EXPERIMENT 4 INTRODUCTION TO AMPLITUDE MODULATION SUBMITTED BY

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1 EXPERIMENT 4 INTRODUCTION TO AMPLITUDE MODULATION SUBMITTED BY NAME:. STUDENT ID:.. ROOM:

2 INTRODUCTION TO AMPLITUDE MODULATION Purpose: The objectives of this laboratory are:. To introduce the spectrum analyzer as used in frequency domain analysis. 2. To identify various types of linear modulated waveforms in time and frequency domain representations. 3. To implement theoretically functional circuits using the Communications Module Design System (CMDS). Equipment List. PC with Matlab and Simulink 2

3 I. Spectrum Analyzer and Function Generator. This section deals with looking at the spectrum of simple waves. We first look at the spectrum of a simple sine wave. To Start Simulink: Start Matlab then type simulink on the command line. A Simulink Library Window opens up as shown in figure. Figure. Spectrum of a simple sine wave: - Figure.2 shows the design for viewing the spectrum of a simple sine wave. Figure.2 3

4 Table Simulink Library Model components Time scope Spectrum scope Sine wave Simulink Library Browser DSP Blockset DSP sink Download from website (ce.kmitl.ac.th) DSP Blockset DSP sources Setting the sine wave with Khz frequency (fig..4 b) then draw Figure.3 from the timedomain scope and spectrum scope. The frequency domain spectrum is obtained through a buffered-fft scope, which comprises of a Fast Fourier Transform of 28 samples which also has a buffering of 64 of them in one frame. The property block of the B-FFT is also displayed in figure.4 (a) Time scope spectrum scope Figure.3 4

5 This is the property box of the Spectrum Analyzer fig.4 (b) fig..4 (a) From the property box of the B-FFT scope the axis properties can be changed and the Line properties can be changed. The line properties are not shown in the above. The Frequency range can be changed by using the frequency range pop down menu and so can be the y-axis the amplitude scaling be 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 in this case has been set to /5. Note: The sampling frequency of the B-FFT scope should match with the sampling time of the input time signal. Also as indicated above the FFT is taken for 28 points and buffered with half of them for an overlap. Note: The signal analyzer if chosen with half the scale, the spectrum is the single-sided analyzer, so the power in the spectrum is the total power. Similar operations can be done for other waveforms like the square wave, triangular. These signals can be generated from the signal generator block. 5

6 II Waveform Multiplication (Modulation) Instructions. Open the wavformmul.mdl file shown in figure.5 2. The equation y = k m cos 2 (2Π(,)t) was implemented as in fig. B peak to peak voltage of the input and output signal of the multiplier was measured. Then km can be computed as k m Vpp(2kHz) = *2 = Vpp(kHz).5 / 2*2 The spectrum of the output when km= was shown below. =.5 figure.5 6

7 3. set the sine wave as the following figure: 4. The following figure demonstrates the waveform multiplication. A sine wave of khz is generated using a sine wave generator and multiplied with a replica signal. 5. Draw the output spectrum and the output of multiplier in the following signal graph output of multiplier 7

8 output of spectrum NOTE: it can be seen that there are two side components in spectrum. The components at fc + fm and (fc + fm) can be seen along with a central impulse. If a DC component was present in the input waveform, then y = k m *(cos(2π(,)t) + Vdc) 2 = k m *(cos 2 (2Π(,)t) + 2cos(2Π()t*Vdc + Vdc 2 ) The effect of adding a dc component to the input has the overall effect of raising the amplitude of the 2KHz component and decreases the 2KHz component. However, for a value of Vdc =.V, the KHz component reduces and for any other increase in the Vdc value, the KHz component increases. Magnitude Frequncy 8

9 III Double Side-Band Suppressed Carrier Modulation:. The experiment set up of a DSB-SC signal is connected shown in the following figure. The Signals are at khz and khz. 2. Draw the output of FFT scope in the figure below. It can be seen that the output consists of just two side bands at +(fc + fm) and the other at (fc + fm), i.e. at 9kHz and khz. 9

