Faculty of Engineering

Electronics and Communication Department

Spring 2025

Academic year: First

2024-2025
 Faculty of Engineering
Electronics and Communication Department

Spring 2025

Table of Content
Experiment (1) Filters (LPF-HPF-BPF-BRF) ………………….

Experiment (2) DSBSC modulation & demodulation………….

Experiment (3) DSBWC modulation & demodulation…………

Experiment (4) SSB modulation & demodulation………………

Experiment (5) Frequency modulation &

demodulation…………
objectives:
The objective of this experiment is to simulate the effect of
different types of filters on communication systems.

Procedures:

1.​ Draw the circuit shown in Figure1 on LabVIEW COMM Program:

Figure 1

A.​ From Search bar in program , extract Generate Wave to generate three
signal sources and two add to sum three signals as shown in Figure2.

Figure 2

B.​ You can use Power Spectrum Density (PSD) to show the spectrum of each
signal and summation of waves. Let Signal Frequencies are 1KHz, 3KHz and
5Hz.
Draw The Spectrum of signal 1:

Draw the spectrum of signal 2:

Draw the spectrum of signal 3:

Draw the spectrum of two waves after summation:

Draw the spectrum of three waves after summation:

Then you can study the effects of different types of filters by putting
different types after summation block as shown in Figure 3.

Figure 3
2- The block diagram of Figure 3 may be plotted by LabView Comm Prog
as shown in Figure 4. Let's the cutoff frequency of LPF is equal 2KHz, and
4KHz for HPF, the cutoff low and cutoff high for BPF and BSF are 2KHz
and 4KHz respectively.

Figure 4
Draw LPF output:

Draw HPF output:

Draw BPF output:

Draw BRF output:

3- change the cutoff frequency of LPF to be 3KHz and show the

spectrum of it. also change cutoff to be 5KHz. Draw the spectrum in two
cases
Draw LPF output:

Draw HPF output:

4- change the cutoff frequency of HPF to be 1KHz and show the

spectrum of it. also change cutoff to be 5KHz . Draw the spectrum in
two cases.
Draw LPF output:

Draw HPF output:

5- Change the low and high cutoff frequency of BPF to be 4KHz and
6KHz. Show the spectrum of it. also change cutoff to be 0Hz and 7KHz.
Draw the spectrum in two cases.
Draw BPF output:

Draw BPF output:

6- Repeat step 5 for BRF and Draw the spectrum.

Draw BRF output:

Draw BRF output:

You can add tunable filter (LPF, HPF, BPF and BSF) by controlling the
cutoff frequency for each filter:
Tunable LPF output:
Tunable HPF output:
Tunable BPF output:

Tunable BRF output:

Also, you can add notch filter output here:
1- SOFTWARE: LABVIEWCOMM
Objective:
The objective of this experiment is to simulate Double side band suppressed carrier
modulation and demodulation (DSB-SC).

Procedures:
1-draw the circuit diagram as shown in the figure:
2-draw the modulated and demodulated signals DSB-SC
Hardware (EMONA):

DSBSC modulation & demodulation

In this experiment you will create a DSBSC signal and gain insight into the meaning of
“Suppressed Carrier”. As well you will again use product demodulation to recover the
message and examine the effect of phase and frequency errors on the recovery
process.

DSBSC is an acronym for Double Sideband-Suppressed Carrier. It is the most basic

form of modulation composing of a simple multiplication of two sinusoids: one being
the message and the other the carrier. The relevance of the ‘suppressed carrier” term
should be evident if you have completed the earlier lab on Amplitude Modulation.

Learning Objectives

After completing this lab, you should be able to complete the following activities.

1.​ Generate a real DSBSC signal

2.​ Examine a real DSBSC signal with scope and compare it to its original message
3.​ Use multiple message sources in your DSBSC examination
4.​ Describe the term “depth of modulation”
5.​ Describe the effects and meaning of over and under modulation of the carrier
6.​ Compare original and demodulated signals
7.​ Describe distortion in the recovered signal
8.​ Identify the effect of phase and frequency errors on the demodulation process
9.​ Explain the term “product modulation”
Required Tools and Technology
Platform: NI ELVIS III Instruments ✔​ Install Instruments:
used in this lab:
http://www.ni.com/documen
●​ Oscilloscope-Time
tation/e
●​ Oscilloscope-FFT
n/ni-elvis-iii/latest/getting-
●​ Function Generator
started/installing-the-soft-fro
nt- panel/
✔​ Access
instrumentshttps://measure
mentsliv e.ni.com
✔​ View User
Manualhttp://www.ni.co
m/en-
us/support/model.ni-elvis
-iii.html
✔​ View tutorials
https://www.youtube.com/p
laylist?li
st=PLvcPIuVaUMIWm8ziaSx
v0gwt shBA2dh_M

Hardware: Emona Communications ✔​ View User

Board Components used in this lab:
Manualhttp://www.ni.com/en-
●​ Four BNC to 2mm banana-plug
leads us/support/model.emona-
●​ Assorted 2mm banana-plug communications-board-for-ni-elvis-
patch leads iii.html
●​ Set of headphones or earbuds

Expected Deliverables

In this lab, you will collect the following deliverables:

 ✔​ Calculations

✔​ Data from measurements

✔​ Observations

Your instructor may expect you complete a lab report. Refer to your instructor for
specific requirements or templates.
Section 1: DSBSC modulation

1.1​Theory and Background

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 shows a simple message signal and an unmodulated carrier. It also shows
the result of modulating the carrier with the message using DSBSC.
.

Figure 1:DSBSC signals

So far, there doesn’t appear to be much difference between AM and DSBSC.

However, consider Figure 2. It is the DSBSC signal at the bottom of Figure 1 but with
dotted lines added to track the signal’s envelopes (that is, its positive peaks and
negative peaks). If you look at the envelopes closely you’ll notice that they’re not the
same shape as the message as is the case with AM (see Experiment 4, Figure 2 for an
example).
 ​

Figure 2:DSBSC envelopes

Instead, alternating halves of the envelopes form the same shape as the message as
shown in Figure 3.

Figure 3:DSBSC message

Another way that DSBSC is different to AM can be understood by considering the

mathematical model that defines the DSBSC signal:

DSBSC = the message × the carrier

Do you see the difference between the equations for AM and DSBSC? If not, look at the
AM equation in Lab 4.

When the message is a simple sinewave (such as in Figure 2) the equation’s solution
(which necessarily involves some trigonometry) tells us that the DSBSC signal
consists of two sinewaves:
▪​ One with a frequency equal to the sum of the carrier and message frequencies

▪​ One with a frequency equal to the difference between the carrier and
message frequencies
Importantly, the DSBSC signal doesn’t contain a sinewave at the carrier frequency.
This is an important difference between DSBSC and AM.

That said, as the solution to the equation shows, DSBSC is the same as AM in that a
pair of sinewaves is generated for every sinewave in the message. And, like AM, one
is higher than the unmodulated carrier’s frequency and the other is lower. As message
signals such as speech and music are made up of thousands of sinewaves,
thousands of pairs of sinewaves are generated in the DSBSC signal that sit on either
side of the carrier frequency. These two groups are called the sidebands.

So, the presence of both sidebands but the absence of the carrier gives us the name
of this modulation method - double-sideband, suppressed carrier (DSBSC).

The carrier in AM makes up at least 66% of the signal’s power but it doesn’t contain
any part of the original message and is only needed for tuning. So by not sending the
carrier, DSBSC offers a substantial power saving over AM and is its main advantage.

1.2​Implement: DSBSC modulation

You will now build a model of the system being studied and explore its performance.
Powering up the ELVIS III + EMONA Communications Board

1.​ Ensure that the NI ELVIS III Application Board power button at the top
left corner of the unit is OFF (not illuminated).

2.​ Carefully plug the Emona Communications board (ECB) into the NI ELVIS III
ensuring that it is fully engaged both front and back.

3.​ Ensure that you have connected the NI ELVIS III to the PC using the USB
cable and that the PC is turned on.

4.​ Turn on the Application Board Power button by pressing it once and confirm
that it is illuminated. The LEDs on the ECB should also be illuminated. If
they are not, then switch the unit off immediately and check for connection
or insertion errors.

5.​ Open the Instrument Launcher software in your browser and select
the required instruments.

Table 1 Scope Configuration

Channel Voltage range 2 V/div

Horizontal Time base 50us/div

Trigger Analog Edge, Chan 1, Rising

Probe Attenuation 1x

6.​ Connect the setup shown in Figure 4.

Note: Insert the black plugs of the oscilloscope leads into a ground (GND) socket.
 Figure 4:Patching for DSBSC

This set-up can be represented by the block diagram in Figure 5. It implements the
entire equation: DSBSC = the message × the carrier.
Figure 5:Block diagram for DSBSC
With values, the equation on the previous page becomes:

DSBSC = 4Vp-p 2.08kHz sine × 4Vp-p 100kHz sine.

7.​ Adjust the scope’s Timebase control to view two or so cycles of the
Master Signals module’s 2.08kHz SINE output.

8.​ Activate the scope’s Channel 2 input to view the DSBSC signal out of
the Multiplier module as well as the message signal.

9.​ Set the scope’s Channel 1Scale control to the 1V/div position and the
Channel 2 Scale control to the 2V/div position (if it’s not already).

10.​ Capture a screenshot of the scope and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.

Display layout tips:

●​ Position the message signal in the upper half of the display and the
DSBSC signal in the lower half.

11.​ If they’re not already, overlay the message with the DSBSC signal’s
envelopes to compare them using the scope’s Channel 1 Position control.

12.​ Set the scope’s Channel 1 Scale control to the 1V/div position and the
Channel 2 Scale control to the 2V/div position (if it’s not already).

1-1​ 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.
1-2​ The DSBSC signal is a complex waveform consisting of more than one
signal. Is one of the signals a 2.08kHz sinewave? Explain your answer.
1-3​ For the given inputs to the Multiplier module, how many sinewaves does
the DSBSC signal consist of, and what are their frequencies?

1-4​Why does this make DSBSC signals better for transmission than AM signals?

1.3​Implement: 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.

1.​ Disconnect the plugs to the Master Signals module’s 2.08kHz SINE output.

2.​ Connect them to the Speech module’s output as shown in Figure 6.

Remember: Dotted lines show leads already in place.

 Figure 6:DSBSC using speech as a message

3.​ Set the scope’s Timebase control to the 1ms/div position.

4.​ Hum and talk into the microphone while watching the scope’s display.

1-5​ Why isn’t there any signal out of the Multiplier module when you’re not
humming or talking?

1.4​Implement: Investigating depth of modulation

It’s possible to modulate the carrier by different amounts. This part of the experiment
lets you investigate this.

1.​ Return the scope’s Timebase control to the 100µs/div position.

2.​ Locate the Amplifier module on the board and set its 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’s face).
3.​ Modify the set-up as shown in Figure 7.
 Figure 7:Patching for DSBSC with 2.08kHz message

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

Figure 8:Block diagram for DSBSC with 2.08kHz message

Note: At this stage, the Multiplier module’s output should be the normal DSBSC
signal that you sketched earlier.
Recall from Experiment 4 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 in that
previous labbefore going on to the next step.

4.​ Vary the message signal’s amplitude a little by turning the Amplifier module’s
Gain control left and right a little. Notice the effect that this has on the
DSBSC signal’s P and Q dimensions.

1-6​ Based on your observations in Step 4 above, when the message’s

amplitude is varied, which dimensions are affected?

On the face of it, determining the depth of modulation of a DSBSC signal is a

problem. The modulation index is always the same number regardless of the message
signal’s amplitude. This is because the DSBSC signal’s Q dimension is always zero.

