OPTICAL FIBER

TRANSMISSION AND
RECEPTION

MANUAL

C/ Del Agua, 14. P.I. San José de Valderas. 28918 LEGANES (Madrid) SPAIN. Tlf:+34-916199363 FAX:+34-
916198647edibon@edibon.com www.edibon.com
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TABLE OF CONTENTS

7 PRACTICES MANUAL ............................................................................................................. 2

7.1 INTRODUCTION.......................................................................................................................... 2
7.1.1 Use of this handbook .................................................................................................................................. 2
7.1.2 Using the practice board EDICOM-6 ........................................................................................................ 3
7.1.3 Power supply .............................................................................................................................................. 3
7.1.4 Faults study ................................................................................................................................................ 4
7.2 TRANSMISSION AND RECEPTION WITH OPTICAL FIBER ............................................ 8
7.2.1 Fiber Optics Fundamentals ........................................................................................................................ 8
7.2.2 FM Frequency Modulation ........................................................................................................................ 9
7.2.3 PWM Pulse Width Modulation ................................................................................................................. 10
7.3 GENERAL DESCRIPTION OF THE EDICOM-6 BOARD................................................... 12
7.3.1 Main features ............................................................................................................................................ 12
7.3.2 Block diagram description........................................................................................................................ 13
7.4 LABORATORY PRACTICAL EXERCISES ........................................................................... 21
7.4.1 Practice 1: Amplitude modulation of an analog signal ............................................................................ 21
7.4.2 Practice 2: Amplitude modulation of a digital signal ............................................................................... 26
7.4.3 Practice 3: Frequency modulation ........................................................................................................... 30
7.4.4 Practice 4: Pulse width modulation ......................................................................................................... 36
7.4.5 Practice 5: Digital signals transmission using optical fibers using EDICOM-4 ..................................... 40
7.4.6 Practice 6: Faults simulation ................................................................................................................... 42
7.5 APPENDIX A: SOLUTIONS FOR TEACHER ......................................................................... 1
7.5.1 Practice 1: Amplitude modulation of an analog signal .............................................................................. 1
7.5.2 Practice 2: Amplitude modulation of a digital signal ................................................................................. 3
7.5.3 Practice 3: Frequency modulation ............................................................................................................. 5
7.5.4 Practice 4: Pulse width modulation ........................................................................................................... 7
7.5.5 Practice 6: Faults simulation ..................................................................................................................... 8
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7 PRACTICES MANUAL

7.1 INTRODUCTION

This manual will guide you to the basic concepts about communications. It
is accompanied by a practice board where you will be asked to perform a series of
assemblies to complete your studies.

The aim of the manual is the study of one of the most common
transmission techniques at present. For this, a series of recommended practices for
the student are included that will improve the student’s knowledge.

7.1.1 Use of this handbook

This handbook describes how to use the EDICOM-6 board properly and
how to perform a series of practical exercises. This manual is divided in three clearly
differentiated parts:

- A theoretical compendium on the subject being studied.

- A detailed description of the different blocks of the board.

- A set of practices that allows the student to understand the concepts being studied.

Even you already have some knowledge about the subject to be studied, it
is always advisable to take a look at the theoretical introduction before doing the
practices.
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7.1.2 Using the practice board EDICOM-6

The following accessories will be needed for the proper use of this board:

 A set of connecting leads. (Supplied with the unit).

 A multimeter.

 An oscilloscope.

 A power supply that can be a EBC-100 base unit, FACO or an independent power
supply.

Note: When carrying out practices, it is necessary to be careful when you are
manipulating the leads, because they could provoke short circuits between their
terminals and the different components that are assembled on the board. An
example would be if the same end of a lead or of a probe touched two pins of an
integrated circuit.

7.1.3 Power supply

7.1.3.1 Working with the EBC-100 base unit or FACO

To start working with the EDICOM-series it is necessary to supply. To do

this, follow this procedure:

1. Check that the network voltage at which the Base Unit will be connected is the
same as it is indicated on the back of the unit ( there are only two possibilities 220
V and 127V A.C.). Then, connect the power cable supplied to the Base Unit or
FACO.

2. Make sure that the Base Unit or FACO are switched off and the board does not
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have links or leads connecting the test points.

