ELE 3203
Communication systems
Lab 4: Amplitude Demodulation
Student Name ID Number
Faculty Name: Mr. Yacine Adane
Lab Instructor: Ms. Rajalakshmi
Submission Date: October 10, 2020
Prepared By: Rajalakshmi S Menon, ADW
Objective
To study the function of Amplitude Demodulation. Also, to vary the modulation index and see
how it affects the output signal.
Apparatus Required
a) Hardware Tools: Computer system
b) Software Tool: MATLAB 7.0 and above version.
Prepared By: Rajalakshmi S Menon, ADW
INTRODUCTION
The purpose of this experiment is to further expose you to applications of digital Fourier
analysis using MATLAB. These topics will be explored in the context of sampling audio
signals for the design and simulation of the modulation and demodulation processes used in
amplitude modulation. So, we are getting to perform Amplitude Demodulation.
Demodulation is recovering the first message signal from a modulated carrier signal.
Telecommunication receivers are made for the aim of demodulation of a selected or different
type of signal. To reach the Amplitude Demodulation we have to use a block that will help us
to reach the following formula Demodulated signal = 𝑆𝑖𝑛(2𝜋𝑓𝑐𝑡) + 𝑚2 sin(2𝜋𝑓𝑐𝑡 − 2𝜋𝑓𝑐𝑡) −
𝑚 2 sin(2𝜋𝑓𝑐𝑡 + 2𝜋𝑓𝑐𝑡) . This can be determined while using a simple block such as Sine
wave, constant, scope, product, adder blocks, and filter (Buffer). But they must organize in a
specific way to reach the goal.
Prepared By: Rajalakshmi S Menon, ADW
THEROY
Carrier signal= Vcsin(2πfct)
Message signal= Vmsin(2πfmt) # fm must be smaller than fc
When carrier amplitude is altered with respect to message signal,
Modulated Signal = (Vc+ Vmsin(2 πfmt))*sin(2 πfct)
In terms of modulation index (m=Am/Ac) the equation becomes
Demodulated signal= 𝑆𝑖𝑛(2𝜋𝑓𝑐𝑡) + 𝑚2 sin(2𝜋𝑓𝑐𝑡 − 2𝜋𝑓𝑐𝑡) − 𝑚 2 sin(2𝜋𝑓𝑐𝑡 + 2𝜋𝑓𝑐𝑡) .
Where,
Vc = Carrier signal amplitude
Vm = Message signal amplitude
fc= Carrier frequency
fm =Message frequency
Prepared By: Rajalakshmi S Menon, ADW
PROCEDURE
The following blocks are required to draw the block diagram as shown in the Figure 1.
1. Carrier Signal Source
2. Message Signal Source
3. Blocks for viewing the signals – Scope
4. Product Block
5. Filter
We can find these blocks in the following locations of Simulink Library.
Carrier, Message, Constant blocks
Simulink –> Sources –> Sine wave
Simulink –> Sources –> Constant
View Block
Simulink –> Sink –> Scope
Product and Summer Block
Simulink –> Math Operations–> Product
Simulink –> Math Operations–> Summer
Analog Filter Design
Simulink–> DSP System Toolbox–> Filtering–> Filter Implementations–> Analog
Filter Design
Prepared By: Rajalakshmi S Menon, ADW
Figure 1. Amplitude Modulation and Demodulation Block Diagram
Block parameters can be changed by selecting the block and parameter to be used are given
below.
Carrier Signal frequency = 2*pi*25 and sampling time=1/5000
Message Signal frequency = 2*pi and sampling time=1/5000
Amplitudes of Message Signal is 1 and Carrier Signal is 3
Analog Filter Design Filter Order= 4
To study the Amplitude Demodulation, we need the Amplitude modulation. So, first, we
develop the Amplitude modulation, which is a combination of several blocks to develop the
following formula; Modulated signal=Vc (1+ msin(2 πfmt))*sin(2 πfct). After that we are
going to multiply the carrier signal and inter the filter (buffer). The sine wave will display in
the Scope. Which gives us a Demodulation signal. So, the Demodulated signal is dependent
on the filter.
