This document describes an experiment on amplitude modulation. The objectives are to demonstrate AM in the time and frequency domains, determine modulation index from plots, and examine how modulation index affects sideband levels. The experiment uses a circuit to multiply a carrier and modulating signal, producing an AM carrier viewed on an oscilloscope in the time domain and a spectrum analyzer in the frequency domain. For a modulation index of 1, the sideband voltage is half the carrier voltage as expected. Changing the modulating signal amplitude produces a lower modulation index as seen in the modulated carrier plot.

1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY
Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite
Experiment No. 5
AMPLITUDE MODULATION
Lopera, Jericho James August 09. 2011
Signal Spectra and Signal Processing/BSECE 41A1 Score:
Engr. Grace Ramones
Instructor

2. OBJECTIVES:
1. Demonstrate an amplitude-modulated carrier in the time domain for different modulation indexes
and modulating frequencies.
2. Determine the modulation index and percent modulation of an amplitude-modulated carrier from
the time domain curve plot.
3. Demonstrate an amplitude-modulated carrier in the frequency domain for different modulation
indexes and modulating frequencies.
4. Compare the side-frequency voltage levels to the carrier voltage level in an amplitude-modulated
carrier for different modulation indexes.
5. Determine the signal bandwidth of an amplitude-modulated carrier for different modulating signal
frequencies.
6. Demonstrate how a complex modulating signal generates many side frequencies to form the
upper and lower sidebands.

3. SAMPLE COMPUTATIONS:
Step 3
Step 4
Step 7
Step 8
Step 9
Step 9 Q
Step 10
Step 10 Q
Step 12
Step 13
Step 13 Q
Step 16
Step 17

4. Step 19
Step 20
Step 21 Q
Step 26
Step 28
Step 30
Step 35

5. DATA SHEET:
Materials
Two function generators
One dual-trace oscilloscope
One spectrum analyzer
One 1N4001 diode
One dc voltage supply
One 2.35 nF capacitor
One 1 mH inductor
Resistors: 100 Ω, 2 kΩ, 10 kΩ, 20 kΩ
Theory:
The primary purpose of a communications system is to transmit and receive information such as audio,
video, or binary data over a communications medium or channel. The basic components in a
communications s ystem are the transmitter communications medium or channel, and the receiver.
Possible communications media are wire cable, fiber optic cable, and free space. Before the information
can be transmitted, it must be converted into an electrical signal compatible with the communications
medium, this is the purpose of the transmitter while the purpose of the receiver is to receive the
transmitted signal from the channel and convert it into its original information signal form. Is the original
electrical information signal is transmitted directly over the communications channel, it is called baseband
transmission. An example of a communications system that uses baseband transmission is the telephone
system.
Noise is defined as undesirable electrical energy that enters the communications system and interferes
with the transmitted message. All communication systems are subject to noise in both the communication
channel and the receiver. Channel noise comes from the atmosphere (lightning), outer space (radiation
emitted by the sun and stars), and electrical equipment (electric motors and fluorescent lights). Receiver
noise comes from electronic components such as resistors and transistors, which generate noise due to
thermal agitation of the atoms during electrical current flow. In some cases obliterates the message and
in other cases it results in only partial interference. Although noise cannot be completely eliminated, it can
be reduced considerably.
Often the original electrical information (baseband) signal is not compatible with the communications
medium. In that case, this baseband signal is used to modulate a higher-frequency sine wave signal that
is in a frequency spectrum that is compatible with the communications medium. This higher-frequency
sine wave signal is called a carrier. W hen the carrier frequency is in the electromagnetic spectrum it is
called a radio frequency (RF) wave, and it radiates into space more efficiently and propagates a longer
distance than a baseband signal. W hen information is transmitted over a fiber optic cable, the carrier
frequency is in the optical spectrum. The process of using a baseband signal to modulate a carrier is
called broadband transmission.
There are basically three ways to make a baseband signal modulate a sine wave carrier: amplitude
modulation (AM), frequency modulation (FM), and phase modulation (PM). In amplitude modulation (AM),
the baseband information signal varies the amplitude of the higher-frequency carrier. In frequency
modulation (FM), the baseband information signal varies the frequency of the higher-frequency carrier
and the carrier amplitude remains constant. In phase modulation (PM), the baseband information signal
varies the phase of the high-frequency carrier. Phase modulation (PM) is different fork of frequency
modulation and the carrier is similar in appearance to a frequenc y-modulated signal carrier. Therefore,
both FM and PM are often referred to as an angle modulation. In this experiment, you will examine the
characteristics of amplitude modulation (AM).
In amplitude modulation, the carrier frequenc y remains constant, but the instantaneous value of the
carrier amplitude varies in accordance with the amplitude variations of the modulating signal. An
imaginary line joining the peaks of the modulated carrier waveform, called the envelope, is the same
shape as the modulating signal with the zero reference line coinciding with the peak value of the
unmodulated carrier. The relationship between the peak voltage of the modulating signal (Vm) and the
peak voltage of the unmodulated carrier (Vc) is the modulation index (m), therefore,