However, this isn’t the problem that it seems. One of the main reasons for calculating
an AM signal’s modulation index is so that the distribution of power between the
signal’s carrier and its sidebands can be calculated. However, DSBSC signals don’t
have a carrier (remember, it’s suppressed). This means that all of the DSBSC signal’s
power is distributed between its sidebands evenly. As such, there’s no need to
calculate a DSBSC signal’s modulation index.

The fact that you can’t calculate a DSBSC signal’s modulation index might imply that
you can make either the message or the carrier as large as you like without worrying
about over-modulation. This isn’t true. Making either of these two signals too large
can still overload the modulator resulting in a type of distortion that you’ve seen
before. The next part of the experiment lets you observe what happens when you
overload a DSBSC modulator.

5.​ Set the Amplifier module’s 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.

6.​ Capture a screenshot of the scope and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.

1-7​ What is the name of this type of distortion?

Section 2: DSBSC Demodulation

2.1​Theory and Background

Lab 5 shows how the envelope detector can be used to recover the original message
from an AM signal (that is, demodulate it). Unfortunately, the envelope detector cannot
be used to demodulate a DSBSC signal.

To understand why, recall that the envelope detector outputs a signal that is a copy of
its input’s envelope. This works well for demodulating AM because the signal’s
envelopes are the same shape as the message that produced it in the first place (that
is, as long as it’s not over-modulated). However, recall that a DSBSC signal’s
envelopes are not the same shape as the message.

Instead, DSBSC signals are demodulated using a circuit called a product detector
(though product demodulator is a more appropriate name) and its basic block diagram
is shown in Figure 9. Other names for this type of demodulation include a
synchronous detector and switching detector.
 Figure 9:Block diagram for DSBSC demodulation

As its name implies, the product detector uses multiplication and so mathematics are
necessary to explain its operation. The incoming DSBSC signal is multiplied by a pure
sinewave that must be the same frequency as the DSBSC signal’s suppressed carrier.
This sinewave is generated by the receiver and is known as the local carrier.

To see why this process recovers the message, let’s describe product detection
mathematically:

DSBSC demodulator’s output = the DSBSC signal × the local carrier

Importantly, recall that DSBSC generation involves the multiplication of the message
with the carrier which produces sum and difference frequencies. That being the case,
this information can be substituted for the DSBSC signal and the equation rewritten
as:

DSBSC demodulator’s output = [(carrier + message) + (carrier – message)] × carrier

When the equation is solved, we get four sinewaves with the following frequencies:

▪​ Carrier + (carrier + message)

▪​ Carrier + (carrier - message)

▪​ Carrier - (carrier + message) which simplifies to just the message

▪​ Carrier - (carrier - message) which also simplifies to just the message

(If you’re not sure why these sinewaves are produced, it’s important to remember that
whenever two pure sinewaves are multiplied together, two completely new sinewaves
are generated. One has a frequency equal to the sum of the original sinewaves’
frequencies and the other has a frequency equal to their difference.)

Importantly, notice that two of the products are sinewaves at the message frequency.
In other words, the message has been recovered. As the two message signals are in
phase, they simply add together to make one larger message.

Notice also that two of the products are non-message sinewaves. These sinewaves are
unwanted and so a low-pass filter is used to reject them while keeping the message.

2.2​Implement: DSBSC Demodulation

To experiment with DSBSC demodulation you need a DSBSC signal. The first part of
this experiment gets you to set one up. This procedure is identical to that in
Implementation 1 above.

1.​ Ensure that the NI ELVIS III power switch at the back of the unit is off.

2.​ Carefully plug the Emona Communications application board into the NI
ELVIS III.

3.​ Power up the ELVIS III and connect to the PC.

4.​ Power up the application board using the Application Board power button at
the top left corner of the ELVIS III.

5.​ Run the NI launcher software and open the instruments you need.
6.​ Configure the scope using the configuration below:
Table 2 Scope Configuration

Channel Voltage range 2 V/div

Horizontal Timebase 50us/div

Trigger Analog Edge, Channel 1, Rising

Probe Attenuation 1x

7.​ Connect the set-up shown in Figure 10.

Note: Insert the black plugs of the oscilloscope leads into a ground (GND) socket.

Figure 10:Patching for DSBSC signal

This set-up can be represented by the block diagram in Figure 11. It generates a
100kHz carrier that is DSBSC modulated by a 2.08kHz sinewave message.
 Figure 11:Block diagram for DSBSC

8.​ Adjust the scope’s Timebase control to view two or so cycles of the
Master Signals module’s 2.08kHz SINE output.

9.​ Activate the scope’s Channel 2 input to view the DSBSC signal out of
the Multiplier module as well as the message signal.

Note: If the Multiplier module’s output is not a DSBSC signal, check your
wiring.

10.​ Set the scope’s Channel 1Scale control to the 1V/div position and the
Channel 2Scale control to the 2V/div position.

2.3​Implement: Recovering the message using a product detector

1.​ Locate the Tuneable Low-pass Filter module on the board and set its Gain
control to about the middle of its travel.

2.​ Turn the Tuneable Low-pass Filter module’s Frequency Adjust control fully
clockwise.
3.​ Modify the set-up as shown in Figure 12.
 Figure 12:Patching for product demodulation

The additions to the set-up can be represented by the block diagram in Figure 13. The
Multiplier and Tuneable Low-pass Filter modules are used to implement a product
detector which demodulates the original message from the DSBSC signal.

Figure 13: Block diagram for product demodulation

The entire setup is represented by the block diagram in Figure 14. It highlights the fact
that the modulator’s carrier is “stolen” to provide the product detector’s local carrier.
This means that the two carriers are synchronised which is a necessary condition for
DSBSC communications.
 Figure 14:Block diagram for complete system

4.​ Capture a screenshot of the scope and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.

Tip: Position the message signal in the upper half of the graph and the DSBSC
signal in the lower half.

2-1​ Why must a product detector be used to recover the message instead
of an envelope detector? Tip: If you’re not sure, refer to the preliminary
discussion

2.4​ Implement: Investigating the message’s amplitude on

the recovered message

1.​ Locate the Amplifier module on the board and turn its Gain control to about
a quarter of its travel.
2.​ Disconnect the plugs to the Master Signals module’s 2.08kHz SINE output.

3.​ Use the Amplifier module to modify the set-up as shown in Figure 15.
 Figure 15:Patching for the complete system
The addition to the set-up can be represented by the block diagram in Figure 16. The
amplifier’s variable gain allows the message’s amplitude to be adjustable.

Figure 16: Block diagram for amplitude adjustment

4.​ Vary the message signal’s amplitude up and down a little (by turning
the Amplifier module’s Gain control left and right a little) while watching
the demodulated signal.

2-2​ What is the relationship between the amplitude of the two message signals?
5.​ Slowly increase the message signal’s amplitude to maximum until the
 demodulated signal begins to distort.

2-3​ What do you think causes the distortion of the demodulated signal? Tip: If
you’re not sure, connect the scope’s Channel 1 input to the DSBSC modulator’s
output and set the Trigger Sourceto Channel 2.

Section 3: Transmitting and recovering speech using DSBSC

This experiment has set up a DSBSC communication system to “transmit” a 2.08kHz
sinewave. The next part of the experiment lets you use it to modulate, transmit,
demodulate and listen to speech.

1.​ If you moved the scope’s Channel 1 input and adjusted its Trigger Source
control to help answer Question 2-3, return them to their previous positions.

2.​ Disconnect the leads to the Amplifier module and modify the set-up as
shown in Figure 17.

Figure 17:Patching for DSBSC with speech as message

3.​ Set the scope’s Timebase control to the 2ms/div position.

4.​ Turn the Amplifier module’sGain control fully anti-clockwise (minimum gain).

5.​ Without wearing the headphones, plug them into the Amplifier module’s
 headphone socket.

6.​ Put the headphones on.

7.​ As you perform the next step, set the Amplifier module’s Gain control to
a comfortable sound level.

8.​ Hum and talk into the microphone while watching the scope’s display
and listening on the headphones.

Section 4: Carrier synchronisation - phase and frequency errors

Crucial to the correct operation of a DSBSC communications system is the

synchronisation between the modulator’s carrier signal and the product detector’s
local carrier. Any phase or frequency difference between the two signals adversely
affects the system’s performance.

4.1​Implement: The effect of phase errors

Recall that the product detector generates two copies of the message. Recall also that
they’re in phase with each other and so they simply add together to form one bigger
message. However, if there’s a phase error between the carriers, the product detector’s
two messages have a phase error also. One of them has the sum of the phase errors
and the other the difference. In other words, the two messages are out of phase with
each other.

If the carriers’ phase error is small (say about 10°) the two messages still add together
to form one bigger signal but not as big as when the carriers are in phase. As the
carriers’ phase error increases, the recovered message gets smaller. Once the phase
error exceeds 45° the two messages begin to subtract from each other. When the
carriers’ phase error is 90° the two messages end up 180° out of phase and
completely cancel each other out.

The next part of the experiment lets you observe the effects of carrier phase error.

1.​ Turn the Amplifier module’sGain control fully anti-clockwise again.

2.​ Return the scope’s Timebase control to about the 100µs/div position.
3.​ Locate the Phase Shifter module on the board.

4.​ Set the Phase Shifter module’s Phase Adjust control to about the middle of
its travel.

5.​ Disconnect the leads to the Speech output and modify the set-up as shown
in Figure 10 below.

Figure 18:Patching for phase adjustment

The set-up in Figure 18 can be represented by the block diagram in Figure 19. The
Phase Shifter module allows a phase error between the DSBSC modulator’s carrier and
the product detector’s local carrier to be introduced.
Figure 19: Block diagram for phase adjustment
6.​ Slowly increase the Amplifier’s module’s gain until you can comfortably
hear the demodulated 2.08kHz tone.

7.​ Vary the Phase Shifter module’s Phase Adjust control left and right
while watching and listening to the effect on the recovered message.

8.​ Turn the Phase Shifter module’s Phase Adjust control until the recovered
message is smallest.

4-1​ Given the size of the recovered message’s amplitude, what is the likely phase
error between the two carriers? Tip: If you’re not sure about the answer to this
question (and the next one), reread the notes.

9.​ Verify your answer to Question 4-1 by connecting the scope’s Channel 1
input to the Master Signals module’s 100kHz SINE output, its Channel 2
input to the Phase Shifter module’s output and setting its Timebase control to
the 5µs/div setting.

10.​ Adjust the Phase Shifter module’s soft Phase Adjust control until the
two signals are in phase.

4-2​ Given the two carriers are in phase, what should the amplitude of the
recovered message be?

11.​ Verify your answer to Question 4-2 by reconnecting the scope’s Channel 1
input to the Master Signals module’s 2.08kHz SINE output, reconnecting its
Channel 2 input to the Tuneable Low-pass Filter module’s output and setting
its Timebase control back to the 100µs/div setting.
4.2​Implement: The effect of frequency errors

When there’s a frequency error between the DSBSC signal’s carrier and the product
detector’s local carrier, there is a corresponding frequency error in the two products
that usually coincide. One is at the message frequency minus the error and the other is
at the error frequency plus the error.

If the error is small (say 0.1Hz) the two signals will alternately reinforce and cancel each
other which can render the message periodically inaudible but otherwise intelligible. If
the frequency error is larger (say 5Hz) the message is reasonably intelligible but fidelity
is poor. When frequency errors are large, intelligibility is seriously affected.

The next part of the experiment lets you observe the effects of carrier frequency error.

