HALEEMA SAADIA

21-EE-02(B1)

EXPERIMENT No. 6

DSBSC MODULATION
Objective
To implement DSBSC Modulation on Emona Datex Trainer

Introduction

This modulation technique is implemented with Two sidebands. Carrier is not transmitted separately.

Theory

DSBSC is a modulation system similar but different to AM Like AM, DSBSC uses a microphone or some
other transducer to convert speech and music to an electrical signal called the message or baseband
signal. The message signal is then used to electrically vary the amplitude of a pure sinewave called the
carrier. And like AM, 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
modulating the carrier with the message using DSBSC.

FIGURE 6.1 simple message signal and an unmodulated carrier

 LAB SESSION
The experiment

For this experiment you'll use the Emona DATEx to generate a real DSBSC signal by implementing its
mathematical model. This means that you'll take a pure sinewave (the message) that contains absolutely
no DC and multiply it with another sinewave at a higher frequency (the carrier). You'll examine the
DSBSC signal using the scope 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 carrier's depth of modulation. You'll also observe the effects of modulating
the carrier too much. I tshould take you about 50 minutes to complete this experiment.

Equipment
• Personal computer with appropriate software installed

• NIELVISII plus USBcable and power pack

• Emona DATE x experimental add-in module

• TwoBNCto2mmbanana-plugleads

• Assorted2mmbanana-plugpatchleads

Experimental Procedure

Part A - Generating a DSBSC signal using a simple message

1. Ensure that the NI ELVIS II power switch at the back of the unit is off.
2. Carefully plug the Emona DATE x experimental add-in module into the NIELVIS II.
3. Set the Control Mode switch on the DATEx module (top right corner) to PC Control.
4. Connect the NI ELVIS IIto the PC using the USB cable.
Note: This may already have been done for you.
5. Turn on the NI ELVIS II power switch at the rear of the unit then turn on its Prototyping Board Power
switch at the top right corner near the power indicator.
6. Turn on the PC and let it boot-up.
7. Launch the NI ELVIS mx software.
8. Launch and run the NI ELVIS II Oscilloscope virtual instrument (VI).
9. Set up the scope per the procedure in Experiment 1 (page 1-12) ensuring that the Trigger Source
control is set to CH 0.
10. Launch the DATEx soft front-panel (SFP).
11. Check you now have soft control over the DATEx by activating the PCM Encoder module's soft
PDM/TDM control on the DATEx SFP. Note:If you're set-up is working correctly, the PCM Decoder
module's LED on the DATEx board should turn on and off.
12. Connect the set-up shown in Figure 4 below. Note: Insert the black plugs of the oscilloscope leads
into a ground (GND) socket.

FIGURE 6.2 set up for Generating a DSBSC signal

This set-up can be represented by the block diagram in Figure 5 below. Itimplements the entire equation:
DSBSC = the message x the carrier

FIGURE 6.3 BLOCK DIAGRAM FOR DSBSC

With values, the equation on the previous page becomes:

DSBSC = 4Vp-p 2kHz sine x 4Vp-p 100kHz sine.

13. Adjust the scope's Time base control to view two or so cycles of the Master Signals module's
2kHzSINEoutput.

14. Activate the scope's Channel 1 input (by checking the Channel 1 Enabled box) to view the DSBSC
signal out of the Multiplier module as well as the message signal.

15. Set the scope's Channel 0 Scale control to the 500mV/div position and the Channel 1 Scale control to
the 1V/div position (if it's not already).

16. Draw the two waveforms to scale in the space provided below. Tip: Draw the message signal in the
upper half of the graph and the DSBSC signal in the lower half

If they're not already, overlay the message with the DSBSC signal's envelopes to compare them using the
scope's Channel 0 Position control.
Question1
What feature of the Multiplier module's output suggests that it's a DSBSC signal? Tip: If you're not sure
about the answer to the questions, see the preliminary discussion.

The key feature in the output of a Multiplier module that suggests it's a DSBSC (Double-Sideband
Suppressed Carrier) signal is the presence of both upper and lower sidebands while the carrier
component is either completely absent or significantly suppressed. This results in a symmetric spectrum
around the point where the carrier frequency would be. Additionally, the amplitude of the signal follows
the shape of the modulating signal, indicating amplitude modulation. These characteristics collectively
indicate the presence of a DSBSC signal.

Question2
The DSBSC signal is a complex waveform consisting of more than one signal. Is one of the signals a
2kHz sine wave? Explain your answer.
A DSBSC (Double-Sideband Suppressed Carrier) signal typically consists of more than one signal,
including the modulating signal and the upper and lower sidebands. In the context of your question, if
a DSBSC signal contains a 2kHz sine wave, it would likely be the modulating signal. The modulating
signal is responsible for shaping the amplitude variations of the DSBSC signal. So, if a 2kHz sine wave
is present within the DSBSC signal, it is most likely acting as the modulating signal that imparts its
amplitude characteristics onto the carrier signal, resulting in the DSBSC waveform.

Question3

For the given inputs to the Multiplier module, how many sine waves does the DSBSC signal consist of
,and what are their frequencies?
The DSBSC (Double-Sideband Suppressed Carrier) signal generated by a Multiplier module typically
consists of three sine waves:

1. Carrier Frequency: The carrier frequency is the frequency at which the carrier signal would have
been if it were not suppressed. In this case, the carrier frequency is not specified in your question, but it
would be a single frequency.