Prepared By: Rajalakshmi S Menon, ADW
RESULTS AND DISCUSSIONS:
1. Comment on your results.
To begin with the, Am demodulation is a concept, which is to return the Modulated signal to
a Message signal. Which is known as AM-Demodulated. Also, from the result we can see
that the message signal and Demodulated signal are approximately similar, but with different
amplitudes. Moreover, it’s on the positive side because of the filter (Buffer). This can be seen
in Figure 1. Therefore, there is a phase shift in a Demodulated signal. This is because of the
amplitude of the message signal and carrier signal. Where the message signal amplitude less
𝑉𝑚𝑎𝑥" 𝑉𝑚$𝑛
than the carrier signal (Vm < Vc). In addition, the index modulation is 0.5; m= =
𝑉𝑚𝑎𝑥$𝑉𝑚$𝑛
%.5"(.5
%.5$(.5 = 0.5.
Figure 1: General Amplitude Demodulation
Prepared By: Rajalakshmi S Menon,
ADW
2. Upload your Simulink file.
Figure 2: Simulink
Figure 3
3. Explain your result. (Identify each block function)
A Message signal is a sine wave that carries information such as voice, and data.
Also, the Carrier signal is a sine wave, but it’s worked to modulated for transmission by
AM.
m Block is a modulation index that shows how the Amplitude Modulation changed.
Moreover, Product 1 is the multiplication of the message signal with the modulation
index (0.5).
Then, the Adder used to add the constant (1) with the message signal parameters.
However, Product 2 is used to multiply the message signal parameters after being added
with the constant, with the carrier signal. Which will lead to modulate the signal.
Therefore, Product 3 is used to multiply the output from Product 2 with the carrier signal.
Lastly, the output from product 3 is interred into the Filter (Buffer), which is
Demodulated the Amplitude modulation to return back as the message signal.
Finally, the Display (scope) has been used to see or display the output result.
Prepared By: Rajalakshmi S Menon, ADW
4. What will happen to the waveform when the modulation index m=1. Attach the
scope results and Comment on your result.
As we know the range of the normal index modulation is between 0 and 1. In this case, as we
have to use the index modulation of 100% and the Amplitude modulation becomes perfect
modulated. This can be seen in figure 4. The Demodulation signal in this case has less curve
at the beginning of the waveform. Also, it starts from zero and it’s doesn’t reach the negative
values. However, the demodulated waveform it’s looks like the message signal with different
amplitudes. So, if we see them without the amplitude number, we will think they are one
signal.
Figure 4: perfect modulated; m= 100%
Prepared By: Rajalakshmi S Menon, ADW
5. What will happen to the waveform when the modulation index m<1. Attach the
scope results and Comment on your result.
In this second case, as we decrease the index modulation more than one, the Amplitude
modulation becomes under modulated. This can be seen in figure 5. The Demodulation signal
in this case has more curve at the beginning of the waveform. Also, it starts from zero and it’s
doesn’t reach the negative values. However, it’s in a specific amount of amplitude. If we see
from figure 5, the range of its amplitude between 5.5 and 3.5. Moreover, this range was very
small which is about increasing/ decreasing for 2 units only.
Figure 5: Under Modulated
Prepared By: Rajalakshmi S Menon, ADW
6. What will happen to the waveform when the modulation index m>1. Attach the
scope results and Comment on your result.
In this case, when the Amplitude modulation becomes over modulated which can be seen in
figure 6. Also, the demodulated signal reaches negative values. This means when we increase
the modulation index, the demodulation signal effected. Moreover, by increases the index
modulation the amplitude of the Demodulation increases also. However, the demodulation
still has the message signal waveform shape.
Figure 6: Over modulated; m>1
Prepared By: Rajalakshmi S Menon, ADW
CONCLUSION AND OBSERVATION
Demodulation is the reverse process where the received signal is transformed to their original form.
Amplitude Modulation (AM) is the process of changing the amplitude of a relatively high-frequency
carrier signal in proportion with the instantaneous value of the modulating signal. A high-frequency
carrier signal is also termed the radio-frequency (RF) signal because it is at a high-enough frequency
to be transmitted through free space as a radio wave. Low-frequency information signal is also term
such as modulating signal, intelligence, and audio signal.
Prepared By: Rajalakshmi S Menon, ADW
