EEP306: Amplitude modulation


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communication engineering lab report, 5th sem, electrical engineering, IIT Delhi, eep306

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EEP306: Amplitude modulation

  1. 1. EEP306 LAB REPORT AMPLITUDE MODULATION SUBMITTED BY:GROUP3 Umang Gupta (2010EE50564) IndraBhushan (2010EE50548) VivekMangal (2010EE50566) Jitendra Kumar Meena (2009EE50483) 12-9-12 Experiment
  2. 2. AMPLITUDE MODULATION Objective: Modelling of an AM signal .Setting and measuring the depth of modulation. Study of AM waveform and spectra. Demodulation of AM signals by envelope detection. Introduction: Let m(t) represent an arbitrary waveform that is the message to be transmitted, and let the constant M represent its largest magnitude: The message might be just a simple audio tone of frequency: A carrier wave is modeled as a sine wave: In which the frequency in Hz is given by: The constants and represent the carrier amplitude and initial phase, and are introduced for generality. For simplicity, their respective values can be set to 1 and 0. It is assumed that and that Amplitude modulation is formed by the product: A represents the carrier amplitude, which is a constant that demonstrates the modulation index. y (t) can be written in the form Therefore, the modulated signal has three components: a carrier wave and two sinusoidal waves (known as sidebands), whose frequencies are slightly above and below Demodulation methods The simplest form of AM demodulator consists of a diode which is configured to act as envelope detector. Matlab Code for the Experiment: clc fc=2000; fm=100; k=[0.5,1,1.5]; t=(0.00005:.00005:2/fm);
  3. 3. am_sig=zeros(3,400); dem_sig=zeros(3,400); m= sin(2*pi*t.*fm); c= cos(2*pi*t.*fc); figure plot (t,m) gridon title ('message signal') xlabel ('time') ylabel ('amplitude') fori = 1:3, am_sig(i,:)=c.*(1+k(i).*m); figure plot (t,am_sig(i,:)); gridon; title ('amplitude modulated signal'); xlabel ('time'); ylabel ('amplitude'); end fori=1:3, dem_sig(i,:)=hilbert(am_sig(i,:)); figure plot (t,abs(dem_sig(i,:))-1); gridon; title ('demodulated message signal'); xlabel ('time'); ylabel ('amplitude'); end dsbsc_sig=m.*c; figure plot (t,dsbsc_sig); gridon title ('DSBSC modulated signal'); xlabel ('time'); ylabel ('amplitude'); y=hilbert(dsbsc_sig); figure plot (t,abs(y)); gridon; title ('demodulated DSBSC signal'); xlabel ('time'); ylabel ('amplitude');
  6. 6. FIG: AM SIGNAL m=1
  8. 8. FIG: DEMODULATED DSBSC SIGNAL Observation and conclusion: Modulation: The Channel 1 we observed the AM signal,channel 3 is message signal and channel 2 output of adder when dc component is added.
  9. 9. Fig: AM SIGNAL m<1
  10. 10. FIG: AM SIGNAL m=1
  11. 11. FIG: AM SIGNAL m>1 Conclusion: On changing the value of m the waveform is changing. For values of m>1 the envelope is not a copy of AM signal. Demodulation:
  12. 12. The Channel 1 we observed the AM signal,channel 3 is message signal and channel 2 demodulated signal of envelope detector. FIG:m<1 FIG: m=1
  13. 13. Fig: m>1 Conclusion: The output signal is distorted when m>1.This was due to distortion of envelope of AM signal which was fed to envelope detector. DSBSC Demodulation: DSBSC DEMODULATION CONCLUSION: We cannot use envelope detector for DSBSC signal demodulation as the output signal and frequency double of message signal.
  14. 14. We need a coherent detector for demodulating DSBSC signal. One can use an envelope detector if one has an idea of what is the sign of the signal initially