Analog Modulation Kes

7/27/2019 Analog Modulation Kes

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ANALOG MODULATION


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BASEBAND TRANSMISSION

Is the original information signal either in a

digital or analog form.

Transmission of original information whether

analog or digital, directly into transmission

medium is called baseband transmission


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Modulation

Is a process of modifying a carrier wave (radio

wave) systematically by the modulating signal

(audio).

To makes the signal suitable for transmission

and compatible with the channel.

Other words process of changing/varying one

of the parameters of the carrier wave by the

modulating signal


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TYPES OF MODULATION

Analog modulation : where the parameter of thecarrier is change proportionally with themodulating signal.

Amplitude modulation (AM) Frequency modulation (FM)

Phase modulation (PM)

Pulse Modulation: where the carrier is a periodic

pulse train and one of the pulseparameters(either amplitude, width or position)changes according to the audio signal


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NEED FOR MODULATION

To generate modulated signal that is suitable fortransmission and compatible with the channel

To allow efficient transmission

To overcome hardware limitations- the physical size of

some electronic components depend on the range offrequencies that are used in the circuit. The higher thefrequencies the physical size of the components maybe reduced

To allow frequency assignment. Multiplexing- sending multiple signals simultaneously.

Transmission speed and distance can be increased.


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AMPLITUDE MODULATION(AM)

Process of varying the amplitude of the radiofrequency (RF) carrier wave by the amplitudevariations of a modulating signal which

consists of a range of audio frequencies. Thefrequency of the carrier is not affected.

Application of AM:- radio broadcasting, TVpicture(video), facsimile transmission.

Frequency range for AM- 535kHz -1600kHz

Bandwidth 10kHz


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MODULATION PROCESS

AM MODULATOR

MODULATING SIGNAL

CARRIER

MODULATED WAVE

The data you are willing to send through any

medium.

The wave which actually carries your data

through the medium. carrier waves are mostly

constant in their own amplitude and frequency


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AM SIGNAL ANALYSIS

Carrier signal

Modulating Signal


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Information signal

Modulated carrier


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The amplitude of the modulating signal should be less than the amplitude of carrier. When

The amplitude of the modulating signal is greater than the amplitude of the carrier, distortionwill occur, causing incorrect information to be transmitted..

The peak value of the modulating signal be less than the peak value of the carrier/


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MODULATION INDEX AND PERCENTAGE OF MODULATION


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Effects of Modulation Index

m = 1 m > 1

In a practical AM system, it usually contains many

frequency components. When this is the case,

22

2

2

1 ... nT mmmm


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SIDEBANDS AND FREQUENCY DOMAIN Whenever a carrier is modulated by an information signal,

new signals at different frequencies are generated as part ofthe process.

the sidebands occur at frequencies that are the sum and

difference of the carrier and modulating frequencies


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Frequency of AM wave for single frequency

modulating signal

The carrier component is a constant amplitude and frequencies does not carry any of the

Information signal .

The complete AM wave takes up more frequency bandwidth that it necessary . Both sidebands

carry same information


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Sideband calculation

OR


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OR


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EXAMPLE:


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AM Spectrum

ffc

Ec

fusf

mEc/2mEc/2

flsf

fmfm

fusf= fc + fm ; flsf= fc - fm ; Esf= mEc/2

Bandwidth, B = 2fm

EXAMPLE 2


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EXAMPLE 2


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EXAMPLE 3


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PULSE MODULATION


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QUIZ 1

For an AM modulator with a carrier frequencies fc =

100kHz and a maximum modulating signal frequencies

fm(max) = 5kHz, determine

frequencies limits for upper and lower sidebands

Bandwidth

Upper and lower side frequencies produced when the

modulating signal is a singlefrequency 3kHz toneDraw the output spectrum


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solution


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AM Power

Total average (i.e. rms) power of the AM signal

is: PT = Pc + 2Psf, where

Pc

= carrier power; and Psf

= side-frequency

power

If the signal is across a load resistor, R, then:

Pc

= Ec

2/(2R); and Psf

= m2Pc

/4. So,

)2

1(2

mPP cT


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AM Current

The modulation index for an AM station can

be measured by using an RF ammeter and the

following equation:

21

2mII o

where I is the current with modulation and

Io is the current withoutmodulation.


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Complex AM Waveforms

For complex AM signals with many frequency

components, all the formulas encountered

before remain the same, except that m is

replaced by mT. For example:

21);

21(

22T

oT

CT

mII

mPP


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Block Diagram of AM TX


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Transmitter Stages

Crystal oscillator generates a very stable

sinewave carrier. Where variable frequency

operation is required, a frequency synthesizer

is used.

