WIRELESS TRANSCEIVERS:

WALKIE TALKIE

OUTLINE----------------------------

• Brief History

• General Walkie Talkie Structure

• Modulation and Modulation Types

A LITTLE HISTORY----------------------------------------

• Some Similarities Between Communication Methods in History

• The Story of The Walkie-Talkie

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.

• 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.

• 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.

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.

• 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.

• 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.

General Structure of Walkie Talkie

----------------------------------------• HOW TO WORK ?

• TYPES OF COMMUNICATION

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 :

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

• 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.

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

• 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

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

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

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

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)

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.

• 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

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 

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.

.

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.

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

• 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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