6. Multiplying the modulation index (m) by 100 gives the percent modulation. W hen the peak voltage of the
modulating signal is equal to the peak voltage of the unmodulated carrier, the percent modulation is
100%. An unmodulated carrier has a percent modulation of 0%. W hen the peak voltage of the modulating
signal (Vm) exceeds the peak voltage of the unmodulated carrier (Vc) overmodulation will occur, resulting
in distortion of the modulating (baseband) signal when it is recovered from the modulated carrier.
Therefore, if it is important that the peak voltage of the modulating signal be equal to or less than the
peak voltage of the unmodulated signal carrier (equal to or less than 100% modulation) with amplitude
modulation.
Often the percent modulation must be measured from the modulated carrier displayed on an oscilloscope.
W hen the AM signal is displayed on an oscilloscope, the modulation index can be computed from
where Vmax is the maximum peak-to-peak voltage of the modulated carrier and Vmin is the minimum peak-
to-peak voltage of the modulated carrier. Notice that when Vmax = 0, the modulation index (m) is equal to 1
(100% modulation), and when Vmin = Vmax, the modulation index is equal to 0 (0% modulation).
when a single-frequency sine wave amplitude modulates a carrier, the modulating process causes two
side frequencies to be generated above and below the carrier frequency be an amount equal to the
modulating frequency (fm). The upper side frequency (fuse) and the lower side frequency (fluff) can be
determine from
A complex modulating signal, such as a square wave, consists of a fundamental sine wave frequenc y and
many harmonics, causing many side frequencies to be generated. The highest upper side frequenc y and
the lowest lower side frequency are determined be the highest harmonic frequency (fm (max), and the
highest lower side frequency and the lower upper side frequency are determined by the lowest harmonic
frequency (fm (min)). The band of frequencies between (fC + fm (min)) and (fC + fm (max)) is called the upper
sideband. The band of frequencies between (fC – fm (min)) and (fC – fm (max)) is called the lowers sideband.
The difference between the highest upper side frequency (fC + fm (max)) and the lowest lower side
frequency (fC – fm (max)) is called the bandwidth occupied by the modulated carrier. Therefore, the
bandwidth (BW ) can be calculated from
This bandwidth occupied by the modulated carrier is the reason a modulated carrier is referred to as
broadband transmission.
The higher the modulating signal frequencies (meaning more information is being transmitted) the wider
the modulated carrier bandwidth. This is the reason a video signal occupies more bandwidth than an
audio signal. Because signals transmitted on the same frequency interfere with one another, this carrier
bandwidth places a limit on the number of modulated carriers that can occupy a given communications
channel. Also, when a carrier is overmodulated, the resulting distorted waveshape generates a harmonic
that generate additional side frequencies. This causes the transmitted bandwidth to increase and interfere
with other signals. This harmonic-generated sideband interference is called splatter.
W hen an AM signal on an oscilloscope, it is observed in the time domain (voltage as a function time). The
time domain display gives no indication of sidebands. In order to observe the sidebands generated by the
modulated carrier, the frequency spectrum of the modulated carrier must be displayed in the frequenc y
domain (sine wave voltage levels as a function of frequency) on a spectrum analyzer.
A sine wave modulated AM signal is a composite of a carrier and two side frequencies, and each of these
signals transmits power. The total power transmitted (PT) is the sum of each carrier power ((PC) and the
power in the two side frequencies (PUSF and PUSB). Therefore,