3. If you are working with the EBC-100, introduce the board in the base box slot
situated in the front part of the box, or let it drop gently from the upper part.

4. Connect the ribbon power supply cable that is shipped with the EBC-100 or
FACO to the board.

5. The board is now ready to use. Switch on the power supply when you are asked
to.

7.1.1.1 Working with an independent power supply

In order to use the EDICOM-6 practice board independently of its base

unit, an independent power supply (or more than one) is needed to supply voltages
that should be connected as follows:
 5 volts, at point 5v
 12 volts, at point l2v
 -12 volts, at point -l2v
 0 reference, at point GND

In all the exercises it will be taken for granted that you are working with a
EBC-100 base unit or FACO. If this is not the case, when you are read to supply the
board by pressing the switch of the base unit, you should do the same with the
independent power supply you are using.

7.1.4 Faults study

In most of the study circuits, faults can be simulated. The student must find
out what is happening and why the circuit does not work properly. These fault
simulations can be of various types: from damaged components to an incorrect circuit
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assembly.

These faults are simulated through one or several switch matrices that are
on a block called SWITCHED - FAULTS. This block tends to be located in the
lower part of each printed circuit board. In the matrices, each switch has two
positions: one is for normal operation (all the switches in the position OFF, which is
printed on the board) and the other is for introducing the fault (position ON).

NOTE: In all circuits, the proper work of the EDICOM-6 board is with the
switches in the OFF position. Thus, before starting to work, check that all the
switches are in this position.

Figure 1. Switches Fault OFF position

Figure 2. Switches Fault ON position

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Figure 3. Aspect of the board EDICOM-6.

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Figure 4. Schematic diagram view of the board EDICOM-6.

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7.2 TRANSMISSION AND RECEPTION WITH OPTICAL FIBER

7.2.1 Fiber Optics Fundamentals

The science of fiber optics deals with the transmission or guidance of light
(rays or waveguide modes in the optical region of the spectrum) along transparent
fibers of glass, plastic, or a similar medium. The phenomenon responsible for the
fiber or light-pipe performance is the law of total internal reflection.

The Total Internal Reflection law says that a ray of light, incident upon the
interface between two transparent optical materials having different indices of
refraction, will be totally internally reflected (rather than refracted) if

(1) the ray is incident upon the interface from the direction of the more dense
material and

(2) the angle made by the ray with the normal to the interface is greater than some
critical angle, the latter being dependent only on the indices of refraction of the
media (see figure 5).

Figure 5. Refraction and reflection

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7.2.2 FM Frequency Modulation

In frequency modulation, the frequency of the sinusoidal carrier wave is

made to change according to the momentary level of the information signal
(modulator signal), while the amplitude of the carrier stays constant, see figure 6.

Any transmitted communication signal will be affected by noise when it

passes through the transmission medium. This unwanted noise is superimposed on
the AM signal received.

In amplitude modulation, the information is contained within the amplitude

of the transmitted signal. Any additional amplitude variation, due to noise, will
appear at the output of the AM demodulator, together with the demodulated
information. This leads to a reduction in the quality of the audio signal to the output
of the receiver.

However, in frequency modulation, the information is contained in the

frequency of the transmitted signal, and not the amplitude. In the receiver, this
information is extracted from the FM signal through a FM demodulator (or
"frequency discriminator").

If the FM receiver is designed not to be sensitive to the amplitude

variations, it should be capable of reproducing the audio signal without being
affected by noise. This is the most important advantage of the FM communication
over AM.

To generate FM, a frequency modulator is used to vary the frequency of

the sinusoidal carrier signal, in synchronization with the modulator signal. In the
following practices, we will use the EDIBON ED-CAM FM to study two specific
types of frequency modulators.
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Figure 6. FM signal

7.2.3 PWM Pulse Width Modulation

In pulse width modulation , the width of individual pulses in the pulse train
is varied from its default value in accordance with the instantaneous amplitude of the
modulating signal at sampling intervals. The amplitude and position of the pulses is
kept constant, see figure 7.

