ECE 4117
Experiment 2
Amplitude Modulation
ECE 4117 – Telecommunications Laboratory
Fall 2012
Purpose
In this lab, the concept of Amplitude Modulation (AM) is investigated. Implementation of AM is
primarily done through the use of GNU Radio and the USRP. Both transmission and receiving is
investigated.
Introduction
In communication, modulation is the process of varying one or more properties of a highfrequency periodic waveform, called the carrier signal, with a modulating signal which typically
contains information to be transmitted, called the baseband. A modulator is used to modify a
carrier signal’s parameter (frequency, amplitude, phase, etc.) in accordance to the baseband
signal.
Figure 1: Block diagram of modulating a signal
Modulation is necessary in the transmission of data, such as an audio signal, over a long range.
Baseband signals are inherently limited in range because their frequency range is typically in a
lower range and their power makes the signal unsuitable for radio transmission. Modulation
gives several advantages, such as reducing the size of transmitting antenna, better usages of
limited bandwidth, and much more.
There are three types of basic modulation (analog), amplitude (AM), frequency (FM), and phase
modulation. The latter two are similar and belong to the class of modulation known as angle
modulation. For this lab, we will only cover AM.
Amplitude Modulation (AM) is a method to modulating the carrier signal, where the amplitude
of the carrier signal is varied in line with the variations in intensity of the baseline (information)
wave. Thus, the overall amplitude or envelope of the carrier is modulated to carry the audio
signal. Below, we see the layout of a signal and the modulated carrier signal.
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Figure 2: Audio Signal and Amplitude Modulated Carrier Signal
A GRC file will be included for this lab, where the process of amplitude modulation and
demodulation is demonstrated.
In this lab, you will build a transmitter and receiver layout within GNU radio for AM radio,
based on the GRC file provided. The USRP will then be used to transmit information from one
computer to another using GNU radio.
GRC File
For this example, refer to am_tx_rx.grc
The purpose of this file is to mimic and show the process of transmitting and receiving an AM
radio signal. The file takes a WAV file and modulates it. Then it sends it to the receiving side
where it is demodulated and played through a scope and the computer’s speakers. A more
thorough explanation is provided below.

First, the WAV file is added to a constant value of 1, this shifts the signal by one. This
shift is needed in order to achieve classic amplitude modulation, where the carrier is
available in the final product. This signal is now 1+m(t), where m(t) is the baseband
signal. The sampling rate throughout the layout is 44.1 KHz, based on the initial rate
from the WAV file.
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



Next, the signal’s data type is converted to complex format. The data does not change,
but the data can now be used in complex operations.
The signal is now multiplied with constant value of 0.1. This is done to shrink the data
to fit the -1 to 1 amplitude requirements of the USRP. It is not necessary in this
example.
The signal is multiplied again with a high frequency cosine signal. This is the carrier
signal. This signal’s frequency is the frequency at which the USRP or any radio would
broadcast a signal. This is the final step in Transmission and the signal would then be
transmitted through the air.
The signal arrives at the receiving side and gets multiplied by another cosine signal. This
acts as a frequency shift or tuner, in order to center carrier signal at 0 Hz. Since the
carrier signal is at 10 KHz, the frequency of the frequency shift is negative, -10 KHz.
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
If the signal was among an array of other signals, a filter would be needed to select a
signal and filter out the rest. This is called a channel filter. Since a tuner is used to bring
the wanted carrier signal to 0 Hz, a low pass filter can be used to filter out any unwanted
signals. The cutoff frequency at 5 KHz is standard and is in line with the available
bandwidth with standard AM radio (10 KHz).

