AN-NAJAH NATIONALUNIVERSITY
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
Communications LAB
For
Telecommunication Engineering 69328
Electrical Engineering 63473
Last prepared 2012/2013
Dr. Ahmed Masri
Inst. Khadija Dweikat
Eng. Nuha Odeh
1
‫جامعة النجاح الوطنية‬
An-Najah National University
‫كلية الهندسة‬
Faculty of Engineering
‫قسم هندسة االتصاالت‬
Department of Telecommunication Engineering
Communications lab (63473/69328)
Experiment name and number:______________________________________
Instructor Name
Registration number:
Student name:
1234Academic Year:
Semester:
Credit Hours: 1
Date:
Description
Day :
Time:
Report mark:
Marks
Student Total Grade:
2
ILO’s
ILO’s %
Grade
Contents
EXP #1: Signal Source................................................................................................... 1
EXP #2: Tuned Circuits ................................................................................................. 5
EXP #3: Amplitude Modulation .................................................................................. 10
Matlab Tutorial of Experiment 3 ............................................................................. 16
EXP #4: AM – Detection and Demodulation .............................................................. 18
Matlab Tutorial of Experiment 4 ............................................................................. 25
EXP #5: Single-Sideband Transmission ...................................................................... 28
Matlab Tutorial of Experiment 5 ............................................................................. 32
EXP #6: FM Modulation-Demodulation ..................................................................... 34
EXP #7: Sampling........................................................................................................ 39
EXP #8: Delta and Sigma – Delta Modulation ............................................................ 45
EXP #9: Pulse Code Modulation ................................................................................. 50
EXP #10: Data formats & Noise in Digital Systems ................................................... 59
EXP #11: Amplitude Shift Keying (ASK)................................................................... 66
EXP #12: FSK modulation/demodulation ................................................................... 73
i
GENERAL ELECTRICAL SAFETY GUIDELINES
1) Be familiar with the electrical hazards associated with your workplace.
2) You may enter the laboratory only when authorized to do so and only during
authorized hours of operation.
3) Be as careful for the safety of others as for yourself. Think before you act, be
tidy and systematic.
4) Avoid bulky, loose or trailing clothes. Avoid long loose hair.
5) Food, beverages and other substances are strictly prohibited in the laboratory
at all times. Avoid working with wet hands and clothing.
6) Use extension cords only when necessary and only on a temporary basis.
7) Request new outlets if your work requires equipment in an area without an
outlet.
8) Discard damaged cords, cords that become hot, or cords with exposed wiring.
9) Before equipment is energized ensure, (1) circuit connections and layout have
been checked by a laboratory technician and (2) all colleagues in your group give
their assent.
10) Know the correct handling, storage and disposal procedures for batteries, cells,
capacitors, inductors and other high energy-storage devices.
11) Experiments left unattended should be isolated from the power supplies. If for
a special reason, it must be left on, a barrier and a warning notice are required.
12) Equipment found to be faulty in any way should be reported to the laboratory
technician immediately and taken out of service until inspected and declared safe.
13) Never make any changes to circuits or mechanical layout without first
isolating the circuit by switching off and removing connections to power supplies.
14) Know what you must do in an emergency, i.e. Emergency Power Off
i
Electrical Emergency Response
The following instructions provide guidelines for handling two types of electrical
emergencies:
1. Electric Shock:
When someone suffers serious electrical shock, he or she may be knocked
unconscious. If the victim is still in contact with the electrical current,
immediately turn off the electrical power source. If you cannot disconnect the
power source, depress the Emergency Power Off switch.
IMPORTANT:
Do not touch a victim that is still in contact with a live
power source; you could be electrocuted.
Have someone call for emergency medical assistance
immediately. Administer first-aid, as appropriate.
2. Electrical Fire:
If an electrical fire occurs, try to disconnect the electrical power source, if
possible. If the fire is small and you are not in immediate danger; and you have
been properly trained in fighting fires, use the correct type of fire extinguisher to
extinguish the fire. When in doubt, push in the Emergency Power Off button.
NEVER use water to extinguish an
electrical fire.
ii
General Lab Report Format
Following the completion of each laboratory exercise, a report must be written
and submitted for grading. The purpose of the report is to completely document
the activities of the design and demonstration in the laboratory. Reports should be
complete in the sense that all information required to reproduce the experiment is
contained within. Writing useful reports is a very essential part of becoming an
engineer. In both academic and industrial environments, reports are the primary
means of communication between engineers.
There is no one best format for all technical reports but there are a few simple
rules concerning technical presentations which should be followed. Adapted to
this laboratory they may be summarized in the following recommended report
format:
 ABET Cover Page
 Title page
 Introduction
 Experimental Procedure
 Experimental Data
 Discussion
 Conclusions
Detailed descriptions of these items are given below.
Title Page:
The title page should contain the following information
 Your name
 ID
 Experiment number and title
 Date submitted
 Instructors Name
Introduction:
It should contain a brief statement in which you state the objectives, or goals of
the experiment. It should also help guide the reader through the report by stating,
for example, that experiments were done with three different circuits or consisted
of two parts etc. Or that additional calculations or data sheets can be found in the
appendix, or at the end of the report.
The Procedure
It describes the experimental setup and how the measurements were made.
Include here circuit schematics with the values of components. Mention
instruments used and describe any special measurement procedure that was used.
Results/Questions:
This section of the report should be used to answer any questions presented in the
iii
lab hand-out. Any tables and /or circuit diagrams representing results of the
experiment should be referred to and discussed / explained with detail. All
questions should be answered very clearly in paragraph form. Any unanswered
questions from the lab hand-out will result in loss of points on the report.
The best form of presentation of some of the data is graphical. In engineering
presentations a figure is often worth more than a thousand words. The reare some
simple rules concerning graphs and figures which should always be followed. If
there is more than one figure in the report, the figures should be numbered. Each
figure must have a caption following the number. For example, “Figure
1.1:DSB-SC ” In addition, it will greatly help you to learn how to use headers
and figures in MS Word.
The Discussion
It is a critical part of the report which testifies to the student’s understanding of
the experiments and its purpose. In this part of the report you should compare the
expected outcome of the experiment, such as derived from theory or computer
simulation, with the measured value. Before you can make such comparison you
may have to do some data analysis or manipulation.
When comparing experimental data with numbers obtained from theory or
simulation, make very clear which is which. It does not necessarily mean that
your experiment was a failure. The results will be accepted, provided that you can
account for the discrepancy. Your ability to read the scales may be one limitation.
The value of some circuit components may not be well known and a nominal
value given by the manufacturer does not always correspond to reality. Very
often, however, the reason for the difference between the expected and measured
values lies in the experimental procedure or in not taking into account all factors
that enter into analysis.
Conclusion:
A brief conclusion summarizing the work done, theory applied, and the results of
the completed work should be included here. Data and analyses are not
appropriate for the conclusion.
Notes
Typed Reports are required. Any drawings done by hand must be done with
neatness, using a straightedge and drawing guides wherever possible.
Freehand drawings will not be accepted.
iv
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP # 1: Signal Source
Objectives
Introducing the Signal Source model and showing how its output frequency is
controlled by its input voltage, and the dB scale of attenuation.
Equipment
Signal source, power supply, oscilloscope, and frequency meter.
Tasks Explain with drowning how CRO is used to measure each of the following
1. Voltage (both AC and DC).
2. Frequency.
3. Phase shift.
1.1 Use of Internal control input
Note before starting this experiment you have to:
1. Calibrate of the voltage controlled oscillator
2. Connect the circuit as shown in Fig 1.1
3. Connect the signal source to the power supply and turn the variable attenuator
to 0 dB and the control C to 5.
4. Switch on the power supply, adjust the oscilloscope to have stable display
5. Draw to scale the output of oscilloscope. Compare the frequency meter
reading and the measured value on RCO.
6. Turn the frequency control C and make a note of what happens.
7. Use the time base of the oscilloscope to estimate the range of frequency
available as the frequency control is moved between 0.1 and 10.0 on the C
scale.
8. Measure the exact frequency and tabulate your results in table 1.1
9. What is the scale setting of the frequency control which gives 455 kHz
output.
1
Fig 1.1
Table 1.1
Scale Setting
F (KHz)
(Frequency meter)
F (KHz)
(CRO)
0.1
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Exercise 1
Based upon the figures recorded in your table, plot a graph on a linear graph paper, of
frequency against scale setting.
KEEP THIS FIGURE FOR FUTURE REFERENCE
2
1.2 Use of External control inputs
The external input terminals. Ax and Bx can be used to control the output frequency of
the Signal source.
