Objective: This experiment uses MATLAB to simulate DSB and AM modulation methods and the corresponding demodulation schemes.
Prepare for the experiment: Read Matlab example 5.1/page 174 and Matlab examples 6.1 to 6.4 /page 216-219.
Procedure: Part 1. Modulation
a - DSB. Write the code for an m-file (script) to generate a DSB signal. The modulating (message) signal is a single tone signal with frequency 1kHz and the carrier frequency is 30kHz. Time Vector: 3001 points over range from 0 to 3ms (3 cycles of the modulating signal). Plot your original message signal both in time and its spectrum. (Note: the Matlab examples 6.1 and 6.2 will help, but use the cosine functions for your signals instead of sine as in the example). In your report you should include the plots for the modulated signal in time and for the spectrum of the modulated signal.
b - AM. For the same message signal and same carrier as in part a. write the code for an m-file to generate an AM signal. Do simulations for the following percentages of modulation: 1. 25% 2. 50% 3. 100% and 4. 125%. In your report you should include the plots for the modulated signal in time and for the spectrum of the modulated signal for each of the 4 cases. Discuss the results relative to the modulation index m.
c - SSB. Use Matlab to implement a 4 pole-pair Butterworth band-pass filter to select the lower frequency component of the DSB modulated signal in part a. Plot the signal at the output of the filter in time and its spectrum. (Example 5.1 should be of help).
Part 2. Demodulation
For each of the signals generated in Part 1 in this second part demodulation scheme should be implemented. (Hint: it might be better to implement demodulation right after the modulation since you want to use the modulated signal of part 1 as your initial signal of part 2.) For demodulation you need first to multiply the modulated signal of part 1. (which is considered now to be the received signal assuming no noise was added and no other degradation of the signal happened during transmission = ideal channel) with the carrier. It might help to do the hand calculations too, to know exactly what to expect from your simulations. Use a Butterworth low-pass filter of your choice to select the desired component. You should recover this way your original modulating signal (eventually scaled in amplitude). For AM case do the demodulation part only for case 2, m=0.5.
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