Experiment # 3 Amplitude Modulation and Demodulation

Transcription

1 ECE 316 c 2005 Bruno Korst-Fagundes Fall 2005 Experiment # 3 Amplitude Modulation and Demodulation Message Signal + X AM DSB DC Carrier 1 Purpose Amplitude modulation (AM), still widely used in commercial radio today, is one of the simplest ways that a message signal can modulate a sinusoidal carrier wave. The purpose of this lab is for you to gain familiarity with the concepts of amplitude modulation and demodulation. This will be done in two main steps: First, an amplitude modulation system will be created and simulated using Simulink. Second, this system will be implemented on a TMS320C6711 DSP platform. Similarly to an AM radio station, the modulating signal will be an audio signal. If time allows, the AM signal will be demodulated by an envelope detector and low-pass filter combination, which you will implement with discrete analog components on a prototype-board. At each of these steps, you will be required to modify various system parameters and observe the resulting changes. 2 Background Reading and Preparation Amplitude modulation is one type of continuous-wave modulation, covered in [1]. Before coming to the lab, you are encouraged to read [1], and also the principles of operation of the diode envelope detector circuit shown below (see, e.g., [2]). Vi(t) C R Vo(t) A diode envelope detector 1

2 The preparation work should lead you to identify the equation for an AM-modulated signal, to understand the concept of overmodulation, to understand the time domain and frequency domain representation of an AM-modulated signal and to choose appropriate values for R and C values for the envelope detector circuit. 3 Experiment The experiment is divided in two main parts, and possibly one extra, which is optional. These are: the design and simulation of an AM modulator in Simulink; the downloading and testing of the designed system on a DSP platfom; and, time allowing, the demodulation of the signal with analog, discrete, components. All results that will be marked are to be reported in the proper spaces provided in this outline. 3.1 Designing and Simulating an AM Modulator Based on the block diagram that you have reviewed for an AM modulator, build your model using the blocks found on the ECE416 blockset. Your AM Modulator should look like the figure below. Figure 1: Amplitude Modulator For the initial simulation, use a DSP Sine generator as input, and for output use one time scope and one Freq.Domain Scope, as described in Chapter 1. For your message signal, use a 1.5KHz, 0.5 peak Voltage (1V pp ) sinusoid. As your block diagram should indicate, the input signal must be added to a DC component and this combined signal will be multiplied by the carrier frequency. Use a DC component of 1. The carrier frequency is generated by another discrete-time sine wave generator, with an amplitude of 1 (peak value) and 12KHz. You may choose add gain blocks at different locations in the signal 2

3 paths. Run your simulation, and observe the result on the Simulink scopes. Adjust the setting of your scopes appropriately (i.e, you do not need to display all the way to -150dB). You should observe an AM signal similar to the one you drafted on your lab preparation sheet. The picture below shows the time-domain amplitude modulated signal that you should observe. Explain the resulting voltage values. Figure 2: Amplitude Modulation, Time Domain Figure 3 shows the frequency domain plot that your simulation should produce. Explain both the magnitude and frequency values. Yes, explain the numbers. 3

4 Figure 3: Amplitude Modulation, Frequency Domain Modify your system to obtain a modulation index of 100%. Draw and explain the magnitude values of the frequency domain plot. Bonus Mark: Change the frequency of the sinusoidal message signal to 1000Hz. Why has the frequency domain amplitude changed if the time domain has not? (hint: this has nothing to do with AM) 4

5 Now modify your system to obtain a modulating index greater than 1 (i.e., greater than 100%), and sketch the resulting signal. Assuming you only have an envelope detector available for receiving your signal, what is the consequence of overmodulation? Design an AM-DSB-SC system (sinusoidal input). Show the block diagram, the time-domain and frequency domain results. Would this method be preferable over AM-DSB? Why? Suppose, hypothetically, that you have a 3KHz square wave signal to transmit using AM. Which of the methods would you prefer to use: AM-DSB, AM-DSB-SC or AM-SSB? Why? Remember: bandwidth is at a premium, always. 5

6 3.2 Building and Running Your Model As you have done in previous experiments, you will now build your model on a DSP platform. You can choose to either copy and paste the AM modulator on one of the channels of the model provided in c:/ece416/template/two chan.mdl. When using the template, do not forget to save your system with a different name, so that the template remains unchanged for future use. Your new system should have one of the channels untouched and the other modulated in AM. You must pay attention to the following: since the DSP target will operate with frames, you should set your DSP sine wave to 256 samples per frame. Also, set the output of the DSP constant to be Frame-based, and fill in a frame period of 256/ Having done all that, press the build all button. Another interesting point: after you run your system on the target hardware, you will notice that the modulated signal will not have the expected amplitude. This is due to the fact that there is an analog filter at the output of the CODEC, which starts to roll off at approximately 10KHz. Since your modulated signal presents a carrier at 12KHz, you can expect your output to be off by a certain margin (question for thinking only: how would you determine this margin?). Answer the questions below, pertaining to the system you are running. Use a 1KHz, 1V pp sine wave as input (make sure your Agilent signal generator is adjusted to see a high-impedance load at the output). 1. Time Domain Results Sketch the time domain results. Yes, these should be similar to the first ones you got in your simulation. As you sketch your output, report the values that you believe are relevant for someone who reads it to understand what you have done. Why is the amplitude different than what you expected? (Read the text above and remember it for future experiments). If you change your DC value to zero, what is the consequence? (avoid changing it directly in the code; change in the model and rebuild it). Draw the signal in time domain and explain what you see. 6

7 2. Frequency Domain Results Use the FFT capability of your scope to obtain the Frequency Domain display. Sketch the Frequency Domain results both with and without DC. Explain the difference. Change your system to AM-DSB-SC, and use now a 1KHz, 1V pp square wave as input. Sketch the results. Now when you look at your results, think of it this way: you have mixed a carrier with a stream of bits. Your 1KHz square wave mimmics alternating bits at 2Kbps. Here is the main question: how wide is the bandwidth needed to transmit this signal? If you decrease your bit rate, what happens to the bandwidth? What if you increase the rate? Note: in communications, mixing is the multiplication between two signals; in audio it is their addition! 7

8 3.3 If Time Allows: Demodulating the AM Signal Your modulator runs fine, but your intention is to transmit the desired signal and receive it after it goes through modulator, amplifiers, antenna, channel and receiving antenna. You must demodulate the received signal to extract the modulating signal, which is the signal containing the information (in your case, this information is a 1KHz, 1V pp sine wave, or tone). You are now to build and test a simple AM demodulator which is made of the envelope detector you have designed in your preparation sheet. The components used may not have the same values as the ones you have calculated during your preparation. Your task is to transmit the output signal of the DSP target onto the input of the envelope detector you have assembled on a prototype board, and display the output of your envelope detector to the oscilloscope. Sketch the resulting waveform at the output of the envelope detector. modulating signal? Did it recover the 4 Accomplishments In this experiment, you were presented with the key issues involved in designing, simulating and implementing an AM modulator and demodulator. This was done by using a simulation model, a DSP platform in which the model was implemented and an envelope detector for the demodulation of the signal. The experiment intended to guide you through the steps necessary to achieve a practical understanding of the concepts studied in the theory of Amplitude Modulation. References [1] S. Haykin Communication Systems, 4th Edition. Toronto: John Wiley & Sons, Inc., [2] B. P. Lathi, Modern Digital and Analog Communication Systems, 3rd Edition. New York: Oxford University Press, [3] The Mathworks, Inc., Using Simulink. [4] The Mathworks, Inc., Real-Time Workshop. 8