Pulse width modulation is also known as pulse length modulation (PLM)

or pulse duration modulation (PDM). Pulse width modulation is mostly used in
control applications as the average value of the signal is proportional to the pulse
width. PWM receiver design is very simple as simple low pass filtering of the signal
results in decoded signal.
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Figure 7. PWM signal

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7.3 GENERAL DESCRIPTION OF THE EDICOM-6 BOARD

7.3.1 Main features

The EDICOM-6 board allows the user to study the basics concepts about
fiber optics transmissions. It is designed to illustrate the modulation techniques of a
light source and the subsequent recuperation of the original signal. It also introduces
the concepts of FM (Frequency Modulation) and PWM (Pulse Width Modulation).
The board allows us to check the advantages of digital signals over analog ones,
when they are transmitted using optical fibers as the medium. The key features are:

 Transmitter and receiver integrated in the same board.

 Two independent channels with optical fiber medium.

 Optical fiber connectors of minimal complexity, to aid the user’s task.

 Admits different types of modulation:

- Frequency modulation.

- Pulse width modulation.

 Internal generation of two signals, one sinusoidal and the other square, both with a
frequency of 1KHz.

 Possible use of the board together with the EDICOM 4.

 Fault simulation block.

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7.3.2 Block diagram description

To work easily with the EDICOM-6, the board is organized in different

blocks. The block structure of the EDICOM-6 module is shown in the figure 8. A
detailed description of each block is included in the following lines.

Fi

Figure 8. Block diagram of the EDICOM-6

7.3.2.1 Function generator

The function generator block, figure 9, generates two signals: a digital and
a sinusoidal signal of variable amplitude. The frequency of both signals is 1Khz. The
amplitude of the sinusoidal signal can be adjusted by turning the potentiometer
located below the socket of this signal.
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Figure 9. Function generator block

7.3.2.2 RS-232 Interface

Figure 10. RS-232 Interface block

This block is a deprecated block and it will be no longer available.

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7.3.2.3 Frequency Modulator

Figure 11. Frequency modulator block

The Frequency Modulator block, figure 11, is used to generate a FM

signal. FM signals contain their information on its frequency. This block consists of a
VCO (Voltage Controlled Oscillator). A VCO is an electronic oscillator designed to
be controlled in oscillation frequency by a voltage input. The frequency of oscillation
is varied by the applied DC voltage.

7.3.2.4 Pulse Width Modulator

Figure 12. Pulse Width modulator block

The Pulse Width Modulator block is used to generate a PWM signal. PWM
signals contain the information on the width of the pulse of a square signal, figure 12.
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7.3.2.5 Emitter Circuits

Figure 13. Emitter block

The figure 13 shows the emitter circuits. The emitter circuits are based on
an IR LED ( InFrared Light Emitter Diode) that is a led that emits radiation whose
wavelength is longer than that of visible light (400-800 nm). There is a switch
ANALOG/ DIGITAL to select the type of the signal to be transmitted. Each emitter
is provided with a socket to introduce the fiber optic cable. The transmitting diodes
are taken to work with the receiving diodes as a set, as they have the same
wavelength. The emitter diode selected is a SFH 485 diode from OSRAM. The main
characteristics of this IR LED are summarized in the following table:

PARAMETER VALUE

Forward Current 100mA

Reverse Voltage 5V

Power dissipation 200mW

WaveLength at Peak 880nm

emission

Table 1. Characteristics OSRAM SFH 485

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Note: Close attention must be paid to the instructions concerning the position of
the switches that appear on the board.

7.3.2.6 Receiver Circuits

The receiver circuits are shown in the figure 14. The receiver circuits are
based on an photodiode. A photodiode is a electronic device that generates energy
proportional to the incident radiation. Each receiver is provided with a socket to
introduce the fiber cable.

Figure 14. Receiver block

The photodiodes are taken to work with the transmitting diodes as a set, as
they have the same wavelength. The photodiode selected is a SFH 203 photodiode
from OSRAM.
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PARAMETER VALUE

Photocurrent 9.5uA

Dark Current 1nA

WaveLength of max. 850 nm

sensitivity

WaveLength at Peak 880nm

emission

Table 2. Characteristics OSRAM SFH 203

7.3.2.7 Comparator Circuits

The board is provided with two comparators for comparing two signals.
There is a potentiometer on each comparator to adjust the voltage level at the
inverting input of the comparator.