1. Set the main frequency control C to give a frequency of 100 kHz, and then
connect the +1 volt ref signal to Ax.
2. Insert a wire link in the A signal path (output) and observe what happens as
you adjust the A sensitivity control.
3. Make a note of the amount by which the 1 V signal changes the frequency
then tabulate your result in table 1.2.
Table 1.2
Sensitivity control
Frequency with no +1 V Frequency with + 1 V Frequency change
setting
signal (KHz)
signal (KHz)
(KHz)
Using input A
0.0
1.0
0.5
Using input B (transfer the + 1 V ref to terminal Bx)
0.0
0.1
0.05
Q1) What is the most sensitive knob A, B or C? Explain?
1.3 Calibrating the attenuator
An attenuator is a device for reducing the magnitude of a signal and it is measured in
decibel (dB), which is a logarithmic measure of the amount of reduction.
The signal source module contains two attenuators, one variable by controlling knob
marked (0 to -20), the other having an attenuation which is fixed by whichever output
socket is chosen. The output attenuation will be the sum of the two values, for
example, if the knob is marked at -8 and the output socket -24 dB the overall
attenuation is 32 dB smaller.
1. Connect the oscilloscope to the 0 dB socket, the variable attenuator is till at 0 dB.
See that the socket Ax and Bx. are disconnected, set the frequency control C to give,
approximately, 500 kHz.
2. Measure the peak to voltage from the CRO screen. Record your results as in table
1.3 below.
3. Use the fixed and variable attenuators to achieve any attenuation needed
4. Take the output voltage at the 0 dB setting as the reference voltage V˳.
3
Table 1.3
Attenuator
setting (dB)
0.0
-3
-6
-9
-12
-15
-18
-21
-24
-27
-30
-33
𝑽
𝑽𝟎
Output volts
p-p (m.v)
1.0
𝒍𝒐𝒈𝟏𝟎
0.0
𝑽
𝑽𝟎
𝟐𝟎𝒍𝒐𝒈𝟏𝟎
0.0
𝑽
𝑽𝟎
(REF)
Note 0 dB means zero attenuation NOT zero output.
Exercise 2
1. Get the output voltage from 0 dB socket when the variable attenuator is set to
-12.
2. Get the output voltage from -6 dB socket when the variable attenuator is set to
-6.
(This confirms that the dB values can be added together)
Use logarithmic-linear scale to plot the dB values against 𝑽𝑽
𝟎
Q2) Compare the value in the last column with the dB values in the first column.
Q3) Give a short description about VCO, draw the basic circuit, and explain?
Q4) Give a short description about the function of signal source.
Q5) What is dB? Where is it used?
Q6) What is the importance of 455 KHz frequency in communication?
4
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP # 2: Tuned Circuits
Objectives
The main objectives are to show that:
1. The parallel combination of inductance and capacitance forms a “tuned”
circuit which resonates at particular frequency.
2. Tuned circuit can be used to respond selectivity to signal of particular
frequencies.
3. Their selectivity depends on the “damping” or energy losses associated with
the circuit.
Equipment signal source, tuned circuit, power supply, and 10-k ohm resistance
oscilloscope, frequency meter, function generator.
2.1 Damped Oscillation
1) Connect the circuit as shown in Fig 2.1 and set the function generator to 3KH
square ware.
2) Set the oscilloscope to display correctly
3) Turn the knob of the tuned circuit model to vary the capacitance and notice
that the frequency of the oscillation changes.
Fig 2.1
5
Fig 2.2
Exercise 2.1 Use the time base to estimate the maximum and minimum
frequencies and record your results.
Q1) Explain from circuit analysis point view how Fig 2.2 is formed?
4) Set the capacitor of tuned circuit to a central position. Then estimate, from the
oscilloscope, the ratio by which each oscillation is smaller than the one before,
and write the results in table2.1
5) Repeat the previous step 5 when 10 KΩ resistance is connected at the output
of tuned circuit across the HI and Low sockets.
6) Draw the results on the oscilloscope with and without a resistance.
Q2) What is the effect of resistance on frequency and amplitude explain\
Table 2.1
Amplitude without
resistance
Ratio An
An+1
Amplitude with
resistance
Ratio An
An+1
1st osscill
2nd osscill
3rd osscill
4th osscill
5th osscill
6th osscill
2.2 Frequency Response
1. Connect the Signal Source and Tuned circuit as shown in Fig 2.3.
2. Set the attenuator to 0 dB the D control to mid-scale and adjust the control C
to give 455 KHz frequency (use control B for fine tuning)
3. Adjust the tuned circuit until the output on oscilloscope is maximum
amplitude and DO NOT touch it again during the rest of experiment.
Q3) Explain that the output is sinusoidal while the input is square wave?
6
4. Change the frequency of signal source as indicated in table 2.2
5. Measure the amplitude and calculate the attenuation in table 2.2
6. Plot the relation between attenuation and frequency.
Fig 2.3
Q4) Calculate the bandwidth (B) from your result.
Q5) Calculate the Q-factor from your results ( 𝑄 =
𝑓0
𝐵
)
𝑓0 : Frequency with max amplitude.
2.3 Swept Frequency Display
A way to draw the frequency response on oscilloscope, the frequency of a signal
is repeatedly swept through the range of interesting frequencies, and the spot on
the screen is deflected in the X direction similarly. Meanwhile the response of the
circuit to it deflects the spot on the Y- axis.
Fig 2.4 shows the time base output, signal source frequency and tuned circuits
output
Table2-2
Frequency (KHz) Amplitude
Amp/Amax
20log(Amp/Amax)
435
440
445
450
455
460
7
465
470
480
Fig 2.4
Fig 2.5
8
1. Connect the circuit as shown in Fig 2.5
2. On signal source, set the attenuation to 0 dB, Ax and Bx controls to zero, also
adjust the frequency to 445 KHz.
3. Switch oscilloscope to XY operations, dc coupling all switches
4. Adjust the output of the function generator to suitable amplitude and
frequency about 40 Hz sine wave. [or 20 Hz triangle waveform]
5. On the tuned circuit adjust the tuned module to resonance shown by maximum
Y-amplitude on the oscilloscope.
6. On the signal source slowly increase the Ax or Bx sensitivity controls and the
display should change shapes. Adjust the C control until the peak amplitude
occurs in the middle of the screen like Fig 2.4c
7. Draw the output.
8. Connect 10K-ohm resistance across the output of tuned circuit.
Q5) What is the effect of resistance on the peak response?
Q6) What is the effect of resistance on the response at frequencies away from the
peak?
Q7) What is the effect of resistance on the bandwidth?
Q8) How can we use tuned circuit in communication?
9
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #3: Amplitude Modulation
Objectives
To see that Amplitude Modulation
1. Is causing the amplitude of the carrier to vary in accordance with the
modulating signal.
2. Is a multiplicative process which produces side frequencies.
3. The original signal can be recovered by further modulation process.
Equipment required
Signal source, filters (ACS2956C), double balanced modulators, power supply, (2)
620 ohm, 10-k ohm, 4.7 nf, voltmeter, function generator, oscilloscope.
3.1
Introduction
Before starting this experiment you must remember the AM equation and how it is
represented in time and frequency domains. The AM signal can be written as:
VAM(t) = A(1+m(t))cos(wct)
If m(t), the modulating signal, is a single tone, then the AM signal is represented by
VAM(t) = A(1+µcos(wmt))cos(wct)
AM-signal in the time domain is shown in Fig 3.A
Fig 3.A
3.2
Multiplying Action of a Modulator
1. Connect the circuit shown in Fig 3.1
2. Adjust the oscilloscope, so that two square wave forms should be seen. Set Y
channels for dc coupling
3. Set the signal source to 0 attenuation and the frequency about 455 KHz.
4. Turn knob A fully clockwise.
5. Adjust control A to give dc voltage from 0 to 1 volt in steps of 0.2 v. For each
setting record the peak-to-peak value of the output voltage as shown in table
3.1
Q1) Is the output in phase with the input? Draw the result.
10
Fig 3.1
DC signal (V)
Table 3.1
Output signal V p-p
(dc input to a)
Output signal V p-p
(dc input to 𝑎̅ )
0.0
0.2
0.4
0.6
0.8
1.0
Q2) What difference is caused by using 𝑎̅ terminal instead of a?
Q3) Plot the peak to peak output voltage against the dc input counting the dc
input as negative when applied to the a terminal. The result should show that the
output is proportional to the input, provided that the input signal is not too large.
Let it below 0.5 v.
1. Adjust the dc voltage until the input and output voltage have the same
amplitude, as seen on the oscilloscope. Then vary the ac output of signal
source. Using its attenuator.
