Transcript high level modulation transmitters

Government Engineering
College, Godhra
SUBJECT :
Audio and Video
System
GEC GODHRA

1.
2.
3.
Prepared By :
Chaudhary shailesh N.
Chauhan Rajani V.
Thakor Pruthviraj P.
130600111013
130600111016
130600111053
Guided By: Krisnkant Sir
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Block diagram of communication
system
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TRANSMITTER
Information or
Message
Transducer
Transmitter
Communication
Channel or
Medium
Information
in Electrical form
• It takes the information to be communicated
in electrical form and convert it into an
electronic
signal
compatible
with
the
communication medium .
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TRANSMITTER

In this block diagram of communication
system, the upper section is called the
transmitting section.
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TRANSMITTER



The main parts of transmitter are explained
as follows :
Microphone : It converts sounds into
electrical signals in wires. It is the opposite
of a loudspeaker.
Modulator : The audio signal is modulated
into the radio frequency carrier in this
modulator stage.
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TRANSMITTER


Frequency generator : The frequency
generation stage will decide the frequency
on which the transmitter will operate.
RF power amplifier : The power
amplification of the radio signal is carried
out in the final stage. It makes the signal
stronger so that it can be transmitted
through the channel over long distances.
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TRANSMITTER

An antenna is a transducer which
converts electrical signals into
electromagnetic waves.
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BASIC BLOCKS OF TRANSMITTER

Modulator

RF oscillator

Power amplifier
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BASIC FUNCTIONS OF TRANSMITTER

Modulation
Carrier generation
Amplification (Power)

It is an electronic unit which accepts the


information
signal
to
be
transmitted
and
converts it into an RF signal capable of being
transmitted over long distances . GEC GODHRA
BASIC FUNCTIONS OF TRANSMITTER
Every transmitter has three basic functions as
follows:

The transmitter must generate a signal of correct
frequency at a desired point in the spectrum.

Secondly it must provide some
modulation to modulate the carrier.

Third it must provide sufficient power
amplification in order to carry the modulated
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signal to a long distance.
form
of
CLASSIFICATION OF RADIO
TRANSMITTERS
1.
According to the type of modulation used.
2.
According to service involved.
3.
According to the frequency range involved.
4.
According to the power used.
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CLASSIFICATION BASED ON
TRANSMITTED FREQUENCY

Low frequency (LF) transmitters (30 KHZ300KHZ)

Medium frequency (MF) transmitters ( 300 KHZ3 MHZ)

High frequency (HF) transmitters (3 MHZ30MHZ)
GEC GODHRA
CLASSIFICATION BASED ON
TRANSMITTED FREQUENCY

Very high frequency (VHF) transmitters
(30MHZ-300 MHZ)

Ultra high frequency (UHF) transmitters
(300 MHZ- 3GHZ)

Microwave transmitters (>3GHZ)
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CLASSIFICATION BASED ON TYPE
OF SERVICE INVOLVED

Radio broadcast transmitters.

Radio telephony transmitters.

Radio telegraph transmitters.
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CLASSIFICATION BASED ON TYPE
OF SERVICE INVOLVED

Television transmitters.

Radar transmitters.

Navigational transmitters.
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CLASSIFICATION BASED ON TYPES
OF MODULATION

CW Transmitters

AM Transmitters
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FM Transmitters
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SSB Transmitters
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CONTINIOUS WAVE (CW)
TRANSMITTERS

The CW Transmitter is the simplest type of
transmitter.

It is a simple crystal oscillator circuit.

This oscillator generates a carrier signal of the
desired frequency.
GEC GODHRA
CONTINIOUS WAVE (CW)
TRANSMITTERS

Information to be transmitted is expressed in a
special form of code using dots and dashes to
represent letters of the alphabet and numbers.

The information transmitted in this way is called
as continuous wave (CW) transmission.
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CONTINIOUS WAVE (CW)
TRANSMITTERS

The key is a simple hand operated switch
connected in emitter of the transistor.

By closing the key we can turn on the crystal
oscillator on and by opening the key the
oscillator is turned off.
GEC GODHRA
CONTINIOUS WAVE (CW)
TRANSMITTERS

When the key is closed, the oscillator produces
a sinusoidal signal at a frequency equal to the
crystal frequency, whereas with the key open,
the output of oscillator is zero.

The key is opened and closed in order to
produce zero output and dots or dashes.
GEC GODHRA
CONTINIOUS WAVE (CW)
TRANSMITTERS

Dots correspond to the short duration output
whereas a dash corresponds to a long
duration output.

The required messages can be transmitted
using different combinations of dots and
dashes for different alphabets and letters.
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ADVANTAGES OF CW TRANSMITTER

Simple to construct

Compact and portable

Can be operated on batteries
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DISADVANTAGES

A skilled operator is required to convert the
message to be sent into a coded form of dots
and dashes.

Long distance communication is not possible.

Voice or picture can not be sent.
GEC GODHRA
AM TRANSMITTER
Amplitude modulation technique is used in AM
transmitters, here the amplitude of carrier is varied
in proportion with the amplitude of the modulating
signal, keeping its frequency and phase constant.