Figure 15. Comparator block

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7.3.2.8 Phase-Locked Loop Detector

A phase-locked loop or phase lock loop (PLL) is a control system that

generates a signal that has a fixed relation to the phase of a "reference" signal. A
phase-locked loop circuit responds to both the frequency and the phase of the input
signals, automatically raising or lowering the frequency of a controlled oscillator
until it is matched to the reference in both frequency and phase. The most common
uses of the PLL are for generating stable frequencies, recovering a signal from a
noisy communication channel, or distributing clock timing pulses in digital logic
design.

In the EDICOM- 6 the PLL is used for recovering the signal transmitted
from the channel.

Figure 16. PLL block

7.3.2.9 Low Pass Filter & AC Amplifier Circuit

The signal received can be filtered for the reconstruction of the original
signal using a low-pass filter. There is also an AC amplifier to obtain the desired
output level signal by adjusting the gain using the potentiometer located closed to the
amplifier.
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Figure 17. Low Pass Filter & AC Amplifier

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7.4 LABORATORY PRACTICAL EXERCISES

7.4.1 Practice 1: Amplitude modulation of an analog signal

7.4.1.1 Introduction

The aim of this exercise is to see how the amplitude modulation of a beam
of light is carried out. For this, as for the modulating signal, we shall first use an
analog signal and then a digital one. Thus, we can check the advantages of digital
signals over analog ones.

7.4.1.2 Carrying out the practice

1. First of all make sure that the board is properly connected to the power supply as
it is explained in the Power supply chapter.

2. Remember that for the proper working of all the circuits, the fault switches must
be in the position OFF (see Faults study chapter ).

3. Carry out the assembly specified in figure 18 using the optical fiber cable that is
30cm long.

4. Set the mode switch of the emitting circuit in the position ANALOG.

5. Using an oscilloscope, observe the sinusoidal signal to be transmitted, turn the

potentiometer gain of the sinusoidal signal until the output is about 2Vpp.

6. Follow the signal by observing the TP5, 6, 9, 10, 19, 20, 27, and also the signal
recuperated at the output of the filter, TP28. If the signal received is very weak,
turn the potentiometer of the output amplifier until you obtain the desired
amplitude.
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7. Observe and record the waveforms at TP5(CH1) and TP10(CH2) by using an

oscilloscope (Oscilloscope 1).

Oscilloscope 1

8. Refer to Oscilloscope 1. The signal at the TP10 is :

a. a noisy signal due to channel distortion.

b. a frequency-modulated signal.

c. a PWM signal.

d. none of above.

9. Observe and record the waveforms at TP10(CH1) and TP20(CH2) by using an

oscilloscope (Oscilloscope 2).
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Oscilloscope 2

10.Refer to Oscilloscope 2. The signal at the TP20 :

a. is the received signal after being filtered to add the channel distortion.

b. is the received signal after being demodulated to remove the channel

distortion.

c. is the received signal after being filtered to remove the channel distortion.

d. none of above.

11.Continue observing the output signal while, at the same time, increasing and
decreasing the gain of the input signal.

12. Move the optical fiber cable with your hand and notice how this affects the
quality of the signal received. This is due to the fact that the transmission quality
of analog signals through optical fibers depends on the curvature radius of the
fiber.

Note: On carrying out this exercise, take care that the curvature radius of the
optical fiber cable is never less than 15mm. A radius inferior to this could made
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the fiber to break.

13.Substitute the fiber cable for the one of 1m long. Check that the amplitude of the
output signal has decreased. This is because the amplitude of analog signals
decreases, when they are transmitted using an optical fiber medium, in relation to
the fiber length.
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Figure 18. Diagram of the practice1

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7.4.2 Practice 2: Amplitude modulation of a digital signal

7.4.2.1 Introduction

We have seen that analog signals have a series of limitations when they are
transmitted through an optical fiber channel. In the following experiment we shall see
the advantages of digital signals over analog ones. We shall see that there are no
attenuation problems of the signal due to the fiber length, and that there is no quality
loss of the signal received with a different curvature radius of the fiber.

7.4.2.2 Carrying out the practice

1. First of all make sure that the board is properly connected to the power supply as
it is explained in the Power supply chapter.

2. Remember that for the proper working of all the circuits, the fault switches must
be in the position OFF (see Faults study chapter ).

