This document describes an experiment on amplitude modulation. The objectives are to demonstrate AM in the time and frequency domains for different modulation indexes and frequencies. The experiment uses a circuit to mathematically multiply a carrier signal with a modulating signal. Key findings include: - For a 5 kHz modulating signal, the modulation index was 100% and sideband frequencies were 5 kHz from the 100 kHz carrier. - Reducing the modulating signal to 0.5 V reduced the modulation index to 51%, as expected based on the signal amplitudes. - Sideband voltage levels were half the carrier voltage for 100% modulation, matching theoretical calculations.

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

2. OBJECTIVES:
Demonstrate an amplitude-modulated carrier in the time domain for different modulation indexes
and modulating frequencies.
Determine the modulation index and percent modulation of an amplitude-modulated carrier from
the time domain curve plot.
Demonstrate an amplitude-modulated carrier in the frequency domain for different modulation
indexes and modulating frequencies.
Compare the side-frequency voltage levels to the carrier voltage level in an amplitude-modulated
carrier for different modulation indexes.
Determine the signal bandwidth of an amplitude-modulated carrier for different modulating signal
frequencies.
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
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).

6. 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 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.

7. 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 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.

8. 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.
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°

9. 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.
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 difference is 0% . They have the same values

10. 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
The difference between the values is 0.82%.
How did the frequency of the center spectral line compare with the carrier frequency?
The difference is 0% . They have the 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?
The difference between the values 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

11. Questions: How did the calculated voltage values compare with the measured values in on the spectral line?
The difference between the values 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?
The side frequency voltages are one-half of the carrier voltage. Yes, this is expected for
modulation index of 1.
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?
The difference between the values is 2%.
How did this percent modulation compare with the percent modulation in Step 3 and 4? Explain any
difference.
Compare to Step 3 and 4, the result was decreased by one-half. The percent modulation is
directly proportional to the amplitude of the modulating signal, that is why when the
modulating signal was reduced by half, so was the per cent modulation.

12. 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.
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?
The difference between the values 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?
The side frequency is 1/4 of the carrier. But the modulation also decreased by half, that is why
it is 50% less than the result in Step 6.

13. 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?
They have a 0.1 × 10-6 difference. It is almost 0.
How did this waveshape compare with the previous amplitude-modulated waveshapes? Explain any
difference.
The waveshape is the carrier signal waveshape. There is a difference because the modulating
signal is zero.
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.

14. 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?
The difference is 0% . They have the same values
How did the voltage amplitude of the spectral line compare with the carrier amplitude?
The difference between the values is 0.16%.
How did this frequency spectrum compare with the previous frequency spectrum?
There is only carrier frequency and no side frequencies this time.
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.
Question: How did this waveshape differ from the waveshape for a 5 kHz modulating frequency in Step 2?
The frequency of the modulating signal increased by 5Hz therefore, it has greater number of
cycles per second compare to other waveshapes.
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.

15. 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?
The bandwidth is twice of the 5 kHz. It 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.
The difference between the values is 41.458 mV. As the modulating signal increases, it will
affect the amplitude of side frequencies. It will decrease.
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 by 20 kHz or twice of the modulating frequency.
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?

16. 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
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.
Because it is complex wave, it generates many side frequencies.
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.
The difference between the values is 0.002 kHz. The bandwidth is almost equal to Step 6.

17. 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.
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.
The difference is that the modulating index was decrease by one-half, because the amplitude of
the modulating signal was also decrease by one- half

18. CONCLUSION:
, Therefore, I conclude that, in amplitude modulation or AM, varies the value of the carrier
amplitude and remain the carrier frequency as constant.
The oscilloscope displays the time-domain curve of the amplitude modulation. The modulation
index is the relationship of the voltage of the modulating signal and the voltage of the unmodulated carrier.
It is directly proportional to the modulating amplitude and inversely proportional to the carrier amplitude.
The percent modulation can also be determined through the maximum and the minimum values of the
modulating signal displayed on an oscilloscope. The envelope of the modulated signal is as the shape of the
curve of the modulating signal.
The spectrum analyzer displays the frequency domain of the circuit. The voltage of each frequency
is half of the carrier voltage multiplied by the modulation index. Therefore, as the carrier voltage or the
modulation index increases, so as the voltage of each side band, otherwise the voltage of each side
frequency decreases.
In square wave, which is a complex wave generates many side frequencies. This is because square
wave consists of a fundamental sine wave frequency and many harmonics.