Unit 3 Lab – Amplitude Modulation and demodulation
Objectives: After completing this lab you should be able to,
1. Verify the operation of full ca
ier AM modulator and demodulator.
2. Measure the modulation index of an AM signal.
3. Use spectrum analyzer to measure the bandwidth of an AM signal.
Procedures:
1. The following modulator is refe
ed as Square-law modulator or diode modulator. Build the following circuit using Multisim.
Modulating signal: Double-click to open the settings:
Frequency: 1kHz
Amplitude: 4Vp
Oscilloscope
Ca
ier signal: Double-click to open the settings:
Frequency: 100kHz
Amplitude: 8Vp
2. Double click on the Multiplier function and change the Output gain (K) to 0.1 as shown below.
Multiplier function settings
3. Run the simulation and double click on the oscilloscope. You should be able to observe the following signal. You need to adjust the scope to observe the following.
Envelope of AM signal is the modulating signal
Cursors
Amplitude Modulated signal
4. Place the cursor 1 and 2 at the maximum and minimum level of the AM signal as shown below. Cursors are located on the left side of the oscilloscope display as indicated above.
Since 1st cursor is placed at the max. point which has 3.566V and 2nd cursor is placed at min. point has a voltage of 1.5952V
Cursor settings
5. Record the max. and min. voltage values in the table below under Vmax and Vmin respectively. 1st row completed as an example.
Modulating signal Amplitude
Vmax
Vmin
Measured modulation index (%)
4Vp
3.566
1.592
38.27%
5Vp
7Vp
8Vp
9Vp
Modulation index calculations
6. Calculate the percentage modulation index using the formula below and record the result under measured modulation index (%) in table above. 1st one completed as an example.
Modulation index m (%) = * 100
7. Change the amplitude of the modulating signal using XFG1 to the list of values shown in the table above and repeat steps 4 through 6.
8. Now, set amplitude of the modulating signal to 12Vp, then answer the following questions.
a. Does the AM signal envelope resemble the message signal?
. Does this condition cause over modulation?
c. Provide the screenshot of the oscilloscope signal here with timestamp.
9. Now set the amplitude of modulating signal back to 4Vp on XFG1.
10. Now connect the spectrum analyzer as shown below.
Spectrum
Analyze
11. Double click on the spectrum analyzer and set it to the following. After entering the values click on Enter.
Spectrum analyzer settings
12. Now run the simulation. You should be able to see the following representation of the AM signal on the spectrum analyzer with ca
ier at the center and two sidebands above and below the ca
ier.
USB
LSB
Ca
ie
13. Using the cursor, measure the frequency of the ca
ier, USB and LSB. Record the results in table below.
Message frequency
Ca
ier Frequency
USB
Frequency
LSB frequency
Bandwidth
1kHz
2kHz
3kHz
AM signal frequency measurements
14. Calculate the bandwidth of the AM signal by subtracting fLSB from fUSB. Record the result in the table above under bandwidth.
15. Change the modulating signal frequency using XFG1, to the listed values in the table above and repeat steps 12 and 13.
16. Based on the results from the table above, what is the relation between the message/modulating signal frequency to the bandwidth?
17. Increase the amplitude of the modulating signal using XFG1. What happened to the amplitude of the side bands?
Demodulation using Diode/Envelope detector:
18. In this part, the message/modulating signal will be demodulated using a simple circuit known as Diode/Envelope detector. Build the following circuit using Mulisim next to the modulator.
Demodulato
AM Modulator and Demodulato
19. Run the simulation. Open the oscilloscope. Does the modulating signal seem to be recovered?
20. Place the screenshot of the oscilloscope image with timestamp here.
XFG1
COM
XFG2
COM
A1
0.1V/V 0V
Y
X
R1
100Ω
R2
100Ω
R3
100Ω
XSC1
A
B
Ext Trig
+
+
_
_
+
_
XSA1
TIN
XFG1
COM
XFG2
COM
A1
0.1V/V 0V
Y
X
R1
100Ω
R2
100Ω
R3
100Ω
XSC1
A
B
Ext Trig
+
+
_
_
+
_
D1
1N4149
R4
1kΩ
C1
50nF
XFG1
COM
XFG2
COM
A1
0.1V/V 0V
Y
X
R1
100Ω
R2
100Ω
R3
100Ω
XSC1
A
B
Ext Trig
+
+
_
_
+
_
Unit 3
Lab
–
Amplitude Modulation and demodulation
Objectives:
After completing this lab you should be able to,
1.
Verify the operation of full ca
ier AM modulator and demodulator.
2.
Measure the modulation index of an AM signal.
3.
Use
spectrum
analyzer to measure the bandwidth of an AM signal.
Procedure
s
:
1.
Th
e following
modulator
is
efe
ed as Square
-
law modulator or diode modulator.
Build
the following circuit using Multisim.
XFG1
COM
XFG2
COM
A1
0.1V/V 0V
Y
X
R1
100O
R2
100O
R3
100O
XSC1
A
B
Ext Trig
+
+
_
_
+
_
Modulating signal: Double
-
click to open the settings:
Frequency
: 1kHz
Amplitude: 4Vp
Ca
ier signal: Double
-
click to open the settings:
Frequency: 100kHz
Amplitude: 8Vp
Oscilloscope
Unit 3 Lab – Amplitude Modulation and demodulation
Objectives: After completing this lab you should be able to,
1. Verify the operation of full ca
ier AM modulator and demodulator.
2. Measure the modulation index of an AM signal.
3. Use spectrum analyzer to measure the bandwidth of an AM signal.
Procedures:
1. The following modulator is refe
ed as Square-law modulator or diode modulator. Build
the following circuit using Multisim.
XFG1
COM
XFG2
COM
A1
0.1V/V 0V
Y
X
R1
100O
R2
100O
R3
100O
XSC1
A
B
Ext Trig
+
+
_
_
+
_
Modulating signal: Double-click to open the settings:
Frequency: 1kHz
Amplitude: 4Vp
Ca
ier signal: Double-click to open the settings:
Frequency: 100kHz
Amplitude: 8Vp
Oscilloscope