UNIVERSITY OF BALAMAND

FACULTY OF ENGINEERING

AMPLITUDE MODULATION

By

Dany Ishak, Tony Hayek

Chady Hamod, Francis Nader

Spring 2011

Copyright © Team1

All Rights Reserved

Table of contents

1- Introduction………………………………………………………..p3-4

2- Chapter 1: Amplitude modulation…………………………………p5-201.1 Double sideband large carrier (DSB-LC)…………………………………..p6-

111.2 Double sideband suppressed carrier (DSB-SC)…………………………p11-161.3 Single sideband (SSB)

………………………………………………………………p16-191.4 Conclusion………………………………………………………………………

……...p202 Chapter 2: Labview

Design………………………………………………………………….p21-302.1 Genreal

Introduction………………………………………………………………..p212.2 Project Implemantation………………………………………………………

p22-272.3 Graphical

Interface…………………………………………………………..p27-302.4 Conclusion………………………………………………………………………

……p303 Chapter 3: Telecommunication

experiments…………………………………………...p31-483.1 The Generation of an AM

signal…………………………………………………p31-373.2 Double sideband modulation……………………………………………………

p37-433.3 Single sideband generation………………………………………………………

p43-484 Conclusion……………………………………………………………………p48

2

Introduction

One of the most important inventions that mankind has reached is Telecommunications which consist of the communication between people over significant distances by electronic means. It allows the transmission of words, sounds, images, or data in the form of electronic or electromagnetic signals or impulses. Telecommunication dominates the major part of human life, in terms of consolidating the social relationship, contributing on educational mean, and providing many other services that help reduce the distances between people and makes the world very small.

The analog communication systems were the fundamental step toward the evolution of this field. Even though the communication systems have been greatly evolved since then, the analog systems, in particular AM and FM communication systems, still inevitable for transmitting the signal over a long distance.

We propose an educationally designed software application that exposes the students to the fundamental concept of AM communications and propels them to explore its characteristics, strengths and weaknesses.

Thus, our BS project is about Amplitude modulation/demodulation or AM, which is a type of common analog modulation techniques, and it is a part of telecommunications.

We have done our project using important software which is LabView. The reasons of choosing this software are its important and efficient benefits; some of them are:

It is powerful, flexible and scalable design. It is easy to learn, use, maintain and upgrade. It supports wide variety of data acquisition and embedded control devices.

It has ability to solve and execute complex algorithms in real time.

Same tools are used in academia and industry.

Our project has an educational benefit. It helps students in general and telecommunication students specifically, to experience and understand the amplitude modulation. We will analyze AM, DSB-SC and SSB. We will provide a simple graphical interface easy to understand and use.

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Our report is divided into three chapters; each chapter contains necessary ideas and important subjects in order to develop more the knowledge about AM and the main benefits and goals of this issue.

The first chapter is dedicated to the theoretical part about the amplitude modulation and demodulation; it includes brief description about the three types of modulation that we are going to simulate.

The second chapter is devoted about LabView design and interface; it includes information about each block diagram: definition, importance, and role.

The third chapter of our project involves three experiments one on each type of modulation simulated by our developed software application on LabView, in order to illustrate all information and theoretical knowledge about AM communication.

The first experiment is AM-LC The second one is AM-SC The last experiment is AM-SSB

4

Chapter 1 – Amplitude Modulation

1.1 - Introduction:

There are different types of amplitude modulation, such as commercial

amplitude modulation or AM (DSB-LC), DSB-SC (double sideband suppressed

carrier), and SSB (single sideband).

Amplitude modulation (AM) is used since the first days of the 20th century mainly

for transmitting voice and signals through the conventional broadcast band like the

long-, medium- and shortwave bands because of its easy and cheap way of realization.

Amplitude modulation is a technique used in electronic communication, most

commonly for transmitting information via a radio carrier wave.

This chapter is divided into 3 main parts:

1.1) Double Sideband-Large Carrier.

1.2) Double SideBand-Suppresed Carrier.

1.3) Single Sideband.

These three elements will be discussed in details with their formulas and block

diagrams. In each part, there are detailed paragraphs about modulation, demodulation,

power and detectors for each type.

