
30GHz
1cm
to 10cm
Broadband indoor
systems, microwave
links, satellite
communications
Attenuation due to rain, snow and fog
EHF 30GHz
to
300GHz
1mm
to 10mm
LOS communication
(short distance or
satellite)
Attenuation due to rain, snow and gases
26. • The carrier waves frequencies for radio broadcasting are assigned by
Federal Communications Commission (FCC)
FREQUENCY ALLOCATION
• Use of radio frequency bands of the electromagnetic spectrum is regulated
by governments in most countries, in a spectrum management process
known as frequency allocation or spectrum allocation.
• Radio propagation does not stop at national boundaries. Giving
technical and economic reasons, governments have sought to harmonise
the allocation of RF bands and their standardization.