Used in radio & TV broadcasting.
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AM TRANSMITTER

In AM Transmitter, AM signal is transmitted
by a transmitter. The information is contained
in its amplitude variation.
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TYPES OF AM TRANSMITTERS

Low Level modulation transmitters.

High Level modulation transmitters.
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LOW LEVEL MODULATION
TRANSMITTERS
Signal
Source
(Modulating
Signal)
Low Level
Modulator
Power
Amplifier
(Linear)
RF Carrier
Oscillator

The generation of AM wave takes place at a low power
level.

The generated AM signal is then amplified using a chain
of linear amplifier ( A , AB or B).
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LOW LEVEL MODULATION TRANSMITTERS
Antenna
STABILISED
RF
OSCILLATOR
AF
modulating
signal
Class A
Buffer
amplifier
Audio
processing &
filtering
Modulator
Linear
Amplifiers
Power
Amplifiers
Class A AF
amplifier
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LOW LEVEL MODULATION
TRANSMITTERS


The RF oscillator produces the carrier signal.
The RF oscillator is stabilized in order to
maintain the frequency deviation within the
prescribed limit. The carrier frequency is
equal to the transmitter frequency.
Usually the transmitter operates on assigned
frequencies or channels. Crystal provides the
best way to obtain the described frequency
with good stability.
GEC GODHRA
LOW LEVEL MODULATION
TRANSMITTERS

We cannot use the LC oscillator because
they have low frequency stability.

The carrier signal from the crystal oscillator is
applied to the modulator with a modulating
signal. At the output of the modulator we get
the AM wave.
GEC GODHRA
LOW LEVEL MODULATION
TRANSMITTERS

The modulating signal is obtained from a
source such as a microphone and applied to
a buffer processing unit.

The buffer is a class A amplifier which
isolates the AF source from the rest of high
power circuit and amplifies it to an adequate
level.
GEC GODHRA
LOW LEVEL MODULATION
TRANSMITTERS

The amplified modulating signal is applied to
the modulator along with the carrier. At the
output of the modulator we get the AM wave.

The AM signal is then amplified using a chain
of linear amplifier to raise the power level.
GEC GODHRA
LOW LEVEL MODULATION
TRANSMITTERS

The linear amplifier can be class A, AB or B
type amplifiers. The linear amplifier are used
in order to avoid the wave form distortion in
AM wave.

The amplitude modulated signal is then
transmitted using transmitted antenna.
GEC GODHRA
LOW LEVEL MODULATION
TRANSMITTERS

The transistorized modulator circuits can be
used for low level modulator due to the low
power which is to be handled.

The low level transmitter does not require a
large AF modulator power so its design is
simplified.
GEC GODHRA
LOW LEVEL MODULATION
TRANSMITTERS

Overall efficiency is much lower compared to
high level modulation . This reduce to the
use of less efficient linear amplifiers.
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AUDIO PROCESSING
 The
AF modulating signal is passed
through an audio processing unit before
applying it to the modulator.
 This block carries out some form of
“speech processing” in the form of filtering
and amplitude control.
 The weak signals amplified automatically
with a higher gain and strong signals are
amplified with smaller gain. This will bring
all the signals to a sufficient level. GEC GODHRA
HIGH LEVEL MODULATION
TRANSMITTERS
Signal
Source
(Modulating
Signal)
Wide Band
Power
Amplifier
High Level
Modulator
RF Carrier
Oscillator

Narrow Band
Power Amplifier
The generation of AM wave takes place at high power
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levels.
HIGH LEVEL MODULATION
TRANSMITTERS

Highly efficient class C amplifier are used in
high level modulation.

Efficiency is more than low level modulation.
GEC GODHRA
HIGH LEVEL MODULATION TRANSMITTERS
Antenna
Stabilized RF
crystal
oscillator
Class A RF
amplifier
AF
modulating
signal
Audio
processing &
filtering
Class C RF
power
amplifier
Class A AF
amplifier
High Level
Modulator
Class B AF
power
amplifier
GEC GODHRA
HIGH LEVEL MODULATION
TRANSMITTERS

Many of the AM transmitters use the high level
modulation technique.

The crystal oscillator produces the required
carrier signal. The class A amplifier following the
oscillator acts as a buffer which isolates the
oscillator from the high power circuit.
GEC GODHRA
HIGH LEVEL MODULATION
TRANSMITTERS


The output of this class A amplifier is applied
to a class C power amplifier. It raises the
power level of the carrier to an intermediately
high value.
The AF modulating signal is applied to the
audio processing unit which processes this
signal as discussed in the previous section.
GEC GODHRA
COMPARISION BETWEEN LOW-LEVEL
AND HIGH-LEVEL MODULATION
1.

2.

Power level :
Modulation is carried
out at low power level.
Amplifier stages:
Need lesser amplifier
stages.

Modulation is carried
out at high power
level.