5

1.2- DOUBLE SIDEBAND / LARGE CARRIER (DSB-LC):

1.2.1 –Modulation:

  DSB-LC system is commonly known as AM radio, or AM broadcast. The signal fed into the

mixer can be made always positive by adding a DC offset to m(t):

ϕ ( t )=[m ( t )+A ]cosW ct

The modulated carrier can be re-expressed as:

ϕ ( t )=m ( t ) cosW ct +AcosW ct

which is equivalent to adding a carrier component to a DSB-SC signal, hence the name DSB-LC

(large carrier).

Fig.1.1- Modulation

In DSB-LC, high energy at the carrier frequency is relieved. This energy comes from

the amplitude ‘A’ being modulated by the carrier frequency. This results in the modulated signal

with envelope similar to the message signal.

1.2.2 - Modulation Index:

  The modulation index m is a dimensionless number, defined as the ratio of the sideband energy to the carrier energy:

6

m = modulation index ¿peak modulating voltagepeak varrier voltage

= AmAc

Alternatively, percentage modulation is obtained by multiplying m by 100% to obtain the number in

percentage form:

Percentage modulation = m x 100%

  The modulation index m should always > 0 and < 1. If m = 0, the resultant modulated

waveform is just a constant envelope of amplitude A, the amplitude of the carrier frequency. There is

no modulation of the carrier wave. If m > 1, the resultant waveform is over modulated and is

distorted.

Trapezoidal display

Among the methods presented for determining the modulation index, the trapezoidal method

is probably the most common. When the modulating signal is voice or music, the modulation index

is constantly changing, but the trapezoid pattern provides a uniform display and allows meaningful

measurements.

Modulation index: m= (A-B) / (A+B)

When the peak amplitude of the message signal equals the peak amplitude of the unmodulated

carrier, 100% modulation is obtained. Figure 1.2 shows the AM waveforms and trapezoidal patterns

for m equal to 0.5 and 1.0.

7

Fig 1.2- AM waveforms and trapezoidal patterns for m equal to 0.5 and 1.0.

Overmodulation (m>1) occurs when the modulating signal has a peak amplitude greater than that of

the unmodulated carrier. The modulation index is directly related to power and efficiency in AM

communications. At 100% modulation each sideband frequency has amplitude equal to one-half of

the carrier amplitude. When overmodulation occurs both sides of the modulation envelope cross over

the zero reference line. This causes distortion in the receiver and interference with other stations,

because frequencies outside the assigned bandwidth of the over modulating station are produced.

Figure 1.3 illustrates overmodulation and shows the extra sideband frequencies which are produced.

8

Fig 1.3 overmodulation

1.2.3 - Sideband Power and transmission efficiency:

  There are three components to any DSB-LC waveform, the upper and lower sidebands and

the carrier frequency. If the powers in all three of these are added up, the total power in the

modulated signal will be

  PT = Pc + PSB ; where PSB = Pusb + Plsb

The fraction of the total power that is contained in the sidebands is a measure of the

transmission efficiency (μ). In equation form this can be expressed as μ= PSB / PT . Since PSB is

directly related to the modulation index (m), the ratio PSB / PT, and the theoretical efficiency, can be

determined from the modulation index using the following equation:

μ= m2

2+m2

The maximum DSB-LC efficiency is obtained at m = 1. For a 100% percentage

modulation, it is found that μ = 33%. This means that the maximum transmission efficiency only

results in one-third of the power being used to transmit the information. The remaining 67% of the

power is "wasted" in sending the carrier.

1.2.4 - Demodulation of AM :

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Amplitude modulation, AM, is one of the most straightforward ways of modulating a radio

signal or carrier. The process of demodulation, where the audio signal is removed from the radio

carrier in the receiver is also quite simple as well. The easiest method of achieving amplitude

demodulation is to use a simple diode detector. This consists of just a handful of components: a

diode, resistor and a capacitor.

Fig.1.5-AM Diode Detector

 

In the circuit above, the diode rectifies the signal, allowing only half of the alternating waveform

through. The capacitor is used to store the charge and provide a smoothed output from the detector,

and also to remove any unwanted radio frequency components. The resistor is used to enable the

capacitor to discharge. If it was not there and no other load was present, then the charge on the

capacitor would not leak away, and the circuit would reach a peak and remain there.

