Memorial University of Newfoundland Faculty of Engineering and Applied Science. Lab Manual

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1 Memorial University of Newfoundland Faculty of Engineering and Applied Science Engineering 6871 Communication Principles Lab Manual Fall 2014

2 Lab 1 AMPLITUDE MODULATION Purpose: 1. Learn how to use Matlab Simulink Toolbox for simulating communication system. 2. Use the Simulink to analyze signals in time and frequency domain. 3. Identify DSB-SC, Large Carrier (LC) amplitude modulated waveforms in time and frequency domain representations. 4. Implement theoretically functional circuits using the Communications Module Design System (CMDS). Equipment List: 1. PC with Matlab (Version R2012a or higher) and Simulink Prelab: 1. Signal Analysis in Time and Frequency Domain Review the Fourier transform for aperiodic and periodic signals. P1. Find the Fourier transform of m(t)=cos(2 *200t) and sketch its Fourier spectrum. 2. Simulink Simulink is a program for simulating signals and dynamic systems. Simulink has two phases of use: model definition and model analysis. A typical session starts by either defining a new model or by recalling a previously defined model, and then proceeds to analyze that model. In order to facilitate the model definition, Simulink has a large library of blocks. Models are created by combining proper blocks from the library and edited in the model window principally using mousedriven operations. An important part of mastering Simulink is to become familiar with manipulations of various model components in these windows. After you create (or define) a model, you can analyze it either by choosing options from the Simulink menus in the model window or by entering commands in the Matlab command window. The progress of an ongoing simulation can be viewed while it is running, and the final results can be made available in the Matlab workspace when the simulation is complete. To Start Simulink: Start Matlab then type simulink on the command line. A Simulink Library Window opens up as shown in figure 1.1. In the lower left part, all the blocks directories are listed, some directories may have subdirectories. If you choose any directory or subdirectory, all the subdirectories or blocks can be seen in the right part of the library window.

3 Figure 1.1 Example: Design a model to analyze the signal m(t)= cos(2 *200t) in time and frequency domain. Steps: 1. Once Matlab is loaded, type simulink at the Matlab prompt. The Simulink library window will appear. 2. In the library window, click on the directory Simulink\Sources, all the signal generator blocks will be listed in the right part of the library window. 3. Choose a block (here Sine Wave), and click the right button of the mouse, a pop-up menu will appear. Select Add to a new model and the OK, you can create a new model window and add the sine wave generator source block in the model window. You can name and save the model as.mdl file.

4 4. In your model window, using the left mouse button, double-click on the sine wave generator block. A new window appears that displays all the properties of your selected block, and you can adjust the block (signal or system) parameters. Here in this example, specify the model fields as follows: Frequency: 2*pi*200 Phase shift: pi/2 Sample time: 1/4000 (it should be less than half of the message signal period at least) Then, the Sine Wave block will produce a signal sin(2 *200t+ /2)=cos(2 *200t). 5. Next, we want to check the Fourier spectrum of the output of the signal generator. You can use a Spectrum Scope block from the directory DSP System Toolbox\ Sinks by dragging it from the library window to your model window. Double-click on the Spectrum Scope block to open the property window shown in Figure 1.2. Figure 1.2 Under Scope Properties : check Buffer input and Specify FFT length boxes For Spectrum Units, choose dbw/hertz