Q4) What relationship is there between the output and the input at the b terminal?
7. Your results should show that c=k ab that is the output is a product of two
inputs.
11
3.3
Amplitude modulation
1. Connect the circuit shown in Fig 3.2
2. Set the function generator to give zero output voltage and select sine wave at 1
Hz.
3. Set the dc bias to 0.5 v, using knob A.
4. Slowly increase the output voltage of the function generator and carefully
watching the oscilloscope, you will see the amplitude of the output signal
oscillating as a result of the carrier being amplitude modulated by a 1 Hz
signal.
5. Rise the signal frequency (from the function generator) to 50 KHz and the
output voltage to 0.5 v, then adjust your oscilloscope or the signal frequency
and you will see an AM signal as shown in Fig 3.3.
Q5) Calculate the modulation indexes, m, for your signal.
6. Increase the signal as amplitude gradually, so that the modulation index
increase.
Q6) Why might be problems in recovering the original signal if the modulation index
exceeds 1?
7. To removing the side band spectrum. Connect a band pass filter (AC2956C)
between modulator and Oscilloscope as shown in Fig 3.4
Fig 3.2
12
Fig 3.3
8. On the function generator reduce the frequency to 1 KHz.
9. Adjust the knob C on the signal source so that the frequency is accurately in
the center of the filter’s pass band.
10. On the function generator, increase the signal frequency.
11. Draw the output of the oscilloscope.
Q7) What happens to the filter’s output as the frequency is raised, and why?
KEEP ALL CONFIGURATIONS FOR THE NEXT SECTION
Fig 3.4
13
3.4
Demodulation using balanced modulator
The AM signal can be demodulated (recovering the base band signal) by using
balanced demodulator.
cos(wc t )
1. Connect the circuit as shown in Fig 3.5
2. Draw, to scale, the output of the oscilloscope, this should be the demodulated
waveform
3. Connect a 4.7nF capacitor in parallel with the demodulator load. The capacitor
will remove the high frequency components (modulator module works as a
demodulator).
4. Draw to scale the waveform of the oscilloscope.
Q8) Calculate the ratio of power of the output signal of the demodulator to input
signal of the function generator.
Fig 3.5
3.5
Suppressed carrier modulation
The AM suppressed carrier modulation is given by AM-SC = m(t) cos(wct)
We can get this signal by disconnecting the + 1 v ref. Socket .
Draw to scale the modulated and demodulated waveforms.
14
Q9) What happens to the modulated and demodulated waveform, do you have a shift
in the dc level?
Q10) Does the demodulation process need the carrier in the modulated signal?
3.6
Square law detection
A(1  m(t )) cos(wct )
V0 (t )
Q11) Analyze the block diagrams shown above.
You can see that if the AM signal is used to modulate itself, the effect is to produce a
signal which is the square of the received signal.
1. Remove the carrier (the signal from the signal source) from the demodulator
and connect it’s a terminal instead to the modulated signal (so that a and b
signals are the same).
2. Reconnect the lead to the 1 v ref. Socket. Do you achieve demodulation?
3. See the effect of the modulation index, m, on the output by increasing and
decreasing the carrier, using the A control.
4. Sketch the input and output waveform s of the demodulator for the
modulation index 0.1 and 0.9
Q12) Comment on your results?
15
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXPERIMENT #3-Matlab
Matlab Tutorial of Experiment 3
Objectives:
To simulate the experiment using “Simulink”, this is available in MATLAB
Software.
How to start with Simulink:
To start you need to install the package of MATLAB with will contain in the
Simulink tool.
After making sure that MATLAB is running of your computer follow the steps
below:
1. Run MATLAB on your computer.
2. In order to turn on the Simulink tool type simulink then click Enter or
from the start menu in MATLAB, choose Simulink then Library Browser.
3. On the library browser table, choose file
New
Model, or click on
the first icon on the tool bar (Create a new model), or simply use the
shortcut Ctrl+N.
Needed Blocks:
Experiment 3 is about the Amplitude Modulation (AM), so the simulation of this case
will need the following Blocks:
 From the library browser table, choose “source” then choose two “signal
generators” and drag them using the mouse to the model page, then
choose “constant” and drag it to the model page.
 From the library browser table, choose “Math Operations” then choose
“Sum” and drag it to the model page, then choose “Product” and drag it
to the model page.
 From the library browser table, choose “Sinks” then choose three
“Scopes” and drag them to the model page.
 From the library browser table, choose “Signal Processing blockset” then
choose “Signal Processing Sinks” then choose “Spectrum Scope” and
drag it to the model page.
16
Procedure:
1. We add the output of the first generator with the constant in order to simulate
(constant +M(t)).
2. The result of the addition will be multiplied with the input of the second
function generator which represents the carrier signal.
a. Make sure that the frequency of the carrier signal is set to be larger than
the frequency of the message signal (why??).
b. From the “Product” properties, set the sampling time from -1 to 0.001.
3. The output of the product will be connected to the scope in order to see the
modulated signal in time domain; also it will be connected to the spectrum
scope in order to see the modulated signal in frequency domain.
a. Make sure to enable the “buffer input” at the spectrum scope
properties.
b. You can change the axis properties of the scopes as you wish.
The model will look like:
You can rename any block your want by the name which is suitable for the situation.
To simulate the DSB-SC, simply set the value of the constant to zero,
run model again and notice the difference.
Is there Any Others ways to simulate the AM in Matlab?
17
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #4: AM – Detection and Demodulation
Objectives
To show that
1. A diode may be used for detecting (demodulating) a normal amplitude
modulated signal.
2. To avoid distortion of the signal, the signal level and time constants associated
with the circuit must be carefully chosen
3. A dc component in the detected signal. Dependent on the carrier amplitude,
can be used for automatic gain control AGC.
Equipment required
(100 K-ohm, 220 nF), Signal source, Detector, Tuned circuit, Double balanced
modulators, power supply, function generator, oscilloscope, voltmeter.
4.1 Diode Detector
1. Connect the circuit shown in Fig 4.1
2. On the signal source set the frequency to approximately_455kHz, the
attenuator to 0 dB and potentiometer A to about its mid position.
3. On the function generator set the frequency to 300 Hz. Sinusoidal
waveform and minimum output (zero) amplitude.
4. Adjust the tuned circuit for maximum response.
5. Adjust potentiometer A to give 2 V pp on Y1 trace.
6. Increase, the output of the function generator until the modulation index
(m) is just less than 1.
Sketch the two waveforms you see on the oscilloscope.
Q1) What frequency components do you think will be presented in the detector output
waveform?
7. To remove high-frequency components, connect the 5nf capacitor C4 in
8.
9. Sketch to scale the detector output waveform.
Q2) What is the effect of capacitor on the output?
Q3) What is the efficiency with the parallel R.C load knowing that the efficiency of a
diode detector is defined as output to input, where the output is the peak to peak
value of the recovered signal and the input is the range of variation of the peak carrier
voltage (Vmax - Vmin of the positive cycle).
18
1. See the effect of changing the load on the waveform. Tabulate your result in
table4.1
Fig 4.1
R
(kΩ)
4.7
4.7
100
100
C
(nf)
RC
(µs)
Table 4.1
Output Max.
ripple p-p (mv)
Remarks
5
22
5
22
Q4) What do you notice from the result you have and the waveform of the
oscilloscope. Can you explain the non-sinusoidal waveforms?
19
2. The carrier component in the AM signal corresponds to the dc terminal a of
the modulator. Reduce it, by turning the A control on the signal source
anticlockwise.
Q5) Is a simple diode detector capable of demodulating a signal with suppressed
carrier properly? What happens?
Distortion occurs when the rate of discharge through R is less than that required to
follow the input signal amplitude. The effect is known as diagonal peak clipping, Fig
4.2e
3. Change the frequency of m(t) from 100 Hz to 10 kHz. Draw the output to
scale and show that the diagonal peak clipping is frequency dependent.
Q6) Does the diagonal peak clipping disappear? Is it a function of the modulation
index?
4. With the signal frequency of m(t) 300 Hz, R=100KΩ, C=22nf, decrease the
amplitude of the modulating signal, m(t), from the function generator. (Adjust
your oscilloscope properly).
Exercise 1: If modulating frequency increases, does it affect the diagonal peak
clipping. To avoid this problem, do you have to increase or decrease the time
constant? Test that.
The detected output signal has a dc value which must be removed.
5. For dc-blocking, set the frequency of m(t) to 1kHz, add a 100K-Ohm resistor
and a 220nf capacitor as shown in Fig3.4 to the output of the detector. Draw
the output before and after dc-blocking.
