CNET304 Lab 4 AM & ASK Modulation (1)

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Feb 20, 2024

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Lab 4: AM & ASK Modulation Lab 4: AM & ASK Modulation Prepared by: Dr. Sattar Hussain,  Centennial College, 2022 Name Student ID Signature* *By signing above, you attest that you have contributed to this submission and confirm that all work you have contributed to this submission is your work. Any suspicion of copying or plagiarism in this work will result in an investigation of Academic Misconduct and may result in a “0” on the work, 1 School of Engineering Technology and Applied Science (SETAS) Information and Computing Engineering Technology (ICET) CNET304 - Wireless Technology Course Lead: Dr. Sattar Hussain Section No. Mark Obtained (out of 25) Due Date

Lab 4: AM & ASK Modulation Learning Objectives Upon completion of this lab, students will be able to:  Generate an amplitude-modulated signal and display it as a waveform and spectrum  Analyze the time domain and frequency domain of a generated AM signal  Vary the modulation percentage by varying the amplitude of the modulating signal  Measure the frequencies of the lower and upper sidebands  Calculate the bandwidth of the generated AM signal  Generate an ASK signal and display it in the time-domain and frequency domain. Equipment Required R&S RTB2002B Digital oscilloscope. Pre-Lab Assignment [ 2 marks ] 1. Does the carrier of an AM signal contain any information? Explain. The carrier of an AM signal does not inherently contain any information. It serves as a reference for demodulation, while the actual data resides in the sidebands surrounding the carrier frequency 2. What percentage of the total power in an AM signal with m=100% is in the carrier? One sideband? Both sidebands? In an AM signal with 100% modulation, the total power is distributed as follows: Carrier Power: The carrier signal accounts for 25% of the total power. Each Sideband Power: Both the upper and lower sidebands each contribute 37.5% of the total power 1. The remaining 25% is shared between the two sidebands. This distribution ensures that the total power in the AM signal remains constant, even when modulation occurs. 2

Lab 4: AM & ASK Modulation Procedure Important Notes: a. For this lab, some of the parameter values are assigned based on group number. Such assignment is referred to by the letter x. Always replace the x (whenever it is found) by your group number. b. All screenshots must show the time stamp at the right-bottom corner. All hand sketches must br correctly labeled with horizontal and vertical scales or values. c. Note: You are required to print your name and your lab partner name on every single screenshot submitted within this lab using the Annotation tools of the RTB2004B Oscilloscope. 1. To start, click the [Preset] key to reset the instrument to the scope mode and default state. 2. Click the [ Gen ] key on the front panel to open the " Function Generator " menu, where you can create various waveforms. 3. Generate AM signal with a sinewave carrier signal of f C = 9x.0 kHz, amplitude V C = 0.5 Vpp, modulated by sinewave modulating signal with f m = 5 kHz at 75% modulation index (depth). Output 1 Function Sine Frequency (Carrier Frequency) 9x.0 kHz Amplitude 500 mVpp Offset 0 V Noise 0% Modulation 1 Modulation Type AM Function Sine Frequency (Modulating Frequency) 5 kHz AM Depth 75% 4. Make the following adjustment and set up for CH1 of the oscilloscope: 3

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Lab 4: AM & ASK Modulation 5. You may need to synchronize the oscilloscope with the internal triggering level if the AM signal is not stable or clear. To do that, move the trigger level (TL) Marker up or down until you obtain the signal shown in the screenshot below. You may also press the [ Single ] key to freeze the Oscilloscope display and get a single shot. Remember to unfreeze the display for the next reading by clicking the [ Run/Stop ] key. 6. [ 2 mark ] Sketch or print and paste a screenshot for the Oscilloscope display below. 4 Coupling DC Vertical Scale 100 mV Time Scale 50 µs

Lab 4: AM & ASK Modulation 7. Calculate the AM modulation index using the formula: 5