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1.3 - DOUBLE SIDEBAND/SUPPRESSED CARRIER (DSB-SC):

Because there are two pieces of sidebands; this type of AM is called double sideband (DSB)

modulation. Also, because the carrier frequency is absent from the modulated signal, and then it is

called suppressed carrier (SC) modulation. This type of modulation is therefore called double

sideband, suppressed carrier amplitude modulation (DSB-SC AM). The two pieces of sidebands are

named the upper sideband, USB, and the lower sideband, LSB.

1.3.1- Modulation:

Note that a(t) is the modulated signal that results from a cosine wave being multiplied (or

modulated) by a modulating baseband signal that contains the information to be transmitted, or m(t).

Fig 1.6-Modulation of DSB -SC

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In the Figure below, the carrier is assumed to have amplitude of 1 and the modulating signal

is assumed to have the amplitude Am. When these two are multiplied, or modulated, the amplitudes

of the resulting signals are half of the product of the two individual amplitudes, or A2

.

Fig 1.7 Spectrum of DSB-SC

Properties of DSB-SC Modulation:

(a) There is a 180 phase reversal at the point where +a(t)=+m(t) goes negative. This is typical of

DSB-SC modulation.

(b) The bandwidth of the DSB-SC signal is double that of the message signal, that is,

BWDSB-SC =2B (Hz).

(c) The modulated signal is centered at the carrier frequency Wc with two identical sidebands

(double-sideband) – the lower sideband (LSB) and

the upper sideband (USB). Being identical, they both convey the same message component.

(d) The spectrum contains no isolated carrier. Thus the name suppressed carrier.

(e)The 180 phase reversal causes the positive (or negative) side of the envelope to have a shape

different from that of the message signal, see Figure

1.8 (a) and (b). This is known as envelope distortion, which is typical of DSB-SC modulation.

(f) The power in the modulated signal is contained in the two sidebands.

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Fig 1.8 DSB-SC Modulation

1.3.1-   Sidebands:

 

The sidebands are always symmetric about the carrier frequency:

ωusb = ωc + ωm or fusb = fc + fm

ωlsb = ωc - ωm or flsb = fc - fm

1.3.2- DSB-SC Average Power:

Fig.1.9-DSB-SC Average Power

13

 In Figure 1.9 the peak voltage of the modulated signal is half the peak voltage of the

modulating signal. There are two sidebands and it is important to use rms voltages for

computing power. The expression for the average power of a DSB-SC modulated

signal is hence

P = 2.(Am/ )2 / R = Am2/4R

1.3.4-Demodulation of DSB-SC:

In DSB-SC the carrier frequency is suppressed, extra circuitry is required to

locate and track the carrier and hence increase the cost of the receiver. A coherent

demodulator must be used. The local oscillator present in the demodulator generates a

carrier which has same frequency and phase as that of the carrier in the modulated

signal.

Fig 1.10-Demodulationof DSB-SC

If the demodulator has constant phase, the original signal is reconstructed by passing v(t) through an

LPF.

14

1.3.5- Efficiency of DSB-SC:

For AM signals, maximum RF power occurs at 100% modulation. At this point, sideband power is

maximum but it represents only one-third of the total RF power. The other two-thirds of the total

power are “wasted” in the carrier. However, the total RF power for a DSB signal consists entirely of

sideband power because there is no carrier power or (very little) in the RF signal. Yet, the bandwidth

requirements still the same. Therefore, we moved to SSB modulation

1.4- SINGLE SIDEBAND (SSB) :

1.4.1 Modulation:

The concept of SSB modulation can be represented as shown in the figure below. The

spectrum of an SSB signal can be theoretically obtained in the manner suggested by first removing

the carrier from the AM signal to produce the DSB spectrum shown in fig 1.11 (b). Then, by

removing one of the two sidebands from the DSB signal, one of the SSB spectra of fig 1.11 (c) will

be obtained. The advantages of SSB modulation are:

1-There is no carrier present in the spectrum of an SSB signal.

2- Only half the frequency bandwidth is required for communication since only one sideband is

transmitted.

Therefore, SSB offers efficient power utilization and economic bandwidth use. These advantages are

offset, however, by the fact that transmission and reception equipment is much more complex.

Fig 1.11- The concept of SSB modulation

15

In practice it is simpler and easier to start with DSB, since this type of modulation is obtained

directly when a message signal is combined with an RF carrier through a balance mixer. To produce

an SSB signal, all that remains to do is filter out one of the sidebands before the RF signal is

transmitted.

