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Course Number : EEE 310 Student No.: 1006035

Group : 01

Experiment No. 1(a) & 1(b)

Name of the Experiment :

DOUBLE SIDEBAND AM GENERATION AND DETECTION & SINGLE SIDEBAND AM

GENERATION AND DETECTION

Submitted by : S. M. Shafiul Hasan

Section: A2 Level: 3-II

Partners St. Number :

1006034,1006036,1006037,1006038,

1006039,1006040,1006041.

Date of Performance : 12-07-14

Date of Submission : 25-07-14

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Exercise 1(a)

Part A: Double Sideband AM Generation

Objective:

1. To investigate the generation of double sideband amplitude modulated (AM) waveforms.

2. To examine the effects of changing audio frequency and amplitude on carrier suppression.

Part B: Double Sideband AM Detection

Objective:

1. To investigate the detection of double sideband amplitude modulated (AM) waveforms.

2. To examine the effects of changing audio frequency and amplitude on carrier suppression.

Exercise 1(b)

Part A: Single Sideband AM Generation

Objective:

1. To investigate the generation of single sideband amplitude modulated (AM) waveforms.

2. To examine the effects of changing audio frequency and amplitude on carrier suppression.

Part B: Single Sideband AM Detection

Objective:

1. To investigate the detection of single sideband amplitude modulated (AM) waveforms.

2. To examine the effects of changing audio frequency and amplitude on carrier

Suppression.

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Modulation:

In many telecommunications systems, it is necessary to represent an information-bearing signal

with a waveform that can pass accurately through a transmission medium. This assigning of a

suitable waveform is accomplished by modulation, which is the process by which some

characteristic of a carrier wave is varied in accordance with an information signal, or modulating

wave. The modulated signal is then transmitted over a channel, after which the original

information-bearing signal is recovered through a process of demodulation.

Classification:

There are 3 basic types of continuous wave modulation:

1) Amplitude Modulation (AM)

2) Frequency Modulation (FM)

3) Phase Modulation (PM)

Amplitude modulation:

In AM radio communication, a continuous waveradio-frequency signal (a sinusoidalcarrier

wave) has its amplitudemodulatedby an audio waveform before transmission. The audio

waveform modifies the amplitude of the carrier wave and determines the envelope of the

waveform. In the frequency domain, amplitude modulation produces a signal with power

concentrated at the carrier frequency and two adjacent sidebands. Each sideband is equal

inbandwidthto that of the modulating signal, and is a mirror image of the other. Standard AM is

thus sometimes called "double-sideband amplitude modulation" (DSB-AM) to distinguish it

from more sophisticated modulation methods also based on AM.

Consider a carrier wave (sine wave) of frequency fcand amplitude A given by:

.

Let m(t) represent the modulation waveform. For this example we shall take the modulation to be

simply a sine wave of a frequency fm, a much lower frequency (such as an audio frequency)

than fc:

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,

where M is the amplitude of the modulation. We shall insist that M

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MATLAB codes and figures:

clear all;

close all;

clc;

%amplitude modulation

fs=20000;

T=1/fs;

L=fs;

t=0:T:1;

m_t=sin(2*pi*400.*t); %Message singal

A=abs(min(m_t))+2;

mod_sig=(A+m_t).*sin(2*pi*5000.*t) %Modulated signal

figure(1);

plot (t(1:500),m_t(1:500),'ro')

hold on;

plot (t(1:500),mod_sig(1:500),'b-')

hold off;

title('Amplitude modulated signal and the message signal');

NFFT=2^nextpow2(L);

Y=fft(mod_sig,NFFT)/L; %Normalized amplitude spectrum

f= fs/2*linspace(0,1,NFFT/2+1);

figure(2);

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plot(f,2*abs(Y(1:NFFT/2+1)));

title( 'Single sided amplitude spectrum');

xlabel( 'Frequency (Hz)');

ylabel( '|Y(f)|');

axis([4000 6000 0 .25])

%DSBSC

s_t= m_t.*sin(2*pi*5000.*t);

figure(3);

plot (t(1:500),s_t(1:500),'b');

hold on;

plot (t(1:500),m_t(1:500),'r');

title('Double side band suppressed career');

NFFTds=2^nextpow2(L);

Yds=fft(s_t,NFFTds)/L; %Normalized amplitude spectrum

fds= fs/2*linspace(0,1,NFFTds/2+1);

figure(4);

plot(f,2*abs(Yds(1:NFFTds/2+1)));

title('DSBSC amplitude spectrum');

axis([4000 6000 0 .25])

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Fig: Modulated signal and message signal

Fig: Single sided amplitude spectrum

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Virtues of AM:

Simplicity of implementation

Necessary equipment are cheap and available

Limitations:

Wasteful of power

High vulnerability to noise

Double Side Band Suppressed Carrier AM(DSBSC):

In DSB-SC only the upper and lower side bands are transmitted. A DSB-SC signal can be

obtained by multiplying the message signal m(t) with the carrier signal c(t).