Buffer isolates the crystal oscillator from any

load changes in the modulator stage.

Frequency multiplier is required only if HF or

higher frequencies is required.


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Transmitter Stages (contd)

RF voltage amplifier boosts the voltage level of

the carrier. It could double as a modulator if

low-level modulation is used.

RF driver supplies input power to later RF

stages.

RF Power amplifier is where modulation isapplied for most high power AM TX. This is

known as high-level modulation.


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AM Modulator Circuits


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Impedance Matching Networks

Impedance matching networks at the output

of RF circuits are necessary for efficient

transfer of power. At the same time, they

serve as low-pass filters.

Pi networkT network


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by H Chan, Mohawk College

Generating SSB - Filtering Method

The simplest method of generating an SSB

signal is to generate a double-sideband

suppressed-carrier (DSB-SC) signal first andthen removing one of the sidebands.

BPF or

AF

Input

Balanced

Modulator

Carrier

Oscillator

DSB-SCUSB

LSB


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Suppressed-Carrier AM Systems

Full-carrier AM is simple but not efficient in

terms of transmitted power, bandwidth,

and SNR. Using single-sideband suppressed-carrier

(SSBSC or SSB) signals, since Psf= m2Pc/4,

and Pt=Pc(1+m2

/2

), then at m=1, Pt= 6 Psf. SSB also has a bandwidth reduction of half,

which in turn reduces noise by half.


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Filter for SSB

Filters with high Q are needed for

suppressing the unwanted sideband.

fa = fc - f2

fb = fc - f1

fd = fc + f1

fe = fc + f2

f

dBXantifQ c

4

)20/log( where X = attenuation ofsideband, andf = fd - fb


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AM Receivers

Basic requirements for receivers:

ability to tune to a specific signal

amplify the signal that is picked upextract the information by demodulation

amplify the demodulated signal

Two important receiver specifications:sensitivity and selectivity


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Amplitude Modulation Receiver Circuit


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Tuned-Radio-Frequency (TRF) Receiver

The TRF receiver is the simplest receiver thatmeets all the basic requirements.


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Block Diagram of SSB RX


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SSB Receiver (contd)

The input SSB signal is first mixed with the

LO signal (low-side injection is used here).

The filter removes the sum frequencycomponents and the IF signal is amplified.

Mixing the IF signal with a reinserted

carrier from a beat frequency oscillator(BFO) and low-pass filtering recovers the

audio information.


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SSB RX (contd)

The product detector is often just a

balanced modulator operated in reverse.

Frequency accuracy and stability of the BFOis critical. An error of a little more than 100

Hz could render the received signal

unintelligible. In coherent or synchronous detection, a

pilot carrier is transmitted with the SSB

signal to synchronize the BFO.


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AM POWER (extra notes)

Total transmitted power

W k h


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We know that:


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Example


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Please Read and Explain!


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Example:

???

Example:


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Example:


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Single sideband modulation


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Waveforms for Balanced Modulator

V1, fc

V2, fm Vo

f

fc+fmfc-fm

Si l id b d i l SSB


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Single sideband signal SSB


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CONT..


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Disadvantages of DSB and SSB


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Peak Envelope Power

SSB transmitters are usually rated by the

peak envelope power (PEP) rather than the

carrier power. With voice modulation, thePEP is about 3 to 4 times the average or

rms power.

L

p

R

VPEP

2

2

where Vp = peak signal voltage

and RL = load resistance


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Example 1:


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Example 1:

Example 2:


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Example 2:


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Generating SSB - Phasing Method

This method is based on the fact that the lsf and

the usf are given by the equations:

cos(c-m)t = (cosct cosmt + sinct sinmt)cos(c+m)t = (cosct cosmt - sinct sinmt)

The RHS of the 1st equation is just the sum of two

products: the product of the carrier and the

modulating signal, and the product of the sametwo signals that have been phase shifted by 90o.

The 2nd equation is similar except for the (-) sign.


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Diagram for Phasing Method


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Phasing vs Filtering Method

Advantages of phasing method :

No high Q filters are required.

Therefore, lower fm can be used.

SSB at any carrier frequency can begenerated in a single step.

Disadvantage:

Difficult to achieve accurate 90o phase shift

across the whole audio range.


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Trapezoidal Pattern

Instead of using the envelope display to

look at AM signals, an alternative is to use

the trapezoidal pattern display. This is

obtained by connecting the modulatingsignal to the x input of the scope and the

modulated AM signal to the y input.

Any distortion, overmodulation, or non-linearity is easier to observe with this

method.


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Trapezoidal Pattern (contd)

Improper

phase-Vp>+Vp

minmax

minmax

VV

VVm

m1

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