3. Carry out the assembly specified in figure 19 using the optical fiber cable that is
30cm long.

4. Set the mode switch of the emitting circuit in the position DIGITAL.

5. Observe the square signal to be transmitted with an oscilloscope. Follow the

signal by observing the TP5, 6, 9, 10, 14, 15, 19, 20, 27, as well as the signal
recuperated at the output of the filter, TP8. Use the corresponding potentiometer
in the comparator block to obtain a clear output.

6. Observe and record the waveforms at TP5(CH1) and TP10(CH2) by using an

oscilloscope (Oscilloscope 3).
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Oscilloscope 3

7. Observe and record the waveforms at TP10(CH1) and TP15(CH2) by using an

oscilloscope (Oscilloscope 4).

Oscilloscope 4

8. Refer to Oscilloscope 3. Choose the correct sentence:

a. In fiber-optic communication, analog signals are preferred over digital signals

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because they are not affected by channel attenuation.

b. In fiber-optic communication, digital signals are preferred over analog signals

because the amplitude and frequency are affected by the channel attenuation
effect.

c. In fiber-optic communication, digital signals are preferred over analog signals

because the information is not affected by the channel attenuation effect.

d. none of above.

9. Refer to Oscilloscope 4. The signal at TP10 is:

a. passed through a comparator to amplified the signal.

b. passed through a comparator to filter the signal.

c. passed through a comparator to conform to the requirements of digital signals.

d. none of above.

10. Move the optical fiber cable with your hand and see how this does not affect the
quality of the signal received.

Note: On carrying out this exercise, take care that the curvature radius of the
optical fiber cable is never less than 15mm. A radius inferior to this could make
the fiber to break.
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Figure 19. Diagram of the practice2

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7.4.3 Practice 3: Frequency modulation

7.4.3.1 Introduction

The aim of this exercise is to see how the frequency modulation of a beam
of light is carried out. For this, as modulating signal, we shall use a sinusoidal signal.
This signal will pass through a voltage-controlled oscillator (VCO). This oscillator
will give us a digital signal whose frequency is proportional to the voltage of the
input signal. This digital signal will be transmitted. Once received, the signal must be
demodulated. To do this, it will be passed through a phase loop link (PLL), which
will carry out the opposite operation to the one done by the VCO.

7.4.3.2 Carrying out the practice

1. First of all make sure that the board is properly connected to the power supply as
it is explained in the Power supply chapter.

2. Remember that for the proper working of all the circuits, the fault switches must
be in the position OFF (see Faults study chapter ).

3. Carry out the assembly specified in figure 20 using the optical fiber cable that is
30cm long.

4. Set the mode switch of the emitting circuit in the position DIGITAL.

5. Observe the sinusoidal signal to be transmitted with an oscilloscope, turn the gain
potentiometer of the sinusoidal signal until its output is about 4Vpp.

6. Follow the signal by observing the TP1, 2, 5, 6, 9, 10, 14, 15, 19, 20, 23, 24, 25,
26, 27, as well as the signal recuperated at the output of the filter, TP28. Turn the
corresponding potentiometers of the comparator block and output amplifier until
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the desired signal is achieved.

7. Observe and record the waveforms at TP1(CH1) and TP2(CH2) by using an

oscilloscope (Oscilloscope 5).

Oscilloscope 5

8. Refer to Oscilloscope 5. The signal at TP2 is:

a. a signal whose frequency varies depending on the instantaneous value of the

frequency of the signal in TP1.

b. a signal whose frequency varies depending on the instantaneous value of the

amplitude of the signal in TP1.

c. an amplitude modulated signal.

d. none of above.
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9. Refer to Oscilloscope 5. The signal at TP2 has:

a. the information that we want to transmit contained in the frequency of the

signal.

b. the information that we want to transmit contained in the amplitude of the

signal.

c. the information that we want to transmit contained in the frequency and the
amplitude.

d. none of above.

10.Refer to Oscilloscope 5. Choose the correct sentence about Frequency-Modulated

signals (FM):

a. FM and Amplitude-Modulated (AM) signals are equally affected by noise.

b. AM signals are less affected by noise than FM signals.

c. FM signals are less affected by noise than AM signals.

d. none of above.