Need more amplifier
stages.
GEC GODHRA
COMPARISION BETWEEN LOW-LEVEL
AND HIGH-LEVEL MODULATION
3.

4.

Power efficiency :
After modulation linear
amplifiers can only be
used. This gives lower
power efficiency.
Power losses :
Power losses in
amplifiers is higher,
the cooling problem is
severe.

Non linear amplifiers
can also be used. This
leads to higher power
efficiency.

Power losses is less,
the cooling problem is
not severe.
GEC GODHRA
COMPARISION BETWEEN LOW-LEVEL
AND HIGH-LEVEL MODULATION
:
5.
Applications

Used as higher power
broadcast transmitters.

Used in TV
transmitters.
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FM TRANSMITTERS

Frequency modulation technique is used.

In FM frequency of the carrier is varied in
proportion with the amplitude of the modulating
signal keeping its amplitude constant.
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FM TRANSMITTERS

Used in radio, TV sound broadcasting &
police wireless transmission.

In FM transmitter the FM signal is transmitted
by a transmitter. The information is contained
in its frequency variation.
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FM TRANSMITTERS

The FCC has assigned a band of 20 MHz to
the commercial FM broad cast service.

This band extends from 88 MHz to 108 MHz.
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FM TRANSMITTERS

This 20 MHz band is divided in 100 channels,
each having a bandwidth of 200 KHz.

For providing high quality reliable music the
maximum frequency deviation allowed is 75
KHz,
with
a
maximum
modulating
frequency of 15 KHz.
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signal
METHODS OF FM GENERATION
Methods of FM Generation
Direct Methods
Indirect Methods
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DIRECT FM

In direct FM generation the frequency of the
carrier is changed directly in proportion with the
modulating signal amplitude.
Types of Direct FM

Reactance modulator

Varactor diode modulator
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DIRECT FM

These methods use a varactor diode or a
reactance transistor for presenting a variable
reactance across the frequency determining
circuit of an oscillator.
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DIRECT FM
Oscillator Tank
Circuit
Modulating
Signal
Variable reactance device
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DIRECT FM

When the variable reactance of the device
varies with the modulating signal the
oscillator generator the corresponding FM
signal.
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DIRECT MODULATORS

Few other direct modulators are:

Varactor diode modulator.
Reactance modulator.
V.C.O. modulator.
Stabilized reactance modulator.
Cross by direct FM transmitters.
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REACTANCE MODULATOR



In the reactance modulator, a transistor or
FET is operated as a variable reactance
(inductive or capacitive) device.
This device is connected across the tuned
circuit of an oscillator.
As the instantaneous value of modulating
voltage changes, the reactance offered by
the transistor or FET will change
proportionally.
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VARACTOR DIODE

Here the voltage applied across the varactor diode
varies in proportion with the modulating voltage.

This will vary the junction capacitance of the varactor
diode. The varactor diode appears in parallel with the
oscillator tuned circuit. Hence the oscillator frequency
will change with change in varactor diode capacitance
and FM wave is produced.
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ADVANTAGES OF DIRECT FM
MODULATION

The
main
advantage
of
direct FM generation is the
simplicity of the modulators
and their low cost.
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DISADVANTAGES OF DIRECT
METHOD

In the direct method of FM generation we have
to use the LC oscillator. The LC oscillator
frequency is not stable.

Therefore its not possible to use such oscillator
for communication or broadcast purpose.
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DIRECT FM

Therefore we have to use a scheme in which
we can use the crystal oscillator to control
the carrier frequency.

Therefore we have to use the automatic
frequency control system.
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INDIRECT METHOD ( ARMSTRONG
METHOD)

Here FM is obtained through phase modulation.

A Crystal oscillator is used and hence the
frequency stability is very high.

A narrow band PM signal is generated via DSBSC signal and a 90° phase shifted sub carrier
signal from a crystal oscillator.
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INDIRECT METHOD ( ARMSTRONG
METHOD)

The integration of the modulating signal
makes the NBPM signal to NBFM signal.

This NBFM signal is applied to a harmonic
generator (frequency multiplier) which will
increase both the carrier frequency and the
maximum deviation to the required.
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COMPARISION OF AM AND FM
BROADCASTING



AM Broadcasting
It requires smaller
transmission
bandwidth.
It can be operated in
low, medium and high
frequency bands.
It has wider coverage.



FM Broadcasting
It requires larger bandwidth.
It needs to be operated in very
high and frequency bands.
Its range is restricted to 50 km.
GEC GODHRA
COMPARISION OF AM AND FM
BROADCASTING

The demodulation is
simple.


The stereophonic
transmission is not
possible.
The system has poor
noise performance.



The process of
demodulation is
complex.
In this, stereophonic
transmission is
possible.
It has an improved
noise performance.
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REVISION






What is Transmitter
Classification of Transmitters
 Based on Type of modulation
 Based on Transmitted frequency
AM Transmitters
Types of AM transmitters

Low Level modulation transmitter
 High Level modulation transmitter
FM Transmitters
Types of FM generation
 Direct method of FM generation
 Indirect method of FM generation
GEC GODHRA