6. Reverse R,C as shown in Fig4.4, what is the effect on diagonal peak clipping
and is the output signal a dc signal?
Exercise 2: At what modulation index does the distortion start? Does it vary with
frequency?
Exercise3: Show that the distortion is avoided if m > R3/(R+R3).
For AM-SC disconnect +1 ref.
4.2 Automatic Gain Control
AGC is used in communication systems to keep the variation of output of received
signal constant, while the input signal is not. To vary the gain, we can use doublebalanced modulator since its output is proportional to each of its input, if one input is
the signal to be received and the other input is a control signal, the gain applied to the
signal will be proportional to the control signal. The dc component of the detector
output provides a positive-going dc voltage which is a measure of the signal strength,
a block diagram in Fig4.5 represent the system.
20
Exercise 4: Analyze the block diagram shown in Fig 4.5
1. Connect circuit shown in Fig4.6
The output of the signal modulator is passed into the other product modulator in the
module, which will now act as a gain controlled amplifier. Its gain is initially set by
the dc voltage applied to input a.
2.
3.
4.
5.
6.
7.
8.
9.
Adjust dc voltage to 0.25v using voltmeter.
At first leave the AGC lead unconnected.
Set the oscilloscope to externally triggered from the function generator.
On the signal source set the frequency to 455KHz and the level to -18dB.
Check the tuning circuit to give maximum signal on Y1.
On the function generator set the output to be sine wave at 1 KHz.
Set the channels of oscilloscope to suitable values.
By changing the amplitude of the function generator signal, set the modulation
index to a level which avoids distortion of the modulation envelope.
10. On the detector module, set the gain control of the dc amplifier to “min” and
move the slide switch to the left (do not apply -12 offset).
11. Complete the table below with and without AGC.
You will notice that the AGC makes a noticeable difference to the variation in output
signal.
Exercise 5: Plot on a single logarithmic/linear graph paper two graphs of your peak
to peak output voltage figures against dB settings of source attenuators, label the
graph clearly. (X-axis signal level and Y-axis output V pp). This graph should make it
clear that AGC makes the output more constant).
21
Fig 4.2
22
Fig 4.3
Fig 4.4
Block Diagram
Fig 4.5
23
Fig 4.6
Carrier
attenuator
-12
-15
-18
-21
-24
-27
-30
Table 4.2
Output voltage V p-p without
AGC
24
Output voltage V p-p with AGC
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXPERIMENT #4-Matlab
Matlab Tutorial of Experiment 4
Objectives:
To simulate the demodulation process of the AM-LC and AM-SC using Simulink.
Introduction:
In this tutorial, we will recover the message signal that we already modulated in the
previous experiment, AM signal could be Large Carrier “LC” or Suppress Carrier
“SC” and each one of these two types had its own demodulation technique. Envelope
detection is used in the first case while square detection is used for the second.
Needed Blocked and Procedure:
We will use almost the same blocks that we used in the previous tutorial in order to
simulate the AM, A new block will be used which is the “Transfer function”, which
we can obtain from the library browser then we choose “continuous” then we choose
“transfer fcn” and drag it the model page.
In order to simulate the envelope detection, we will do the following:
1. We generate Amplitude Modulation signal with large carrier as we did it the
last tutorial.
2. We multiply the AM signal with the same carrier that we used in order to
modulate the original signal.
3. The result will enter to a low-pass filter which we can simulate using the
transfer fcn block as the following:
The transfer function of the low pass filter is the following:
𝑜𝑢𝑡𝑝𝑢𝑡
1
=𝐾
𝑖𝑛𝑝𝑢𝑡
1 + 𝑆𝑇
Where “T” is the time constant, T=RC.
And:
𝑓𝑐 =
1
1
=
2𝜋𝑇 2𝜋𝑅𝐶
Or equivalently (in radians per second):
1
1
=
𝑇 𝑅𝐶
Since MATLAB deals with radians and with some manipulation, the transfer function
will be:
𝑂𝑢𝑡𝑝𝑢𝑡
𝑤𝑐
=𝐾
𝐼𝑛𝑝𝑢𝑡
𝑤𝑐 + 𝑆
𝑤𝑐 =
25
In order to simulate this in simulink we double-click on the transfer fcn block and
this window will appear:
We change both the coefficients of the numerator to “w” by typing [2*pi*f] where “f”
is the cut off frequency of the filter and the denominator to “s + w” by typing
[12*pi*f].
As we selected the frequency of the message signal to be 1Hz, we choose the cut off
frequency to be 5Hz.
In order to remove distortion we cascade two low pass filters in order to get our
message signal.
For the case of demodulation of AM-SC we used the square law detection which is
same as the previous method expect that we will multiply the AM signal with itself
instead of the carrier.
The model will look like in the case of envelope detection:
26
The model will look like in the case of square law detection:
27
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #5: Single-Sideband Transmission
Objective
The main objective is to understand
1. The term signal-sideband transmission
2. A method of producing and receiving a signal sideband transmission
3. The importance of synchronizing the carrier signal used for modulation
processes at transmitter and receiver
5.1
Single-Sideband Transmitter
1. Connect the circuit as shown in Fig5.1 (the sending end of the communication
channel).
2. Set the amplitude of the signal source to 0 dB and carefully adjust its
frequency to 450 KHz using Control Bx and C.
3. Set the function generator output to 0.5 Vpp, 2KHz sine wave.
4. Set the oscilloscope with external triggering from the function generator.
Fig 5.1
Y1 trace should display the characteristics waveform of a suppressed-carrier AM
signal in which the top and bottom of the envelope cross each other. Y2 is a high
frequency sine wave (you should see the carrier, open the range to see that).
Q1) What is the frequency of Y2 waveform?
Q2) Explain this value theoretically.
Q3) Why are the two waveforms displayed different?
28
5.2 The Local Oscillator
At the receive we need a source of carrier-frequency to demodulate the transmitted
signal (local oscillator).
Connect the receiver circuit as shown in Fig5.2 with Y1 sensitivity set to 1 v/div.
Q4) What are the main parts of the local oscillator?
Verify that the circuit is oscillating, (producing a high frequency output)
Fig5.2
5.3
Single-Sideband Receiver
1. Complete receiver circuit as shown in Fig5.3, the frequency meter will be used
later.
2. The oscilloscope is connected to show the input and output of the system, (the
modulating signal at the transmitter and the recovered, demodulated and
filtered, output signal of the receiver).
3. Set the low-pass filter to 2.8 KHz cut-off frequency.
4. The oscilloscope is externally triggered from the function generator
5. Y1 should be quite steady, showing the 2KHz input signal, Y2 is not
important now.
6. Connect the frequency meter to the TTL of signal source and adjust it very
accurately to 450KHZ.
7. Connect the frequency meter to the output of the receiver’s local oscillator
(Fig 5.2) and adjust it very accurately to 450KHz.
8. With very sensitive movement, only a touch, adjust very accurately, the local
oscillator frequency to see that Y2 displays a waveform like that of Y1.
9. Check that, the oscilloscope display will be equally steady as you alter the
frequency of the signal from the function generator over the range of 1 to 3
KHz.
10. Replace the function generator with an Audio modules as shown in Fig 5.4
11. Set the oscilloscope to internal triggering.
29
You can talk into the microphone and hear from speaker.
Q5) What is the quality of the heard signal if the frequencies of the sender and
receiver do not exactly match?
Q6) What happens to the received signal if the frequencies of the carrier at the
transmitter and the local oscillator at the receiver are very slightly different?
If the required audio band is 1 to 3 KHz, what happens to the received signal if the
frequency of the local oscillator at the receiver is 453 KHz? To answer, complete
table 5.1 below.
Fig 5.3
Signal (KHz)
1
3
Table 5.1
Transmitted (KHz)
450
453
30
Received (KHz)
Fig 5.4
31
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXPERIMENT #5-Matlab
Matlab Tutorial of Experiment 5
Objectives:
To simulate the modulation process of the signal side band using Simulink.
Introduction:
Single Sideband “SSB” is a modulation technique depends on suppressing both
carrier and one side band in the same time, sending one band of the modulated signal
in order to save power and bandwidth.
The Figure shows the block diagram of Weaver’s method for generating SSB
modulated waves. The message (modulating) signal m(t) is limited to the band
ƒa ≤ |ƒ| ≤ ƒb
The auxiliary carrier applied to the first pair of product modulators has a frequency fv,
which lies at the center of this band, as shown by
𝑓𝑎 + 𝑓𝑏
𝑓0 =
2
The low-pass filters in the in-phase and quadrature channels are identical, each with a
cutoff frequency equal to (ƒa - ƒb)/2. The carrier applied to the second pair of product
modulators has a frequency fc that is greater than (ƒa - ƒb)/2.