1.​ Launch and run the NI ELVIS III Function Generator Instrument.

2.​ Adjust the function generator’s soft controls for an output with the
following specifications:

▪​ Waveshape: Sine

▪​ Frequency: 100kHz exactly

▪​ Amplitude: 4Vpp

▪​ DC Offset: 0V

3.​ Disconnect the leads to the Phase Shifter module and modify the set-up
as shown in Figure 20.
 Figure 20:Patching for frequency error

The entire set-up can be represented by the block diagram in Figure 21. The function
generator allows the local oscillator to be completely frequency (and phase)
independent of the DSBSC modulator.

Figure 21:Block diagram for frequency error

 4.​ If you’re not doing so already, listen to the recovered message using the
headphones.

5.​ Compare the scope’s frequency measurements for the original message and the
recovered message.

Note 1: You should find that they’re very close in frequency.

Note 2: You’ll notice that the volume of the recovered messages varies. This is due to
the phase error between the two carriers and should be ignored for the following
steps.

6.​ Reduce the function generator’s output frequency to 99.8kHz.

7.​ Give the function generator a moment to achieve the correct frequency and
note the change in the tone of recovered message.

Tip: If you can’t remember what 2.08kHz sounds like, set function generator’s output
to 100kHz for a moment then return it to 99.8kHz.

8.​ Experiment with other local carrier frequencies around 100kHz and listen to the
effect on the recovered message.

9.​ Return the function generator’s output to 100kHz.

10.​ Disconnect the plugs to the Master Signals module’s 2.08kHz SINE output and
connect them to the Speech module’s output.

11.​ Hum and talk into the microphone to check that the whole set-up is still working
correctly.

Vary the function generator’s frequency again and listen to the effect of an unsynchronised
local carrier on speech.
1- SOFTWARE: LABVIEWCOMM
Objective:
The objective of this experiment is to simulate Double side band with carrier modulation
and demodulation (DSB-WC).
Procedures:
1-draw the circuit diagram as shown in the figure:

2-draw the modulated and demodulated signals DSB-WC at modulation index (μ=1 )
,where μ=mp/Ac
3- draw the modulated and demodulated signals DSB-WC at modulation index (μ<1 )
4- draw the modulated and demodulated signals DSB-WC at modulation index (μ>1 )
2- HARDWARE: EMONA
Amplitude modulation and Demodulation (DSBWC)

In this Lab you will Create an amplitude modulated signal from a variety of message
sources, calculate the modulation index and confirm the frequency spectrum of this
signal type.

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 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: Amplitude Modulated signal

Figure 2 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: Message envelopes

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

▪​ One with a frequency equal to the sum of the carrier and message frequencies

▪​ One with a frequency equal to the difference between the carrier and message frequencies

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 also known as double-
sideband, full carrier (DSBFC).

Importantly, it’s clear from this discussion that the AM signal doesn’t consist of any
2
signals at the message frequency. This is despite the fact that the AM signal’s
envelopes are the same shape as the message.

3
Section 1: Amplitude Modulation

For this experiment you’ll use the Emona board 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. 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 scope.

Learning Objectives

After completing this lab, you should be able to complete the following activities.

1.​ Generate a real AM signal

2.​ Examine a real AM signal with scope and compare it to its original message
3.​ Use multiple message sources in your AM examination
4.​ Describe the term “depth of modulation”

4
Required Tools and Technology

Platform: NI ELVIS III ✔​ Install Instruments:

Instruments used in this http://www.ni.com/documentatio
lab: n/ en/ni-elvis-iii/latest/getting-
●​ Oscilloscope-Time started/installing-the-soft-front-
●​ Oscilloscope-FFT panel/
●​ Function Generator
✔​ Access
instrumentshttps://measurementsl
ive.ni.com
✔​ View User
Manualhttp://www.ni.com/en-
us/support/model.ni-elvis-iii.html
View tutorials
https://www.youtube.com/playlist
?
list=PLvcPIuVaUMIWm8ziaSxv0g
wtshBA2dh_M
Hardware: Emona Communications
Board Components used in this lab:
●​ Four BNC to 2mm banana-plug ✔​ View User
leads
Manualhttp://www.ni.com/en
●​ Assorted 2mm banana-plug
- us/support/model.emona-
patch leads
communications-board-for-ni
●​ Set of headphones or earbuds
- elvis-iii.html

Software: NI ELVIS III Function Generator ✔​ Access instrument

File used in this lab (available in lab folder): https://measurementslive.ni.co
●​ ECB_positive1V_DC.csv m

5
Expected Deliverables

In this lab, you will collect the following deliverables:

✔​ Calculations

✔​ Data from measurements

✔​ Observations

Your instructor may expect you complete a lab report. Refer to your instructor for
specific requirements or templates.

6
1.1​Implement: Generating Amplitude modulation (AM)

Powering up the ELVIS III + EMONA Communications Board

1.​ Ensure that the NI ELVIS III Application Board power button at the top
left corner of the unit is OFF (not illuminated).

2.​ Carefully plug the Emona Communications board into the NI ELVIS III ensuring
that it is fully engaged both front and back.

3.​ Ensure that you have connected the NI ELVIS III to the PC using the USB
cable and that the PC is turned on.

4.​ Turn on the Application Board Power button by pressing it once and confirm
that it is illuminated. The LEDs on the board should also be illuminated. If
they are not, then switch the unit off immediately and check for connection or
insertion errors.

5.​ Open the Instrument Launcher software in your browser and select
the required instruments.

Table 1 Scope Configuration

Channel Voltage range 2 V/div

Horizontal Timebase 50us/div

Trigger Analog Edge, Chan 1, Rising

Probe Attenuation 1x

7
6.​ Use the ELVIS III Function Generator output Channel 2 to create a DC voltage
of about 1V by loading the Custom waveform file “ECB_positive1V_DC.csv”.

7.​ Connect the set-up shown in Figure 3.

Figure 3: Create DC voltage

8.​ Locate the Adder module on the board and turn its G control fully
anti- clockwise.

9.​ Adjust the Adder module’s g control to obtain a 1V DC output (as measured
by the Scope).

10.​ Connect the set-up shown in Figure 4.

Note: Insert the black plugs of the oscilloscope lead into a ground (GND)
socket.

8
 Figure 4: Add DC to the message

This set-up can be represented by the block diagram in Figure 5. It implements the
highlighted part of the equation: AM = (DC + message) × the carrier.

Figure 5: Block diagram for addition

9
11.​ Set up the scope with the following settings:

▪​ Channel 1Coupling control to the DC position

▪​ Channel 1Scale control to the 500mV/div position

▪​ Trigger Level control to the 1V position instead of 0V

Adjust the Trigger level and Source to have a stable signal to view.

12.​ While watching the Adder module’s output on the scope, turn its G
control clockwise to obtain a 1Vp-p sinewave.

The Adder module’s output can now be described mathematically as:

AM = (1VDC + 1Vp-p 2.08kHz sine) × the carrier

1-1​In what way is the Adder module’s output now different to the signal out of
the Master Signals module’s 2.08kHz SINE output?

13.​ Modify the set-up as shown in Figure 6.

10
 Figure 6: Multiply the baseband message by the carrier

This set-up can be represented by the block diagram in Figure 7. The additions that
you’ve made to the original set-up implement the highlighted part of the equation:

AM = (DC + message) × the carrier.

Figure 7: Block diagram for AM

With values, the equation on the previous page becomes:

AM = (1VDC + 1Vp-p 2kHz sine) × 4Vp-p 100kHz sine.

11
14.​ Adjust the scope’s Timebase control to view only two or so cycles of the
message signal i.e.: 100us/div or even 50us/div for one cycle. Change
the Volts per division control for Channel 2 to 2 V.

15.​ Activate the scope’s Channel 2 input to view the Multiplier module’s output
as well as the offset message signal.

16.​ Capture a screenshot of the scope and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.

17.​ Use the scope’s Channel 1Position control to overlay the message with the
AM signal’s upper envelope then lower envelope to compare them.

18.​ Capture a screenshot of the scope and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.

1-2​What feature of the Multiplier module’s output suggests that it’s an AM signal?

Tip: If you’re not sure about the answer to the questions, see the preliminary
discussion.

1-3​The AM signal is a complex waveform consisting of more than one signal. Is one
of the signals a 2.08kHz sinewave? Explain your answer.

12
1-4​For the given inputs to the Multiplier module, how many sinewaves does the
AM signal consist of, and what are their frequencies?

13
1.2​Implement: 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.

1.​ Disconnect the plug on the Master Signals module’s 2.08kHz SINE output
that connects to the Adder module’sA input.

2.​ Connect it to the Speech module’s output as shown in Figure 8.

Remember: Dotted lines show leads already in place.

Figure 8: Using speech as a message

3.​ Set the scope’s Timebase control to the 1ms/div position.

4.​ Hum and talk into the microphone while watching the scope’s display.

14
1-5​Why is there still a signal out of the Multiplier module even when you’re
not humming (or talking, etc.)?

1.3​Implement: Investigating depth of modulation

It’s possible to modulate the carrier by different amounts. This part of the experiment
lets you investigate this.

1.​ Return the scope’s Timebase control to the 100µs/div position.

2.​ Disconnect the plug to the Speech module’s output and reconnect it to
the Master Signals module’s 2.08kHz SINE output.

Note: The scope’s display should now look like the screen captures done
previously in this Lab.

3.​ Vary the message signal’s amplitude a little by turning Adder

module’sG control left and right and notice the effect on the AM signal.

1-6​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

15
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

16
for several reasons including calculating the distribution of the signal’s power
between the carrier and sidebands.

Figure 9 shows two key dimensions of an amplitude modulated carrier. These two
dimensions allow a carrier’s modulation index to be calculated.

Figure 9: Modulation index dimensions

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

4.​ Adjust the Adder module’sG control to return the message signal’s amplitude
to 1Vp-p.

5.​ Measure and record the AM signal’s P dimension. Record your

measurement in Table 1.

6.​ Measure and record the AM signal’s Q dimension.

7.​ Calculate and record the AM signal’s depth of modulation using the
equation below.

m=P−Q
P+Q

P dimension Q dimension m

17
Table 1: Modulation index measurements

18
A problem that is important to avoid in AM transmission is over-modulation. When the
carrier is over-modulated, it can upset the receiver’s operation. The next part of the
experiment gives you a chance to observe the effect of over-modulation.

8.​ Increase the message signal’s amplitude to maximum by turning the

Adder module’sG control to about half its travel then fully clockwise and
notice the effect on the AM signal.

9.​ Set the scope’s Channel 1Scale control to 1V/div and the Channel
2Scale control to 2V/div.

10.​ Use the scope’s Channel 1Position control to overlay the message with the
AM signal’s envelopes and compare them.

1-7​What is the problem with the AM signal when it is over-modulated?

1-8​ What do you think is a carrier’s maximum modulation index without

over- modulation?

11.​ Capture a screenshot of the scope and append to your report. Annotate your

19
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.

20
1.4​Implement: Investigating the Frequency Spectrum of AM

It’s possible to modulate the carrier by different amounts. As you discovered in the
previous section, this affects the relative levels of sideband to carrier in the signal. This
part of the experiment lets you investigate this.

1.​ Maintain the setup from the previous section with a 2.08kHz message.

2.​ Enable the FFT mode of the Oscilloscope instrument. Change the
scope’s timebase to 1ms/div. This increases the resolution of the FFT
display.

3.​ Set the frequency span for the FFT displayed from say 90kHz to 110kHz for
a closeup of the frequency domain of interest.

4.​ Set the modulation index of your signal to m = 1 and examine the spectrum.

