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Usman Institute of Technology Department of Electrical Engineering COMMUNICATION SYSTEMS (EL-321) Fall- 2016 Engr. Waseem Zeeshan Engr. Tahzeeb Jamal Engr. Muhammad Umair EXPERIMENT # 7: Observe the Characteristics of Amplitude Modulation Name of Student: __________________________________________________________ Roll No.: ______________________________Group& Section: ____________________ Date of Experiment : ________________________________________________ Report Submitted on : ________________________________________________ Marks Obtained : Remarks if any : _________________________________________________ Signature : _________________________________________________ Amplitude Modulation: An Amplitude Modulated signal is composed of both low frequency and high frequency components. The amplitude of the high frequency (carrier) of the signal is controlled by the low frequency (modulating) signal. The envelope of the signal is created by the low frequency signal. If the modulating signal is sinusoidal, then the envelope of the modulated Radio Frequency (RF) signal will also be sinusoidal. This would be the case in a common AM radio. The low frequency signal would be an audio signal and the high frequency would be the transmitting frequency of the AM radio station. Shown in Fig. 1 is an example of an AM signal in the time domain. Fig. 1 Amplitude Modulated Signal The mathematical representation for this waveform is as follows: f(t)= A[1+m cos (2fmt)] cos (2fct) Where, A=Amplitude of carrier m = modulation index fm = modulating frequency fc = carrier frequency Figure 2 shows the physical interpretation of the mathematical equation given above. In this diagram, the quantities A and mp are indicated. The circuit for generating an AM modulated waveform must produce the product of the carrier and the modulating signal. After receiving an AM signal, it can be demodulated to recover the low frequency signal. One of the simplest types of AM demodulating circuits is the envelope detector. Fig. 2 AM Modulated Signal Showing Values Modulation Index: It is the ratio of the amplitudes of modulating signal and carrier. A complementary figure to modulation index is also used for amplitude modulation signals known as the modulation depth, it is typically the modulation index expressed as a percentage Where, M=Amplitude of message signal A=Amplitude of carrier Typically the modulation index of a signal will vary as the modulating signal intensity varies. However some static values (given below) will enable the various levels to visualize more easily. Fig. 3a Under-modulation Fig. 3b Exact-modulation Fig. 3c Over-modulation Under-modulation is achieved when the modulation index, m is 0<m<1 Exact-modulation is achieved when the modulation index, m=1 Over-modulation is achieved when the modulation, m>1 The amplitude modulation index is key parameter for any AM transmission as it is necessary to keep the index or depth within limits to reduce distortion and interference Objective: 1 To observe the AM wave with respect to the variation in modulation index on MATLAB MATLAB is used as a tool to observe the change in shape of AM modulated signal by varying modulation index Type in the following code in an m-file MATLAB Code clear all close all clc fm=1000; fc=10*1000; Ac=2; T=1/fm; t=0:T/1000:6*T; %Frequency of message/modulating signal %Frequency of carrier signal %Amplitude of carrier signal %Time period of message signal m=input('Enter the value of modulation index: '); Am=Ac*m; %Amplitude of message signal Vm=Am*cos(2*pi*fm*t); Vc=Ac*cos(2*pi*fc*t); Mod=Ac*(1 + m*cos(2*pi*fm*t)).*cos(2*pi*fc*t); figure, subplot(311) %Message signal %Carrier signal %Modulated signal plot(t,Vm) grid on xlabel('Time'); ylabel('Amplitude'); title('Modulating Signal') subplot(312) plot(t,Vc) grid on xlabel('Time'); ylabel('Amplitude'); title('Carrier Signal') subplot(313) plot(t,Mod) grid on xlabel('Time'); ylabel('Amplitude'); title('Modulated Signal') Procedure: 1. Open MATLAB editor and type in the code 2. Run the m-file and enter the modulation index values at 0.5, 1 and 1.5. Sketch the modulated waveforms below and write your comments 3. Observe and write what will happen if the modulation index exceeds 1 Objective: 2 To observe the AM wave by varying modulating and carrier signal voltage levels Procedure: 1. Provide power supply to the trainer containing AM block. 2. Apply the modulating and carrier voltages with frequencies 500 Hz and 5 KHz at J5 and J4 respectively and observe the waveform at J6 3. Fill the following observation column: S.No. Modulating Signal Voltage (Vp-p) Carrier Signal Voltage (Vp-p) 1 6 2 2 6 4 3 6 6 4 6 8 5 6 12 Modulation Index Waveform Result: …………………………………………………………………………………… …………………………………………………………………………………… …………………………………………………………………………………… …………………………………………………………………………………