2. Upper Sideband: The upper sideband is a copy of the modulating signal, shifted in frequency by
an amount equal to the frequency of the modulating signal. If the modulating signal is a 2kHz sine wave,
then the upper sideband frequency would also be 2kHz higher than the carrier frequency.

3. Lower Sideband: The lower sideband is another copy of the modulating signal, but it is shifted in
frequency by an amount equal to the negative of the modulating signal's frequency. Therefore, if the
modulating signal is a 2kHz sine wave, the lower sideband frequency would be 2kHz lower than the
carrier frequency.
Question4

Why does this make DSBSC signals better for transmission than AM signals?
DSBSC (Double-Sideband Suppressed Carrier) signals are often considered better for transmission than
conventional AM (Amplitude Modulation) signals because they are more bandwidth-efficient. In AM,
the entire carrier signal is transmitted along with both sidebands, resulting in a wider bandwidth
requirement. DSBSC, on the other hand, eliminates the carrier component, effectively cutting the
required bandwidth in half while still carrying the same information in both upper and lower
sidebands. This efficient use of bandwidth makes DSBSC signals more suitable for efficient spectrum
utilization, reducing interference and allowing more signals to be transmitted within the available
frequency spectrum.
0utput

FIGURE 6.4 DSBSC SIGNAL

Part B - Generating a DSBSC signal using speech
This experiment has generated a DSBSC 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 a DSBSC signal looks like when modulated by speech.
18. Disconnect the plugs to the Master Signals module's 2kHz SINE output.
19. Connect them to the Speech module's output as shown in Figure 6 below. Remember: Dotted lines
show leads already in place.

FIGURE 6.5 connections for Generating a DSBSC signal using speech

20. Set the scope's Timebase control to the 1ms/div position.
21. Hum and talk into the microphone while watching the scope's display.

Question 5
Why isn't there any signal out of the Multiplier module when you're not humming or talking? There
is no signal out of the Multiplier module when not humming or talking because the DSBSC
modulation process relies on modulating the message signal (speech in this case) onto the carrier
signal. If there is no input message signal, there is nothing to modulate

0utput

FIG 6.5 DSBSC signal using speech

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.
22. Return the scope's Timebase control to the 100µs/div position.
23. Locate the Amplifier module on the DATEx SFP and set its soft Gain control to about a quarter of its
travel (the control's line should be pointing to where the number nine is on a clock'sface).
24. Modify the set-up as shown in Figure 7 below.

FIGURE 6.6 connections for Investigating depth of modulation

The set-up in Figure 7 can be represented by the block diagram in Figure 8 below. The Amplifier allows
the message signal's amplitude to be adjustable.

FIGURE 6.7 block diagram

Note: At this stage, the Multiplier module's output should be the normal DSBSC signal that you sketched
earlier.

Recall from Experiment 5 that an AM signal has two dimensions that can be measured and used to
calculated modulation index (m). The dimensions are denoted P and Q. If you've forgotten which one is
which, take a minute to read over the notes at the top of page 5-14 before going on to the next step.

25. Vary the message signal's amplitude a little by turning the Amplifier module's soft Gain control left
and right a little. Notice the effect that this has on the DSBSC signal's P and Q dimensions
Question 6
Based on your observations in Step 25, when the message's amplitude is varied
D neither dimensions P or Q are affected.
D only dimension Q is affected.
D only dimension P is affected.
D both dimensions P and Q are affected

D both dimensions P andQ are affected.

26. Set the Amplifier module's soft Gain control to about half its travel and notice the effect on the
DSBSC signal.
Note 1: Resize the display as necessary using the scope's Channel 1 Scale control.
Note 2:If doing this has no effect, turn up the gain control a little more. 27
. Draw the new DSBSC signal to scale in the space provided below

Question 7
What is the name of this type of distortion?

The type of distortion observed when the message signal's amplitude is too high in a DSBSC modulator is
called "over-modulation" or "clipping distortion

FIGURE 6.8 effect of amplifier

module control gain on dsbsb

signal
 Discussion of Results
In this experiment, we successfully implemented Double-Sideband Suppressed Carrier (DSBSC)
modulation using the Emona DATEx Trainer. DSBSC offers transmission advantages over AM, reducing
power consumption and bandwidth usage. When modulating with speech, the Multiplier module only
produced output during speech input. We explored depth of modulation by adjusting message signal
amplitude, affecting both P and Q dimensions. Over-modulation led to distortion, known as clipping
distortion.

Summary
In summary, a Double-Sideband Suppressed Carrier (DSBSC) signal effectively suppresses the carrier
component, conserving bandwidth and reducing interference. While it offers bandwidth efficiency and
power savings, its demodulation can be more complex, making it suitable for applications where these
benefits outweigh the complexity.

References(IEEE Method)

 Proakis, J. G., & Salehi, M. (2002). Communication Systems Engineering. Pearson.

 Lucky, R. W. (1965). Automatic equalization for digital communication.
 Electronics Hub. (n.d.). Modulation Techniques in Communication Systems.