5 For Spectrum Type, choose Two-sided ((-Fs/2 Fs/2)) Under Axis Properties : Set Minimum Y-limit as -50, Maximum Y-limit as 0, For Y-axis label, choose Magnitude-squared, db Note: The frequency domain Fourier spectrum is obtained through the Spectrum Scope block, which comprises of a Fast Fourier Transform of 128 samples which also has a buffering of 64 of them in one frame. From the property box of the Spectrum Scope the axis properties can be changed and the Line properties can be changed. The Frequency range can be changed by using the frequency range pop down menu and so can be the y-axis the amplitude scaling changed to either real magnitude or the db (log of magnitude) scale. The upper limit can be specified as shown by the Min and Max Y- limits edit box. The sampling time of the B-FFT scope should match with the sampling time of the input time signal. 6. In order to show the spectrum of the signal, we need to connect the Sine Wave block to Spectrum Scope block. To connect them, first click on block Sine Wave, hold down the control key, and then click on block Spectrum Scope. A connection line will appear between block Sine Wave and block Spectrum Scope. 7. Select the Simulation menu and then run the simulation by clicking Start, and a new window will be opened automatically to show the simulation result. For this example, you will see two impulses which corresponds to the spectrum of a cosine signal. 8. To check the output of the signal generator in time domain. You can add a Scope block from the directory Simulink\ Sinks by dragging it from the library window to your new model window. Double-click on the Scope block to open the result-displaying window, click the second toolbar named parameters to open its property window. Under General, specify: Time range: 0.05 (means showing result for time interval s, you can change it) Sampling: set Sample time: 0 9. Select the Simulation menu and then run the simulation again. After finishing simulation, you can double-click the Scope block to view the signal in time domain. P2. Print your model file and simulation results of signal in time and frequency domain, compare your result with that in P Save your model file to disk as a Matlab.mdl file by selecting Save from the file menu in your model window. This file can be redrawn and simulated on the screen for further editing. Similar operations can be done for other waveforms like the square wave, triangular. 3. Amplitude Modulation a. Double-Sideband Suppressed Carrier (DSB-SC) AM

6 The DSB-SC signal can be written as φ(t)=m(t)cos(ω c t), where m(t) is the message signal, ω c is the carrier frequency. b. Large Carrier (LC) AM The LC AM signal is φ(t)= [m(t)+a] cos(ω c t), where A is the carrier amplitude. The modulation index is defined as μ=m p /A, where m p is the maximum value of m p. Procedure: I. Double-Sideband Suppressed Carrier (DSB-SC) Amplitude Modulation P3. Use Simulink to create a model for implementing DSB-SC amplitude modulation for message signal m(t)=2cos(2 1000t) with a carrier of frequency of 5 khz. Print your model file, both the message signal and modulated signal in time and frequency domain based on Matlab simulation. Explain the DSB-SC effect. Hint: You need to use the Product block under the directory Simulink\Math Operations. You also need to set the waveform parameters to appropriate values. 1 The effect of different carrier frequency P4. Repeat P3 by varying the carrier frequency to 10 khz. How does the modulated signal change in time and frequency domain? 2 The effect of different modulating frequency and amplitude P5. Repeat P3 by varying the message signal m(t) s frequency to 2 khz and amplitude to 20. How does the modulated signal change in time and frequency domain? 3 Change the modulating signal to a square wave P6. Repeat P3 by varying the message signal to a square wave with amplitude to 2, frequency f 0 =1 khz and width of T0/2. How does the modulated signal change in time and frequency domain? Hint: You need to use the Pulse Generator block under the directory Simulink\Sources. You also need to set the waveform parameters to appropriate values (especially pulse type as sample based). II. Large Carrier (LC) Amplitude Modulation P7. Use Simulink to design a model for implementing LC amplitude modulation for message signal m(t)=2cos(2 1000t) with a carrier of frequency of 5 khz and modulation index μ=1. Calculate the value of A. Print your model file, both the message signal and modulated signal in

7 time and frequency domain based on Matlab simulation. Explain the LC AM effect. Is the carrier component suppressed in the frequency domain? Hint: You may need to use the Constant block under the directory Simulink\Sources and the Sum block under the directory Simulink\Math Operations. You also need to set the block parameters to appropriate values. P8. Repeat P7 by varying the modulation index to μ=0.5. How does the modulated signal change in time and frequency domain? P9. Repeat P7 by varying the message signal to a square wave with amplitude to 2 and frequency f 0 =1 khz (width of T0/2). How does the modulated signal change in time and frequency domain? P10. Repeat P7 by varying the message signal to a square wave with amplitude to 1 and frequency f 0 =1 khz (width of T0/2). What is the value of A for which LC AM signal can be converted into a DSB-SC signal? Conclusion: