FACULTY OF ENGINEERING
LAB SHEET
ETN2016
ANALOG COMMUNICATIONS
TRIMESTER 2 (2016/2017)
TM201
Amplitude Modulation
Note: Students are advised to read through this lab sheet before doing experiment. On-thespot evaluation will be carried out during or at the end of the experiment. Your performance,
teamwork effort, and learning attitude will count towards the marks.
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1. Objectives
Experiment Part 1: Amplitude Modulation (AM)
 To examine a message signal, a carrier signal and an AM modulated waveform in
time domain.
 To measure the modulation index of an AM signal.
 To examine a message signal, a carrier signal and an AM modulated waveform in
frequency domain.
Experiment Part 2: Demodulation of AM Signal
 To demonstrate the demodulation of AM signal.
2. Introduction
a) Modulation
To communicate over long distance we can send a radio frequency (RF) signal between
two antennas, one at the transmitting end and the other at the receiver. Frequencies used by
AM transmissions are typically between 200 kHz and 25 MHz. A typical radio frequency
of say, 1 MHz is much higher than the audio frequencies present in the human voice.
We appear to have two incompatible requirements. A radio system uses frequencies like 1
MHz to transmit over long distance, but we wish to send typical voice frequencies of
between 300 Hz and 3.4 kHz which are quite impossible to transmit by radio signals. This
problem can be overcome by using a process called “modulation”.
A radio system can easily send high frequency signals between a transmitter and a receiver
but this, on its own, conveys no information. Now, if we were to alter the high frequency
signals (in amplitude or frequency or phase or any combination of them) in
correspondence with the variation of the amplitude of the message signal, we could use it
to send information.
Modulation is a process where a “carrier wave” is systematically altered (in amplitude or
frequency or phase or any combination of them) in correspondence with the variation of a
modulating signal, the message signal. The resulting modulated signal will carry the
message information in its amplitude or frequency or phase or any combination of them.
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b) Amplitude Modulation (AM)
In Amplitude Modulation, the carrier signal sc (t )  Ac cos c t is modulated in amplitude
s AM (t )  Ac 1  m(t ) cosc t
by m(t) to produce:
Figure 1: AM
Ac
Carrier
10
8
6
4
2
0
-2
-4
-6
-8
0
20
40
60
80
100
120
Message signal or modulating waveform
20
15
10
Ac
5
0
-5
-10
-15
-20
0
20
40
60
80
100
120
Modulated AM waveform
c) Depth of modulation
Let us consider the following signals:
sc (t )  Ac cos c t
Carrier:
m(t )  m cos m t
Modulating signal:
Modulated signal:
s AM (t )  Ac 1  m cosmt  cosc t
The amount by which the amplitude of the carrier wave increases or decreases depends on
the amplitude of the information signal and is called the modulation index.
A  Ac Amax  Amin Amax  Amin
A B
Modulation index, m  max



Ac
2 Ac
Amax  Amin A  B
Percentage of modulation  m  100%
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i) Undermodulation, m < 1
Figure 2: m < 1
A
1.5
1
B
0.5
Ac(min)
Ac(max)
Ac
0
-0.5
-1
-1.5
0
5
10
15
20
25
30
35
40
45
Under this modulation condition, a simple noncoherent demodulation technique such
as envelope detector can be used to recover the original message signal without
distortion.
ii) Overmodulation, m > 1
Figure 3: m > 1
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
0
5
10
15
20
25
30
35
40
45
Under this modulation condition, noncoherent demodulation will not be able to
recover the original message signal without distortion.
When the message signal is constantly changing, as in a voice or music signal, the
frequency of the modulating envelope and the percentage of modulation are
constantly changing. This constant change makes it practically impossible to
determine the modulation index from the waveform displayed on an oscilloscope.
A solution to the above problem is to set the oscilloscope display to X-Y mode. In this
method, the X input is Channel 1 (the message signal) and the Y input is Channel 2
(the modulated signal).
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A
B
Figure 4 : XY mode oscilloscope display
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The modulation index is given by: m 
A B
A B
d) Frequency spectrum
Figure 5 : amplitude of spectral components of AM signal (1-sided plot)
AM
modulation
m2Ac
m1Ac m3Ac
lower
sideband
carrier
Ac
m2Ac/2
m3Ac/2 m1Ac/2
upper
sideband
m2Ac/2
m1Ac/2 m3Ac/2
f
fc-f2
fc+f2
fc
fc-f1
fc+f1
fc-f3
fc+f3
The amplitude spectrum of the following signal m(t)
0
f1
f2 f3

m(t )  Ac m1 cos1t  m2 cos2 t  m3 cos3t
and the corresponding AM signal are shown in Figure 5.

Note that each modulating frequency component produces its own upper and lower side
frequencies around the carrier frequency. All the upper side frequencies are grouped
together and referred to as the upper sideband (USB) and all the lower side frequencies
form the lower sideband (LSB).