m(t )
cos(2f 0t )
cos(2f c t )
sin(2f 0t )
sin(2f ct )
32
Needed blocks and Procedure:
In order to build this model you will need:
Function generators, summer, Products, spectrum scopes, low pass filters and scopes.
The model will look like:
33
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #6: FM Modulation-Demodulation
Objectives
To show that
1. A carrier may be modulated by changing its frequency in accordance with the
signal.
2. A frequency-modulated signal has many side bands. Dependent on the
amplitude as well as the frequency of the signal
3. FM provides improved immunity from interfering signals, with increasing
bandwidth compared with AM.
4. How to adjust and use FM detector.
5. Equipments required signals source, filter, tuned circuit, power supply, 10 k
ohm, oscilloscope, frequency meter, voltmeter, 2 function generator.
Introduction
The FM signal for base band modulating signal, m(t), is given by
1
VFM = Ac cos(2πfct + 2π kr ∫0 𝑚(𝑡)𝑑𝑡)
If m(t) is a single tone equal to Am cos(2π ƒm t), then the FM signal is written as
V FM= Ac cos(2πfct + β sin(2πfmt))
When β is the modulation index equal to frequency deviation divided by the
modulation frequency (β = ∆f/ƒm ) where ∆f = Kf Am .
An FM signal represented in time and frequency domains is shown in Fig 6.1
Jn(β) is Bessel function coefficients. The number of side bands spectrums depends on
the modulation index β:
To calculate the band with FM signal using number of side bands
BW = 2nfm, Where n is the number of significant side band spectrums.
Fig 6.1
34
6.1
Setting up a Frequency Modulation Signal
1. Connect circuit shown in Fig 6.2
2. Set the function generator on square wave with 0.1 Hz and 0 amplitude.
3. Set the frequency meter to a sampling rate of one sample per second.
4. Set the signal source modulate to 0 dB and control B to maximum
sensitivity.
5. Using the frequency meter, and control C, set the frequency of the signal
source to 455 KHz.
6. Increase function generator amplitude to 6 Vpp.
Fig 6.2
Q1) What is happening now to the frequency, as shown on the oscilloscope and
frequency meter, record maximum and minimum frequencies obtained?
7. Adjust the function generator output so that the oscilloscope measure is as
6 Vpp (measure is at 100 Hz, temporary).
8.
With 0.1 Hz. Square wave, adjust control B until the deviation becomes
exactly 60 kHz (455±60) kHz. Then disturb the B control.
9. Change the peak and frequency of the signal as tabulated in Table 6.1
Peak
value.
6
6
6
4
4
4
2
2
2
Frequency Hz
0.1
1.0
10
0.1
1.0
10
0.1
1.0
10
Table 6.1
Frequency deviation on
frequency meter(±kHz)
60?
Shape on oscilloscope
40?
20?
Q2) Comment on your results, is the frequency deviation depend on amplitude or
frequency, explain?
Q3) Does the deviation change with waveform, if the peak signal voltage remains the
same?
35
11.2
Display of the side Frequencies
SET THE FUNCTION GENERATOR TO SINE WAVE
1. Connect the circuit shown in Fig 6.3, where the modulation generator is a sine
wave function generator used to generate the modulating signal.
2. Connect the frequency meter to the signal source module.
3. 3.Set the oscilloscope for XY operation
4. Set the modulation generator to sine wave 20 kHz and its output to 0 voltage.
5. Set the sweep generator output to 5 Vpp, 3 kHz triangle wave form.
6. On the signal source, set the A control at 0.2 and the variable attenuator to 0
dB. DO NOT disturb the B signal.
7. Both tuned circuits should be set approximately to 455 kHz.
8. Adjust both tuned circuits for maximum response (regardless of the shape)
9. Adjust the second tuned circuit and the neutralizing capacitor of the crystal
filter together to get a response which is symmetrical as possible.(You should
get the response at the center of the trace)
10. Increase-slowly the modulation generator output to 2 Vpp. You should see a
range of side frequencies as appear in Fig 6.4
Fig 6.3
36
Fig 6.4
Q4) With 2 Vpp input, what value of deviation have you set up (refer to Table 6.1)
Q5) Count the number of pairs of side frequencies?
Complete Table 6.2 below.
Signal
freq.
p(KHz)
Input
voltage
Vpp
20
20
20
2
4
6
11.3
Frequency
deviation
in freq.
meter D(=
KHz)
Table 6.2
Deviation
ration D
ρ
No. of
pairs of
side
freq.
Approx
band
width
(KHz)
AM
Bandwidth
FM DETECTION (The phase shift Detector)
The principle of phase shift detector is shown in the block diagram in Fig 6.5
Fig 6.5
Q6) Explain how phase shift detector work?
Q7) Name two different type of FM demodulator?
1. Connect the circuit shown in Fig 6.6, ignore connections with broken lines.
2. Do not disturb the B control and check that the modulator is balanced.
3. Set the output of the function generator to 0.
4. With no signal applied, adjust the height of each Y trace to a different
known height.
5. Set the signal source to 0 dB at 455 kHz, what do you observe on both
channels of the oscilloscope? Comment on the output frequency.
6. Adjust the tuned circuit for maximum input signal, then observe the effect
of varying the signal frequency.
37
7. Sketch to scale the waveforms on oscilloscope.
8. Plug a 4.7 nF capacitor in parallel with the 620-ohm resistance, sketch the
output wave forms.
9. Disconnect Y1.switch the oscilloscope for XY working and make the
connection shown in the broken line in Fig 6.6
10. Set the function generator to triangle wave 5 Vpp, 100 Hz. The
oscilloscope should display the characteristic of the detector.
11. Adjust the tuned circuit so that the graph is at the center of the
oscilloscope.
Q8) If the horizontal scale is 10 kHz per volt, what do you consider the deviation
which this detector can handle within its linear region?
Q9) How could it be extended?
Connect a 620 ohm resistor across the tuned circuit.
Q10) What deviation which this detector can now handle within its linear region?
Fig 6.6
38
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #7: Sampling
Objectives
The main Objectives are
1. To know what sampling is and what is 'sample and hold'.
2. The sampling theorem and aliasing frequencies.
3. Understand the effect of filters in eliminating aliasing.
4. What is Time division multiplexing.
Equipment
Waveform synthesizer, 2 Sample/Hold & Multiplex, Power supply, function
generator, oscilloscope, and frequency meter.
7.1
Sample and Hold
Fig. 7.1
39
1. Connect the circuit as shown in Fig. 7.1
2. Set the clock switch on, the module to 'int', then set the frequency of its
waveform generator to 5 KHz approximately.
3. Set the function generator to 100 Hz, sine wave at 10 Vpp.
4. Set the oscilloscope to externally triggered from the function generator.
You should see both the input sine wave signal and the sampled
approximation as shown in Fig. 7.2
5. Adjust the signal frequency and notice the movement of the sampling steps
along the output waveform.
6. Move the Y2 lead to the output, instead of the input, of the low pass filter.
Note that the output signal now clearly re-sample the input signal.
Fig.7.2
Effects of Sampling Time
In order to see the effect of inadequate sampling time,
1. Alter the circuit as shown in Fig. 7.3 (Signal source and oscilloscope
connections unchanged).
2. On the pulse former, adjust the set pulse widths control until the waveform is
just showing some noticeable distortion.
3. Vary the signal frequency.
What effect on the distortion do you notice as the frequency is varied?
40
Fig 7.3
6.2 Aliasing
Q1) What is the minimum number of samples needed to be able to recover the
original signal?
1.
2.
3.
4.
5.
Reconnect the waveform generator directly to the sample-and-hold, as in Fig
7.1.
Measure the sampling frequency (i.e the frequency of the square-wave output
of the wave generator). It should still be about 5 KHz.
Increase the frequency of the signal, from the function generator, to about this
value.
What you should see is illustrated in Fig 7.4, an aliasing frequency has
extended right down to the lowest values.
Set the signal frequency close to half the sampling frequency. You should a
modulated waveform without distortion.
41
Fig 7.4
Use of an Input Filter
To prevent aliasing, it is necessary to protect the system against unwanted frequency
components by filtering the input signal.
1. Reconnect the input signal from function generator so that it passes
through the other low pass filter on the module to the sample and hold
circuit.
2. To see that the aliasing problem is reduced, (but not eliminated because
the filter is not perfect), check how the system responds as you vary the
signal frequency from 1 KHz to 10 KHz.
Q2) at what signal frequency does the filter reduce the input to the sample and hold
circuit to about 10% of its value at low frequency?