5.​ Set to modulation index to various values including m = 0 and m > 1.

6.​ Capture a screenshot of the FFT and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.

7.​ Confirm that the levels of carrier versus sideband correspond correctly to
the levels you would expect based on the equation for AM and the level of
modulation index, “m” you have set up.

1-9​ For m= 1, what does theory predict the ratio of carrier to sidebands to be?
What have you measured? Explain any differences.

21
Figure 10:Example of FFT display settings for AM

22
Amplitude demodulation
In this Lab you will recover a variety of messages from an amplitude modulated carrier
signal using two methods and develop an understanding of the demodulation process
in the time and frequency domains.

If you’ve completed Experiment 4 then you’ve seen what happens when a 2.08kHz
sinewave is used to amplitude modulate a carrier to produce an AM signal. Importantly,
you would have seen a key characteristic of an AM signal – its envelopes are the same
shape as the message (though the lower envelope is inverted).

Recovering the original message from a modulated carrier is called demodulation and
this is the main purpose of communications and telecommunications receivers. The
circuit that is widely used to demodulate AM signals is called an envelope detector. The
block diagram of an envelope detector is shown in Figure 1 below.

Recovered
AM signal message

Rectified AM signal
Figure 11: Block diagram of AM demodulation

As you can see, the rectifier stage chops the AM signal in half letting only one of its
envelopes through (the upper envelope in this case but the lower envelope is just as
good). This signal is fed to an RC LPF which tracks the peaks of its input. When the
input to the RC LPF is a rectified AM signal, it tracks the signal’s envelope. Importantly,
as the envelope is the same shape as the message, the RC LPF’s output voltage is
also the same shape as the message and so the AM signal is demodulated.
23
A limitation of envelope detector shown in Figure 1 is that it cannot accurately recover
the message from over-modulated AM signals. To explain, recall that when an AM
carrier is overmodulated the signal’s envelope is no-longer the same shape as the
original message. Instead, the envelope is distorted and so, by definition, this means
that the envelope detector must produce a distorted version of the message.

Learning Objectives

After completing this lab, you should be able to complete the following activities.

5.​ Generate a real AM signal

6.​ Examine a real AM signal with scope and compare it to its original message
7.​ Use multiple message sources in your AM examination
8.​ Demodulate an AM signal using two different methods
9.​ Describe the effects and meaning of over- and under-modulation of the carrier
10.​Compare original and demodulated signals
11.​Explain the term” product modulation” as well as the math.

Section 1: Amplitude modulation (AM)

1.1​: Theory and background

For this experiment you’ll use the EMONA Communications board(ECB) to generate an
AM signal by implementing its mathematical model. Then you’ll setup an envelope
detector using the Diodeand RC LPF on the ECB’s Utilities module.

After completing these steps, you’ll connect the AM signal to the envelope detector’s
input and compare the demodulated output to the original message and the AM
signal’s envelope. You’ll also observe the effect that an over-modulated AM signal has
on the envelope detector’s output.

Finally, if time permits, you’ll demodulate the AM signal by multiplying it with a local
carrier instead of using an envelope detector.

24
1.2​ Implement: Setting up the AM modulator

You will now build a model of the system being studied and explore its performance.

Powering up the ELVIS III + EMONA Communications Board

3.​ Ensure that the NI ELVIS III Application Board power button at the top
left corner of the unit is OFF (not illuminated).

4.​ Carefully plug the Emona Communications board into the NI ELVIS III ensuring
that it is fully engaged both front and back.

3.​ Ensure that you have connected the NI ELVIS III to the PC using the USB
cable and that the PC is turned on.

4.​ Turn on the Application Board Power button by pressing it once and confirm
that it is illuminated. The LEDs on the board should also be illuminated. If
they are not, then switch the unit off immediately and check for connection or
insertion errors.

5.​ Open the Instrument Launcher software in your browser and select
the required instruments.

Table 1 Scope Configuration

Channel Voltage Range 2 V/div

Horizontal Timebase 50us/div

Trigger Analog Edge, Channel 1, Rising

Probe Attenuation 1x

25
To experiment with AM demodulation, you’ll need to generate an AM signal. The first
part of the experiment gets you to set one up.

6.​ Use the ELVIS III Function Generator Channel 2output to create a DC voltage of
about 1V by loading the Custom waveform file “ECB_positive1V_DC.csv”. Set
the Update Rate to 100kS/s and click the Run button to start the Generator.

7.​ Connect the Function Generator Channel 2 to input B of the ECB Adder module
on the ECB.

8.​ Connect Channel 1 of the scope to the output of the Adder module on the ECB.

9.​ Turn the Adder module’sG control fully anti-clockwise.

10.​ Adjust the Adder module’s g control to obtain about 1V DC output as measured
by the scope.

11.​ Connect the set-up shown in Figure 2.

Figure 12: Setting up an AM signal

10.​ Set up the scope as per following:

26
 ▪​ Display Channel 1
▪​ Channel 1Coupling to DC
▪​ Channel 1Scale(Volts per Division) control to the 500mV/div position
instead of 1V/div
▪​ Trigger Level control to the 1V position instead of 0V

11.​ Adjust the Adder module’s G control to obtain a 1Vp-p sinewave.

12.​ Adjust the scope’s Timebase control to view about two cycles of the
message signal.

13.​ Activate the scope’s Channel 2 input to view the modulated carrier.

Self check: If the scope’s Scale control for Channel 2 is set to the 1V/div
position, the scope should now display an AM signal with envelopes that are
the same shape and size as the message.

The set-up in Figure 2 can be represented by the block diagram in Figure 3. It

generates a 100kHz carrier that is amplitude modulated by a 2.08kHz sinewave
message.

Figure 13: Block diagram for AM

27
14.​Enable the FFT mode of the Oscilloscope instrument. Adjust the settings as per
the example shown in Figure 4. Be aware that the FFT resolution is linked to the
Scope’s horizontal timebase. Explore this relationship by playing around with the
Time per division settings. You will find that you can either have an optimum time
display or an optimum frequency display but not both at the same time.

Figure 14: Example Scope and FFT capture for AM

The scope and FFT capture report in Figure 4 is obtained by pressing the “camera”
icon in the top right of the Oscilloscope instrument panel. It conveniently records all the
settings as well.

28
1.3​ Implement: Recovering the message using an envelope detector

1.​ Modify the set-up as shown in Figure 5.

Note: Dotted lines show leads already in place.

Figure 15:Using the envelope detector

The additions to the set-up can be represented by the block diagram in Figure 6. As
you can see, it’s the envelope detector explained in the preliminary discussion.

Figure 16: Block diagram for envelope detection

2.​ Adjust the scope’s Scale and Timebase controls to appropriate settings for
the signals.

3.​ Disconnect the scope’s Channel 2 input from the Diode’s output and connect it to the
RC LPF’s output instead.

29
4.​ Capture a screenshot of the scope showing the recovered signal before and
after the LPF and append to your report. Annotate your report appropriately so
as to identify the waveforms captured. Use the cursors to highlight important
levels and transition points in the waveform if necessary.

1-1​What is the relationship between the original message signal and the
recovered message?

1.4​ Implement: Investigating the message’s amplitude on

the recovered message

1.​ Vary the message signal’s amplitude up and down a little by turning the
Adder module’s G control left and right. As you do so, watch the
demodulated signal.

1-2​What is the relationship between the amplitude of the two message signals?

2.​ Slowly increase the message signal’s amplitude to maximum while watching
the demodulated signal.

1-3​What do you think causes the heavy distortion of the demodulated signal?

Tip: If you’re not sure, connect the scope’s Channel 1 input to the AM modulator’s
output.

30
31
1-4​ Why does over-modulation cause the distortion?

1.5​ Implement: Transmitting and recovering speech using AM

This experiment has set up an AM communication system to “transmit” a message

that is a 2.08kHz sinewave. The next part of the experiment lets you use the set-up to
modulate, transmit, demodulate and listen to speech.

1.​ If you moved the scope’s Channel 1 input to help you answer Question
1-4, reconnect it to the Adder module’s output.

2.​ Return the message signal’s amplitude to 1Vpp by adjusting the Adder module’s G
control.

3.​ Modify the set-up as shown in Figure 7. The change is simply moving the input
from the 2.08kHz sine to the Speech output.

Figure 17: Using speech as a message

32
4.​ Set the scope’s Timebase control to the 2ms/div position.

5.​ Connect the output of the RC LPF to the Amplifier modules input. Turn the
Amplifier module’s Gain control fully anti-clockwise to minimum.

6.​ Without wearing the headphones, plug them into the Amplifier module’s
headphone socket.

7.​ Put the headphones on.

8.​ As you perform the next step, set the Amplifier module’s Gain control to
a comfortable sound level.

9.​ Hum and talk into the microphone while watching the scope’s display and
listening on the headphones. Clapping near the microphone gives very clear
distinct signals.

Section 2: The mathematics of AM demodulation

AM demodulation can be understood mathematically because it is uses multiplication

to reproduce the original message. To explain, recall that when two pure sinewaves are
multiplied together (a mathematical process that necessarily involves some
trigonometry that is not shown here) the result gives two completely new sinewaves:

▪​ One with a frequency equal to the sum of the two signals’ frequencies

▪​ One with a frequency equal to the difference between the two signals’ frequencies

The envelope detector works because the rectifier (diode) is a device that multiplies all
signals on its one input with each other. Ordinarily, this is a nuisance but not for
applications like AM demodulation. Recall that an AM signal consists of a carrier, the
carrier plus the message and the carrier minus the message. So, when an AM signal is
connected to a rectifier’s input, mathematically the rectifier’s cross multiplication of all
of its sinewaves looks like:

Rectifier’s Output = Carrier × (Carrier + Message) × (Carrier – Message)

33
If the message signal used to generate the AM signal is a simple sinewave then, when
the equation above is solved, the rectifier outputs six sinewaves at the following
frequencies:

1​ Carrier + (Carrier + Message)

2​ Carrier + (Carrier - Message)
3​ (Carrier + Message) + (Carrier - Message)
4​ Carrier - (Carrier + Message) which simplifies to just the Message
5​ Carrier - (Carrier - Message) which also simplifies to just the Message
6​ (Carrier + Message) - (Carrier - Message)

To make this a little more meaningful, let’s do an example with numbers. The AM
modulator that you set up at the beginning of this experiment uses a 100kHz carrier
and a 2.08kHz message (with a DC component). So the resulting AM signal consists of
three sinewaves: one at 100kHz, another at 102.08kHz and a third at 97.92kHz. Table 1
below shows what happens when these sinewaves are cross-multiplied by the rectifier.

100kHz×102.0 kHz 100kHz×97.92kHz 97.92kHz×102.08kHz

Sum 202.08kHz 197.92kHz 200kHz

Difference 2.08kHz 2.08kHz 4.16kHz

Table 1

Notice that two of the resulting sinewaves are at the message frequency. In other
words, the message has been recovered! And, as the two messages are in phase, they
simply add together to make a single bigger message.

Importantly, we don’t want the other non-message sinewaves so, to reject them but
keep the message, the rectifier’s output is sent to a low-pass filter. Ideally, the filter’s
output will only consist of the message signal. The chances of this can be improved by
making the carrier’s frequency much higher than the highest frequency in the
message. This, in turn, makes the frequency of the “summed” signals much higher
and easier for the low-pass filter to reject.

34
Note: the 4.16kHz sinewave that was generated would pass through the low-pass
filter as well and be present on its output along with the 2.08kHz signal. This is
inconvenient as it is a signal that was not present in the original message. Luckily, as
the signal was generated by multiplying the sidebands, its amplitude is much lower
than the recovered message and can be ignored.

An almost identical mathematical process can be modelled using the ECB’s Multiplier
module. However, instead of multiplying the AM signal’s sinewaves with each other
(the Multiplier module doesn’t do this), they’re multiplied with a locally generated
100kHz sinewave. The next part of this experiment lets you demodulate an AM signal
this way.

2.1 Implement: Product demodulation of AM

1.​ Return the scope’s Timebase control to its earlier setting (probably 200µs/div).

2.​ Disconnect the envelope detector and modify the set-up to return it to just an
AM modulator with a 2.08kHz sinewave for the message as shown in Figure 8.

Figure 18: AM with a 2.08kHz message

3.​ Modify the set-up as shown in Figure 9.

35
 Figure 19: Patching for product demodulation

The additions to the set-up in Figure 9 can be represented by the block diagram in
Figure 10. The Multiplier module models the mathematical basis of AM demodulation
and the RC Low-pass filter on the Utilities module picks out the message while
rejecting the other sinewaves generated.

Figure 20: Block diagram for product demodulation

4.​ Compare the Multiplier module’s output with the Rectifier’s output that you
captured earlier.

36
6-1​Given the AM signal (which consists of 100kHz, 102.08kHz and
97.92kHz sinewaves) is being multiplied by a 100kHz sinewave:

A)​ How many sinewaves are present in the Multiplier module’s output?
B)​ What are their frequencies?

5.​ Disconnect the scope’s Channel 2 input from the Multiplier module’s output
and connect it to the RC LPF’s output instead.

6.​ Compare the RC LPF’s output with the message and the output RC LPF’s that
you captured earlier.

A common misconception about AM is that, once the signal is over-modulated, it’s

impossible to recover the message. However, when the AM signal is generated using
an ideal or near-ideal modulator (such as Figure 3) this is only true for the envelope
detector.

37
Figure 21: Overmodulated AM example recovered without distortion

The AM demodulation method being implemented in this part of the experiment

(called product detection – though it is more accurate to call it product demodulation)
doesn’t suffer from this problem as it’s not designed to recover the message by
tracking one of the AM signal’s envelopes. The final part of this experiment
demonstrates this.
7.​ Connect the scope’s Channel 1 to the AM modulator’s output.

38
 8.​ Set the scope’s Trigger Level to 0V.

Note: The scope will lose triggering but the display will be adequate for
the next steps.

9.​ Slowly increase the message signal’s amplitude to produce a near

100% modulated AM signal by adjusting the Adder module’sG
control.

Note: Resize the AM and demodulated message signals on the screen as

necessary.

10.​ Slowly increase the message signal’s amplitude to produce an AM

signal that is modulated by more than 100% while paying close
attention to the demodulated message signal.

As an aside, the commercial implementation of AM modulation commonly

involves a Class C amplifier for efficiency (that is, to minimise power losses).
When a Class C amplifier is operated at depths of modulation above 100% the
circuit’s operation no- longer corresponds with the model of an AM modulator in
Figure 3. Importantly, in addition to producing an envelope that is not the same
as the original message, the over-modulated Class C circuit produces extra
frequency components in the spectrum. This means that neither the envelope
detector nor the product demodulator can reproduce the message without
distortion
SSB modulation & demodulation

Learning Objectives

After completing this lab, you should be able to complete the following activities.

1.​ Generate a real SSB signal

2.​ Examine a real SSB signal with a scope
3.​ Discuss the differences between SSB and DSBSC
4.​ Demodulate an SSB signal
Required Tools and Technology
Platform: NI ELVIS III ✔​ Install Instruments:
Instruments used in this lab: http://www.ni.com/doc
●​ Oscilloscope-Time ument
●​ Oscilloscope-FFT ation/en/ni-elvis-
●​ Function generator iii/latest/getting-
started/installing-the-s
oft- front-panel/
✔​ Access
instrumentshttps://measur
e mentslive.ni.com
✔​ View User
Manualhttp://www.ni.c
om/en
-us/support/model.ni-elvis
- iii.html
✔​ View tutorials
https://www.youtube.c
om/pl
aylist?list=PLvcPIuVaU
MIW
m8ziaSxv0gwtshBA2d
h_M

Hardware: Emona Communications ✔​ View User

Board Components used in this lab: Manualhttp://www.ni.com/en
●​ Two BNC to 2mm banana-plug -us/support/model.emona-
leads communications-board-for-
●​ Assorted 2mm banana-plug patch ni-elvis-iii.html
leads
●​ Set of headphones or earbuds

Expected Deliverables

In this lab, you will collect the following deliverables:

 ✔​ Calculations

✔​ Data from measurements

✔​ Observations

Your instructor may expect you complete a lab report. Refer to your instructor for
specific requirements or templates.
Section 1: SSB modulation

1.1​Theory and Background

Comparing the two communications systems considered earlier in this manual, DSBSC
offers considerable power savings over AM (at least 66%) because a carrier is not
transmitted. However, both systems generate and transmit sum and difference
frequencies (the upper and lower sidebands) and so they have the same bandwidth for
the same message signal.

As its name implies, the Single Sideband Suppressed Carrier (SSBSC or just SSB)
system transmits only one sideband. In other words, SSB transmits either the sum or
the difference frequencies but not both. Importantly, it doesn’t matter which sideband
is used because they both contain all of the information in the original message.

In transmitting only one sideband, SSB requires only half the bandwidth of DSBSC and
AM which is a significant advantage.

Figure 1 shows a simple message signal and an unmodulated carrier. It also shows the
result of modulating the carrier with the message using SSBSC. If you look closely,
you’ll notice that the modulated carrier is not the same frequency as either the
message or the carrier.
Figure 1: SSB signals
A common method of generating SSB simply involves generating a DSBSC signal then
using a filter to pick out and transmit only one of the sidebands. This is known as the
filter method. However, the two sidebands in a DSBSC signal are close together in
frequency and so specialized filters must be used. This means that the filters for non-
mainstream communications systems can be expensive.

Another way of generating SSB that is becoming increasingly popular is called the
phasing method. This uses a technique called phase discrimination to cancel out one of
the sidebands at the generation stage (instead of filtering it out afterwards).

In telecommunications theory, the mathematical model that defines this process is:

SSB = (message × carrier) + (message with 90° of phase shift × carrier with 90° of
phase shift)

If you look closely at the equation you’ll notice that it’s the sum of two multiplications.
When the message is a simple sinewave the solution of the two multiplications tells us
that four sinewaves are generated. Depending on whether the message’s phase shift is
+90° or -90° their frequencies and phase differences are:

These… Or these…

▪​ Carrier + message ▪​ Carrier + message

▪​ Carrier - message ▪​ Carrier - message

▪​ Carrier + message ▪​ Carrier + message (180° phase shifted)

▪​ Carrier - message (180° phase shifted) ▪​ Carrier – message

Regardless of whether the message’s phase shift is +90° or -90°, when the four
sinewaves are added together, two of them are in phase and add together to produce
one sinewave (either carrier + message or carrier – message) and two of the sinewaves
are phase inverted and completely cancel. In other words, the process produces only a
sum or difference signal (that is, just one sideband).
The block diagram that implements the phasing type of SSB modulator is shown in
Figure 2.

SSB

Figure 2: Block diagram for SSB generation

As SSB signals don’t contain a carrier, they must be demodulated using product
detection in the same way as DSBSC signals (the product detector’s operation is
summarized in the preliminary discussion of DSBSC exp).
1.2​Implement: Generate a SSB signal

For this experiment you’ll use the EMONA Communications board to generate a SSB
signal by implementing the mathematical model for the phasing method. You’ll then
use a product detector (with a stolen carrier) to reproduce the message.

Importantly, you’ll only do so for a sinewave message (that is, you’ll not SSB modulate
then demodulate speech). There’s a practical reason for this. The phase shift
introduced by the Phase Shifter module is frequency dependent (that is, for any given
setting, the phase shift is different at different frequencies). A wideband phase shifting
circuit is necessary to provide 90° of phase shift for all of the sinewaves in a complex
message like speech.
Powering up the ELVIS III + EMONA Communications Board

1.​ Ensure that the NI ELVIS III Application Board power button at the top
left corner of the unit is OFF (not illuminated).

2.​ Carefully plug the Emona Communications board(ECB) into the NI ELVIS III
ensuring that it is fully engaged both front and back.

3.​ Ensure that you have connected the NI ELVIS III to the PC using the USB
cable and that the PC is turned on.

4.​ Turn on the Application Board Power button by pressing it once and confirm
that it is illuminated. The LEDs on the ECB should also be illuminated. If
they are not, then switch the unit off immediately and check for connection
or insertion errors.

5.​ Open the Instrument Launcher software in your browser and select
the required instruments.

Table 1 Scope Configuration

Channel Voltage range 2 V/div

Horizontal Timebase 50us/div

Trigger Analog Edge, Chan 1, Rising

Probe Attenuation 1x

Table 4Function Generator Configuration

Channel 1 Sine

Frequency 10kHz

Amplitude 2Vpp

DC offset 0V
6.​ Connect the set-up shown in Figure 3.
 Figure 3: Patching for phasing setup

This set-up can be represented by the block diagram in Figure 4. It is used to set up two
message signals that are out of phase with each other.
 Figure 4: Block diagram for phasing setup

7.​ Locate the Phase Shifter module on the board and set its Phase Change
control to the 0° position.

8.​ Set the Phase Shifter module’sPhase Adjust control to about the middle of
its travel.

9.​ Set the scopeTrigger Source control to Channel 1.

10.​ Adjust the scope’s Timebase control to view two or so cycles of the
function generator’s output.

11.​ Activate the scope’s Channel 2.

12.​ Check that the two message signals are out of phase with each other.

Note: At this stage, it doesn’t matter what the phase difference is.

14.​ Modify the set-up as shown in Figure 5.

 Figure 5: Partial patching for SSB

This set-up can be represented by the block diagram in Figure 6. It is used to multiply
the two message signals with two 100kHz sinewaves (the carriers) that are exactly 90°
out of phase with each other.
 Figure 6: Block diagram for partial SSB generation

16.​ Use the scope to check that the lower Multiplier module’s output is a
DSBSC signal.

Tip: Temporarily set the scope’s Channel 2 Scale control to the 2V/div position
to do this.

17.​ Disconnect the scope’s Channel 2 input from the lower Multiplier
module’s output and connect it to the upper Multiplier module’s output.

18.​ Check that the upper Multiplier module’s output is a DSBSC signal as well.

19.​ Locate the Adder module and set BOTH its G and g controls to about
the middle of their travel. This is about unity gain.

20.​ Modify the set-up as shown in Figure 7.

 Figure 7: Patching for SSB generation

This set-up can be represented by the block diagram in Figure 8. The Adder module is
used to add the two DSBSC signals together. The phase relationships between the
sinewaves in the DSBSC signals means that two of them (one in each sideband)
reinforce each other and the other two cancel each other out.

Figure 8: Block diagram for SSB

1-1​The signal out of the Adder module is highly unlikely to be an SSB signal at this
stage. What are two reasons for this? Tip: If you’re not sure, one of them can
be worked out by reading the preliminary discussion.

The next part of the experiment gets you to make the fine adjustments necessary to turn
the set-up into a true SSB modulator.

21.​ Deactivate the scope’s Channel 1 input.

22.​ Disconnect the patch lead to the Adder module’sB input.

Note: This removes the signal on the Adder module’sB input from the set-up’s
output.

23.​ Adjust the Adder module’sG control to obtain a 4Vp-p output.

