Download ECE 4117 Experiment 3 Frequency Modulation ECE 4117

ECE 4117
Experiment 3
Frequency Modulation
ECE 4117 – Telecommunications Laboratory
Fall 2012
Purpose
In this lab, the concept Frequency Modulation (FM) is investigated. Implementation of FM 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
There are three types of basic modulation (analog and digital), 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 cover FM.
Frequency Modulation (FM) modulates a carrier signal by varying the frequency of carrier signal
with respect to the message signal. Over time, the frequency would be varied by an arbitrary
factor multiplied the current amplitude of the message signal at that point in time. Since this is
the case, the maximum frequency change in the carrier signal is proportional to the maximum
amplitude of the message signal. Below we see a general layout of a message signal and the
modulated carrier signal
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Figure 2: Audio Signal and Frequency Modulation
In this lab, you will build a transmitter and receiver layout within GNU radio for FM radio,
based on a GRC file provided. The USRP will then be used to transmit information from one
computer to another using GNU radio and files created. The GRC file fm_rx_tx.grc will be
provided to demonstrate FM within GNU radio.
GRC File
This file (fm_rx_tx.grc) will be used to demonstrate the process of transmitting and receiving an
FM signal. The file will take a WAV file and modulates it. It then sends the signal to the
receiving side where it is demodulated and played through a scope and speakers. See the more
thorough analysis below:

The baseband signal is sent to the NBFM Transmit block where it is modulated to a
Narrow-band FM signal. Since this scheme is more complex than AM, GNU Radio
created the block to handle the modulation. The sample rate incoming is 32KHz and the
outgoing sample rate is 256KHz
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
The signal is shifted to its broadcast frequency, where it completes the modulated
transmission signal. The signal would be ready to be broadcast over the air to the
receiver. This step is usually handled in the USRP, so this signal is not necessary in a
transmit layout. It is shown here as an example.

The receiving side starts with the channel filter, the Frequency Xlating (translating) FIR
filter. The filter does two things, it tunes the signal so that the carrier frequency is
translated to 0 Hz and it uses a low pass filter to filter out the unwanted components of
the spectrum of signals. Since our signal is being broadcast at 50KHz, the center
frequency is at -50KHz. The bandwidth of our signal (NBFM) is approximately 2B,
where B is the highest frequency component of the baseband signal. According to the
FFT graph of the signal, this is at about 5KHz. So, the lowpass filter should cut off at
5KHz since the bandwidth runs from -5KHz to 5KHz. The FILT variable box handles the
low pass filter option of FIR filter, which is applied at the taps parameter. The function
indicated in the Taps parameter generates the taps for a low pass filter with a gain of 1 (in
the pass band), a sampling rate equal to 256K, a cutoff frequency of 5KHz (or 40KHz
since the sampling rate is changed from 32K by a factor of 8) and a transition width of
1000.
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
The sampling rate is reduced to a level that the speakers of the computer can handle
(48KHz). It is then demodulated using the NBFM receive block. Again, the process is
complex and is therefore confined to a single block provided by GNU radio. A constant is
multiplied to the signal to increase the volume of the signal and is then outputted to the
audio sink (speakers) and scope sink.
Procedure
1. Run the provided GRC file and analyze the results. Check to see if the output matches the
input WAV file. Do the following to analyze the file:
a. Compare the output of the receiving side to the input of the transmission side
using the scope sink and FFT sink. Take screenshots of these graphs.
b. Compare the modulated signal before and after the Frequency Xlating FIR Filer
using the FFT sink. Again, take screenshots of these graphs.
c. Try replacing the NBFM receive and transmit blocks with WBFM receive and
transmit blocks. This will essentially change this to a Wideband
Transmitter/Receiver. Using Carson’s rule, find the bandwidth of this new signal
and modify the Frequency Xlating FIR filer accordingly. Note that there may be
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error, so a larger transition width within the filter may be needed. Take
screenshots proving this step has been done.
2. Build two separate GRC files based on the receiving and transmitting side of the provided
GRC files. Both sides must be able to transmit/receive either Wideband or Narrowband
FM using the USRP. Therefore, include a Selector (from Misc. category) and a variable
chooser (from Variables category). Combine the two and you should be able to choose
the type of FM you want from a GUI. Remember that USRP must receive 100MHz of
samples and it sends to the computer 100MHz of samples. Use the Rational Resampler
and/or Fractional Interpolator to reduce or increase the sampling rate. Take a screenshot
of both files
3. Transmit a sine wave using two USRPs and the constructed GRC files. Take screenshots
of the sine wave before transmission and after transmission/demodulation using the scope
sink and the FFT sink. (It’s fine if it has some errors/noise)
4. Write a report on Frequency (Angle) modulation, talking about the concepts and
principals and the procedures of the experiment. Include the following in the report:
a. Write about the concepts of FM, including the Wideband and Narrowband FM
schemes.
b. Write what you learned about from the provided GRC file. Include all the
screenshots from the first step. Explain the concepts discussed in greater detail.
c. Compare the steps of demodulation discussed in the class/book, and compare to
what is going on in GNU radio.
d. Describe your experience of broadcasting your waveform.
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