The figure below shows a functional diagram for generating an SSB signal.

Fig 1.12 functional diagram for generating an SSB signal.

The message signal is combined with the Beat Frequency Oscillator (BFO) signal, thus

producing a DSB output centered at the BFO frequency. Sideband selection begins by adjusting the

BFO frequency within its 450-460 kHz range. This causes the frequency contents of the message

signal to be shifted relative to the pass band of the fixed IF filter. Since the 455 kHz IF filter has a

narrow 6 kHz bandwidth and sharp roll-off characteristics, the effect of frequency displacement is to

“push” one of the sidebands outside the pass band of the IF filter. Once the desired sideband has

been selected, the SSB signal is frequency-translated up to the carrier frequency determined by the

Variable Frequency Oscillator.

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1.4.2 Demodulation:

In demodulation technique, the receiver must reverse the operations performed

by the transmitter in order to recover the original information. The figure below shows

the functional block diagram for demodulating SSB signal.

Fig 1.13- functional block diagram for demodulating SSB signal.

Note that VFO and BFO in the receiver must be the same as in the transmitter for correct

demodulation.

17

Conclusion:

There are several advantages of amplitude modulation that is why it is still in widespread use today:

It is simple to implement

it can be demodulated using a circuit consisting of very few components

AM receivers are very cheap as no specialized components are needed.

However, it has many disadvantages:

It is not efficient in terms of its power usage

It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that

of the highest audio frequency

It is prone to high levels of noise because most noise is amplitude based and obviously AM

detectors are sensitive to it.

These disadvantages caused the creation of DSB-SC modulation. It presents power efficiency

over AM modulation since no loss of power in the carrier anymore. Yet, the bandwidth

requirements still the same. Moreover DSB demodulation needs a complex receiver that is more

expensive than the AM receiver. Concerning the bandwidth requirements, the SSB modulation

came to solve this problem. But SSB needs a complex transmitter and receiver.

Chapter 2 – LabView design

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2-1 General introduction:

LabView stands for Laboratory Virtual Instrumentation Engineering Workbench. It is

a graphical programming language that allows Data Acquisition, Instrument Control, Pre /

Post Processing of Acquired Data, and Industrial Automation. It includes various varieties of

platforms such as Microsoft Windows, Linux, Mac OS X, and UNIX. The purpose of

LabView programming is to mechanize the processes used and measure equipments in the

laboratory.

LabView programs are partially similar to some main functions, subroutines, and programs

such as FORTRAN, PASCAL, C, and so on. The Virtual Instrument (VI) includes thwo

important parts namely:

Front Panel: It is a user interface of a Virtual Instrument. It has the capability to stimulate

the ‘Front Panel of a Physical Instrument’.

Block Diagram or Wiring Diagram: It is the source code of Virtual Instrument developed

in LabView Graphical Programming Language named ‘G’. It is described as the ‘Actual

Executable Program’.

2-2 Project implementation :19

Our project consists of simulating amplitude modulation of signals. First, we started

by designing the AM/DSB/SSB Generator. Generating a DSB-LC signal consists of adding a

carrier level (Ac) to the original message signal. Then we multiply it by a carrier frequency

which is a Sine wave function of amplitude 1 and frequency Fc . At the end we multiply the

modulated signal by a constant (called RF Gain) to increase its power because power

attenuation occurs when sending messages over long distances.

The figure below shows the labview schematic of the DSB-LC generation.

Fig

2.1 AM/DSB generation

The waveform graph shows the modulated signal. However, the waveform graph 2 shows the

spectrum of the modulated signal. The VI used before it (called Power) gives the power spectrum

density of a signal. The message signal is got from a VI called based function generator. It gives sine

wave, sawtooth, square wave and triangular wave. However the carrier is got from a unit called sine

waveform.

20

Notice that DSB-SC is generated from this design by giving a very small value (near 0) to the carrier

amplitude (Ac).

