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Hello I am Michael Grady and I am Eduardo Rojas. We are graduate students at the University
of South Florida. And we would like to show you how modulation is
used to convey information.
Do you know how the music is sent through the air to the car radio? How can it travel? For that we use waves.
Imagine that someone is trying to deliver a message in a bottle to another person on a separate island.
The forces that come from the bottle alone cannot get the message across the body of water.
It takes some outside force to carry the message.
In this case the water waves carry the message in the bottle to the other island.
The process of an outside force carrying the message is like the process of modulation.
But we cannot use ocean waves. We use Electromagnetic waves...
For many centuries humanity had been trying to understand the Electromagnetic Field.
James Maxwell postulated four equations that explained the nature of this field.
These equations suggested the existence of an electromagnetic wave that could travel through free space.
Here we see a representation of the electric field radiated by a linear antenna.
As you can see, it resembles the waves in the ocean, but the big difference is that they travel at the speed of light!
Amplitude modulation works by manipulating two signals so that the message behind one signal can be sent using a signal more appropriate for radiation.
One is a low frequency signal that is defined in the time domain by M*sin(w_m*t) and the signal has some resulting representation in the frequency domain.
The second signal is a high frequency carrier signal defined in the time domain by A*sin(w_c*t).
Amplitude modulation multiples these two signals as seen in the following figure.
After multiplication of the signals takes place it can be seen that the amplitude of the carrier signal is modulated or modified by the message signal.
Notice the amplitude follows the same pattern as the message signal.
Hello, this is Eduardo again. We are going to show you a setup where we can see the actual AM modulation working.
So, what we have is a cellphone which is going to be the source of the message. We want to play some music on the cellphone and want to put to the signal generator,
which is this one, and on the oscilloscope we are going to see the signal vs. time, time domain representation of the signals.
So, if I play the music, right here, and connect the cellphone to the input, we can see on the first oscilloscope the voltage of the signal coming out from the cellphone.
It is a low frequency signal that goes up to twenty kilohertz and it cannot be used for radiation.
So, we need a carrier, we need a higher frequency signal.
For that, we use the signal generator, which generates a higher frequency signal, which can be seen here.
Right now is not being modulated.
It is just a pure sinusoidal signal, high frequency signal, and if we click here modulation On,
we can see how now the amplitude the signal is being modulated. So we see the message on the amplitude.
Next, we are going to show you an experiment where we are going to send music with AM modulation.
For this setup, we are using two antennas.
The signal generator on the left produces the modulated signal,
which then will be sent, received, and demodulated using the spectrum analyzer
on the right. This device has a built in speaker.
Hello it's Mike again. And this is our demonstration to show you how modulation can be used to send a signal.
I am holding an antenna and my friend Eduardo has a message for me.
Eduardo is standing about 100 meters away from me and he has a message on his cell phone that he wants to send.
He's hooking his cellphone up to an audio jack and by way of a signal generator and an antenna;
his message is being delivered to me. And my antenna is picking up the signal.
As soon as I turn up the sound, the spectrum
analyzer is demodulating the signal,
and its outputting the signal through the speaker.
(Music plays......Message in the bottleee!!)
And this is how I receive his message in the bottle.
(Music plays......Message in the bottleee!!)