S(t)= m(t)cos(2fct)

The modulated wave undergoes a phase reversal whenever the message signal crosses zero.

MATLAB figure:

Fig: Double side band suppressed carrier modulated signal

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Fig: Amplitude spectrum (DSBSC)

Detection of DSBSC : heterodyne detection

In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with areference "local oscillator" that is set at a close-by frequency, called intermediate frequency (fsig

+ 455 Hz in our experiment). The desired outcome is the difference frequency (455 Hz), which

carries the information (amplitude, phase, and frequency modulation) of the original higherfrequency signal, but is oscillating at a lower more easily processed carrier frequency.

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Advantages of DSBSC modulation:

If we are to tune different radio stations at different frequencies, we might have to use different

amplifiers with different gain bandwidth characteristics, otherwise we had to compromise with

the varying attenuation factors for different frequency signals. Here comes Heterodyne Detection

with a smarter solution.

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Modulation by Local Oscillator transfers all frequency spectrum, irrespective of received signal

frequency, to a prefixed frequency level (intermediate frequency), thus we need to use just

another amplifier which works fine in that IF level.

SSB AM (Sing Side Band Amplitude Modulation):

In radiocommunications, single-sideband modulation (SSB) or single-sideband suppressed-

carrier (SSB-SC) is a refinement of amplitude modulationthat more efficiently

uses transmitterpowerandbandwidth. Amplitude modulation produces an output signal that has

twice the bandwidth of the originalbasebandsignal. Single-sideband modulation avoids this

bandwidth doubling, and the power wasted on a carrier, at the cost of increased device

complexity and more difficult tuning at the receiver.

MATLAB code and figures:

clear all;

close all;

clc;

fs=20000;

T=1/fs;

L=20000;

t=0:T:1;

m_t=sin(2*pi*200.*t);

s_t= m_t.*sin(2*pi*5000.*t);

[b1 a1]=butter(10,[5000 5400]/(fs/2),'bandpass');

y=filter(b1,a1,s_t);

figure (1);

plot(t(300:800),m_t(300:800));

title('Message signal in time domain');

axis([0.015 0.04 -1 1]);

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figure(2);

plot(t(300:800),s_t(300:800));

axis([0.015 0.04 -1 1])

title('Modulated signal in time domain');

figure(3);

plot(t(300:800),y(300:800));

title('Filtered signal in time domain');

axis([0.015 0.04 -0.5 0.5])

NFFT=2^nextpow2(L);

M=fft(m_t,NFFT)/L; %Normalized amplitude spectrum

f= fs/2*linspace(0,1,NFFT/2+1);

figure(4);

plot(f,2*abs(M(1:NFFT/2+1)));

title( 'Frequency spectrum of message signal');

xlabel( 'Frequency (Hz)');

ylabel( '|M(f)|');

axis([0 400 0 0.25]);

S_T=fft(s_t,NFFT)/L;

Y=fft(y,NFFT)/L;

figure(5);

plot(f,2*abs(S_T(1:NFFT/2+1)));

title( 'Frequency spectrum of modulated signal');

xlabel( 'Frequency (Hz)');

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ylabel( '|S(f)|');

axis([4000 6000 0 0.25]);

figure(6);

plot(f,2*abs(Y(1:NFFT/2+1)));

title( 'Frequency spectrum of filtered signal');

xlabel( 'Frequency (Hz)');

ylabel( '|Y(f)|');

axis([5000 5400 0 0.25]);

SSB Generation:

Fig: Message Signal in time domain

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Fig: Modulated Signal in time domain

Fig: Filtered Signal in time domain

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Frequency Domain:

Fig: Message Signal in frequency domain

Fig: Modulated Signal in frequency domain

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Fig: Filtered Signal in frequency domain

Advantage of SSB over DSB:

1. Since the carrier is not transmitted, there is a reduction by 50% of the transmitted power (-

3dBm).

2. Because in SSB, only one sideband is transmitted, there is a further reduction by 50% in

transmitted power (-3dBm (+) -3dBm = -6dBm).

3. Finally, because only one sideband is received, the receiver's needed bandwidth is reduced by

one half--thus effectively reducing the required power by the transmitter another 50% (-3dBm(+) -3dBm (+) -3dBm = -9dBm).

Discussion:

1. For better transmission both the antennas were kept vertical and placed close to each other.

2. The quality of the observed signal degraded as the distance between the transmitter and the

receiver was increased.

3. In diode detection method only DSB signal could be detected. When we lowered the

dc level the output was distorted.