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EEP306: Quadrature amplitude modulation


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

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EEP306: Quadrature amplitude modulation

  1. 1. Umang Gupta (2010EE50564) Indra Bhushan (2010EE50548) Vivek Mangal (2010EE50566) Jitendra Kumar Meena (2009EE5 ) 5-9-12 Experiment Quadrature Amplitude Modulation Aim: To study the working of phase locked loop and identify the lock and capture ranges. Introduction: QAM is an Analog as well as a digital modulation. It can modulate two analog/digital message signals by changing (modulating) of carrier waves, using the amplitude modulation or amplitude shift keying. The two carrier waves usually are sinusoids and out of phase and so called Quadrature amplitude modulation. The modulated waves are summed, and the resulting waveform is a combination of two amplitude modulated signals. QAM is used extensively as a modulation scheme for digital telecommunication systems.
  2. 2. Observations and remarks: Theoritical Results: Matlab code to simulate the qam modulation in two sinusoids: clear all fc=10000; fm1=500; fm2=100; t=(0.000001:.000001:1/fm2); m1=sin(2*pi*t.*fm1); m2=sin(2*pi*t.*fm2); x1=m1.*cos(2*pi*t.*fc); x2=m2.*sin(2*pi*t.*fc); x=x1+x2; figure(1); plot(t,x); title('QAM modulation (fc= 10KHz, fm1=500Hz and fm2=100Hz )'); xlabel('time '), ylabel('signal or voltage'); hold on; grid on; figure(2); plot(t,x1); title('DSBSC modulation (fc= 10KHz and fm=500 Hz)'); xlabel('time '), ylabel('signal or voltage'); hold on; grid on; figure(3); plot(t,x2); title('DSBSC modulation (fc= 10KHz and fm=100 Hz)'); xlabel('time '), ylabel('signal or voltage'); hold on; grid on; fnorm=2000/100000; [b,a] = butter(10, fnorm, 'low'); y1=x.*sin(2*pi*t.*fc); y1l = filtfilt(b, a, y1); y2=x.*cos(2*pi*t.*fc); y2l = filtfilt(b, a, y2); figure(4) plot(t,y1l); title('QAM demodulation');
  3. 3. xlabel('time '), ylabel('message signal 1'); hold on; grid on; figure(5) plot(t,y2l); title('QAM demodulation'); xlabel('time '), ylabel('message signal 2'); hold on; grid on; The outputs of the simulation are as follows- QAM signal for Fc=100KHZ AND 100 AND 500 HZ SIGNAL
  4. 4. Individual demodulated waves. QAM is the sum of the two.
  5. 5. Demodulated signals
  6. 6. Practical observations: (QAM of cont. signal) QAM signal Signal 3 is QAM. Signal 1 is input, signal 2, which is in quadrature is not shown Vector/constellation output of the signal
  7. 7. Demodulated signal
  8. 8. QAM of Discrete signal Modulated signal
  9. 9. Vectored/constellation output Demodulated Signals, note the delay in the output 1 is i/p and 4 is o/p
  10. 10. 2 is i/p and 4 is o/p Conclusion: QAM increases the efiifciency of Amplitude modualtion as two signals can be transmitted over the same bandwidth and hence is popularly preferred over simple amplitude of DSBSC modulation. But however some extra hardware is required to demodulate QAM.