10 By multiplying the carrier signal and the message signal, we achieve modulation. Y*m(t) = [k m cos (2πt)* cos (2πt)] We observe the output to have no KHz component i.e., the carrier is not present. The output contains a band at 9KHz (fc-fm) and a band at KHz (fc+fm). Thus we observe a double side band suppressed carrier. All the transmitted power is in the 2 sidebands. Effect of Variations in Modulating and Carrier frequencies on DSB SC signal. By varying the carrier and message signal frequencies, we observe that the 2 sidebands move according to the equation fc±fm. Now, using a square wave as modulating signal, we see that DSBSC is still achieved. The output from spectrum analyzer was slightly different from the theoretical output. In the result from the spectrum analyzer, there is a small peak at frequency = khz (the carrier frequency) and other 2 peak at and Hz. This may caused by the incorrectly calibrated multiplier. Next, the changes to the waveform parameters have been made and then the outputs have were observed. And here are the changes that have been made Magnitude Frequncy Vary the khz carrier frequency Expected result: Both sidebands are expected to be centered on the new carrier frequency. The real result is as expected. 2 Vary the modulating frequency and amplitude Expected result: The position of the sidebands would have been changed when the modulating frequency is changed. The sidebands would move further from the carrier frequency if the modulating frequency is increased. The peak of the sidebands would be higher if the amplitude of the modulating signal increases The result of the experiment is as expected.

11 3 Change the carrier signal to a square wave. Expected result: There would be the high peaks of the modulating signal around the carrier frequency. Expect for a small peak of the carrier because the time average of the square wave does not equal to zero. The waveform of the signal is expected to be square wave which the amplitude is the sine wave at khz. The result of the experiment is as expected 4 Change the modulating signal to a square wave Expected result: It is likely to see the spectrum of the square wave in the both sidebands around the carrier frequency. The output waveform would be the sine wave, which the amplitude equals to the amplitude of the square wave. The result of the experiment is as expected. IV. Amplitude Modulation:. This experiment is the amplitude modulation for modulation index a = and.5. From the equation of the AM y = km ( + a cos(2π () t) cos(2π () t The representation of the signal in both time-domain and frequency domain when k m = for a= and a=.5 were found to be as shown in figures. 2. The experimental set up for generating an AM signal looks like this: - The

12 .5 Spectrum of AM waveform when a= Magnitude Frequncy The input waveform 5% modulated is shown in figure 3. Draw the output spectrum in the figure below It must be noted here that the A.M signal can be converted into a DSB-SC signal by making the constant =. The waveforms at various levels of modulation are shown in the following figures. 2

13 2 AM waveform when a=.5.5 Magnitude time (sec) x -3.5 AM waveform when a=.5.5 Amplitude time (sec) x -3 3

14 .5 Spectrum of AM waveform when a=.5 Magnitude Frequncy The results from the experiment were shown. The results from the experiment are pretty much the same as in the theoretical ones except there are 2 other peaks at and khz. This is the same as earlier experiment. The cause of this problem is probably the multiplier. 4

15 Appendix PRE LAB INTRODUCTION TO AMPLITUDE MODULATION 5

16 I Sketch the time and frequency domain representations(magnitude only) of the following: A. Cos 2Πft f = khz Sine Wave SCOPE Spectrum Analyzer The time and frequency domain of the input signal is shown as below. 3 2 Amplitude Time domain 2 Amplitude Freq domain subplot(2,,); x = -5:.:5; t = :/4:; time = cos(2*3.4**t); y = cos(2*3.4**x); plot(x,y) axis([ ]); 6