24.​ Reconnect the Adder module’sB input and disconnect the patch lead to its A
input.

Note: This removes the signal on the Adder module’sA input from the set-up’s
output.

25.​ Adjust the Adder module’s g control to obtain a 4Vp-p output.

26.​ Reconnect the patch lead to the Adder module’sA input.

The gains of the Adder module’s two inputs are now nearly the same. Next, the correct
phase difference between the messages must be achieved.

27.​ Slowly vary the Phase Shifter module’sPhase Adjust control left and right
and observe the effect on the envelopes of the set-up’s output.

Note: For most of the Phase Adjust control’s travel, you’ll get an output that
 looks like a DSBSC signal. However, if you adjust the control carefully, you’ll
find that you’re able to flatten-out the output signal’s envelope.

3.​ Set the scope’s Channel 2Scale control to the 500mV/div position.

29.​ Adjust the Phase Shifter module’s Phase Adjust control to make the
envelopes as “flat” as possible.

The phase difference between the two messages is now nearly 90°.

30.​ Tweak the Adder module’sG control to see if you can make the
output’s envelopes flatter.

31.​ Tweak the Phase Shifter module’sPhase Adjust control to see if you can
make the output’s envelopes flatter still.

Once the envelopes are as flat as you can get, the gains of the Adder module’s two
inputs are very close to each other and the phase difference between the two
messages are very close to 90°. That being the case, the signal out of the Adder
module is now SSBSC.

1-2​ How many sinewaves does this SSB signal consist of? Tip: If you’re not sure,
see the preliminary discussion.

1-3​ For the given inputs to the SSB modulator, what two frequencies can this signal
be?
32.​ Keep all settings the same as for the flattest envelope i.e.: the best SSB
signal possible, for this next step.
33.​ To further confirm the reinforcing and cancellation effect between the
two DSBSC signals, view each DSBSC signal on Channel 1 and
Channel 2 respectively. You will notice that they are similar in form but
not aligned.

34.​ Turn on the MATH function of the Oscilloscope and display the sum of
channels 1 & 2 i.e.: MATH channel = Ch1 + Ch2. It should be a signal with a
flat envelope. Confirm for yourself visually that you understand how this
signal comes about.

354.​ Connect Channel 3 of the Oscilloscope to the actual SSB signal on the board
at the output of the ADDER module. Now you can view 4 signals on the
scope: 3 real and 1 calculated. The calculated signal and the real SSB signal
should be the same.

365.​ Capture a screenshot of the scope and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.
Figure 9: Example of addition of DSBSC signals to form SSB

1.3​Implement: Spectrum analysis of an SSB signal

The next part of this experiment let’s you analyse the frequency domain
representation of the SSB signal to see if its spectral composition matches your
answers to Questions 1-2 and 1-3.

1.​ Launchthe NI ELVIS III FFT mode on the Oscilloscope.

2.​ Use the cursors to measure the frequency of the visible sideband.

1-4​ Based on your measurement for the step above, which sideband does your
SSB modulator generate?
3.​ Align cursors with some of the other significant sinewaves close to
this sideband and note their frequencies.

Note: You should find that there’s a sinewave at the carrier frequency and
another at the frequency for the other sideband. Importantly, despite
appearances, these signals are very small relative to the significant sideband
(the scale used for the Y-axis is decibels which is not a linear unit of
measurement).

1-5​ Give two reasons for the presence of a small amount of the other sideband.

4.​ Tweak the Phase Shifter module’s Phase Adjust control and note the effect
on the size of the carrier and other sideband.

Note: Give the signal analyzer’s display time to update after each adjustment.

1-6​ Why doesn’t varying the Phase Shift module’s Phase Adjust control affect the
size of the carrier in the SSBSC signal?

5.​ Adjust the two controls to obtain the smallest size for the insignificant sideband.

6.​ Capture a screenshot of the FFT and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.
7.​ Close the FFT mode and view the oscilloscope.

8.​ Note whether there is any improvement in the SSB signal’s envelope (that
is, note whether the envelope is any flatter).

Section 2: Using the product detector to recover the message

9.​ Reactivate the scope’s Channel 1 input and return the Channel 2Scale
control to the 1V/div position.

10.​ Locate the Tuneable Low-pass Filter module on the board and set its Gain
control to about the middle of its travel.

11.​ Turn the Tuneable Low-pass Filter module’s Cut-off Frequency Adjust control
fully clockwise.

12.​ Modify the set-up as shown in Figure 9.

Figure 10: Product demodulation of SSB

The additions to the set-up shown in Figure 10 can be represented by the block diagram in
Figure 11. The Multiplier and Tuneable Low-pass Filter modules are used to implement a
product detector which demodulates the original message from the SSB signal.

Figure 11: Block diagram for product demodulation

13.​ Use the scope to compare the original message with the recovered message.

2-1What is the relationship between the original message and the recovered message?
1.SOFTWARE (LABVIEW):
First: Frequency Modulation
Method one:
Sfm(t)=Accos(2πfct+2πKf ∫ m(t) dt )) =Accos(2πfct)cos(2πKf) ) -Acsin(2πfct)sin(2π Kf∫ m(t)dt )
Method Two:

Generation of FM using phase modulator

Procedure:
1.​ draw the circuit as shown in the following figure:
Bessel Function

Bessel function of the first kind, is a solution of the differential equation:

Spectrum of Frequency Modulated Signal

Since frequency modulation is a nonlinear process, an exact description of the
spectrum of an frequency-modulated signal for an arbitrary message signal is
more complicated than linear process. However if s(t) is sinusoidal, then the
instantaneous frequency deviation of the angle-modulated signal is sinusoidal and
the spectrum can be relatively easy to obtained.

If we assume m(t) to be sinusoidal then

.
Secondly:Frequency Demodulation
procedures:
1.​ draw the circuit diagram as shown in the figure

2.​ draw the demodulated signal

Report
Due time: one week later.

Please submit it to your TA at the beginning of your next session.

The report must contain:

1.​ The designed and the implemented circuits explanation of selecting this
type of modulation or demodulation circuit.
2.​ The requested plots,and answers to the questions.
3.​ Comments and Conclusions.
4.​ The datasheet of the used components.
 FM Modulation and De Modulation
In this lab you will generate a frequency modulated signal using a variety of message
sources and measure the power and bandwidth of this FM signal by viewing the signal
in the time and frequency domains. As well you will calculate the frequency deviation
of the modulator circuit.

Learning Objectives

After completing this lab, you should be able to complete the following activities.

1.​ Generate a real FM signal using multiple messages

2.​ Examine a real FM signal with scope and compare it to its original message
3.​ Calculate the power in the FM signal
4.​ Describe the bandwidth of an FM signal
5.​ Calculate the frequency deviation of the FM modulator

Prerequisites

You should have completed Lab 1 and Lab 2 and be familiar with the equipment, its use
and the handling precautions for the equipment.
Required Tools and Technology

Platform: NI ELVIS III ​ Install Instruments:

Instruments used in this http://www.ni.com/documentati
lab: o n/en/ni-elvis-iii/latest/getting-
●​ Oscilloscope-Time started/installing-the-soft-front-
●​ Oscilloscope-FFT panel/
●​ Function Generator
​ Access
instrumentshttps://measurement
slive.ni.com
​ View User
Manualhttp://www.ni.com/en-
us/support/model.ni-elvis-iii.html
​ View tutorials
https://www.youtube.com/playlis
t
?list=PLvcPIuVaUMIWm8ziaSxv
Hardware: Emona Communications 0gwtshBA2dh_M
Board Components used in this lab:
●​ Four BNC to 2mm banana-plug
leads ​ View User
●​ Assorted 2mm banana-plug Manualhttp://www.ni.com/en-
patch leads us/support/model.emona-
●​ Set of headphones or earbuds communications-board-for-ni-
elvis-iii.html

Expected Deliverables

In this lab, you will collect the following deliverables:

✔​ Calculations

✔​ Data from measurements

✔​ Observations

Your instructor may expect you complete a lab report. Refer to your instructor for
specific requirements or templates.
Section 1: FM Modulation

1.1​Theory and Background

A disadvantage of the AM, DSBSC and SSB communication systems is that they are
susceptible to picking up electrical noise in the transmission medium (the channel). This
is because noise changes the amplitude of the transmitted signal and the demodulators
of these systems are designed to respond to amplitude variations.

As its name implies, frequency modulation (FM) uses a message’s amplitude to vary the
frequency of a carrier instead of its amplitude. This means that the FM demodulator is
designed to look for changes in frequency instead. As such, it is less affected by
amplitude variations and so FM is less susceptible to noise. This makes FM a better
communications system in this regard.

There are several methods of generating FM signals but they all basically involve an
oscillator with an electrically adjustable frequency. The oscillator uses an input
voltage to affect the frequency of its output. Typically, when the input is 0V, the
oscillator outputs a signal at its rest frequency (also commonly called the
free-running or center frequency). If the applied voltage varies above or below 0V, the
oscillator’s output frequency deviates above and below the rest frequency. Moreover,
the amount of deviation is affected by the amplitude of the input voltage. That is, the
bigger the input voltage, the greater the deviation.

Figure 1 shows a bipolar square wave message signal and an unmodulated carrier. It
also shows the result of frequency modulating the carrier with the message.
 ​

Figure 1: Sketch of FM signals

There are a few things to notice about the FM signal. First, its envelopes are flat –
recall that FM doesn’t vary the carrier’s amplitude. Second, its period (and hence its
frequency) changes when the amplitude of the message changes. Third, as the
message alternates above and below 0V, the signal’s frequency goes above and below
the carrier’s frequency. (Note: It’s equally possible to design an FM modulator to cause
the frequency to change in the opposite direction to the change in the message’s
polarity.)

Before discussing FM any further, an important point must be made here. A

squarewave message has been used in this discussion to help you visualise how an
FM carrier responds to its message. In so doing, Figure 1 suggests that the resulting
FM signal consists of only two sinewaves (one at a frequency above the carrier and
one below). However, this isn’t the case. For reasons best left to your instructor to
explain, the spectral composition of the FM signal in Figure 1 is much more complex
than implied.

This highlights one of the important differences between FM and the modulation
schemes discussed earlier. The mathematical model of an FM signal predicts that
even for a simple sinusoidal message, the result is a signal that potentially contains
many
sinewaves. In contrast, for the same sinusoidal message, an AM signal would consist of
three sinewaves, a DSBSC signal would consist of two and an SSBSC signal would
consist of only one. This doesn’t automatically mean that the bandwidth of FM signals is
wider than AM, DSBSC and SSBSC signals (for the same message signal). However, in
the practical implementation of FM communications, it usually is.

There’s another important difference between FM and the modulation schemes

discussed earlier. The power in AM, DSBSC and SSBSC signals varies depending on
their modulation index. This occurs because the carrier’s RMS voltage is fixed but the
RMS sideband voltages are proportional to the signals’ modulation index. This is not
true of FM. The RMS voltage of the carrier and sidebands varies up and down as the
modulation index changes such that the square of their voltages always equal the
square of the unmodulated carrier’s RMS voltage. That being the case, the power in
FM signals is constant.

Finally, when reading about the operation of an FM modulator you may have recognised
that there is a module on the Emona Communications board that operates in the same
way - the VCO output of the function generator. In fact, a voltage-controlled oscillator is
sometimes used for FM modulation (though there are other methods with advantages
over the VCO).
1.2​Implement: Modulating the VCO with two discrete levels

For this experiment you’ll generate a real FM signal using the VCO module on the
board. First you’ll set up the VCO module to output an unmodulated carrier at a known
frequency. Then you’ll observe the effect of frequency modulating its output with a
squarewave then speech. You’ll then use the NI ELVIS III FFT mode on the
Oscilloscope to observe the spectral composition of an FM signal in the frequency
domain and examine the distribution of power between its carrier and sidebands for
different levels of modulation.