Q3) based on your previous answer, what sampling frequency would enable alias
response to be kept below 10%?
6.3 Pulse Amplitude Modulation (PAM) Multiplex
The signals have been generated in effect, modulate the amplitude of a pulse train
with the value of the input signal. This is called Pulse Amplitude Modulation (PAM).
By interleaving two PAM signals you will be able to transmit two signals.
42
Fig 7.5
1. Connect the circuit as shown in Fig. 7.5, but do not make the connections to
the output filters shown by the broken lines.
2. On the ACS2956H waveform synthesis module.
3. Set the clock set frequency to "max", set the output switch to "normal", it's
amplitude to "max" and adjust the slider controls to produce a waveform
which is not like a sine wave.
4. Set the function generator to 5 Vpp, sine wave at 200 Hz, and adjust its
frequency so that its waveform can be seen as it drifts across the oscilloscope
screen.
5. Sketch the two waveforms you can see, after adjusting the clock frequency, if
necessary.
6. Move Y1 lead to point 3. Sketch the new waveform. You should see the
samples of each the input waveforms. (you may see the sine wave inverted, no
problem).
7. Move Y2 to pints 4 and 5 in turn.
8. Sketch each waveform.
43
9. Connect points 4 and 5 to the output filters. THE WAVEFORMS AT
THESE POINTS WILL BE CHANGED DUE TO THE INPUT R-C
NETWORK IN EACH FILTER.
10. Sketch each waveform.
11. Get the output from points 5 and 7. That is at the output of the low pass filter.
What about its ripples ?
12. Disconnect the lead marked 'A', and connect the pulse former and
multiplexing switch controls to point 8 instead.
13. Set the associated waveform generator clock switch to "int" and set the
frequency control to "max".
14. Sketch the waveform at points 7 and 8.
44
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #8: Delta and Sigma – Delta Modulation
Objectives
1. Understand the process of delta modulation and sigma – delta modulation.
2. Know that each of these systems is a simple form of digital
communication.
3. Understand that digital communication has to distort the signal to some
extent, in order to quantize it, but that this enables subsequent distortion
noise introduced by the channel to be largely rejected.
Equipment required
Sigma – Delta, integrator, Low pass filter, power supply, function generator,
oscilloscope.
Introduction
Delta modulation is called so because each digit sent to the communication
channel is electively an instruction to increase or decrease the signal value by
a fixed increment. Since there will be time when one wants the signal not to
change its value, the best that the system then do is to rise it and lower it
alternately. Provided that the amount is very small, this can be often be
tolerated.
Fig 8.1
45
Fig. 8.1 shows block diagram for Delta modulation system it include
(comparator, clock, data flip-flop, integrator and power amplifier) arranged in
feedback control loop. The output is fed to the integrator, which however has a
DC pious bias also.
The bias is to allow for the fact the output (from the data flip-flop) various
between the two level 0 and +5 volt. The bias allows the integrator to receive
an effective input (+2.5 OR -2.5) volt.
The difference amplifier forms a signal, which changes a sign as the difference
changes polarity.
The comparator detects the changes, which produce a quantize (two value)
signal to the data flip-flop. This passes the comparator signal to its output
terminal every time it is clocked.
The whole is a control loop, which tries to keep the integrator output as close
as possible to the input signal. If the digital signal is connected to an additional
integrator, as indicated by dotted lines in Fig. 8.1, then the two integrators
output should ideally behave in much the same way. So the receiving
integrator should produce the copy of the original signal produced by the
integrator in the loop.
8.1 Delta Modulation
1. Connect the circuit shown in Fig. 8.2.
2. On the Sigma – Delta module set the clock frequency to 'min'
3. Set the function generator to deliver 10v peak to peak sinusoidal
signal at 500 Hz.
4. Switch the audio module as a speaker and adjust its level to give a
comfortable loudness.
5. Switch the low pass filter module to its highest cut off frequency.
On the oscilloscope you should see digital signal switches between two
levels on one channel an approximation to the input 500 Hz signal to
the other channel
6. Move the Y1 lead to the output of receiving integrator in the integrator
module and verify that its waveform is similar.
7. Adjust the frequency of the input signal very slowly either side of 500
Hz.
You should be able to find one or two spot frequencies at which the
trace become steady enough for you to see them clearly. Notice how, at
various times in the period of the input signal. The digital signal
sometimes spends more time at its upper level, and sometimes at its
lower level.
46
Fig 8.2
Q1) when the digital signal is mostly at the upper level, what is the corresponding
features of the integrator output signal?
Slope Distortion
8. Increase the signal frequency gradually to 1 KHz or a little more,
watching a limiting process shown by the two waveforms.
Q2) what aspect of the output signal is limited, and why?
47
Noise due to integrator
9. Reduce the amplitude of the input signal toward zero, increase the
level of the audio module so that you can hear something.
You will find that various odd noises can be heard in the background
Q3) can the low pass filter be used to eliminate the background noise?
10. Try different cutoff frequencies in the low pass filter module, how it
effects?
The effect of the clock frequency
11. Reset the low pass filter to its highest frequency, increase the
amplitude of the signal to 2 vpp.
Q4) how would the waveforms be affected if the clock frequency was increased,
what happens both with and without an input signal?
8.2 Sigma – Delta Modulation
It is somehow like Delta Modulation. Fig. 8.3 shows a control loop in
which a signal is made to match the input signal, and the digital signal is
generated in the process. the essential difference is that what is the
integrated is not the digital signal, but the difference between it and the
input signal.
The Delta modulation system tried to make the input signal and the
integral of the output. the sigma – delta system tries to make equal the
input and the output. The receiver in this system is there for just a low pass
filter to remove the switching steps from the waveform.
Fig 8.3
1- Connect the circuit shown in Fig. 8.4
48
2- On sigma – delta module set the clock frequency to 'min' .
3- Set the function generator to give 4 vpp signal at 500 Hz.
Fig 8.4
4- Switch the audio module as a speaker and adjust its level control to
give suitable loudness.
5- Switch the low pass filter module to its highest cutoff frequency.
You should now see a digital signal which switches between two levels
and the output signal which has considerable noise
6- Increase the clock frequency until the basic sine shape of the output
waveform is recognizable.
7- Vary the signal frequency over the range 0.1 to 1.0 KHz, notice what
happen to the output waveform.
8- Increase the input signal amplitude until limiting occurs.
Q5) what is the limiting amplitude of the signal at 1 KHz and at 300 Hz ?
Q6) what is the obvious difference between this result and that for delta modulation ?
Q7) what determine the limiting amplitude of the output signal ?
49
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #9: Pulse Code Modulation
Objectives
1- To generate Pulse Amplitude Modulation (PAM) and to demodulate
(PAM).
2- To understand sampling theorem and”sample and hold” concept.
3- To understand pulse code modulation concept.
4- Understanding the need of synchronization in PCM.
Equipments required
PCM trainer, low pass filter, audio module, power supply.
Pre-Lab Work:
1- Read the relevant material in your text book.
2- Explain the main result of sampling theorem, and give a simple graphical
proof for it, the define “spectral folding” .
3- A signal g(t)=Sinc2(10πt), is sampled (using informally spaced impulses)
at a rate of (i) 10 Hz; (ii) 20 Hz; (iii) 40 Hz. For each of the tree cases:
a) Sketch the sampled signal and its spectrum. (Hint: the BW of g(t) is
20π rad/sec.).
b) If the sampled signal is passed through an ideal low pass filter of
bandwidth 10 Hz, sketch the spectrum of the output signal.
4- How is PCM different than PAM. Draw a simplified block diagram of a
PCM system?
5- Describe the use of PCM and TDM in modern telephony: give the basic
architecture of modern telephony networks from the user phone to the
central office switch , discuss T1/E1 multiplexing, framing and signaling
bits, etc.
Background:
1-
Pulse Amplitude Modulation (PAM) is a very simple modulation process
which takes samples of the signal value at regular intervals and produces a
stream of pulses as an output, each pulse having an amplitude proportional to its
sample value.
We may express the PAM signal as :
S(t) = ∑∞
𝑛=−∞ 𝑚(𝑛𝑇𝑠)ℎ(𝑡 − 𝑛𝑇𝑠)
Where Ts is the sampling period, and 𝑚(𝑛𝑇𝑠) is the sample value of m(t)
obtained at time 𝑡 = 𝑛𝑇𝑠. h(t) is a standard rectangular pulse of duration T.
This is shown in Fig. 9.1
50
Fig 9.1 Principle of PAM
A PAM signal can be recovered by using a low pass filter, which removes the
sampling frequency and its harmonics.