The final step is the demodulation of the signal. This is done by detecting the envelope or
magnitude of the modulated signal. The block complex to mag does this. It detects the
magnitude of the complex signal and outputs it in float form. This signal is now 1+m(t).
The constant multiple block can be used to control how loud the signal is by changing the
amplitude. Finally, the output can be listened to through the computer speaker and
observed through the scope sink.
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Procedure
1. Run and analyze the GRC file provided. Check to see if the output sound matches the
input wav file. Do the following and take screenshots.
a. Place an FFT sink at the following locations: after the Float To Complex
block, after the first multiply block, after the second multiply block, and at the
output.
b. Place a scope sink at the following locations: After the first add block, after
the first multiply block, after the second multiply block and at the output.
You can only run two graphs at one time in GNU radio, so you will have to do
several runs to obtain the results.
2. Build a separate GRC file based on the receiving side of the am_tx_rx file. This
separate file will be used to receive a signal using the USRP. The input will be a
USRP2 source and the output will be a scope sink and an audio sink. Make the
following changes to the layout:
a. Make the frequency shift have the ability to shift on demand by placing a
variable slider. By using the slider, you have defined a variable identified by
its ID. Using the ID in the frequency field of the signal source enables you to
vary the frequency using the slider. This will enable you to define a variable
that can be shifted from the graph GUI after the layout is executed. Set the
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b.
c.
d.
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default value at 0, the steps at 1000, and minimum and maximum value at 20K to 20K.
Check the incoming signals frequency range and make sure that none of the
wanted signal is being cutoff. If it is, adjust the low pass filter accordingly.
Make the volume variable by placing another variable slider. Try different
values to achieve a good range of volume. Try a default at .5, and making the
range from 0 to 1. Multiply if needed.
Place an FFT sink after the multiply block so as to see the spectrum of the
incoming signal.
The USRP outputs at a sample rate of 100MHz. In order to hear the USRP
signal, the signal needs to be resampled to a slower rate. Two blocks, the
rational resampler and the fractional interpolator can be used to reduce the
sample rate (refer to GNU radio docs for more details on both blocks). Both
are located under the filter menu. Also, several blocks such as the low pass
filter and the USRP source/sink have decimation and interpolation options
built in. The key is to have a large enough sample rate to see what is necessary
in the FFT sink, therefore having a sample rate at around 256K would be ideal
at the FFT stage. After this, the sample rate can be dropped to a rate the
speakers of the computer can pick up (44.1 or 48 KHz). Make sure all of the
blocks are set to the proper sampling rate in their branch.
3. After building the receiving layout, test the layout. This layout can be tested by
implementing a file source instead of the USRP source and using the file named
test_am_usrp.dat. It will work like the USRP in that in it is actually a recording from
the USRP. It contains a spectrum of AM radio signal, where it is centered at 780
KHz. This file can be implemented in place of the USRP source. The only difference
is that the file’s sample rate is 256 KHz, so adjust the sample rates accordingly. Take
a picture of your layout, and derive scope and FFT graphs from one of the broadcasts.
4. Build another GRC file layout based on the transmission side of the am_tx_rx file.
This separate file will be used to transmit signal using the USRP. The input will be a
signal source and the output will be a USRP2 sink. The output needs to have a
sampling rate of 100 MHz. Therefore, you need to use resampling blocks to raise the
sample rate. Also, remove the second signal source as the USRP2 does the
modulation within itself. Just remove the multiply block and the Carrier signal block.
5. Using two computers and two USRPs, send a sine wave signal from one computer to
the next. Get as close as possible to the original waveform. Noise will be a factor in
the receiving of the waveform, therefore try physically moving the USRPs closer.
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Take a picture of outgoing and incoming modulated and demodulated waveforms. If
the signal is too dirty, try replacing the receiving side scope with a file sink. This
produces a binary data file. Using this data, you can check the information on Matlab.
A reference sheet will be able for this method online.
6. Write a report on the concepts of AM, the transmission and receiving of information,
and the procedures done in the experiment. Make sure to include the following:
a. Write in detail about the basics of AM. Also include some of the other
modulation schemes within AM.
b. Write up an analysis of the am_tx_rx file. Explain why the blocks are there
and what purpose they serve. Explain some of the concepts in greater detail.
Make sure to include the waveform screenshots.
c. Write about GNU radio and the USRP TX/RX. Compare to these to schemes
of TX/RX within the book.
d. Describe your personal experience on broadcasting your waveform from both
created files. Be sure to include all data obtained.
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