1) The document describes an experiment on amplitude modulation (AM) involving demonstrating AM signals in the time and frequency domains for different modulation indexes and frequencies. 2) Key aspects of AM are discussed, including how the modulation index is defined and relates to percent modulation. Modulation indexes above 1 cause overmodulation and distortion. 3) AM generates sidebands above and below the carrier frequency by the modulating frequency. The bandwidth occupied depends on the highest modulating frequency components.

1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY
Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite
Experiment No. 5
AMPLITUDE MODULATION
Maala, Michelle Anne C. 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

4. Step 16
Step 17
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 system 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. When 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. When 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

6. 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 frequency-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 frequency 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,
Multiplying the modulation index (m) by 100 gives the percent modulation. When 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%. When 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. When 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 frequency and many harmonics, causing many side frequencies to be
generated. The highest upper side frequency 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

7. 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.
When 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 frequency 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,
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. When 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

8. and PLSF) is one-quarter of the carrier power (PC). When 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. When 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-frequency
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. When 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.

9. 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.

10. 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.
 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?
 There is no difference between the values. They have same values.
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 frequency (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: What was the frequency 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
 There is a small difference between the values which is 0.82%.

11. How did the frequency of the center spectral line compare with the carrier frequency?
 There is no difference between the values. They have same values.
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 frequency
spectrum compare with the calculated bandwidth in Step 8?
 There is a small difference between the values which is 0.82%.
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?
 There is a small difference between the values which is 9.11%.
What was the relationship between the voltage levels of the side frequencies and the
voltage level of the carrier? Was 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.

12. 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?
 There is a small difference between the values which is 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 and the percent
modulation decreased by half because it is directly proportional to the Vm.
Step 14 Bring down the spectrum analyzer enlargement. Run the simulation until
the Resolution Frequency match, then pause the simulation. You have plotted
the frequency spectrum for a modulated carrier. Draw the spectral plot in the
space provided.
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.

13.  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?
 There is a small difference between the values which is 9.17%.
What 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 carrier. This is because the modulation index decreased
by 50% It is half of the result in Step 6.
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.
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.
 m = 0.1 × 10-6
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?
 There is 0.1 × 10-6 difference.

14. 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 frequency of the spectral line compare with the carrier
frequency?
 There is no difference between the values. They have same values.
How did the voltage amplitude of the spectral line compare with the carrier
amplitude?
 There is a small difference between the values which is 0.16%.
How did this frequency spectrum compare with the previous frequency spectrum?
 The carrier frequency is only the signal present. There is no side frequencies.
Step 23 Change the modulating frequency 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.

15. 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 frequency (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.
 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?

16.  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: Was 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 frequency 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 frequency? 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. Because
square wave consists of a fundamental sine wave frequency and many harmonics,
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

17. Step 31 Bring down the spectrum analyzer enlargement. Run the simulation until
the Resolution Frequency match, the pause the simulation. You have plotted the
frequency 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.
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 generates many side frequencies. It is because a square wave consists of a
fundamental sine wave frequency and many harmonics.
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.
 There is a small difference between the values which is 0.002 kHz. The
bandwidth is almost equal to Step 6.
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

18. one full screen display, then pause the simulation. Draw the curve plot in the space
provided.
Step 35 Determine the modulation index (m) and percent modulation from the
curve plot in Step 34.
 m = 0.5 = 50%
Question: What is the difference between this curve plot and the one in Step 29?
Explain.
 In this curve plot, there is a modulating signal that is why the minimum
amplitude is not at 0 V. The modulation index decreased by 50 % because of
the amplitude of the modulating signal was reduced.

19. CONCLUSION:
After conducting the experiment, I conclude that amplitude modulation or
commonly called AM, vary with carrier amplitude. The carrier frequency as remain
constant in AM.
. The modulation index is the ratio of the amplitude of the modulating and carrier
signal. It can also be measured through the maximum and the minimum voltage of the
modulating signal
In addition, the oscilloscope displays the time-domain of the amplitude
modulation on the other hand, the spectrum analyzer displays the frequency domain.
The voltage of each frequency is half of the product of the carrier voltage and the
modulation index. Then it is directly proportional to the carrier voltage and the
modulation index.
Because of a fundamental sine wave frequency and many harmonics of a
square wave, it is considered as a complex wave. This kind of wave generates many
side frequencies unlike the other wave which only consist of two side frequencies.
The bandwidth of AM depends on the frequency of the modulating signal. It is
twice the modulating frequency. Then, the bandwidth is directly proportional to the
modulating frequency.