2-
Pulse Code Modulation differs in from PAM in that, instead of sending a
pulses of corresponding amplitude, it sends numbers which represent the sample
values. The numbers are typically encoded in binary form and sent serially over
a single wire. In order to reassemble the bits correctly, the timing of the transmit
and receive clocks must be synchronized.
Fig 9.2
51
Lab work:
Part 1: Pulse Amplitude Modulation
A. Generation of PAM signal
1-
Connect circuit shown in Fig. 9.3
Fig 9.3
Set the sampling switch to 32 KHz and DC output to 1V,.
3- Observe on the oscilloscope the output on channel 2.
4- Change the value of DC (i.e. increase and decrease).
2-
Q1: what aspect of the output wave form is simply related to the variable DC input
voltage?
Disconnect the input from the Dc source and connect it to function
generator.
6- Set the function generator to frequency 2KHz and 2 Vpp.
7- Observe the output on oscilloscope on channel 2.
8- Change the frequency to 20 KHz, Observe the output on channel 2.
5-
Q2: Does the output is still reasonably related to input?
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B. Demodulation of PAM using filters
1-
Connect the output of Fig. 9.3 to LPF as shown in Fig. 9.4b
m(t )
s(t )
Fig 9.4b
Set the function generator to 2 KHz and 2 Vpp.
Q3: does the output of filter look like the input waveform?
Q4: change the frequency of input signal to :
i10 KHz
ii15 KHz
iii20KHz
Does the output of the filter still look reasonably like the input waveform?
3- Repeat the previous steps using high order LPF as shown in Fig. 9.5
2-
Fig 9.5: Connection for 6th order filter
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Q5: Does the output of higher order filter different from the output of lower
order filter in previous step. Comment and compare your results.
Part two: Sample and Hold
A. Sample and Hold basic operation.
Connect the circuit as shown in Fig.9.6
Fig 9.6
Q6: Does the output on channel 2 go to zero between samples?
Q7: Does the output still look reasonably like the input Sine wave when:
a- The sampling frequency change to 16 KHz
b- The sampling frequency change to 2 KHz
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B- Demodulation of PAM using sample and Hold
Connect the circuit shown in Fig.9.7
Fig 9.7
Q8: Which waveform do you think will need less filtering to recover the
original signal? (Hint: connect the circuit represented by the following block
diagram to answer Q8)
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Part three: Pulse Code Modulation
A. PCM and serial transmission
When we send the bits one after the other, this is called serial transmission.
Fig.9.8 shows how this being done.
Fig 9.8 Serial Transmission
1- Connect the circuit shown in Fig.9.10
Fig 9.10
2- Set the pulse clock control to “auto” and “slow”
3- Set the three slide switches ‘Pulse Code Modulation’ to “In”
4- Concentrate on the output of A\D converter, adjust the potentiometer to
give say 0110
Q9: What is the number appears on the receiver side?
B. Signal transmission using 4-bit PCM
1234-
Connect the circuit shown in Fig.9.11
Set the pulse clock control to “auto” and “fast”
Set the three slides switches ‘Pulse Code Modulation’ to “in”
Set the function generator to sine waveform at 100 Hz and 5 volt peak to
peak
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Q10: Why does the output waveform moves in steps?
Q11: Do you think the quantization noise much reduced by using a 3.4 kHz LPF
a) With 3V peak to peak input?
b) With 10V peak to peak input?
Explain briefly.
Fig 9.11
5- Set the frequency of the generator to 5kHz
Q12: What happens to output as the frequency of input raised to 5 kHz?
C. Transmitting Voice signal using 8-bit PCM
1- Connect the circuit shown in Fig.9.12
2- On the data source set the format switch to ‘8 data bits’ and the data source
switch to ‘ADC’
3- On the data receiver set both switches to the upper positions.
Note: The system is now acting as a one way telephone.
Q13: Does a digital PCM stream of bits representing analog voice consume more or
less bandwidth than the original analog signal?
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Fig 9.12
58
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #10: Data formats & Noise in Digital Systems
Objectives:
1- To generate different waveforms for data format using either unipolar or
bipolar signals.
2- To understand that different format has different bandwidth, suitability for
AC or DC coupled channels and the timing information they contain.
3- To understand that the data format is selected to match the characteristics
of the channel available for the transmission.
4- To know that there are three forms of additive noise which can be
presented in communication system.
5- To understand that in a digital system, corruption “within limits” of the
transmitted signal does not corrupt the output from the receiver.
Equipment required:
Power supply, Oscilloscope, Function generator, Data source, Data receiver,
Data clock regeneration, Audio module.
Pre-lab work:
1- Referring to basic Fourier theory results, explain why the time duration of
a pulse signal and its bandwidth are inversely proportional.
2- Assuming the binary data rate Rb = 1 kbps, plot the waveforms
representing the binary sequence, b={0,1,0,1,0,0,0,0,1,0,0,1,1} using the
following line codes:
a) Alternate Mark inversion
b) Bipolar RZ c) Manchester
(split-phase)
3- Which line codes will generate a waveform with no DC component? Why
is it sometimes important in practice to encode signals for transmission in
such a way as to have no DC component?
4- At the output of the channel, it is necessary to extract a symbol-rate clock
synchronized with the received signal. Such a clock signal indicates when
a specific symbol starts and can therefore be used to trigger sampling of
each received symbol. This operation, termed timing recovery is facilitated
by some line codes, which ones? Why?
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Background:
I-
Data Formats: A given binary signal can be represented by a number of
different formats shown in the following table:
Data formats differ in:
a) The frequency bandwidth they occupy.
b) The degree to which they provide necessary timing information to the receiver.
c) Their requirement for d.c transmission.
II-
Matching the format to the channel : In selecting a data format for a given
communication channel the aim is to maintain an adequate signaling rate
while reducing error to an acceptable level.
Among the factors which have to be considered are the available channel bandwidth,
whether the channel can transmit d.c and the method of synchronization of the
transmitted and received signals.
There are two methods of maintaining synchronism :
a) By sending the clock signal over a separate channel that is independent of
the data signal.
b) By the extraction of a clock signal from the received data signal.
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Lab Work:
Part one: Examination of Different Data Formats
1- Connect circuit shown in Fig. 10.1
Fig 10.1
2- Adjust the oscilloscope to produce a display similar to that shown in Fig. 10.2
Fig 10.2
Q1: Is the bit clock of RZ or NRZ form?
3- On the data source, set the sample word 01011000
4- Connect YZ terminal of the OSC. To the output of the data format at point 1,
2, 3,4,5,6. Draw the output for the data at each case and write the name of the
format.
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5- Repeat step “4” using the following words : “to gain familiarity with each
format”.
a- 01010101
b- 11111111
c- 00000000
d- 11110000
6- Choose the suitable code word which will produce one transition on every
code word, find the code word and sketch the results on the Osc.
Q2: Which formats produce a series of pulses both for “all zeros” and for “all ones” ?
Q3: Which format may be suitable for sending along an a.c- only channel whose
bandwidth is insufficient for RZ of Biphase signals?
Part Two: Extraction of clock signals
1- Connect the circuit shown in Fig. 10.3
2- Set the data source module to “8 data bits” and the data source switch to mid
position, the module will now transmit any 8-bit word selected by the set of
pushbuttons as unipolar NRZ signal.
Fig 10.3
3- On the data source select the following word sample “01011000”.
4- Use the oscilloscope to display the sample word @(pt.1), and biphase format
@(pt. 3) .
5- Adjust the bias setting on data clock regeneration module to display the
transition impulses.(Hint: See Fig. 10.4 as an example).
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6- Repeat step 3, 4 and 5 for the following words :
A- 01010101
B- 11111111
C- 00000000
Fig 10.4
7- Repeat steps 3 to 6 using different kind of formats (Unipolar RZ, Biphase,
Bipolar NRZ, ternary).
 Tabulate your results as in Fig. 10.4
Q4: For the bit pattern 01100000, how many transitions are there:
a) If the word is unipolar NRZ formatted.
b) If the word is unipolar RZ formatted.
Part three: Noise in Digital Systems
Introduction:
In an ideal communication system the received signal will be identical to that sent out
by transmitter. In practice, the communication channel is subjected to the noise in the
form steady or impulsive disturbances. Fig 10.5 shows recovery of data from a noisy
signal.
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Fig 10.5
Lab Work:
In this part of experiment we will simulate the effect of noise on the data by
using the following block diagram seen in Fig. 10.6
Fig 10.6
1- Connect the circuit shown in Fig. 10.7
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Fig 10.7
2- On the data source module set the format switch to “8 data bits” and the data
source switch to its mid position.
3- Set up any 8-bit word on the data source module.
