EEE304 Lab 4 Answer Sheet Name: Date: Lab Description Write a paragraph explaining what you have learned from this lab exercise. Task 1 Open the AM_Mod_coherent.slx file. This program has been pre-coded for you. We have a baseband (message) signal whose amplitude can be varied by the first tunable knob named "Baseband Signal: Amplitude" while the simulation is running in Simulink. The Carrier signal has a fixed amplitude of 2 but its frequency can be modified by the second knob named "Carrier signal: Frequency". The third knob allows you to view the spectrogram of different signals. Figure 1. Modulation and Coherent Demodulation 1. In the "Analog Filter Design" block, the parameters of Butterworth filter have been assigned using variables for the sake of this exercise: "n" is the order of the filter and "cutoff" is the cutoff frequency in rad/s. By looking at the frequency of the "Baseband Signal" (2000Hz), decide on a cutoff value and order for the filter. Assign values to "n" and "cutoff" in the MATLAB command window so that the variables are in the workspace. Run the simulation. On the "Demodulated Signal Scope", the baseband signal and the one that is modulated and demodulated should have the same frequency and amplitude with maybe some phase shift. What is the choice of "n" and "cutoff" that gives you perfect demodulation? Provide screenshot of the "Demodulated Signal Scope" and show that the frequency and amplitude are indeed the same (phase can be different). 2. With the parameters "n" and "cutoff" chosen for the filter in 1, provide screenshots of the spectrum analyzer for all the 5 positions in knob 3. These will show you the spectrogram of the baseband, carrier, modulated signal, demodulated signal and finally the demodulated and filtered signal. If the choice in 1 was correct, the baseband and demodulated+filtered signal should look very similar. Provide side-by-side screenshots of those two to show that is the case. 3. The exercise starts off with the carrier signal at 20,000Hz. Without changing the butterworth filter from 1, vary the carrier frequency signal (knob 2 "Carrier signal: Frequency", or enter it in the frequency parameter inside the "Carrier Signal" block) to 10,000Hz, 5,000Hz and 1,000Hz. Provide screenshots of the spectrogram from the baseband signal and the demodulated+filtered signals (change knob 3) in each case. Discuss your observation. 4. We will now see the effect of modulation index. Keep the carrier frequency at 20,000Hz. Change the baseband amplitude from 1 to 2 and then to 4 by changing the parameter inside the "Baseband Signal" block. For both cases, provide spectrograms of the baseband vs demodulated+filtered signal (change knob 3) and discuss your observations. Do they match with what was taught in Section 3 in the lab manual? Task 2 Open the AM_Mod_incoherent.slx file. This program has been pre-coded to perform amplitude modulation and demodulation using the non-coherent method (envelope detection). Once again, the baseband (message) signal's amplitude can be varied by the first tunable knob "Baseband Signal: Amplitude" while the simulation is running in Simulink. The carrier signal has a fixed amplitude of 2 but its frequency can be modified by the second knob "Carrier signal: Frequency". The third knob allows you to view the spectrogram of different signals. The Butterworth filter in the "Analog Filter Design" block is a 10th order bandpass filter. Figure 2. Modulation and Non-coherent Demodulation (Envelope Detection) The process of non-coherent modulation, requires the baseband signal to be shifted up by the amplitude of the carrier signal so as to get a completely positive signal. The constant block after the baseband signal achieves this purpose. Modulation is done by the carrier signal, and for demodulation a method called envelope detection is performed. Here, the absolute value of the received signal is taken then bandpass filtered to get the demodulated signal. In this case, however, perfect demodulation does not guarantee that the demodulated+filtered signal amplitude will match that of the baseband signal. 1. Start, by setting the Wlo value to 30*2π rad/s and Whi to 5000*2π rad/s by typing in the "Analog Filter Design" parameter box. This will set you up for correct demodulation. Take screenshots from all scopes in the block diagram (Baseband, Carrier Signal, Modulated Signal Scope, Demodulated unfiltered, LPF Scope and Demodulated Signal Scope). Explain in your own words what is happening to the baseband signal at each scope position (what we are looking for is the description of how non-coherent demodulation is happening). 2. Change the Wlo parameter to 1*2π rad/s. What happens to the spectrogram of the demodulated+filtered signal (provide screenshot)? What frequencies do you think are present in there and why? 3. We will now see the effect of modulation index in non-coherent demodulation. Switch Wlo back to 30*2π rad/s. The carrier signal has an amplitude of 2. The first knob allows us to change the amplitude of the baseband signal. All previous exercises had the baseband amplitude at 1. What is the modulation index at this point? 4. Now change the amplitude of the baseband signal so that the modulation index is 1. Is there any difference in the demodulated+filtered signal (show both spectrogram and signal vs time plots). 5. Finally change amplitude of the baseband signal so that the modulation index is 2. Provide screenshots of the spectrogram and any of the scope outputs you think is pertinent, describe what is happening. Task 3 Open the AM_Mod_coherent_tada_inc.slx file. This program has been pre-coded to perform amplitude modulation and demodulation using the coherent method. However, two blocks are missing and the "Dem Gain" block is configured for the wrong gain. When missing blocks are filled in and the correct parameters are chosen for each block, the tada sound should be heard in both the direct path and the modulated/demodulated path. The two paths are chosen via the "Manual Switch" during simulation. Fill in the missing blocks and run the simulation with correct parameters. Switch the positions of the "manual switch" to make sure that you do in fact hear similar sound on both channels. Provide your choices of parameters for the two missing blocks and the corrected gain for the "Dem Gain" block. Justify your choices. Provide a screenshot of the "Demodulated Signal Scope" to show the signals from the two paths. LAB 4: LAB REPORT GRADE SHEET Name: Instructor Assessment Grading Criteria Max Points Points Lost Template Neatness, Clarity, and Concision 2 Lab Description 8 Description of Assigned Tasks, Work Performed & Outcomes Met Task 1 14 Task 2 14 Task 3 12 Points Lost Lab Score (out of 50) Late Lab Lab Score