If the frequency range of the message signal is from 0 to f3, then, the bandwidth of the
message signal, BW m ( t )  f 3 . The corresponding AM modulated signal will occupy a
frequency range from f c  f 3 to f c  f 3 , and the bandwidth of the corresponding AM
modulated signal is given by:
BW AM   f c  f 3    f c  f 3   2 f 3  2 BW m (t ) .
3. Material & Equipment Required
i)
ii)
iii)
iv)
ANACOM 1/1, ANACOM 1/2
Power Supply : +12V, -12V
Dual Trace Oscilloscope
Spectrum Analyzer
4. References
i)
ii)
iii)
LJ Technical System, “An Introduction to Analog Communications – Curriculum
Manual AT02”
LJ Technical System, “An Introduction to Analog Communications – Student
Workbook AT02”
Simon Haykin, "Communication Systems", Wiley, 5th edition, 2009.
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5. Experiment Procedure
5.1 Experiment Part 1: Amplitude Modulation (AM)
5.1.1. Message Signal, Carrier Signal, and AM Signal
i)
Connect the ANACOM 1/1 board to the power supply as shown below:
Figure 6 : Power supply connection
+12V –12V +5V
Power Supply PU2
GND
+12V
0V
-12V
ANACOM 1/1
Board
ii)
iii)
ANACOM 1/1 board set-up
 AUDIO INPUT SELECT is switched to INT position.
 MODE is switched to DSB.
 SPEAKER is switched to OFF.
 In the AUDIO OSCILLATOR both the AMPLITUDE PRESET and the
FREQUENCY PRESET should be set to maximum (fully clockwise).
 In the BALANCED MODULATOR & BANDPASS FILTER CIRCUIT 1, the
BALANCE PRESET should be set to maximum (fully clockwise).
 In the OUTPUT AMPLIFIER, decrease the gain to its minimum value (fully
counter clockwise).
Switch on the power supply.
Oscilloscope probe
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iv)
Connect the oscilloscope Channel 1 probe to tp1 (test point 1), probing the message
signal input of the MODULATOR and trigger on Channel 1. This is a message signal
that is going to be used to amplitude modulate a carrier signal. Set timebase to
50s/div.
Determine the period and peak-to-peak voltage of the signal. Calculate the frequency
of the signal.
Take a photo of the waveform shown on the oscilloscope screen. Insert the photo into
your lab report. Show the period and peak-to-peak voltage of the signal on the photo.
v)
Set the oscilloscope to DUAL display mode. Connect the oscilloscope Channel 2
probe to tp9, probing the carrier signal input of the MODULATOR and trigger on
Channel 2. Set the Channel Coupling to AC. Set the oscilloscope timebase to
0.2s/div. This is a high frequency signal that is going to be used to carry the message
signal. It is called the carrier. Adjust the vertical scale if necessary.
Determine the period and peak-to-peak voltage of the signal. Calculate the frequency
of the signal.
Take a photo of the waveform shown on the oscilloscope screen. Insert the photo into
your lab report. Show the period and peak-to-peak voltage of the signal on the photo.
vi)
Connect the Channel 2 probe to tp3, probing the amplitude modulated (AM) signal
at the output of the MODULATOR. Set the oscilloscope vertical mode to DUAL,
trigger on Channel 1 (the message signal), and timebase to 50s. Adjust the vertical
scale if necessary.
Describe what you observe on the oscilloscope screen.
Take a photo of the waveform shown on the oscilloscope screen. Insert the photo into
your lab report.
vii)
Adjust the AMPLITUDE PRESET and the FREQUENCY PRESET in the AUDIO
OSCILLATOR. Observe if the envelope of the AM signal (Channel 2) changes in
correspondence to the changes in the amplitude and frequency of the message signal
(Channel 1).
Describe your observation.
viii)
Adjust the BALANCE PRESET in the BALANCED MODULATOR & BANDPASS
FILTER CIRCUIT 1.
Display three types of waveforms: over-modulation, 100% modulation, and undermodulation.
Take a photo of each waveform. Insert the photos into your lab report, with clear
labels differentiating between the three.
5.1.2 Modulation Index or Percentage of Modulation
i)
Calculate the modulation index of the AM signal.
Measure the vertical height, A and B (in volts) from the peaks and valleys
respectively. Calculate the modulation index or the percentage of modulation.
Take a photo of the waveform shown on the oscilloscope screen. Insert the photo into
your lab report. Label A and B on the photo.
ii)
Set the oscilloscope to XY mode. (for GOS-652 oscilloscope: set VERT mode to XY, set TRIGGER source to X-Y, and set timebase to X-Y)
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Measure A and B, and calculate the modulation index. The result should be the same
as the one obtained from the previous step.
Take a photo of the waveform shown on the oscilloscope screen. Insert the photo into
your lab report. Label A and B on the photo.
Switch the oscilloscope back to the normal mode.
5.1.3 Power Spectra of the Signals
i)
Keep the ANACOM 1/1 board set-up as before.
ii)
Power on the spectrum analyzer and make the following settings:
 FREQ, CENTER : 5 kHz
 SPAN, WIDTHSPAN : 10 kHz
iii)
Connect an oscilloscope probe to the spectrum analyzer. Probe at tp1 so that the
power spectrum of the message signal can be displayed on the spectrum analyzer.