It should take you about 40 minutes to complete this experiment.

Powering up the ELVIS III + EMONA Communications Board

1.​ Ensure that the NI ELVIS III Application Board power button at the top
left corner of the unit is OFF (not illuminated).

2.​ Carefully plug the Emona Communications board(ECB) into the NI ELVIS III
ensuring that it is fully engaged both front and back.

3.​ Ensure that you have connected the NI ELVIS III to the PC using the USB
cable and that the PC is turned on.

4.​ Turn on the Application Board Power button by pressing it once and confirm
that it is illuminated. The LEDs on the ECB should also be illuminated. If
they are not, then switch the unit off immediately and check for connection
or insertion errors.

5.​ Open the Instrument Launcher software in your browser and select
the required instruments.

Table 1 Scope Configuration

Channel Voltage range 2 V/div

Horizontal Timebase 50us/div

Trigger Analog Edge, Chan 1, Rising

Probe Attenuation 1x
6.​ Connect the set-up shown in Figure 2.

7.​ Set the VCO module’s GAIN to minimum (fully anti-clockwise) and then set
the FREQ control to give an output sinusoid of about 100kHz. This has a
period of 10us, so you can easily fine tune it in the time domain.

8.​ Slowly increase the GAIN to maximum (fully clockwise).

Figure 2: Patching for FM with VCO

This set-up can be represented by the block diagram in Figure 4. The Master Signals
module is used to provide a 2.08kHz squarewave message signal and the VCO is the
FM modulator with an unmodulated center frequency of 100kHz.
Figure 3:Example of FM signal in frequency domain
 Figure 4: Block diagram for FM with VCO

9.​ View both the input and output to the VCO module, and trigger the scope
on the rising edge of the squarewave input.

3.​ Observe the difference in frequency in the output for a 0V input versus a +5 V
input level. This exercise serves to demonstrate how the VCO is modulated
by an input signal. You should use SINGLE mode on the scope to stabilise
the signal for measurements purposes and trigger off of an edge of the input
signal.

1-1 With GAIN at maximum, measure the frequency for both states of the output
signals. What are they ?

11.​ Enable the FFT mode of the Oscilloscope instrument. Change the
scopes timebase to 1ms/div.

12.​ Set the frequency span for the FFT displayed from say 0kHz to 200kHz for
a broad overview of the frequency domain. Vary the VCO GAIN control and
observe the effect on the output spectrum.
13.​ Capture a screenshot of the FFT and append to your report. Annotate your
report appropriately so as to identify the waveforms captured. Use the
cursors to highlight important levels and transition points in the waveform if
necessary.
1.3​Implement: Generating an FM signal using speech

So far, this experiment has generated an FM signal using a squarewave 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 FM signal looks like when modulated by speech.

1.​ Return the scope’s Trigger Level control to 0V.

2.​ Disconnect the plugs to the Master Signals module’s 2.08kHz DIGITAL output.

3.​ Connect them to the Speech module’s output as shown in Figure 5.

Figure 5: Modulating the VCO with speech

4.​ Set the scope’s Timebase control to the 100µs/div position.

5.​ Hum, clap and talk into the microphone while watching the scope’s display.

1.4​Implement: Power in an FM signal

As mentioned earlier, the power in an FM signal is constant regardless of its level of
modulation. This part of the experiment lets you see this for yourself.

1.​ Locate the VCO module.

2.​ Keep the previous set-up from Figure 5

3.​ Or you can change the input to connect to GND. Step 4 makes
this unnecessary.

4.​ Set the VCO module’s GAIN to minimum (fully anti-clockwise) and then set
the FREQ control to give an output sinusoid of about 100kHz. This has a
period of 10us, so you can easily fine tune it in the time domain.

5.​ Open the FFT mode on the scope and view the spectrum.

6.​ Once done, one significant sinewave should be displayed.

7.​ Confirm this is the case.

8.​ Measure the frequency of the sinewave and verify that it’s the VCO’s
rest frequency (that is, 100kHz).

9.​ View the measurement of the signal’s RMS voltage. Record this in Table 1.

10.​ Square and record this voltage.

Table 1
Unmodulated Unmodulated
Carrier VRMS Carrier V 2
RMS

Why square the signal’s RMS voltage? To answer this question, remember that we’re
investigating the power in an FM signal but signal analyzers (and most other test
2
equipment) can’tmeasure power. However, one of the power equations ( P = RMS ) tells
R
us that power and the square of a signal’s ) are proportional
RMS voltage (that is, V 2

values. That being the case, we can investigate power in an FM signal indirectly by
investigating the square of the signal’s RMS voltage because whatever is true of one
must also be true of the other (regardless of R).

11.​ Modify the set-up as shown in Figure 6.

Figure 6:Modulating the VCO with 2.08kHz sinusoid

The carrier will now be frequency modulated by a low level message signal. This
means that the signal analyzer’s display will show about four sidebands. As these
sidebands are small relative to the carrier, they can be better observed by temporarily
setting the Spectrum analyzer’s Units option to dB instead of Linear.

12.​ If you haven’t already done so, return the analyzer’s Units option to Linear.

13.​ Use the VCO module’s Gain control to adjust the modulation of the FM
signal slightly until only five sinewaves are clearly visible in the signal’s
spectrum.
14.​ Use the cursor to measure the RMS voltage of these sinewaves and record
 them in Table 2.

15.​ Square and record the voltages.

16.​ Add and record the squared voltages.

Table 2
Sinewave V
RMS VRMS
2

Total

17.​ Use the VCO module’s Gain control to increase the modulation of the
FM signal until the carrier drops to zero for the first time.

18.​ Repeat these steps for the six most significant sinewaves in the
signal recording your measurements in Table 3.

Table 3
Sinewave V
RMS V2
RMS

2
 3

Total

1-1​How do the totals in Tables 2 and 3 compare with each other and the value in
Table 1?

1-2​What do these measurements help to prove? Explain your answer.

1.5​Implement: Bandwidth of an FM signal

The spectral composition of an FM signal can be complex and consist of many

sidebands. Usually, many of them are relatively small in size and so an engineering
decision must be made about how many of them to include as part of the signal’s
bandwidth. There are several standards in this regard and a common one involves
including all sidebands that are equal to or greater than 1% of the unmodulated
carrier’s
power (or V 2 ). This part of the experiment lets you use
this criterion to measure FM
signal bandwidth.
1.​ Use the VCO module’s Gain control to adjust the modulation of the FM
signal slightly until only five sinewaves are clearly visible in the signal’s
spectrum.

2.​ Use the signal analyzer’s C1 cursor to identify the lowest frequency sinewave
in the FM signal with a V 2 equal to or greater than 1% of the value in
Table 1.

Note: You have to do this by measuring the RMS voltage of the smallest
sinewaves and square the value until equal to
2
you find the first one with a V

or greater than 1% of the value in Table 1.

3.​ Use the signal analyzer’s C2 cursor to identify the highest frequency
sinewave in the FM signal with a voltage equal to or greater than 1% of the
value in Table 1.

4.​ The signal analyzer’s df (Hz) reading is a measurement of the difference in

frequency between its cursors. Following Steps 40 and 41, this reading is
the FM signal’s bandwidth. Record this value in Table 4.

Table 4

1-3​Calculate the bandwidth of a 100kHz carrier amplitude modulated by a

2kHz sinewave.
1-4​How does the FM signal’s bandwidth compare to an AM signal’s bandwidth for
the same inputs?
1-5​How far apart are each of the sidebands ?

5.​ Use the VCO module’s Gain control to increase the modulation of the
FM signal until the carrier drops to zero for the first time

6.​ Repeat steps 40 to 42 recording your measurement in Table 5.

Table 5

1-6​What is the relationship between the message signal’s amplitude and the
FM signal’s bandwidth?
FM Demodulation
In this section you will begin with a frequency modulated signal based on the previous
lab and construct a demodulation process which translates the frequency variations
into voltage variation in a linear manner. The ability to translate between signal
domains is an important principle across many topics. There are several methods to
achieve this and in this experiment you will explore the introductory method of
zero-crossing detection.

Learning Objectives

After completing this lab, you should be able to complete the following activities.

6.​ Describe a zero-crossing detection method

7.​ Discuss frequency to voltage translation
8.​ Recover a variety of FM modulated messages
9.​ Describe the FM spectrum of FM modulated speech

Prerequisites

You should have completed Lab 1 and Lab 2 and be familiar with the equipment, its use
and the handling precautions for the equipment.
Expected Deliverables
In this lab, you will collect the following deliverables:

✔​ Calculations

✔​ Data from measurements

✔​ Observations

Your instructor may expect you complete a lab report. Refer to your instructor for
specific requirements or templates.

FM demodulation

1.1​Theory and Background

There are as many methods of demodulating an FM signal as there are of generating
one. Examples include: the slope detector, the Foster-Seeley discriminator, the ratio
detector, the phase-locked loop (PLL), the quadrature FM demodulator and the zero-
crossing detector. It’s possible to implement several of these methods using the Emona
Communications board, but for an introduction to the principles of FM demodulation,
the zero-crossing detector is used here.

The zero-crossing detector

The zero-crossing detector is a simple yet effective means of recovering the message
from FM signals. Its block diagram is shown in Figure 1.

Figure 7: Block diagram for zero-crossing detection

The received FM signal is first passed through a comparator to clip it heavily,
effectively converting it to a squarewave. This allows the signal to be used as a trigger
signal for the zero-crossing detector circuit (ZCD).

The ZCD generates a pulse of fixed duration every time the squared-up FM signal
crosses zero volts (either on the positive or the negative transition but not both). Given
the squared-up FM signal is continuously crossing zero, the ZCD effectively converts
the squarewave to a continuous rectangular wave with a fixed mark time.

When the FM signal’s frequency changes (in response to the message), so does the
rectangular wave’s frequency. Importantly though, as the rectangular wave’s mark is
fixed, changing its frequency is achieved by changing the duration of the space and
hence the signal’s mark/space ratio (or duty cycle). This is shown in Figure 2 using an
FM signal that only switches between two frequencies (because it has been generated
by a squarewave for the message).

FM signal​ 0V

Comparator's
output

0V

ZCD signal

0V
Figure 8: ZCD operation timing diagram
Recall from the theory of complex waveforms, pulse trains are actually made up of
sinewaves and, in the case of Figure 2, a DC voltage. The size of the DC voltage is
affected by the pulse train’s duty cycle. The greater its duty cycle, the greater the DC
voltage.

That being the case, when the FM signal in Figure 2 switches between the two
frequencies, the DC voltage that makes up the rectangular wave out of the ZCD
changes between two values. In others words, the DC component of the rectangular
wave is a copy of the message signal that produced the FM signal in the first place.
Recovering this copy is a relatively simple matter of picking out the changing DC
voltage using a low-pass filter.

Importantly, this demodulation technique works equally well when the message is a
sinewave or speech.

1.2​Implement: Setting up the FM Modulator

For this experiment you’ll use the Emona Communications board to generate an FM
signal using a VCO. Then you’ll set-up a zero-crossing detector and verify its operation
for variations in the message’s amplitude.

It should take you about 50 minutes to complete this experiment.

Powering up the ELVIS III + EMONA Communications Board

4.​ Ensure that the NI ELVIS III Application Board Powerbutton at the top
left corner of the unit is OFF (not illuminated).