Moving to the SSB generation, it consists of adding Ac to the message signal. Usually Ac should be

near 0 since SSB is designed to have benefits on DSB-SC by reducing the bandwidth of

transmission. Therefore, we do not want to go back to power loss in carrier. After adding Ac, we

multiply the signal by a sine wave signal of amplitude 1 and frequency FBFO. We obtain 2 sidebands

around the BFO frequency. We make the signal then pass by a band pass filter (centered at 455KHZ)

in order to remove one of the sidebands. After that we multiply the signal by another sine wave

signal of amplitude 1 and frequency FVFO in order to move the spectrum to high frequency for

transmission. At the end we multiply the signal by a constant called RF gain for the same reason as

AM modulation. The figure below shows the labview schematic of the SSB generation.

Fig 2.2 SSB generation

21

Now let’s move to the demodulation part. In AM modulation, the message signal corresponds to the

envelope of the modulated signal. Therefore, we used an envelope detector (peak detector) in AM

demodulation. The Envelope Detection was built using the two LabView VI’s: peak detector and

waveform builder. The peak detect can be set to identify either valleys or peaks; for our purpose we

set it to peaks. The waveform builder reconstructs the signal using three variables:

. Peak values: retrieved from the output of the peak detector.

. Start time: set to ‘0’

. Frequency of the signal: from theory, the frequency of the demodulated signal is the same as that of

the carrier frequency.

The figure below shows AM demodulation in Labview.

Fig 2.3 AM demodulation

On the other hand, when talking about DSB modulation, the message signal corresponds to the line

traced through alternate lobes. Then an envelope detector cannot do the demodulation job. We need

a costas loop detector. Designing this loop is very hard in labview and even impossible since it

22

contains some basic components that are not present. Therefore we made use of a DSB demodulator

( a component already existing in labview) to do the job.

Fig 2.4 DSB demodulation

We made the modulated signal pass by a component called (MT-Down convert) in order to make it a

suitable waveform that can be demodulated by the demodulator. We need to specify a pass band

bandwidth for this component greater than the carrier frequency that has been used. Moreover, we

have to push on the DSB button (DSB-SC) to specify that we want DSB-SC demodulation. Finally,

the SSB reception is not other than reverse of the operations performed by the transmitter. Figure

2.5 shows the functional block representation of the manner in which the SSB receiver reverses the

23

modulation process to recover the transmitted information.

Fig 2.5 SSB demodulation

Frequency-translation of the incoming RF signal is fairly straight-forward, being just a matter of

tuning the VFO to exactly the same frequency as that used by the transmitter. The IF signal present

at the output of the IF filter ( filter centered at 455 KHZ-see fig above) represents the transmitted

sideband, either the LSB or the USB. The next step in demodulating is translating the message signal

spectrum back to its proper position in the frequency spectrum. This is done by multiplying it by the

BFO frequency which should be the same as BFO-TX in order to conserve the spectral relationship

between the BFO and the USB that has been established at the transmitter. Finally we put a low pass

filter to remove any glitches that can appear.

2.3 Graphical Interface:

After finishing our design, we worked on the graphical interface. We made our software application

very flexible. The equipments that we designed look exactly as the equipments present in the

telecom lab. The figures below show the result of our work.

24

Fig 2.6 Function Generator

Fig 2.7 AM/DSB/SSB Generator

25

These pictures are taken from the front panel of our application. You can see that any telecom

student can manage easily to use our software to perform amplitude modulation. The rest of the

equipments are shown below.

Fig 2.8 AM/DSB Receiver

Fig 2.9 SSB Receiver

2.4- Conclusion:

26

At the end, the goal of our project can be summarized in 2 main ideas:

1- Making engineering students familiar with the labview software.

2- Presenting an advanced and sophisticated method of showing amplitude modulation.

We hope that you understood all the steps that we passed through in our design. We are going in the

next part to analyze some experiments that can be done using this software application.

Chapter 3 Telecommunication experiments

3.1 Exp1-The Generation of an AM Signal:

1- Pick AM modulation.

Adjust the Function Generator as follows:

Function: Sine Wave

Frequency: Fm=2.5 KHz

Amplitude: Am=2 V

On the AM/DSB/SSB generator choose carrier frequency fc (M) = 500K, carrier level (Ac) of 4V,

and RF gain=1. Run the program.

First observe the graph of the modulated signal and plot it. The plot should be similar to the one

below.

27

Fig 3.1 AM modulated signal

2-After, keeps the RF gain constant while varying the carrier level. Then keep the carrier level

constant and vary the RF gain. Describe the effects that each step has on the waveform.