7. The total power transmitted (PT) can also be determined from the modulation index (m) using the
equation
Therefore, the total power in the side frequencies (PSF) is
and the power in each side frequency is
Notice that the power in the side frequencies depends on the modulation index (percent modulation) and
the carrier power does not depend on the modulation index. W hen the percent modulation is 100% (m =
1), the total side-frequency power (PSF) is one-half of the carrier power (PC) and the power in each side
frequency (PUSF and PLSF) is one-quarter of the carrier power (PC). W hen the percent modulation is 0%,
the total side-frequency power (PSF) is zero because there are no side frequencies in an unmodulated
carrier. Based on these results, it is easy to calculate that an amplitude-modulated carrier has all of the
transmitted information in the sidebands and no information in the carrier. For 100% modulation, one-third
of the total power transmitted is in the sidebands and two-thirds of the total power is wasted in the carrier,
which contains no information. W hen a carrier is modulated by a complex waveform, the combined
modulation index of the fundamental and all of the harmonics determines the power in the sidebands. In a
later experiment, you will see how we can remove and transmit the same amount of information with less
power.
Because power is proportional to voltage squared, the voltage level of the frequencies is equal to the
square root of the side-frequency power. Therefore the side-frequenc y voltage can be calculated from
This means that the voltage of each side frequency is equal to one-half the carrier voltage for 100%
modulation of a sine-wave modulated carrier. W hen a carrier is modulated by a complex waveform, the
voltage of each side frequency can calculated from the separate modulation indexes of the fundamental
and each harmonic.
A circuit that mathematically multiplies a carrier and modulating (baseband) signal, and then adds the
carrier to the result, will produce an amplitude-modulated carrier. Therefore, the circuit in Figure 6-1 will
be used to demonstrate amplitude modulation. An oscilloscope has been attached to the output to display
the modulated carrier in the time domain. A spectrum analyzer has been attached to the output to display
the frequency spectrum of the amplitude-modulated carrier in the frequency domain.

8. XSA1 XSC1
Modulating Signal
XFG1
Ext T rig
+
_
IN T A B
+ _ + _ 0
0 MULTIPLIER SUMMER
3
Y
C
X 2 A 1
1 V/V
4 1 V/V 0 V B 0V
Carrier
1 Vpk
100kHz
0 0°
Procedure
Step 1 Open circuit file Fig 6-1. This circuit will demonstrate how mathematical multiplication of a carrier
and a modulating (baseband) signal, and then adding the carrier to the multiplication result, will
produce an amplitude modulated carrier. Bring down the function generator enlargement and
make sure that the following settings are selected: Sine Wave, Freq=5kHz, Ampl = 1V, Offset=0.
Bring down the oscilloscope enlargement and make sure that the following settings are selected:
Time base (Scale = 50 µs/Div, Xpos= 0, Y/T) Ch A (Scale = 1 V/Div, Ypos = 0, DC) Trigger (Pos
edge, Level = 0, Auto).
Step 2 Run the simulation to one full screen display, then pause the simulation. Notice that you have
displayed an amplitude-modulated carrier curve plot on the oscilloscope screen. Draw the curve
plot in the space provided and show the envelope on the drawing.
Step 3 Based on the function generator amplitude (modulating sine wave voltage, Vm) and the voltage of
the carrier sine wave (Vc), calculate the expected modulation index (m) and percent modulation.
 m=1 or 100%
Step 4 Determine the modulation index (m) and percent modulation from the curve plot in Step 2.

