ESET280_W3_Lab_3_AmplitudeModulationanddemodulation Zachary Trotter

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ECPI University *

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280

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Electrical Engineering

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Dec 6, 2023

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Week 3 Lab – Amplitude Modulation and demodulation Objectives: After completing this lab you should be able to, 1. Verify the operation of full carrier 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 referred 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 100Ω R2 100Ω R3 100Ω XSC1 A B Ext Trig + + _ _ + _ Modulating signal: Double-click to open the settings: Frequency: 1kHz Amplitude: 4Vp Oscilloscope Carrier 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. 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. Envelope of AM signal is the modulating signal Cursors

Cursor settings 5. Record the max. and min. voltage values in the table below under V max and V min respectively. 1 st row completed as an example. Modulating signal Amplitude V max V min Measured modulation index (%) 4Vp 3.566 1.592 38.27% 5Vp 3.935V 1.309V 50.07% 7Vp 4.499V 800.814m V 70.52% 8Vp 4.790V 591.118m V 79.23% 9Vp 5.059V 301.728m V 88.40% Modulation index calculations 6. Calculate the percentage modulation index using the formula below and record the result under measured modulation index (%) in table above. 1 st one completed as an example. Modulation index m (%) = Vmax − Vmin Vmax + Vmin * 100 Since 1 st cursor is placed at the max. point which has 3.566V and 2 nd cursor is placed at min. point has a voltage of 1.5952V

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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? No b. Does this condition cause over modulation? Yes 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.

XFG1 COM XFG2 COM A1 0.1V/V 0V Y X R1 100Ω R2 100Ω R3 100Ω XSC1 A B Ext Trig + + _ _ + _ XSA1 T IN 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 carrier at the center and two sidebands above and below the carrier. Spectrum Analyzer USB LSB Carrier

13. Using the cursor, measure the frequency of the carrier, USB and LSB. Record the results in table below. Message frequency Carrier Frequency USB Frequency LSB frequency Bandwidth 1kHz 100 100.992 99.008 1.982 2kHz 100 101.983 98.017 3.966 3kHz 100 102.975 97.025 5.95 AM signal frequency measurements 14. Calculate the bandwidth of the AM signal by subtracting f LSB from f USB . 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? As the message increases so does the bandwidth. 17. Increase the amplitude of the modulating signal using XFG1. What happened to the amplitude of the side bands? It increases.

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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. 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 AM Modulator and Demodulator 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. Demodulator