Islamic University Technology (IUT)

Department of Electrical and Electronic Engineering (EEE)

EEE 4404: Communication Engineering I

Objectives:

1. Understanding the principle of amplitude modulation (AM).

Part A: Hardware

Apparatus Required:
• Emona ETT-101/C Trainer
• Dual-channel 20 MHz Oscilloscope
• Patch leads and BNC connectors
• Headphones (optional for audio check)

Preliminary Discussion:
Once we are out of the shouting range of another person, we must rely on some communication system to
enable us to pass information. The essential parts of any communication system are the transmitter, a
communication link, and a receiver. In the case of speech, this can be achieved by a length of cable with a
microphone and an amplifier at one end and a loudspeaker and an amplifier at the other.

Figure 1 A Simple Communication System

It is convenient to use a radio communication system for long distances or when it is required to send signals
to many destinations simultaneously.

Amplitude Modulation (AM):

The method that we are going to use is called amplitude modulation. As the name would suggest, we will use
the information signal to control the amplitude of the carrier wave. In an amplitude modulation (AM)
communications system, speech and music are converted into an electrical signal using a device such as a
microphone. This electrical signal is called the message or baseband signal. The message signal is then used
to electrically vary the amplitude of a pure sinewave called the carrier. The carrier usually has a frequency
that is much higher than the message’s frequency.

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By looking at Figure 2, we can see that the modulated carrier wave does appear to contain the information and
the carrier in some way. We will see later how the receiver can extract the information from the amplitude
modulated carrier wave.

Figure 2 AM Modulation

Figure 2 above shows the AM signal at the bottom but with a dotted line added to track the modulated carrier’s
positive peaks and negative peaks. These dotted lines are known in the industry as the signal’s envelopes. If
you look at the envelopes closely, you’ll notice that the upper envelope is the same shape as the message. The
lower envelope is also the same shape but upside-down (inverted). In telecommunications theory, the
mathematical model that defines the AM signal is:

AM = (DC + message) × carrier

Modulation Index (or Depth of Modulation)

The amount by which the amplitude of the carrier wave increases and decreases depends on the amplitude of
the information signal and is called the modulation index. The modulation index can be quoted as a fraction
or as a percentage.
𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛
𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑓 𝑀𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = × 100
𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛
Where Vmax is the Peak-to-Peak Voltage difference in the modulated AM Signal. And Vmin is the minimum Peak-to-
Peak Voltage difference in the same modulated AM signal.
Task

In this experiment you’ll use Emona Telecoms-Trainer 101 to generate a real AM signal by implementing its
mathematical model. This means that you’ll add a DC component to a pure sinewave to create a message
signal then multiply it with another sinewave at a higher frequency (the carrier). You’ll examine the AM signal
using the scope and compare it to the original message.
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Section A: Generating an AM Signal Using a Simple Message

Goal: Create a signal of the form Am(t)+DC

Procedure (Refer to Figure 3):

1. Use the Master Signal module to generate a 2 kHz sine wave.

2. Route this to the Adder Module – Input A.

3. Use the Variable DCV module to use VDC as output to Adder Input B.

4. Adjust the Adder gains to get:

1. Message amplitude: ~1 Vp-p

2. DC offset: ~+ 1 V

5. Observe the combined signal 𝑚(𝑡) = 𝐴𝑚 + 𝐷𝐶 on CH1 of the oscilloscope

Figure 3 Part A

Output Task:

Sketch or capture the oscilloscope view of the Am(t)+DC. Label amplitude and DC bias levels.

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Section B: Multiplying with Carrier (Modulation)

Goal: Multiply the message signal with a high-frequency carrier.

Procedure (Refer to Figure 4):

1. Connect the Adder output to the Multiplier X input.

2. Use 100 kHz sine wave from Master Signal as the carrier, fed to Multiplier Y input.

3. Observe Multiplier output (modulated signal) on CH2 of oscilloscope.

4. Set oscilloscope to DUAL mode and compare CH1 (message) and CH2 (modulated).

Figure 4 Part B

Output Task:

Sketch the modulated waveform. Clearly mark the carrier frequency and message envelope.

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Section C: Investigating Depth of Modulation

Procedure:

1. Vary the Adder gain for the message signal to change amplitude.

2. Observe the change in the AM signal’s envelope.

3. Measure the maximum (P) and minimum (Q) envelope amplitudes.

4. Calculate modulation index:

𝑃−𝑄
𝑚=
𝑃+𝑄

Figure 5 Part C

Output Task:

1. Complete the table:

Sl. P dimension Q dimension m

2. Draw three waveforms:

• Under-modulation (m < 1)

• Critical modulation (m ≈ 1)

• Over-modulation (m > 1)
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Section D: Recovering the Message Using an Envelope Detector

Procedure (Refer to Figure 6):

1. Connect the Multiplier output to the Utilities module’s Rectifier Input.

2. Then the output of the Rectifier comes in as an input to the RC LPF block of the same Utilities Module.

3. Observe the output of the RC LPF block on the oscilloscope.

4. Compare it with the original message signal.

Figure 6 Part D

Output Task:

Plot the recovered message waveform. Comment on any distortions or delays observed.

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Section E: Find a Way to Smoothen the Detected Signal

Goal: Improve the recovered signal by reducing ripple and distortion.

Output Task:

Compare before and after smoothing waveforms.

Result Discussion (To be written by students):

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Report Questions:

1. Explain your understanding on modulation with reference to AM modulation that you have experience in
this experiment.
2. Describe the effects of over-modulation in time and frequency domains. Illustrate with diagrams.
3. Explain why over-modulation should be avoided.
4. What can be said about the phase shift between the signals on the Adder module’s two inputs in Section
A of the experiment?
5. What is the relationship between the message’s amplitude and the amount of the carrier’s modulation?
6. What is the problem with the AM signal when it is over-modulated?
7. Explain in detail about your solution to Section E problem.

References:
This lab sheet has been prepared based upon Curriculum Manual 1 of the Emona 101 Trainer Board.

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 Appendix
ROUND sockets, “ ”, are only for ANALOG signals.

ANALOG signals are typically held near the ETT-101 standard reference level of 4V pk-pk.

SQUARE sockets, “ ”, are only for DIGITAL signals.

Adder

Two analog input signals A(t) and B(t) may be added together in adjustable proportions G
and g. The resulting sum is presented at the output.

Master Signal

• Provides pre-generated analog and digital signals for modulation and coding
experiments.
• Includes 100 kHz signals (sine, cosine, and digital squarewave) as carrier/wireless
signals.

• Includes 2 kHz analog and digital signals as baseband message signals (audible and
ideal for modulation).

• Provides 8 kHz digital squarewave for sampling experiments (e.g., PCM).

• All signals are synchronized, ensuring stable and repeatable oscilloscope displays.

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Multiplier

The MULTIPLIER module is used to multiply two signals in real-time. Two analog input
signals X(t) and Y(t) may be multiplied together. The resulting product is scaled by a factor of
approximately 1.

Utilities

Contains four independent functional blocks:

1. Comparator: Converts analog signals to digital square waves; threshold set via REF
input (must be connected).
2. Precision Halfwave Rectifier: For extracting positive halves of input signals.

3. Diode + RC Lowpass Filter: Simple AM envelope detection; cutoff ≈ 2.6 kHz.

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VARIABLE DC V

The VARIABLE DC module is a stable, bipolar DC voltage source and a stable +5V DC
output.

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