walkie talkie
DESCRIPTION
TRANSCRIPT
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WIRELESS TRANSCEIVERS:
WALKIE TALKIE
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OUTLINE----------------------------
• Brief History
• General Walkie Talkie Structure
• Modulation and Modulation Types
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A LITTLE HISTORY----------------------------------------
• Some Similarities Between Communication Methods in History
• The Story of The Walkie-Talkie
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Similarities between communication methods in history
• 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.
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• 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.
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• 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.
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The Story of The Walkie-Talkie-----------------------------------• 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.
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• 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.
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• 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.
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General Structure of Walkie Talkie
----------------------------------------• HOW TO WORK ?
• TYPES OF COMMUNICATION
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HOW TO WORK ?• 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 :
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amplifier modulator
amplifier
Transducer (Microphone)
Transducer (Loudspeaker)
demodulator
Air medium Channel
Rx antenna
Tx antenna
Block diagram of radio communication system
Source of information
User of information
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• 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.
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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
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• 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
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MODULATION---------------------------------------
• WHAT IS THE MODULATION ?• WHAT IS THE DEMODULATION ?
• TYPES OF MODULATION • AM vs FM
-> Advantages of AM
-> Advantages of FM• CARRIER FREQUENCY BANDS• FREQUENCY ALLOCATION
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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
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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
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TYPES OF MODULATION
Modulation
Analog Digital
Continuous Wave (CW)
(1) Amplitude Modulation (AM) (2) Frequency Modulation (FM)(3) Phase Modulation (PM)
Pulse
(1) Pulse Amplitude Modulation (PAM) (2) Pulse Width Modulation (PWM)(3) Pulse Position Modulation (PPM)
Continuous Wave (CW)
(1) Amplitude Shift Keying (ASK) (2) Phase Shift Keying (PSK)(3) Frequency Shift Keying (FSK)
Pulse
(1) Pulse Code Modulation (PCM) (2) Differential PCM (DPCM)(3) Delta Modulation (DM)
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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.
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• 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
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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
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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.
.
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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.
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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
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• 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.
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