4- Set the function generator to 100 kHz and output voltage to zero.
5- Adjust the lower bias control on the data clock Reg. module to obtain the
correct data word on the data receiver display.
Q5: What will be the effect of setting the bias level?
a) Too low b) Too high
6- Increase gradually the output voltage of the function generator and observe
the data word displayed on the data receiver.
Q6: At what voltage, the data displayed on the receiver start to change.
7- Draw the signal displayed on the oscilloscope, (noisy signal on Y1), and
(Signal after the comparator on Y2) for different value of voltage inputs of
function generator (0.5V, 1V, 2V, 5V, 10V)
Q7: Discuss this effect on “the noise effect on the received signal” when replacing the
data format from unipolar to bipolar format.
65
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #11: Amplitude Shift Keying (ASK)
Objectives
To understand that:
1- In an on-off ASK system the carrier wave is constant and carries no
information.
2- Modulation by a bipolar data signal produces a suppressed carrier system
with a significant improvement in efficiency.
3- A suppressed carrier system requires a local re-generated carrier signal at
the receiver.
Equipments required:
Power supply, oscilloscope, function generator, Data source, Data format,
Data receiver, Data clock regeneration, Data recovery, Tuned circuit, Double
balanced modular.
Pre-Lab Work:
1- The binary sequence b = {1, 0, 1, 0, 0, 1} is to be transmitted through a
bandpass channel at a bit rate of 1 Kbps, and a peak signal amplitude of
IV.
a- Draw the ASK waveform representing the sequence b if the carrier
frequency is 1.28 MHz.
b- Draw the power spectral density of the modulated signal, and justify its
shape.
2- Name three of the coherent and incoherent bandpass OOK receivers.
Background:
Amplitude Shift Keying (ASK) provides the most direct form of modulationby
digital signals, Fig. 11.1 shows on –off ASK.
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Fig 11.1
1- Connect the circuit as shown in Figure 11.2.
2- Set up the bit pattern 01011000 on the data source.
3- From oscilloscope sketch the signal at point (Y1-A), and (Y2-A)
Q1: What does the signal on Y1-A and Y2-A represent ?
Q2: Knowing that the carrier frequency is 1.28MHz, how many cycles of the
carrier will be transmitted for each binary 1 digit at 80000 bit per second.
4- Connect the tune circuit across the line as shown by the dotted links in
Fig. 11.3
5- Connect the oscilloscope to (Y1-B) and (Y2-B).
6- Use the resonance frequency control “or the tuned circuit” to tune this
module for maximum signal [shown in (Y1-B) point] .
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Fig 11.2
7- Adjust the bias of comparator to make each 1 and 0 bit equal width (See
Fig.11.3).
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Fig 11.3
8- Draw the output on oscilloscope for both (Y1-B) point and (Y2-B) point.
Q3: What kind of AM-Demodulation technique used in the receiver?
Q4: Explain how the comparator works?
9- Set the comparator bias too low, how does the output on the comparator
change, Draw the output.
10- Repeat the previous step with setting the bias of the comparator too high.
11- Now, connect the channels of the oscilloscope to point (Y1-A), and (Y2B) to compare between input data and reconstructed data.
Q5: Describe the difference between the two signals.
To control the width of received 1 and 0, and the delay between transmitted
signal and reconstructed signal, we use (delay and width correction) circuit.
12- Connect the leads connections to (Y1-C) and (Y1-C), adjust the width
control, so that the pulses from the second monostable are equal width to
those of the bit clock.
13- Move Y2 back to (Y2-B), and set the delay control so that the gating
pulses from the second monostable are well removed from transitions in
data as shown in Fig.11.4 below.
Fig 11.4
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14- Compare now between signals at (Y1-A) and at (Y1-D), draw the
waveforms, comment on the results.
Part Two : Coherent Demodulation
As alternative to the diode detector used in the above procedure, we can
connect a second balanced modulator so that it acts as coherent
demodulation
The block diagram of the Suppressed carrier ASK is shown in Fig. 11.5
Fig 11.5
1- Connect the circuit shown in Fig.11.6
70
Fig 11.6
2- Set the codeword on data source as 0101100
3- Sketch the output of the modulator, (Y2-B), (the output of the
modulator will be a bi-phase high frequency signal with carrier
suppressed, and sketch (Y1-A). then compare between (Y1-A) and
(Y2-B) and comment on your results.
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Q6: What similarity is there between the modulation and demodulation
processes for suppressed-carrier ASK?
Q7: What is the carrier frequency in your experiment?
Q8: Does the phase on (Y2-B) change at each transmission, show that (sketch
from Osc.) and comment.
4- Connect the Oscilloscope to (Y2-C), sketch the output and compare the
result to the output of data format at (Y2-A) point.
5- Connect the Oscilloscope to (Y2-D), adjust the bias to get the original
signal which is similar to (Y2-A) point.
Q9: By how much the signal at point (Y2-D) delayed to the signal at point
(Y2-A).
6- Repeat the previous steps using the following codeword
11111111, 00000000
Q10: Comment on your results in step 6.
72
FACULTY OF ENGINEERING
TELECOMMUNICATION ENGINEERING DEPARTMENT
EXP #12: FSK modulation/demodulation
Introduction:
In a frequency-modulated system the amplitude of the carrier is constant while
its frequency is altered in accordance with the value of the modulation signal.
When the modulation signal is a binary waveform the transmitted signal is
switched directly from one frequency to another and the system is described as
frequency shifted key (FSK)
A two-level binary signal produces transmitted frequencies of f0 for binary 0
and f1 for binary 1.
Demodulation can be performed by using the frequency-selecting properties of
a tuned circuit to produce either a change in amplitude or phase shift with
frequency. This type of demodulation has largely been superseded by those
using a frequency tracking circuit, such as the phase-locked-loop detector.
Both types of demodulator will be demonstrated in this assignment.
According to figure above, suppose that FSK has frequency f, the output of the
demodulator will be:
 An output at frequency 2f and
 a.d.c term proportional to cos(𝜑) where ᶲ is the phase angle between
the signals.
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We need not to worry about the first term, because we filter it out. The other term is
useful.
Positive for 𝜑 < 90o
Negative for 𝜑 > 90o
So if we choose two frequencies, f0 for the data value 0 and f1 for the data value 1
such that
For f0, 𝜑 >90o
For f1, 𝜑 <90o
Then the multiplied output will be positive for data value 1, negative for 0. This is
easily done by choosing the resonant frequency, ft, of the tuned circuit to be mid-way
between f0, and f1 .
For frequency f0, the phase angle 𝜑, is less than 90o and the dc output is positive. For
frequency f1 the phase angle is greater than 90 and the output is negative. In this
experiment, the actual dc output will be zero for f0, and +5v for f1.
Lab work:
1. Make the connection and switch setting shown in the diagram of figure
below:
2. On the data source module, press the LSB button for one second to set all
bits to zero. The square wave output from the VCO is displayed on
channel Y1.
3. Measure the period of this signal f0, corresponding to binary zero and
calculate the frequency.
4. Now press the MSB button for one second to set all bits to binary 1.
Again, measure the period of this signal f0, corresponding to binary 1.
5. Set the tuned circuit control to a frequency mid-way between f0 and f1.
We shall now examine the output of the frequency discriminator
(demodulator) when an FSK signal is applied.
6. Connect Y1 to the NRZ output of the data source module (Y1-A) In the
figure and Y2 to the output of the balanced modulator (Y2-A).
7. Set the bit pattern 01011000 on the data source module and observe the
oscilloscope traces Y1 will show the binary signal and Y2 the modulated
version with some ripple at twice the carrier frequency.
74
Fig 12.1
8. Adjust the tuned circuit to obtain a well defined binary waveform with
some overshoot at the transition points. Minor adjustments to the balanced
modulator pre-sets A and B may be necessary to increase the amplitude of
the signal.
Observe how adjustments of tuning control alter the amplitude and shape of
the demodulated waveform.
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9. Now connect Y2 to the output of the data squarer (Y2-B). The trace should
be at good replica of the original binary signal with a small delay. Adjust
the data square bios to obtain equal periods for binary 1 and binary 0,
while avoiding any high frequency break-through.
10. Connect Y1 to the clock output socket (Y1-B). If the delay and width
adjustments are correct the clock pulses will be half the width of a data bit
and a positive- going edge will be in the center of each received data bit.
11. Press the recognition word button on the data source modular. The bit
button on the data receiver module display should be identical to that set
up on the data source.
Question 12.1:
If the resonant frequency of the tuned circuit is set higher than the upper frequency of
the FSK signal, how will this affect the dc levels of the demodulated signals ??????
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