ES442 Lab#3
ES442. Lab 3
Amplitude Modulation
I. Objective
1. Understand the concept of AM.
2. Learn to perform amplitude modulation with the multiply function of the Agilent
54622A oscilloscope.
3. Learn to perform amplitude modulation with the Agilent 33220A signal generator.
4. Observe the time domain and frequency domain representations of AM signals
with oscilloscope.
5. Calculate the modulation index based on frequency domain representation of the
signal.
II.
Pre-lab
1- Write down the expression for ordinary AM modulation with fm being the modulating
frequency and fc being the carrier frequency.
2- If we perform DSB-FC over the signals in question 1, write down the Fourier transform
of the modulated signal.
3- How do you calculate the modulation index for AM modulation using Amin and
Amax?
4- How do you measure modulation index using power measurements?
5- How do you calculate the Power in Watts when the load is 50 ohm and the measure
voltage using the spectrum is 10dBV?
6- Plot the spectrum of AM modulation (DSB-FC) and indicate the power of each
component in frequency domain.
II.
Text Equipment
1234-
Ver 2.
Agilent 33220A signal generator
MSO-X Scope with two probes, DEMO port working, and a signal generator option
Memory stick to record your results (bring your own)
Coax cable with clips
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ES442 Lab#3
IV.
Procedures
A. Amplitude Modulation with Function Generator
The Agilent 33220A signal generator can output amplitude modulated signal
directly.
1. Choose the modulation function of the function generator with the following
parameters. Note that the sine waveform is your carrier and AM modulation is
your m(t). In this case µ is the AM modulation depth.
µ
m(t )
Source
fc
Ac
fm
1KHz
1V
100Hz
0.5
sinusoid
Internal
2. With the above parameters, calculate the amplitude of m(t ) . Verify you
calculation by measuring the AM signal with oscilloscope. Capture the display.
3. Measure Amax , Amin , and calculate the percentage modulation through your
measurement and compare the results with the theoretical value.
4. Adjust modulation percentage (also called AM depth) on the function generator
from 0 to 1.3 and observe the change in display, especially when over-modulation
happens. What is the signal when µ = 0? Complete the following table for µ =0
to 1.3 at steps of 0.2. Make sure you include one snap short for each of the
following cases: µ = 0, 0.50, 1, 1.3. Display these figures in a 2x2 table. Make sure
you have description for each case.
5. Plot the Δ as a function of AM depth.
µ
Amin
Amax
Ac
Measured µ
%Δ
B. Frequency domain representation of AM signal
1- Choose the modulation function of the function generator with the
following parameters. Note that the sine waveform is your carrier and
AM modulation is your m(t). In this case µ is the AM modulation depth.
µ
m(t )
Source
fc
Ac
fm
1KHz
1V
100Hz
0.5
sinusoid
Internal
2- Use the FFT function to observe the frequency domain of the signal. What
should be the center frequency in the FFT setting and why? Set the span to
2KHz and Sample rate to 500kSa/s. Capture the display; make sure you
include both the modulated signal and its spectrum on the same screen.
3- By comparing the spectrum with your calculation, measure the value of µ.
Write out the equation that you used for the calculation of µ.
4- Using the measured voltage at different frequencies to calculate the power of
carrier, upper sideband, and lower sideband, and compare your calculations
with theories.
5- Using the measured voltage at different frequencies, calculate the power of
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ES442 Lab#3
carrier, upper sideband, and lower sideband, and compare your calculations
with theories.
6- Measure the modulation index for different values of AM depth: µ = 0.2, 0.3,
0.50, 0.8, 1.0. Display these figures in a 2x2 table. Make sure you have
description for each case. Complete the table below.
7- Plot the Δ (percentage difference) as a function of AM depth.
m
Ec(dBV)
Em(dBV)
Pc(W)
Ps(W)
Pt(W)
Pt/Pc
m
m%
C. Frequency domain representation of AM signal
In this part we are going to use two function generators to perform AM. One
generate the modulating signal and the other one generate carrier.
1. Use function generator 1 to generate carrier signal, and function generator 2 to
generate the modulating signal. The modulating signal should be connected to
Modulation In (At the back of the function generator) of the carrie r function
generator. Generate AM signal with the following parameters. Set the output
impedance of modulating signal to be 75KOhm to match the input impedance of
the carrier signal. The output impedance of the carrier signal remains to be high Z.
Set the modulation index to be 1 at the carrier signal.
m(t )
Source
fc
Ac
fm
Am
1KHz
2V
100Hz
1.2V
sinusoid
External
2. The display on the oscilloscope can be stabilized by using the low frequency
signal as an external trigger. Get a stable display on your oscilloscope.
3. Use the time domain signal to measure the modulation percentage.
4. Use FFT to measure the modulation percentage.
5. Switch back to time domain. Adjust the voltage of the modulating signal until
0 1 , record the voltage of the modulating signal.
6. Adjust the voltage of the modulating signal to observe the effects of
overmodulation.
C. AM with Oscilloscope
The multiplication function of the oscilloscope can be used to generate DSB-SC signal.
1- Connect carrier signal to channel 1, the modulating signal to channel 2 of the
oscilloscope (the output impedance of both function generators should be high
Z). Make sure the probes are set to 10:1 ratio.
2- Set the modulating signal to be m(t) = 2 + 1.2cos(200πt) by using the DC offset
of the function of the function generator.
3- Set the carrier signal to c(t) = 2cos(2000πt).
4- Make sure the two channels have the same scale on the scope. Use the
modulation function of scope to perform multiplication over the two channels.
Capture the display.
5- Write down the time domain expression of the modulated signal, and calculate
the modulation percentage by using the expression.
6- Measure the modulation percentage with the time domain signal on
oscilloscope.
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Δ
ES442 Lab#3
7- Measure the modulation percentage with the frequency domain signal on
oscilloscope. Complete the table below: For each case add the captured signal.
Use a 2x1 table to display your figures.
m (calculated from
the expression)
m (measured from the
time domain)
m (measured from the
frequency domain)
8- Set m(t) = 1 + 0.5cos(200πt) and c(t) = 1.5cos(2000πt). What is the modulation
percentage? Measure it using the time domain display on oscilloscope.
m (calculated from
the expression)
m (measured from time
domain)
m (measured from
frequency domain)
9- With the setting in the above step, Adjust the amplitude of the modulating
signal to get µ =1. What is the amplitude? Does this match the theoretical
value?
8. Adjust the amplitude of the modulating signal to get µ =0.5.
9. Increase the carrier frequency to 10KHz.
10. Change the modulating signal to square waveform with 50% duty cycle, capture
the display in time domain and frequency domain. Complete the table below.
m(t)
signal
Sine
Sine
fm (Hz)
1000
1000
Modulation
Index
1
0.5
Sine
Sine
10000
10000
1
0.5
Square
Square
1000
1000
1
0.5
Measured Em (Vp)
Calculated
Em (Vp)
11. Using a 3x2 table show all the captured figures.
Your Figure Here!
Ver 2.
Case 1: m(t) = Sine, fm =
Case 1: m(t) = Sine, fm =
Case 1: m(t) = Sine, fm =
1000, m=1
1000, m=1
1000, m=1
Case 1: m(t) = Sine, fm =
Case 1: m(t) = Sine, fm =
Case 1: m(t) = Sine, fm =
1000, m=1
1000, m=1
1000, m=1
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