anlog communication lecture 06

8/14/2019 ANLOG COMMUNICATION Lecture 06

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Amplitude Modulation

Lesson 6

EEE352 Analog Communication SystemsMansoor Khan


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Amplitude Modulation

Amplitude Modulation is the process of changing

the amplitude of a relatively high frequency carrier

signal in proportion with the instantaneous value of

the modulating signal (information).

Use in commercial broadcasting of both audio and

video signals.

Also used for two-way mobile radio

communications.


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AM Waveforms


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Time Domain

Signal

Carrier

time

time

time


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Detection of Signal

time

time


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Susceptible to Noise

time


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Frequency Domain

Unmodulated

frequency

SignalCarrier

Baseband

watts

Modulated

frequency

Signal

Carrier

Baseband

watts

Baseband


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AMPLITUDE MODULATION: DOUBLESIDE BAND (DSB)

Modulating signal, base band signal, information

signal

Carrier signal:

with Spectrum

)(tm )(M

)cos()( ccttc

)()()( ccC


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DSB (cont)

Modulation is the product of the base band with thecarrier

)()(2

1)cos()( ccc MMttm


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DSB (cont)

DSB-SC modulation simply shifts the frequency

contents ofm(t) to the carrier frequency


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USB & LSB


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Demodulation

To demodulate we multiply the signal by a

Therefore the FT of this signal is

If we lowpass filter this signal we recover

)cos( twc

)2cos(2

1

2

1)()(cos)()cos()cos()(

2twtmtwtmtwtwtm cccc

)2()2(4

1)(

2

1)2cos(

2

1

2

1)( ccc wMwwMwMtwtm

)(2

1tm


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Demodulation (cont)

We need a carrier of exactly the same frequency and

phase as the carrier used for modulation:

Synchronous Detection or Coherent Detection


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Demodulation (cont)


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Modulators

Multiplier Modulators

Nonlinear Modulators

Switching Modulators

Ring Modulator


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Nonlinear Modulators

Let the IO characteristic of a system to be

Substituting the two inputs

We obtain

)()()(2 tbxtaxty

)()()()()()()(2

22

2

1121 tbxtaxtbxtaxtytytz

)(cos)(1 tmtwtx c )(cos)(2 tmtwtx c

twtbmtamtz ccos)(4)(2)(


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Nonlinear Modulators (cont)

If we pass the signal though a BPF centered at wcwewill get the modulated signal

twtbmtz cBPFTHEAFTER cos)(4)(


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Switching Modulators

In this case we multiply the modulating signal by any periodic

signal of frequency wc

The fourier series of a square pulse train is (Eq. 2.75)

then

....5cos5

13cos31cos2

21)( twtwtwtw ccc

....5cos5

)(3cos

3

)(cos)(

2

2

)()()( tw

tmtw

tmtwtm

tmtwtm ccc


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Switching Modulators (cont)

If this signal is passed trough a BPF centered at wc we

get our modulated signal

.cos)(2

)()( twtmtwtm cBPFTHEAFTER


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Series/Shunt bridge diodemodulator


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Ring Modulator

This is the second kind of switching modulators, in this case

the square wave is bipolar

The fourier series of this pulse train will be given by (Eq. 2.76-

B.P. Lathi)

then

....5cos5

13cos

3

1cos

4)( twtwtwtw ccc

....5cos

5

)(3cos

3

)(cos)(

4)()( tw

tmtw

tmtwtmtwtm ccc


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Ring Modulator (cont)

The desired signal after the BPF is

The input of the BPF does not contain any of the original input

signals, therefore this is an example of a double balanced

modulator

.cos)(4

)()( twtmtwtm cBPFTHEAFTER


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Frequency Mixer or converter

We wanted to change the modulated signal from wcto wI

The productx(t) is

Down conversion if we select

tttmtx mixc coscos)(2)(

tttmtx mixcmixc coscos)()(

Icmix

tttmtx IccIcc cos)(cos)()(

tttmtx IcI 2cos)cos)()(


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Frequency Mixer (cont)

Up conversion if we select

Icmix

tttmtx IccIcc cos)(cos)()(

tttmtx IcI 2cos)cos)()(


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Frequency Mixer (cont)


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AMPLITUDE MODULATION(Transmitted Carrier DSB-TC)

In this case we send the carrier with the signal

We can think as the modulating signal to be

Sidebands

c

Carrier

cAM ttmtAt cos)(cos)(

ttmAt cDCawithsignalulating

AM cos)()(

mod


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AM (cont) The spectrum of this signal is

spectrumCarrier

cc

spectrumSCDSB

ccAM wwwwAwwMwwMw )()(2

1)()(

2

1)(


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EEE 352


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AM (cont)

A is large enough that . The demodulation can be

achieved by a simple envelope detector

Lets consider the peak value of to be . Then the

condition for envelope detection of AM signal is

Which is equivalent to

0)( tmA

)(tm pm

0)( tmA

pmA


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Modulation Index

We define the modulation index as

Therefore we can see that if we want to maintain the

condition

We have

A

mp

pmA

10


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Example 4.4


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Example 4.4 (cont)


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Percentage Modulation

Under modulated (100%)

anlog communication lecture 06

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