9.  m=1 or 100%
Question: How did the value of the modulation index and percent modulation determined from the curve
plot compare with the expected value calculated in Step 3?
 The two values have no difference. They both have the same value.
Step 5 Bring down the spectrum analyzer enlargement and make sure that the following settings are
selected: Frequency (Center = 100 kHz, Span = 100 kHz), Amplitude (Lin, Range = 0.2 V/Div)
Resolution = 500 Hz.
Step 6 Run the simulation until the Resolution Frequency match, then pause the simulation. You have
displayed the frequency spectrum for a modulated carrier. Draw the spectral plot in the space
provided.
Step 7 Measure the carrier frequency (fc), the upper side frequency (fUSF), the lowest side frequenc y
(fLSF), and the voltage amplitude of each spectral line and record the answer on the spectral plot.
 fc = 100 kHz Vc = 999.085 mV
 fLSF = 95.041 kHz VLSF = 458.267 mV
 fUSF = 104.959 kHz VUSF = 458.249 mV
Question: W hat was the frequenc y difference between the carrier frequency and each of the side
frequencies? How did this compare with the modulating signal frequency?
 fc – fLSF = 4.959 kHz fUSF – fc = 4.959 kHz
 They are almost equal. The difference is only 0.82%.
How did the frequency of the center spectral line compare with the carrier frequency?
 The two values have no difference. They both have the same value.
Step 8 Calculate the bandwidth (BW ) of the modulated carrier based on the frequency of the modulating
sine wave.
 BW = 10 kHz
Step 9 Determine the bandwidth (BW ) of the modulated carrier from the frequency spectral plot and
record your answer on the spectral plot.
 BW = 9.918 kHz
Question: How did the bandwidth of the modulated carrier from the frequenc y spectrum compare with the
calculated bandwidth in Step 8?
 They are almost equal. The difference is only 0.82%.

10. Step 10 Calculate the expected voltage amplitude of each side frequency spectral line (VUSF and VLSF)
based on the modulation index (m) and the carrier voltage amplitude
 VUSF = VLSF = 0.5 V
Questions: How did the calculated voltage values compare with the measured values in on the spectral
line?
 They are almost equal. The difference is only 9.11%.
W hat was the relationship between the voltage levels of the side frequencies and the voltage level of the
carrier? W as this what you expected for this modulation index?
 It is one-half of the amplitude of the carrier. This is expected for a 1 modulation index.
Step 11 Change the modulating signal amplitude (function generator amplitude) to 0.5 V (500 mV). Bring
down the oscilloscope enlargement and run the simulation to one full-screen display, then pause
the simulation. Notice that you have displayed an amplitude-modulated carrier curve plot on the
oscilloscope screen. Draw the curve plot in the space provided and show the envelope on the
drawing.
Step 12 Based on the voltage of the modulating (baseband) sine wave (Vm) and the voltage of the carrier
sine wave (Vc), calculate the expected modulation index (m) and the percent modulation.
 m = 0.5 = 50%
Step 13 Determine the modulation index (m) and the percent modulation from the curve plot in Step 11.
 m = 0.51 = 51%
Questions: How did the value of the modulation index and percent modulation determined from the curve
plot compare with the expected value calculated in Step 12?
 They are almost equal. The difference is only 2%.
How did this percent modulation compare with the percent modulation in Step 3 and 4? Explain any
difference.
 It decreased by almost 50%. The modulating signal decreased by half so percent modulation
also decreased by 50%
Step 14 Bring down the spectrum analyzer enlargement. Run the simulation until the Resolution
Frequenc y match, then pause the simulation. You have plotted the frequency spectrum for a
modulated carrier. Draw the spectral plot in the space provided.