First, varying Ac does not affect the envelope of the modulated signal. However, when we

have Ac< Am we will have over modulation. Moreover, when Ac=Am we will have 100%

modulation.

Fig 3.2-a Over modulation case

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Fig 3.2-b 100% modulation

On the other hand, when varying RF gain, the amplitude of the envelope varies proportionally to it

since the formula of the modulation is:

G [(Ac + m (t)) cos2πfct ]

3- Vary the frequency of the sine wave (fm). What changes does it cause in the AM waveform?

The frequency of the AM waveform varies proportionally with the frequency of the sine wave.

4- Now look at the spectrum graph and repeat all the three steps that we did before.

. Amplitude of fc varies proportionally with Ac.

. Amplitude of fc and the two sidebands (fm) vary proportionally with RF gain.

. Increasing fm makes the sidebands move away from the carrier, while decreasing it makes

them approach from the carrier.

5- Vary Am and note the changes on the spectrum.

. When varying Am, the amplitude of the sidebands varies proportionally with it.

6- Give a method of viewing the carrier frequency only.

29

To view the carrier frequency only, we need to give Am a value near 0 (Am=0.1 for ex) and then we

will see on the graph of the modulated signal only the carrier frequency as well as seeing it on the

spectrum also .

Fig 3.1.3-a Carrier signal

Fig 3.1.3-b Carrier spectrum

7- On the function generator, change the sine wave function to square wave, saw

tooth and all the other functions. Describe what you see on the AM waveform

and plot it.

. We will see that the envelope of the AM waveform has the same shape as the

message signal. The plots will be:

30

Fig 3.1.4-a Triangular wave

Fig 3.1.4-b Square wave

Fig 3.1.4-c Saw tooth

31

8- Calculate the modulation index.

m=Am/Ac= 2/4= 0.5. Note that they are other methods of calculating the

modulation index. We talked about some of them in ch1 of the report and we

gave examples. Moreover, the modulation index must be <1. If m=1 100%

modulation (see fig 3.1.2-b). if m>1 over modulation (see fig 3.1.2-a).

9- On the AM/DSB receiver, choose fc(R)= 500KHz. Observe the demodulated

signal for each of the 4 functions(sine wave-triangular…).

. The demodulated signal has the same shape and same frequency as the message

signal for all the functions.

At the end of the experiment, turn all control knobs to their min position.

3.2 Exp2 –Double Sideband Modulation (DSB)

1- Pick DSB modulation.

Adjust the Function Generator as follows:

Function: Sine Wave

Frequency: Fm=2.25 KHz

32

Amplitude: Am=2 V

On the AM/DSB/SSB generator choose carrier frequency fc (M) = 600K, carrier level

Ac=0.01V, and RF gain=1. Finally, on the AM/DSB receiver choose fc(R) = 600K,

and press the AM/DSB button. Run the program.

First observe the modulated signal and plot it.

Fig 3.2.1-a DSB waveform

Fig 3.2.1-b DSB spectrum

33

2- For AM waveform, the envelope of the RF waveform corresponds to the message

signal. What do you notice about the correspondence between the DSB waveform and

the message signal?

. The line traced through alternate lobes of the DSB waveform represents the

message signal.

3- Vary Ac on the AM/DSB/SSB Generator between min and max and return it to the

min position. What effect does it have on the RF waveform?

. First we will have normal DSB, then AM waveform but in over modulation, then

AM waveform with 100% modulation (when Ac=Am=2). At the end, we will have

normal AM waveform when Ac becomes greater than 2 till reaching its maximum

value.

4- Vary Ac between min and max and record the difference in dB between both levels.

This difference corresponds to the carrier suppression in dB.

. First when Ac=0.01 (min), Pc (min) = 4*10-5 w= -43.97 dB

(PdB=10log10(Pw)).

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Fig 3.2.2 DSB spectrum when Ac=0.01

Now when Ac=10 (max) Pc (max) = 45w= 16.53 dB

Fig 3.2.3 AM spectrum when Ac=10

Carrier suppression = Pc (max) – Pc (min) = 60.5 dB.

5- Double the amplitude of the message signal (Am). What changes can you see in the

DSB spectrum? What can u conclude about the RF power?

.when increasing Am, the power of sidebands increase, then the RF power

increases. In AM, 2/3 of the power is wasted in carrier power, however here in DSB

35

the total RF power consists of sidebands power mainly (carrier power very small).

