Experiment 2

DOUBLE SIDEBAND AM AND SINGLE SIDEBAND AM GENERATION AND DETECTION

1. Conventional Amplitude Modulation (DSB-LC) (Alternatively known as Full AM or Double

Sideband with Large carrier (DSB-LC) modulation

2. Double Side Band Suppressed Carrier (DSB-SC) modulation

3. Single Sideband (SSB) modulation

4. Vestigial Sideband (VSB) modulation

What are the different Forms of Amplitude Modulation ?

Conventional Amplitude Modulation (Full AM)

1 The carrier signal is

cccc ftAtc 2 where)cos()(

2 In the same way, a modulating signal

(information signal) can also be expressed as

tAtm mm cos)(

Frequency Spectrum for Full-AM Modulation (DSB-LC)

3 The amplitude-modulated wave can be

expressed as )cos()()( ttsAts cmc

)cos()cos()( ttAAts cmmc

4 By substitution

c

m

A

Am

5 The modulation index.

6 Therefore The full AM signal may be written as

)cos())cos(1()( ttmAts cmc

)]cos()[cos(2/1coscos BABABA

tmA

tmA

tAts mcc

mcc

cc )cos(2

)cos(2

)(cos)(

fC fc+fmfc-fm

2fm

Frequency Spectrum of the above AM signal and Bandwidth Calculation

Frequency Spectrum for a complex input signal with AM

fcfc-fm fc+fm

Spectrum of AM DSB SC

Because it doesn’t have components of the carrier, we call this kind of modulation suppressed carrier

Single-Sideband Modulation

• Band-pass filter to pass only one of the sidebandand suppress the other.

For the generation of an SSB modulated signalto be possible, the message spectrum must havean energy gap centered at the origin.

Single Side Band Modulation (SSB)

• Example of signal with -300 Hz ~ 300 Hz energy gapVoice : A band of 300 to 3100 Hz gives good

articulation• Also required for SSB modulation is a highly selective filter

% Sample the signal 100 times per second, for 2 seconds.Fs = 100;t = [0:2*Fs+1]'/Fs;Fc = 10; % Carrier frequencyx = sin(2*pi*t); % Sinusoidal signal

% Modulate x using single- and double-sideband AM.ydouble = ammod(x,Fc,Fs);ysingle = ssbmod(x,Fc,Fs);

% Compute spectra of both modulated signals.zdouble = fft(ydouble);zdouble = abs(zdouble(1:length(zdouble)/2+1));frqdouble = [0:length(zdouble)-1]*Fs/length(zdouble)/2;zsingle = fft(ysingle);zsingle = abs(zsingle(1:length(zsingle)/2+1));frqsingle = [0:length(zsingle)-1]*Fs/length(zsingle)/2;

% Plot spectra of both modulated signals.figure;subplot(2,1,1); plot(frqdouble,zdouble);title('Spectrum of double-sideband signal');subplot(2,1,2); plot(frqsingle,zsingle);title('Spectrum of single-sideband signal');

Demerits of AM DSB FC

An unmodulated RFcarrier requires narrowbandwidth

Modulation results in creation of a carrier and 2 Sidebands. Thisrequires more power. Moreover carrier contains no information.

Why DSB SC?

The carrier contains no information.

So we can think of avoiding or suppressing carrier.

Why SSB?

The carrier contains no audio information.

The sidebands contains duplicated information