Use the Marker key to activate the marker and use Scroll Key to move the marker
around. The frequency and power (or amplitude) level corresponding to the marker
location are displayed on the upper right hand corner of the screen.
Take a photo of the power spectrum shown on the spectrum analyzer. Insert the photo
into your lab report. On the photo, label the peak frequency (kHz) and the
corresponding power level (mW or dBm).
Hint: The peak frequency is about 3.24 kHz. Is it the same as what you have
previously calculated?
iv)
Set the spectrum analyzer as follows:
 FREQ, CENTER : 1 MHz
 SPAN, WIDTHSPAN : 40 kHz
 Other settings are the same as before.
Probe at tp9 so that the power spectrum of the carrier signal can be displayed on the
spectrum analyzer.
Take a photo of the power spectrum shown on the spectrum analyzer. Insert the photo
into your lab report. On the photo, label the peak frequency (kHz) and the
corresponding power level (mW or dBm).
Hint: The peak frequency should be 1MHz, which should be the same as what you
have previously calculated based on the waveform shown on the oscilloscope.
v)
Keeping the settings of spectrum analyzer as before, probe at tp3 so that the power
spectrum of the AM signal can be displayed on the spectrum analyzer.
How many peaks do you observe? Take a photo of the power spectrum shown on the
spectrum analyzer. Insert the photo into your lab report. On the photo, label the peak
frequencies (kHz) and the corresponding power levels (mW or dBm). Explain your
observations.
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5.2 Experiment Part 2: Demodulation of AM Signal
i)
Connect the ANACOM 1/1 and ANACOM 1/2 boards to the power supply as shown
below:
Figure 7 : Power supply connection
+12V –12V +5V
Power Supply
GND
+12V
0V
-12V
ANACOM 1/1 Board
ii)
iii)
iv)
v)
vi)
vii)
viii)
ix)
+12V
0V
ANACOM 1/2 Board
ANACOM 1/1 Board set-up
 AUDIO INPUT SELECT is switched to INT position
 MODE is switched to DSB
 SPEAKER is switched to OFF
 In the AUDIO OSCILLATOR both the AMPLITUDE PRESET and the
FREQUENCY PRESET should be set to maximum (fully clockwise).
 In the BALANCED MODULATOR & BANDPASS FILTER CIRCUIT 1, the
BALANCE PRESET should be set to maximum (fully clockwise).
 In the OUTPUT AMPLIFIER, increase the gain to its maximum value (fully
clockwise)
 The TX OUTPUT SELECT of ANACOM 1/1 should be set to ANT.
ANACOM 1/2 Board set-up
 In the AUDIO AMPLIFIER, switch the SPEAKER to ON and decrease the
VOLUME preset to its minimum value (fully counter-clockwise)
 The RX INPUT SELECT of ANACOM 1/2 is switched to ANT.
 In the RF AMPLIFIER, switch the TUNED CIRCUIT SELECT to INT (internal)
position and increase the RF AMPLIFIER GAIN CONTROL to maximum (fully
clockwise).
 Set the AGC switch to the IN position.
 Set the DETECTOR switch to the DIODE position.
 Switch the BEAT FREQUENCY OSCILLATOR to the OFF position.
Clip both ends of a wire to the antennas of both ANACOM 1/1 and ANACOM 1/2.
Switch on the power supply.
Increase AUDIO AMPLIFIER’s VOLUME preset until sound is audible from the
speaker.
Adjust the Tuning Control on ANACOM 1/2 until an audio tone can be clearly heard.
This should occur between 55 and 65 on the tuning scale (fine tuning for the strongest
possible signal may be required).
On your dual trace oscilloscope, set both the inputs to AC.
Use Channel 1 of the oscilloscope to monitor the transmitted AM signal at tp3 of
ANACOM 1/1. The AM signal should be under-modulation, else ensure that the
BALANCE PRESET in the BALANCED MODULATOR & BANDPASS FILTER
CIRCUIT 1 is turned to fully clockwise.
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x)
xi)
xii)
Use Channel 1 of the oscilloscope to monitor the message signal at tp1 of ANACOM
1/1. On the oscilloscope, select Channel 1 as the triggering source.
On the ANACOM 1/2, the received signal is passed through a series of circuits
including the RF amplifier, mixer, IF amplifiers, diode detector and AF amplifier. The
demodulated signal can be observed at tp39 of ANACOM 1/2. Use Channel 2 of the
oscilloscope to monitor the demodulated signal at tp39.
Take a photo of the waveforms shown on the oscilloscope screen. Insert the photo into
your lab report. Label the waveforms clearly.
Compare the received signal with the transmitted signal.
 Observe if there is any phase difference between the two signals.
 Examine if there is any distortion on the received signal as compared to the
transmitted signal.
Tune the BALANCE PRESET in the BALANCED MODULATOR & BANDPASS
FILTER CIRCUIT 1 slowly counter clockwise.
Analyze the changes to the received signal. Record your observation for the cases of
100% modulation and over-modulation.
Please prepare your lab report using the given report template.
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