AM Modulation and Demodulation

8/8/2019 AM Modulation and Demodulation

1/46

COMS3100/7100

Introduction toCommunications


2/46

Lecture 9:Amplitude ModulationThis lecture:

1. What is Modulation?

2. AM Signals and Spectra.

3. AM Modulation and Demodulation.

4. Double-Sideband Suppressed Carrier.


3/46

Ref: CCR pp. 152159, 176177, Couch pp. 302312.

COMS3100/COMS7100 Intro to Communications

Lecture 9 - Amplitude Modulation

1 / 24


4/46

Communications Course So Far

x(t)g(t)

Encoder Modulation SignalConditioning

(Filtering /

Ampliication)

Decoderx(t) Demodulation SignalConditioning

(Filtering/

Equalization)

Tx Signal

Noise,

Interference

distortion

Rx Signal

Communicati

ons


5/46


6/46

What is Modulation?

Often, the message is itself a signal, e.g.,an audio signal, and to produce a signal

that is suitable for transmission throughthe channel, we effect some

transformation on the message signal.


Lecture 9 - Amplitude Modulation 3/ 24


7/46

Modulation

0I We call this modulation.0I Modulation is often performed with

respect to another signal, called

the carrier.

0I We say the message modulates

the carrier to produce the

transmitted signal.



8/46


4 / 24


9/46



10/46

Lecture 9 - Amplitude Modulation 5 /24

CW M d l ti


11/46

CW Modulation

g(t) =A(t)cos(2fct+(t))

The two principal subclasses of

CW modulation are:

amplitude modulation, in which the

carrier amplitude is varied with

the message signal andangle modulation, in which the phase

angle of the carrier is varied

with the message signal.COMS3100/COMS7100 Intro to Communications


12/46


E l f CW M d l ti


13/46

Examples of CW Modulation

(1) Carrier wave. (b) Sinusoidal modulating signal.

3) Amplitude-modulated signal. (d) Angle-modulated signal.



14/46


7 / 24

B it f M d l ti


15/46

Bene its of ModulationMake message better suited to the channel

There are three practical bene itsthat result from modulation:1. Modulation can shift the spectral

content of a message signal into a bandwhich is better suited to the channel.0I Antennas only ef iciently radiate and

admit signals whose wavelength issimilar to their physical aperture.

0I Hence, to transmit and receive, say,

voice, by radio we need to shift the

voice signal to a much higher

frequency band.



16/46


Bene its of Modulation


17/46

Bene its of ModulationMultiplexing

2. Modulation permits the use

ofmultiplexing.

0I Multiplexingmeans allowingsimultaneouscommunication bymultiple users on the samechannel.

0I For instance, the radio frequency

spectrum must be shared and

modulation allows users to separate

themselves into bands.



18/46


Bene its of Modulation


19/46

Bene its of ModulationSome immunity to noise/interference

3. Modulation can provide some

control over noise/interference.

0I As we will see, frequency modulation

(FM) permits a tradeoff between

bandwidth and noise.



AM Modulation


20/46

AM ModulationThis lecture focuses on AM Modulation

Next lecture will be on phase modulation (of which FM is a subset of)



21/46


11 / 24

AM Signals and Spectra


22/46

AM Signals and Spectra

(Double-sideband) amplitude

modulation (AM) is a technique from the

verybeginning of CW radio transmissionat the dawn of the 20th century.

0I It is still in use today because of

its simplicity.




23/46

Modulating AM Signals


24/46

Modulating AM Signals

A message signalx(t) is

amplitude modulated as follows:

1. An envelope signal g (t) is obtain

by amplifying and biasing themessage signal, so that

g(t) = gI(t) = Ac[1+x(t)] :

0I The modulation index > 0 is chosen to

ensure that g(t) > 0, and to conserve power.


Lecture 9 Amplitude Modulation


25/46


13 / 24

Modulating AM Signals (2)


26/46


2. The signal is then mixed with the

carrier, a sinusoid of frequency fc,to produce the AM signal

xc(t) = g(t) = gI(t)cos 2fct:




27/46


14 / 24



28/46


3. The AM signal can then be

radiated through the antenna.

AM radio typically uses

0I 148.5 kHz 283.6 kHz (9 kHz channel

spacing) navigational aids + radio in

other countries. (shortwave)

0I 520 kHz 1610 kHz (9 kHz spacing)

(typical commercial radio) (medium wave)




29/46

p

15 / 24

Frequency-Domain Analysis


30/46

Frequency Domain Analysis... of an AM Signal

FT FT

Supposex(t) ! X(f)and g(t) ! G (f).

ThenGf A f f f f ( ) =

1

c

[(c

) +( +c

)]

2 +

A [X

(f

f) + X(f+ f

)] :


31/46

12 c c cCOMS3100/COMS7100 Intro to CommunicationsLecture 9 - Amplitude Modulation

16 / 24

Frequency Domain Analysis (2)


32/46

Frequency Domain Analysis (2)

x(t)

O

/t

Message signal in the time domain.

X(f)

O

22

2

2 /f

W WMessage signal in the frequency domain.

0g (t)

0O g(t)

A M s ign al inG

(th e

f) time domain.

O 1

A c O_ O

2

? ? ? ?

? ? /

fc

fc

AM signal in the frequencydomain.

COMS3100/COMS7100 Intro to Communications Lecture 9

- Amplitude Modulation 17 / 24


33/46

Amplitude Modulation 17 / 24

AM Modulation and


34/46

AM Modulation andDemodulationA System for Amplitude Modulation

Basic AM requires only an ampli ier, a

s u m m e r M essa gex ( t )a n ds igna l a mixer. A Mg( t )S igna l

~

Oscillatorf

signal

c o s t 2 c




35/46


18 / 24

A System for Amplitude


36/46

y pDemodulation

To demodulate the received signal, i.e.,

to recover the original message signal,

we can use an envelope detectorcircuit.


Lecture 9 - Amplitude Modulation 19/ 24


37/46

/ 24

System for AM Demodulation (2)


38/46

y ( )

0I A diode is used to half-wave rectify

the received signal.

0I The R1C1 ilter then smooths torecover an approximation of

the original envelope.

0I R2C2removes the bias.

COMS3100/COMS7100 Intro to Communications Lecture 9 -

Amplitude Modulation 20 / 24


39/46


40/46

DSB-SC AM


41/46

Double-Sideband Suppressed Carrier

0I Instead, in double-sidebandsuppressed carrier (DSB-SC), the

natural envelope is just an ampliication of the message, i.e.,

g(t) = Acx(t):




42/46

Demodulation of DSB-SC AM


43/46

0I A simple envelope detector cannot

be used: a product detector is

needed instead.



23 / 24

Demodulation of DSB-SC AM (2)


44/46

0I The receiver needs accurate phase

information to recover the message)

a carrier recoverycircuit is needed.



Costas Loop


45/46

Shown below is a Costas Loop for carrier recovery in DSB-SC.

0I It is assumed that the phase error e is small.

0I The input to the VCO is roughly proportional to e.




46/46

25 / 24

AM Modulation and Demodulation

Documents

Transcript of AM Modulation and Demodulation