11. Step 15 Measure the carrier frequency (fC), the upper side frequency (fUSF), the lower side frequency
(fLSF), and the voltage amplitude of each spectral line and record the answers on the spectral plot.
 fc = 100 kHz Vc = 998.442 mV
 fLSF = 95.041 kHz VLSF = 228.996 mV
 fUSF = 104.959 kHz VUSF = 228.968 mV
Step 16 Determine the bandwidth (BW ) of the modulated carrier from the frequency spectral plot and
record your answer on the spectral plot.
 BW = 9.918 kHz
Question: How did the bandwidth of the modulated carrier from this frequency spectrum compare with the
bandwidth on the spectral plot in Step 6? Explain.
 The difference is 0%. They have the same values
Step 17 Calculate the expected voltage amplitude of each side frequency spectral line (VUSF) based on
the modulation index (m) and the carrier voltage amplitude (VC).
 VUSF = VLSF = 0.25 V
Questions: How did the calculated voltage values compare with the measured values in on the spectral
plot?
 They are almost equal. The difference is only 9.17%.
W hat was the relationship between the voltage levels of the side frequencies and the voltage level of the
carrier? How did it compare with the results in Step 6?
 It is one-fourth of the amplitude of the carrier. The percent modulation decreased by 50% so
it is half of the result compare to other.
Step 18 Change the modulating amplitude (function generator amplitude) to 0 V (1 µV). Bring down the
oscilloscope enlargement and run the simulation to one full screen display, then pause the
simulation. Draw the curve plot in the space provided.

12. Step 19 Based on the voltage of the modulating (baseband) sine wave (Vm) and the voltage of the carrier
sine wave (Vc), calculate the expected modulation index (m) and percent modulation.
-6
 m = 0.1 × 10
Step 20 Determine the modulation index (m) and percent modulation from the curve plot in Step 18.
 m=0
Questions: How did the value of the modulation index and the percent modulation determined from the
curve plot compare with the expected value calculated in Step 19?
-6
 There is 0.1 × 10 difference.
How did this waveshape compare with the previous amplitude-modulated waveshapes? Explain any
difference.
 It is like the carrier signal waveshape alone, because the amplitude of the modulating signal
is almost 0.
Step 21 Bring down the spectrum analyzer. Run the Resolution Frequencies match, then pause the
simulation. You have plotted the frequency spectrum for an unmodulated carrier. Draw the
spectral plot in the space provided.
Step 22 Measure the frequency and voltage amplitude of the spectral line and record the values on the
spectral plot.
 fc = 100 kHz Vc = 998.441 mV
Questions: How did the frequenc y of the spectral line compare with the carrier frequenc y?

13.  The two values have no difference. They both have the same value.
How did the voltage amplitude of the spectral line compare with the carrier amplitude?
 They are almost equal. The difference is only 0.16%.
How did this frequency spectrum compare with the previous frequenc y spectrum?
 The carrier frequency is only the signal present. There is no side frequencies.
Step 23 Change the modulating frequenc y to 10 kHz and the amplitude back to 1 V on the function
generator. Bring down the oscilloscope and run the simulation to one full screen display, then
pause the simulation. Notice that you have displayed an amplitude-modulated carrier curve
plot on the oscilloscope screen. Draw the curve plot in the space provided and show the
envelope on the drawing.
Question: How did this waveshape differ from the waveshape for a 5 kHz modulating frequency in Step
2?
 Compare to waveshape in Step 2, the waveshape this time is thinner and has lot of cycle per
second.
Step 24 Bring down the spectrum analyzer enlargement. Run the simulation until the Resolution
Frequencies match, then pause the simulation. You have plotted the frequency spectrum for a
modulated carrier. Draw the spectral plot in the space provided.
Step 25 Measure the carrier frequency (fC), the upper side frequenc y (fUSF), and the lower side frequency
(fLSF) of the spectral lines and record the answers on the spectral plot.
 fc = 100 kHz fLSF = 90.083 kHz fUSF = 109.917 kHz
Step 26 Determine the bandwidth (BW ) of the modulated carrier from the frequency spectral plot and
record your answer on the spectral plot.