17 xlabel('time domain'); ylabel('amplitude'); % now create a frequency vector for the x-axis and plot the magnitude and phase subplot(2,,2); fre = abs(fft(time)); f = (:length(fre) - )'*4/length(fre); plot(f,fre); %axis([ - ]); %axis([ ]); xlabel('freq domain'); ylabel('amplitude'); B. Square wave period = msec, amplitude = v SCOPE Square Wave Spectrum Analyzer CODE: subplot(2,,); x = -5:.:5; Fs = 399; t = :/Fs:; time = SQUARE(2*3.4**t); y = SQUARE(2*3.4**x); plot(x,y) axis([ ]); xlabel('time domain'); ylabel('amplitude'); % now create a frequency vector for the x-axis and plot the magnitude and phase subplot(2,,2); fre = abs(fft(time)); f = (:length(fre) - )'*Fs/length(fre); plot(f,fre); %axis([ - ]); %axis([ ]); 7

18 xlabel('freq domain'); ylabel('amplitude'); 3 2 Amplitude Time domain 3 Amplitude Freq domain C. Cos 2 (2Πft) f = khz subplot(2,,); x = -5:.:5; Fs = 699; t = :/Fs:; time = cos(2*3.4**t).*cos(2*3.4**t); y = cos(2*3.4**x).*cos(2*3.4**x); plot(x,y) axis([ ]); xlabel('time domain'); ylabel('amplitude'); % now create a frequency vector for the x-axis and plot the magnitude and phase subplot(2,,2); fre = abs(fft(time)); f = (:length(fre) - )'*Fs/length(fre); plot(f,fre); %axis([ - ]); 8

19 %axis([ ]); xlabel('freq domain'); ylabel('amplitude'); Amplitude Amplitude Cos2(2pift) Time domain Freq domain II A carrier Cos 2Π(5)t is modulated by a single tone Cos 2Π()t. The time and freq domain representation are shown. A. Double side-band suppressed carrier modulation Amplitude Freq domain 9

20 % Modulating the single tone message signal. Ts = 99; subplot(4,,); t = :/Ts:; m = cos(2*3.4**t); plot(t,m); % plot of the carrier signal subplot(4,,2); c = cos(2*3.4*5*t); plot(t,c); % plot of the DSB signal with Suppresed carrier intime domain subplot(4,,3); d = m.*c; plot(t,d); % freq. domain of the DSB signal. subplot(4,,4); fre = abs(fft(d)); f = (:length(fre) - )'*Ts/length(fre); plot(f,fre); %axis([ - ]); axis([ 5]); xlabel('freq domain'); ylabel('amplitude'); B. % AM modulation ( modulation index = ) % Modulating the single tone message signal. Ts = 99; K = ; a = ; subplot(4,,); t = -:/Ts:; m = cos(2*3.4**t); plot(t,m); % plot of the carrier signal subplot(4,,2); c = cos(2*3.4*5*t); plot(t,c); % plot of the DSB signal with Suppresed carrier intime domain subplot(4,,3); d = (K + a*m).*c; plot(t,d); 2

21 % freq. domain of the DSB signal. subplot(4,,4); fre = abs(fft(d)); f = (:length(fre) - )'*4/length(fre); plot(f,fre); %axis([ - ]); %axis([ ]); xlabel('freq domain'); ylabel('amplitude'); %axis([ 2 25]); Amplitude Freq domain C. 5% AM modulation (modulation index =.5) Ts = 99; K = ; a = ; subplot(4,,); t = -:/Ts:; m = cos(2*3.4**t); plot(t,m); % plot of the carrier signal subplot(4,,2); 2

22 c = cos(2*3.4*5*t); plot(t,c); % plot of the DSB signal with Suppresed carrier intime domain subplot(4,,3); d = (K + a*m).*c; plot(t,d); % freq. domain of the DSB signal. subplot(4,,4); fre = abs(fft(d)); f = (:length(fre) - )'*4/length(fre); plot(f,fre); %axis([ - ]); %axis([ ]); xlabel('freq domain'); ylabel('amplitude'); %axis([ 2 25]); Amplitude Freq domain 22