11.Observe and record the waveforms at TP23(CH1) and TP26(CH2) by using an

oscilloscope (Oscilloscope 6).
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Oscilloscope 6

12. Refer to Oscilloscope 6. Choose the correct sentence:

a. When recovering a FM signal the VCO circuit gives a voltage proportional to

the amplitude of the input signal.

b. When recovering a FM signal the VCO circuit gives a voltage proportional to

the frequency of the input signal.

c. When recovering a FM signal the VCO circuit always gives the same signal
regardless of the frequency and amplitude of the input signal.

d. None of above.

13.Continue observing the output signal while, at the same time, increasing and
decreasing the gain of the input signal.

14.Move the optical fiber cable with your hand and notice how this does not affect
the quality of the signal received. It only produces a slight blink of the signal but
does not decrease the amplitude.
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Note: On carrying out this exercise, take care that the curvature radius of the
optical fiber cable is never less than 15mm. A radius inferior to this could cause
the fiber to break.

15.Substitute the fiber cable with the one that is 1m long. Check that the output
signal amplitude does not decrease when this is done because we are in fact
transmitting a digital signal.
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Figure 20. Diagram of the practice3

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7.4.4 Practice 4: Pulse width modulation

7.4.4.1 Introduction

The aim of this exercise is to see how the modulation of an analog signal
into a series of pulses is carried out; where the information is contained in the width
of the mentioned pulses.

7.4.4.2 Carrying out the practice

1. First of all make sure that the board is properly connected to the power supply as
it is explained in the Power supply chapter.

2. Remember that for the proper working of all the circuits, the fault switches must
be in the position OFF ( see Faults study chapter ).

3. Carry out the assembly specified in figure 21 using the optical fiber cable that is
30cm long.

4. Set the mode switch of the emitting circuit in the position DIGITAL.

5. Observe the sinusoidal signal to be transmitted with an oscilloscope, turn the gain
potentiometer of the sinusoidal signal until its output is at maximum.

6. Follow the signal by observing the TP3, 4, 5, 6, 9, 10, 14, 15, 19, 20, 27, as well
as the signal recuperated at the output of the filter, TP28. If the signal obtained
has an unstable trace, adjust it by using the comparator potentiometer until a
reasonable signal is obtained.

7. If the signal received is very weak, turn the potentiometer of the output amplifier
until the desired signal is achieved.
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8. Observe and record the waveforms at TP3(CH1) and TP4(CH2) by using an

oscilloscope (Oscilloscope 7).

Oscilloscope 7

9. Refer to Oscilloscope 7. The signal at TP4 is:

a. an AM signal.

b. an FM signal.

c. a PWM signal.

d. none of above.

10.Refer to Oscilloscope 7. The signal at TP4 has:

a. the information that we want to transmit contained in the amplitude of each

pulse.

b. the information that we want to transmit contained in the width of each pulse.
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c. none of above.

11. Refer to Oscilloscope 7. Choose the correct sentence:

a. To extract the information of the PWM signal is used a low pass filter to obtain
the maximum value.

b. To extract the information of the PWM signal is used a low pass filter to
extract the minimum value.

c. To extract the information of the PWM signal is used a low pass filter to
extract the mean value.

d. None of above.

12.Continue observing the output signal while, at the same time, the gain of the input
signal is increasing and decreasing.
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Figure 21. Diagram of the practice4

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7.4.5 Practice 5: Digital signals transmission using optical fibers using

EDICOM-4

7.4.5.1 Introduction

The board EDICOM-6, can be used as the medium of transmission for any
signal with the only requisite that the signal to be transmitted must be digital. That is,
whenever the signal is a string of pulses it will not be valid for an FSK signal, for
example, in such case, what is actually being transmitted is an analog signal.

To illustrate this concept, we shall make the boards EDICOM-6 and

EDICOM-4 work together, because the last one transmitted a string of digital data.
However, any power source that is capable of generating digital signals can be used.

7.4.5.2 Carrying out the practice

1. First of all make sure that the board is properly connected to the power supply as
it is explained in the Power supply chapter.

2. Remember that for the proper working of all the circuits, the fault switches must
be in the position OFF ( see Faults study chapter ).

3. Carry out the assembly specified in figure 22 using the optical fiber cable that is
30cm long. When using the board EDICOM-4, use the signal in the TP34 as the
signal to be transmitted.

