Dept of Electrical & Electronic Engineering

Course # EEE-307
Experiment # 03
Conventional AM Modulation (DSB)

Preliminary discussion
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.

Figure 1 below shows a simple message signal and an unmodulated carrier. It also shows the
result of amplitude modulating the carrier with the message. Notice that the modulated
carrier’s amplitude varies above and below its unmodulated amplitude.

Figure 1
Figure 2 below shows the AM signal at the bottom of Figure 1 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).

Figure 2

In telecommunications theory, the mathematical model that defines the AM signal is:

AM = (DC + message) × the carrier

When the message is a simple sinewave (like in Figure 1) the equation’s solution (which
necessarily involves some trigonometry that is not shown here) tells us that the AM signal
consists of three sinewaves:

▪ One at the carrier frequency

In other words, for every sinewave in the message, the AM signal includes a pair of
sinewaves – one above and one below the carrier’s frequency. Complex message signals such
as speech and music are made up of thousands of sinewaves and so the AM signal includes
thousands of pairs of sinewaves straddling carrier. These two groups of sinewaves are called
the sidebands and so AM is known as double-sideband, full carrier (DSBFC).

Importantly, it’s clear from this discussion that the AM signal doesn’t consist of any signals
at the message frequency. This is despite the fact that the AM signal’s envelopes are the same
shape as the message.
The experiment
In this experiment you’ll 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 MATLAB and compare it to the original message. You’ll do the same with
speech for the message instead of a simple sinewave.

Following this, you’ll vary the message signal’s amplitude and observe how it affects the
modulated carrier. You’ll also observe the effects of modulating the carrier too much. Finally,
you’ll measure the AM signal’s depth of modulation using a MATLAB.

It should take you about 1 hour to complete this experiment.

Procedure

1. Generate a message signal, a sine wave having frequency of 4kHz. Take 2/3 cycles of
the message signal. Assume the amplitude be 6V.
2. Now add the DC part to the generated message signal, here initially assume the DC
part to be 15V. For analysis, it will be changed later. Add this to the message signal.
3. Now multiply the signal generated from procedure-2 with a carrier signal of 100kHz
to get the DSB-WC signal.
4. In the lab, the same is done with the Emona Telecoms-Trainer 101 as shown below.

AM = (15VDC + 6Vp-p 4kHz sine) × the carrier.

Figure 3
The set-up in Figure 3 can be represented by the block diagram in Figure 4 below. It
implements the highlighted part of the equation: AM = (DC + message) × the carrier.

Figure 4

Figure 5

Question 1
In what way is the modulated output now different to the signal out of the 4kHz
SINE output?
The set-up in Figure 5 can be represented by the block diagram in Figure 6 below. The
additions that you’ve made to the original set-up implement the highlighted part of the
equation:
AM = (DC + message) × the carrier.

Figure 6

With values, the equation is:

AM = (15VDC + 6V-4kHz sine) × Vp-p 100kHz sine.

Question 2
What feature of the final output (DSB-WC signal) suggests that it’s an AM
signal?
Tip: If you’re not sure about the answer to the questions, see the preliminary
discussion.

Question 3
The AM signal is a complex waveform consisting of more than one signal. Is one of
the signals a 4kHz sinewave? Explain your answer.

Question 4
For the given inputs to the Multiplier module, how many sinewaves does the AM
signal consist of, and what are their frequencies?
Part B - Generating an AM signal using speech
This experiment has generated an AM signal using a sinewave for the message. However, the
message in commercial communications systems is much more likely to be speech and
music. The next part of the experiment lets you see what an AM signal looks like when
modulated by speech.

Procedure:
1. Download a voice file from the internet (mp3 file preferred) or record your voice for 3
or 4 seconds or use the voice file as given for experiment-1.
2. Now, cut about 500samples from the voice file (from the middle) and use it as the
message signal.
3. Modulate it with a 100kHz sine wave in MATLAB. You should choose proper value
of DC offset so that modulation index is close to 1 but don’t reach 1.
4. In the physical lab, the same thing is done in the following way shown below.

Figure 7
Part C – Investigating depth of modulation
It’s possible to modulate the carrier by different amounts. This part of the experiment let’s
you investigate this.

Procedure:

1. First use the same carrier and message signal as used in part-A of this experiment.
Copy that same code in another file.
2. Change the value of the message signal amplitude and see the effect on the DSB-WC
signal.
3. Change the value of the DC offset and see the effect on the DSB-WC signal.

Question 5
For the message signal of Part-A, what value of DC offset would you suggest?
What is the modulation index at that value?

Question 6
What is the relationship between the message’s amplitude and the amount of the
carrier’s modulation?

You probably noticed that the size of the message signal and the modulation of the carrier are
proportional. That is, as the message’s amplitude goes up, the amount of the carrier’s
modulation goes up.

The extent that a message modulates a carrier is known in the industry as the modulation
index (m). Modulation index is an important characteristic of an AM signal for several
reasons including calculating the distribution of the signal’s power between the carrier and
sidebands.

Figure 8 below shows two key dimensions of an amplitude modulated carrier. These two
dimensions allow a carrier’s modulation index to be calculated.

Figure 8
The next part of the experiment lets you practise measuring these dimensions to calculate a
carrier’s modulation index.

P −Q
m=
P +Q
 Question 7
What is the modulation index for the values of message signal and DC offset in
part-A?

A problem that is important to avoid in AM transmission is over-modulation. When the

Procedure:
4. Increase the message signal’s amplitude so that it is greater than the DC offset
and notice the shape of the DSB-WC signal. Find the modulation index in this
case.

Question 8
What is the problem with the AM signal when it is over-modulated?

Question 9
What do you think is a carrier’s maximum modulation index without over-
modulation?

Post-Lab Report:
1. Calculate the modulation index and power of the modulated signal using the
experimental data of part-A. Calculate the power at each frequency present in the
DSB-WC signal and draw Power Spectrum Density (PSD)
2. Why envelop detection is not implementable with DSB-SC? Explain
3. “Though DSB is an inefficient modulation method in terms of power, because of easy
detection, it’s preferred over DSB-SC in AM modulation”- Justify the statement in
your own words.
4. Why the DC value should be higher than the amplitude of message signal? What
happens if it lower?
5. Calculate the power of carrier, power of upper side band and power of lower side
band if the DC offset value is 10V and message signal amplitude is 4V. From this,
find the efficiency.
6. Does efficiency depend on frequency of message or carrier signal?
7. If message signal amplitude (4V) is not given in 5, but the modulation index is given
as 0.6, how can the powers at different sideband and efficiency can be calculated?
Explain and find the power in this case.