Therefore, when increasing Am the total power increases in greater proportion than in

AM case.

6- Return all controls to their original position as in the beginning of the experiment.

Look now at the demodulated signal using the DSB detector. You will see it exactly

the same as the original message signal but shifted because of a little delay in time

(signal passes through steps before demodulation).

Fig 3.2.4-a Message signal

36

Fig 3.2.4-b Demodulated signal

7- Try the demodulation for the other functions (square wave-saw tooth...)

. When using square wave or saw tooth, there is some distortion in the demodulated

signal since their frequency spectrum represent infinite pulses. Then the detection of

the signal is harder than the sine wave case.

8- Push off the DSB button. Plot the result of the demodulation.

. You will obtain only the positive part of the message signal with a frequency

approximately twice of fm.

37

Fig 3.2.5-a Demodulated signal

Fig 3.2.5-b Demodulated spectrum

9- Look at the spectrum of the DSB waveform. Does it have bandwidth advantage

over AM?

. DSB modulation does not have bandwidth advantages over AM since we still

have 2 sidebands and the distance between them is 2 fm exactly as in AM

modulation.

At the end of the experiment, turn all control knobs to their min position.

38

3.3 Exp3-Single sideband Modulation (SSB)

1- Pick SSB modulation.

Adjust the Function Generator as follows:

Function: Sine Wave

Frequency: Fm=2.25 KHz

Amplitude: Am=2 V

On the AM/DSB/SSB generator choose fBFO = 452.75K, fVFO=4355 KHz, carrier level

Ac=0.01V. Run the program.

Observe the mixer output (3) and plot it. Describe what you see.

Fig 3.3.1 The frequency spectrum at Mixer output

39

For FBFO=452.75 KHz, the upper sideband is located at f=455 KHz and the lower at f=

450.5 KHz.

2- Observe the frequency spectrum of the signal at the IF Output (4). Describe it and

explain what happened.

. The IF filter has a pass band of: 452 KHz-458 KHz. After filtering, only the upper

sideband is passed. The lower sideband is filtered. (see fig below).

Fig 3.3.2 The frequency spectrum at IF output.

3- Adjust fBFO to 455 KHz using the BFO tuning control. Explain what happened to

the frequency spectrum.

. For fBFO= 455 KHz, we have fusb= 457.25 and flsb= 452.75. These two frequencies

are located in the pass band of the IF filter, then we will have a DSB signal at the IF

output. (See fig. below).

40

Fig 3.3.3 The frequency spectrum at IF output.

4- With fBFO=455 KHz, increase fm to 4 KHz. Explain the changes in the frequency

spectrum.

. In this case, we have fUSB= 458.5 KHz, and fLSB= 451.5 KHz. Both are outside the

range of the filter, then we have only fBFO that passes. USB and LSB are filtered.

Fig 3.3.4 The frequency spectrum at IF output.

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5- Go back to initial specifications (as in problem 1). Moreover, on the AM/DSB/SSB

Generator choose fVFO=4355 KHz, and RF Gain2 = 2. Observe the SSB spectrum and

plot it.

. The plot should be similar to the one below.

Fig 3.3.5 SSB spectrum

6- On the SSB receiver, choose fVFO=4355 KHz, fBFO=452.75 KHz, and IF

amplifier=12. Observe the demodulated signal.

. The demodulated signal has the same shape and same frequency as the message

signal.

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Fig 3.3.6-a Demodulated signal

Fig 3.3.6-b Demodulated spectrum

7 - Vary fm. What happens to fdem?

. Fdem varies proportionally with fm.

8- On the SSB receiver, choose fBFO= 457250 instead of the previous value. Repeat

the previous two steps.43

. For the new value of fBFO, the demodulated signal has same shape and

frequency of the message signal. However, a sideband reversal happened at this

case. We were sending upper sideband, but we demodulated lower sideband.

Therefore when increasing fm, fdem decreases and vice versa.

At the end of the experiment, turn all control knobs to their min position.

Conclusion

At the end, we hope that you like our software application. We did our best to make it

as simple as possible. There are many experiments that can be performed using it.

What we present was only few ideas of what can be done.

Finally we want to thank our advisor, and the two moderators for all the help that they

gave us during the semester.

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