4. Observe the output signal and adjust it using the potentiometer of the comparator
used in the board EDICOM-6.
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Figure 22. Diagram of the practice5

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7.4.6 Practice 6: Faults simulation

7.4.6.1 Introduction

The board EDICOM-6 has a block (switched faults) for the error
simulation referred to the normal working of the board. This block is made up of a
series of eight switches, each of which causes a fault in the normal operation when
the switch is in the ON position. The simulated errors are described in the following
paragraphs.

7.4.6.2 Carrying out the practice

1. First of all make sure that the board is properly connected to the power supply as
it is explained in the Power supply chapter.

2. To set ON a switch see the Faults study chapter.

3. For each fault, a practice should be carried out before the fault can be applied. The
next table indicates the practice that should be carried out for each switch fault.
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Switch Fault Practice

1 Practice 1

2 Practice 1

3 Practice 3

4 Practice 4

5 Practice 4

6 Practice 2

7 Practice 3

8 Practice 4

4. Set ON the fault switch and find out where the fault is (block).
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7.5 APPENDIX A: SOLUTIONS FOR TEACHER

The following pages contain the solutions to the questions of the practices.

7.5.1 Practice 1: Amplitude modulation of an analog signal

Oscilloscope 1

8. Refer to Oscilloscope 1. The signal at the TP10 is:

a. a noisy signal due to channel distortion.

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Oscilloscope 2

10.Refer to Oscilloscope 2. The signal at the TP20 is:

c. is the received signal after being filtered to remove the channel distortion.
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7.5.2 Practice 2: Amplitude modulation of a digital signal

Oscilloscope 3

Oscilloscope 4
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8. Refer to Oscilloscope 3. Choose the correct sentence:

c. In fiber-optic communication, digital signals are preferred over analog

signals because the information is not affected by the channel attenuation
effect.

9. Refer to Oscilloscope 4. The signal at TP10 is:

c. passed through a comparator to conform to the requirements of digital

signals.
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7.5.3 Practice 3: Frequency modulation

Oscilloscope 5

Oscilloscope 6
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8. Refer to Oscilloscope 5. The signal at TP2 is:

a. a signal whose frequency varies depending on the instantaneous value of the

frequency of the signal in TP1.

9. Refer to Oscilloscope 5 . The signal at TP2 has

a. the information that we want to transmit contained in the frequency of the

signal.

10. Refer to Oscilloscope 5. Choose the correct sentence about Frequency-Modulated

signals (FM):

c. FM signals are less affected by noise than AM signals.

12. Refer to Oscilloscope 6. Choose the correct sentence:

b. When recovering a FM signal the VCO circuit gives a voltage proportional

to the frequency of the input signal.
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7.5.4 Practice 4: Pulse width modulation

Oscilloscope 7

9. Refer to Oscilloscope 7. The signal at TP4 is:

c. a PWM signal.

10.Refer to Oscilloscope 7. The signal at TP4 has:

b. the information that we want to transmit contained in the width of each pulse.

11. Refer to Oscilloscope 7. Choose the correct sentence:

c. To extract the information of the PWM signal is used a low pass filter to extract
the mean value.
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7.5.5 Practice 6: Faults simulation

Fault simulation SWT1

With this fault, the sinusoidal signal of 1KHz is cancelled. This error,
therefore, affects all the operation modes, except the amplitude modulation of digital
signals.

Fault simulation SWT2

This error produces the elimination of the output amplifier gain and will,
thus, affect all practices.

Fault simulation SWT3

The incidence of this fault shows up in the frequency modulator and affects
the VCO, provoking its incorrect operation.

Fault simulation SWT4

This fault affects the output filter and thus the effect of this error also
affects all operation modes.

Fault simulation SWT5

With this error we provoke a fault in the superior channel. It appears in one
of the internal amplifiers of the mentioned channel.

Fault simulation SWT6

Through this fault we obtain a malfunction of the upper comparator circuit.

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Fault simulation SWT7

This error affects the PLL, so when we are in the frequency modulation
mode, we will obtain a faulty signal.

Fault simulation SWT8

With this fault we cause an error in the working of the pulse-width

modulator, thus obtaining a faulty output.