14.  BW = 19.834 kHz
Question: How did the bandwidth of the modulated carrier for a 10 kHz modulating frequency compare
with the bandwidth for a 5 kHz modulating frequency in Step 6?
 Compare with the bandwidth in Step 6, the bandwidth this time increased by 10 kHz.
Step 27 Measure the voltage amplitude of the side frequencies and record your answer on the spectral
plot in Step 24.
 Vc = 998.436 mV VLSF = 416.809 mV VUSF = 416.585 mV
Question: W as there any difference between the amplitude of the side frequencies for the 10 kHz plot in
Step 24 and the 5 kHz in Step 6? Explain.
 There is 41.458 mV difference. As the frequency of the modulating signal increases, the
amplitude of each sideband decreases
Step 28 Change the modulating frequenc y to 20 kHz on the function generator. Run the simulation until
the Resolution Frequencies match, the pause the simulation. Measure the bandwidth (BW ) of the
modulated carrier on the spectrum analyzer and record the value.
 BW = 39.67 kHz
Question: How did the bandwidth compare with the bandwidth for the 10 kHz modulating frequenc y?
Explain.
 The bandwidth of the waveshape increased twice of the modulating frequency that explains
why it increases 20 kHz
Step 29 Change the modulating signal frequency band to 5kHz and select square wave on the function
generator. Bring down the oscilloscope enlargement and run the simulation to one full screen
display, then pause the simulation. Notice that you have displayed a carrier modulated by a
square wave on the oscilloscope screen. Draw the curve plot in the space proved and show the
envelope on the drawing.
Question: How did this waveshape differ from the waveshape in step 2?
The waveshape is complex wave. The modulating signal is square wave which consists of a
fundamental sine wave frequency and many harmonics, so there are lot of side frequencies
generated.
Step 30 Determine the modulation index (m) and percent modulation from the curve plot in Step 29.
 m=1
Step 31 Bring down the spectrum analyzer enlargement. Run the simulation until the Resolution
Frequenc y match, the pause the simulation. You have plotted the frequenc y spectrum for a
square-wave modulated carrier. Draw the spectral plot in the space provided. Neglect any side
frequencies with amplitudes less than 10% of the carrier amplitude.

15. Step 32 Measure the frequency of the spectral lines and record the answers on the spectral plot. Neglect
any side frequencies with amplitudes less than 10% of the carrier amplitude.
 fc = 100 kHz
 fLSF1 = 95.041 kHz fLSF2 = 85.537 kHz
 fUSF1 = 104.959 kHz fUSF2 = 114.876 kHz
Question: How did the frequency spectrum for the square-wave modulated carrier differ from the
spectrum for the sine wave modulated carrier in Step 6? Explain why there were different.
 it creates many side frequencies. It is because it is a complex wave
Step 33 Determine the bandwidth (BW ) of the modulated carrier from the frequency spectral plot and
record your answer on the spectral plot. Neglect any side frequencies with amplitudes less than
10% of the carrier amplitude.
 BW = 9.506 kHz
Question: How did the bandwidth of the 5-kHz square-wave modulated carrier compare to the bandwidth
of the 5-kHz sine wave modulated carrier in Step 6? Explain any difference.
 Difference is 0.002 kHz.
Step 34 Reduce the amplitude of the square wave to 0.5 V (500 mV) on the function generator. Bring
down the oscilloscope enlargement and run the simulation to one full screen display, then pause the
simulation. Draw the curve plot in the space provided.

16. Step 35 Determine the modulation index (m) and percent modulation from the curve plot in Step 34.
 m = 0.5 = 50%
Question: W hat is the difference between this curve plot and the one in Step 29? Explain.
 The amplitude of the modulating signal is reduced by half. That explains why the minimum
amplitude is not at 0 V. And also it is only 50 % modulation.

17. CONCLUSION:
An amplitude modulation is one of the type of modulation that can produced by a circuit which is a
modulator that performs a mathematical multiplication of the carrier and the information signals. In this
type of modulation, the amplitude of the carrier is changed depending with the characteristics of the
modulating signal while the carrier frequency remains constant. Oscilloscope shows the time domain
while the spectrum analyzer is frequency domain. The envelope is like the shape of the modulating
signal. Modulation signal is the ratio of the peak voltage of the modulating signal to the peak value of the
carrier. The carrier frequency is at the center. The amplitude of the sidebands is half the modulation index
and the carrier voltage. So the amplitude of the sidebands is directly proportional to the modulation index
and the carrier voltage Complex modulating signal such as square wave produces lot of side bands
because of the fundamental sine wave frequency and many harmonics.