Lab 4: AM & ASK Modulation To measure A max and A min , click on the [ Cursor ] key and adjust the Cursor Type to Horizontal. Locate Cursor 1 at the top of the AM signal envelope to read A max and Cursor 2 at the minimum point of the AM envelope to read A min . Ans) Amax = 222.66mV Amin = -220.7 8. [ 5 marks ] Vary the modulation Depth to 25%, 50%, and 100% and repeat step 9 above. Tabulate your result below. If the Oscilloscope is at single-shot mode, remember to unfreeze the Oscilloscope display by clicking the [ Run/Stop ] key to update the Oscilloscope display. [ 2 mark ] Compare the measured modulation index with the AM Depth used. Are there any discrepancies? 9. Click [ FFT ] key and adjust the FFT screen as follow: Start Frequency 0 Hz 6 AM Depth A max A min Modulation index, n a% 25% 155.27 94.73 24.216 50% 186.52 67.38 46.92 75% 219.77 37.11 71.11 100% 252.93 4.84 96.25

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Lab 4: AM & ASK Modulation Stop Frequency 200 kHz Span 200 kHz RBW 1 kHz Source C1 FFT Window Flat Top Vertical Scale dBm 10. Click the [ Cursor ] key and choose V-Marker for Type and Spectrum for Source. Use the Next Peak/Prev. Peak to select a peak in the spectrum. Two vertical cursors will appear. You can use any one of them. Observe the readings on the bottom of the oscilloscope screen. 11. Record in the Table below the Carrier power (P C ) and the two sidebands power P USB and P LSB . 12. Record also the carrier frequency and the frequency of each of the sidebands then calculate the AM signal Bandwidth (BW=f USB -f LSB ). 13. Calculate the modulation index using the formula: Where  P is the difference between a sideband power and the Carrier: ∆P(dB) = Pusb(dBm)-P c (dBm) 7  P

Lab 4: AM & ASK Modulation [ 5 marks ] P C (dBm) -8.16 P USB (dBm) -14.16 P LSB (dBm) -14.16 m% -6s Carrier Frequency (kHz) 93.90 Upper Sideband Frequency (kHz) 98.19 Lower Sideband Frequency (kHz) 88.11 AM Signal Bandwidth (kHz) 10.08 14. [ 2 mark ] Compare the modulation index obtained in the previous step with the one calculated in step 10 for Modulation Depth of 75%. Are there any discrepancies? Pc = -8.18 Pusb = -16.66 Plsb = -16.66 ASK Modulation 1. Switch back to the Time-domain view. Adjust the modulation Depth to 100%. Change the AM modulating signal to a Rectangle waveform (keep the carrier signal as a sine wave and change only the modulating signal to a rectangle). 2. [ 2 marks ] Sketch or print and paste the Oscilloscope display below. 8

Lab 4: AM & ASK Modulation 9

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Lab 4: AM & ASK Modulation 3. Click the [ FFT ] key to display the frequency-domain of the ASK spectrum. Adjust the FFT display as follow: 4. [ 2 marks ] Sketch or print and paste the Oscilloscope display below. 10 Start Frequency 0 Hz Stop Frequency 500 kHz Span 500 kHz RBW 1 kHz Source C1 FFT Window Flat Top Vertical Scale dBm

Lab 4: AM & ASK Modulation Discussion 1. [ 3 marks ] From the results obtained in step 13 for the carrier and sidebands power, calculate the total power of the AM signal in milliwatts. P total mW = P mW,carrier + P mW,usb + P mW,lsb P total mW = 0.133+0.038+0.038 P total mW ≈ 0.209mW The total power of the AM signal, calculated from the given dBm values for the carrier and sidebands, is approximately 0.209 milliwatts. 2. [ 2 mark ] Comment on the number of sidebands obtained and the bandwidth of the ASK signal compared to the AM modulation. There are only two sidebands produced in ASK (Amplitude Shift Keying) modulation: one above and one below the carrier frequency. When compared to AM (Amplitude Modulation), where two sidebands are normally formed on either side of the carrier frequency, this results in a narrower bandwidth. As a result, ASK signals often have a lower bandwidth than AM signals. 11