5.​ Carefully plug the Emona Communications board(ECB) into the NI ELVIS III
ensuring that it is fully engaged both front and back.

3.​ Ensure that you have connected the NI ELVIS III to the PC using the USB
cable and that the PC is turned on.

4.​ Turn on the Application Board Power button by pressing it once and confirm
that it is illuminated. The LEDs on the ECB should also be illuminated. If
they are not, then switch the unit off immediately and check for connection
or insertion errors.

5.​ Open the Instrument Launcher software in your browser and select
the required instruments.

Table 1 Scope Configuration

Channel Voltage range 2 V/div

Horizontal Timebase 10us/div

Trigger Analog Edge, Chan 1, Rising

Probe Attenuation 1x

6.​ Use the ELVIS III Function Generator output Channel 2 to create a DC voltage
of about 1V by loading the Custom waveform file “ECB_positive1V_DC.csv”.
 Figure 9: Setting up DC voltage using Function Generator

7.​ Connect the set-up shown in Figure 4.

 Figure 10: Patching to create variable DC voltage

8.​ Locate the Adder module on the board and turn its G control fully
anti- clockwise. This branch is not used.

9.​ Adjust the Adder module’s g control to obtain a 2V DC output (as

measured using the Scope).

10.​ Patch the 2V DC voltage out of the ADDER into the VCO input and set
the VCO GAIN control to minimum (fully anti-clockwise).

11.​ Activate the scope’s Channel 2 input to view the FM signal on the VCO’s
output as well as the DC message signal on Channel 1. Trigger the scope on
Channel 2 so the sinewave is stable (set Trigger Source to Channel 2).

12.​ Adjust the VCO carrier frequency to 85kHz using the FREQ control,
while ensuring that the VCO’s GAIN is minimum. This sets the FM
modulator’s resting frequency to be 85kHz.

This set-up can be represented by the block diagram in Figure 5. The positive voltage
output from the Function Generator and the Adder module are being used to provide a
simple DC message and the VCO implements the FM modulator with a carrier
frequency of 85kHz.

Figure 11: DC input into VCO module

13​ Vary the VCO input’s GAIN control from minimum (zero) and confirm that the
.​ VCO’s output frequency changes accordinglyi.e.: increases with voltage

14​ Vary the VCO input’s GAIN control to give an output frequency of about
.​ 100kHz for the input control voltage of +2V. Do NOT vary this GAIN
control from now on.

1.3​Implement: Setting up the zero-crossing detector

1.​ Locate the Tuneable Low-pass Filter module on the board and turn its GAIN
control fully clockwise (maximum gain).

2.​ Turn the Tuneable Low-pass Filter module’s TUNE control fully
clockwise (maximum passband).

3.​ Modify the set-up as shown in Figure 6.

 Figure 12: Patching for FM modulator and demodulator

The additions to the set-up can be represented by the block diagram in Figure 7. The
Comparator module is used to clip the FM signal at the zero-volt level, effectively
turning it into a squarewave which is independent of amplitude. The Comparator
module also has a built-in PULSE output which triggers a fixed length pulse
(monostable pulse) on the negativeedge of the squarewave output. This implements
the zero-crossing detector function (ZCD)in Figure 7. To complete the FM demodulator,
the RC Low-pass Filter module and Tuneable Low-pass Filter module combination is
used to pick-out the changing DC component of the ZCD’s PULSEoutput.

Figure 13: Block diagram for FM demodulation

The entire set-up can be represented by the block diagram in Figure 8.

4.​ View the output signal from the RC LPF (point E in Figure 6). Its input is a
narrow pulse repeating at approximately 98kHz.

1-1​ Why does the output from the RC LPF still contain high frequency
components from the squarewave input?
Figure 14: Block diagram for FM modulation and demodulation
You will now use the Tuneable Low-pass Filter to isolate the DC component of the ZCD
for the voltage range of the input signal we plan to use.

5.​ Keep the Vin voltage at +2V DC with the VCO output frequency at
about 100kHz.

6.​ Set the scope’s Channel 2Scale control to the 1V/div position and connect it
to the output of the RC LPF module.

Note: You should now see a 250mVp-p wave with a DC offset of

approximately 2V. This is the filtered pulse train out of the ZCD by the RC LPF
only.

7.​ Now move the Channel 2 scope lead to view the output of the TUNEABLE
LPF (at point F in Figure 6). Confirm that you now see a steady DC voltage.
Any variations have been attenuated by this filter.

Note 1: Youhave now eliminated all non-DC components from this signal.
That said, the filter will still allow the message signal to pass because only the
higher frequency components from the squarewave have been eliminated.

Note 2:You do not need to reduce the cutoff frequency of the TUNEABLE LPF
because its maximum passband frequency is still much lower than the
components from the squarewave.

8.​ Vary the ADDERs GAIN control between minimum and maximum
which changes the Vin voltage to the VCO between 0V and about +2V.

Note 1: As you do, you should notice that the DC voltage out of the
Tuneable Low-pass Filter module varies as well. This variation is slight so
look closely. The duty cycle of the ZCD output will vary as well.

Note 2: If this doesn’t happen, check that the scope’s Channel 1 Coupling
control is set to the DC position.
9.​ Now to investigate the VCO output for a negative going Vin voltage.
10.​ Using the ELVIS III Function Generator, output Channel 2 which is currently
loaded with “ECB_positive1V_DC.csv”. To create a negative voltage out of the
Function Generator set the Function Generator Gain setting to -1. This will
output a -1V DC voltage into the ADDER module. Set the ADDER module’s
GAIN control to change the Vin voltage to the VCO to about -2V.

11.​ Confirm that the VCO output frequency is now lower than its resting
frequency of 85kHz and is about 70kHz. As you would expect, a negative
going Vin causes the VCO frequency to reduce to below its resting frequency.

12.​ Return to a positive voltage for Vin, by resetting the Gain setting in
the Function generator window to +1.

1.4​ Implement: Investigating the operation of the

zero-crossing detector

The next part of the experiment lets you verify the operation of the zero-crossing
detector.

1.​ Rearrange the scope’s connections to the set-up as shown in Figure

9.Connect scope Channels 1 and 2 to point C and the Comparator OUT port.
 Figure 15: Patching for FM modulator and demodulator

The new scope connections can be shown using the block diagram in Figure 10.

Figure 16: Block diagram for viewing Comparator

2.​ Vary the ADDER GAIN which changes the Vin voltage.
 Note: This will cause small but noticeable changes in the FM signal’s
frequency.

3.​ As you vary the FM signal’s frequency, pay close attention to the
mark-space ratio (that is, the duty cycle) of the Comparator’s output.

Tip: You may find it helpful to adjust the scope’s Vertical Position controls to
separate the signals on the display.

1-2​Does the mark-space ratio of the signal on the Comparator’s output change?

1-3​ What does this tell us about the DC component of the comparator’s output?

4 Rearrange the scope’s connections to the set-up as shown in Figure 11.Connect

. the scope Channels 1 and 2 to point D and the Comparator OUT port.
Figure 17:Patching for FM modulator and demodulator

The new scope connections can be shown using the block diagram in Figure 12.

Figure 18: Block diagram for viewing Zero Crossing

Detector
5.​ Vary the ADDER GAIN which changes the Vin voltage to model and changing
message voltage in.

6.​ As you perform the step above, note how the frequency of the two
signals changes.

7.​ Turn on the scope’s cursors.

8.​ Use the scope’s cursors to measure the width of the ZCD output’s mark
and space for different DC input voltages.

Note: The time difference between the two cursors is displayed directly
above the Channel 1 & 2 measurements and is denoted as dT.

Tip: You may find it helpful to turn the scope’s Channel 1 off as you do this and
set its Timebase control to 10µs/div when measuring the mark’s width.

1-4​ As the FM signal changes frequency so does the ZCD’s output.

What aspect of the ZCD’s output signal changes to achieve this?

1-5​What does this tell us about the DC component of the comparator’s output?

9.​ If you deactivated the scope’s Channel 1 then reactivate it and return its
Timebase control to 50µs/div.
10​ Rearrange the scope’s connections to the set-up as shown in Figure 13.
IMPORTANT: Use the scope Channels 1 and 2 to view points D and F of
Figure 13.
Figure 19: Patching for FM modulator and demodulator

The new scope connections can be shown using the block diagram in Figure 14.

Figure 20: Block diagram for viewing Filtering

11​ If you’ve adjusted the scope’s Channel 2Vertical Position control, re-zero it.
.

12​ Vary the ADDERs GAIN control in small steps again to model an FM signal’s
changing frequency.

13​ As you perform the step above, compare the outputs from the ZCD (at
the PULSE output terminal) and the Tuneable Low-pass Filter module.

1-6​ Why does the Tuneable Low-pass Filter module’s DC output go up as the
mark- space ratio of the ZCD’s output goes up?

1-7​ If the original message is a sinewave instead of a variable DC voltage, what

would you expect to see out of the Tuneable Low-pass Filter module and why?

Section 2: Transmitting and recovering a sinewave using FM

This experiment has set up an FM communication system to “transmit” a message

that is a DC voltage which varies from -2V to +2V. The next part of the experiment
lets you use the set-up to modulate, transmit and demodulate a test signal (a
sinewave).

1.​ If it’s not already, turn the Tuneable Low-pass Filter module’s Gain control
 fully clockwise.

2.​ Modify the set-up as shown in Figure 15.

 Figure 21: Patching for sinewave input signal

This modification to the FM modulator only, can be shown using the block diagram in
Figure 16. Notice that the message is now provided by the Master Signals module’s
2.08kHz SINE output. View this signal at point B with Channel 1 on the scope.

Figure 22: Block diagram for sinewave input to modulator

3.​ Make the following adjustments to the scope’s controls:

▪​ Scale control for Channel 0 to 1V/div and to 1V/div for Channel 1

▪​ Input Coupling control for both channels to AC
▪​ Trigger Type to Analog Edge
▪​ Trigger Source to Channel1
▪​ Timebase control to 200µs/div
4.​ Without needing to vary the TUNEABLE LPFs cutoff frequency you will
already be viewing the demodulated 2.08kHz message sinewave with an
amplitude of approximately 500mVp-p (at point F)

2-1 What does the FM modulator’s output signal tell you about the ZCD signal’s duty
cycle?

Section 3: Transmitting and recovering speech using FM

The next part of the experiment lets you use the set-up to modulate, transmit and
demodulate speech.

5.​ Disconnect the plugs to the Master Signals module’s 2.08kHz SINE output.

6.​ Modify the set-up as shown in Figure 18. You will replace the sinewave
input signals with a Speech signal from the microphone module. Continue
to view the input message at point B, as well as the output at point F of
Figure 18.
Figure 23: Example signals for FM, ZCD and demodulated output
 Figure 24: Patching for speech input to modulator

7.​ Set the scope’s Time base control to the 2ms/div position.

8.​ Locate the Amplifier& Headphone Output module on the board and turn its
Gain control fully anti-clockwise to minimum.
9.​ Without wearing the headphones, plug them into the Amplifier &
Headphone Output module’s headphone socket.

10​ Put the headphones on.

.
11​ As you perform the next step, set the Amplifier & Headphone Output module’s
.​ Gain control to a comfortable sound level.

12​ Hum and talk into the microphone while watching the scope’s display and
.​ listening on the headphones.

13​ Once you have completed viewing the signal with the scope, open the FFT
.​ mode on the scope to view the spectrum of the frequency modulated speech
(at point C of Figure 18). Try whistling into the microphone. This will help you
to see the difference between single tones and speech during modulation.