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EEP306: Frequency modulation


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

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EEP306: Frequency modulation

  1. 1. 29-8-12Umang Gupta, 2010EE50564 VivekMangal, 2010EE50566 IndraBhushan, 2010EE50548 Jitendra Kumar Meena, 2009EE50483 Experiment Frequency Modulation and Demodulation Aim: To study and observe the frequency modulation and demodulation using VCO and PLL. Introduction: In communication system, it is often that one has to send a speech/video signal of lower frequency content. It is very difficult to transmit the signal at low frequency through the medium. Hence the signal is modified to high frequency signal so that it can be sent through the medium. Once the signal is modified on the receiver’s end it can be sent through the medium. Now the receiver on receiving the signal applies demodulation techniques to get the message signal. One such method is frequency modulation. Let the message signal be m(t). We define frequency modulated signal, x(t)=cos(2πfct+ ) fcis the carrier frequency. K is constant. This is called frequency modulation, as the information content is encoded in frequency. Now this can be demodulated in many ways by using a multiplier and applying Hilbert transform or by PLL using VCO Observations and remarks: Matlab Outputs: CODE: clearall fc=10000; fm=500; t=(0.000001:.000001:2/fm); x=cos(2*pi*t.*fc+15*sin(2*pi*t.*fm)); //frequency modulated signal figure(1); plot(t,x); title('frequency modulation (fc= 10KHz and fm=500 Hz)'); xlabel('time '), ylabel('signal or voltage'); holdon; gridon; y=10*demod(x,fc,10*fc,'fm'); //function to demodulate the signal figure(2); plot (t,y); title('demodulated signal(fc= 10KHz and fm=500 Hz)'); xlabel('time '), ylabel('signal or voltage'); holdon; gridon; Taking fc=10KHz and fm=500Hz We get following outputs Modulated Signal for two msg periods. Note the change in frequncy.
  2. 2. Demodulated signal. Note that there is a DC component which needs to be filtered Experimental Obeservations: The modulated signal for step square wave is as:
  3. 3. For a sinusoid, the modulated wave output is: Demodulated signal o/p is as follows:
  4. 4. There are some distortions in the demodulated signal. This may because of the tuning of the PLL. Conclusion The results of FM modulations are well within the predicted boundaries. But it is not an efficient way of transmission as it takes up larger bandwidth. The PLL implementation of FM is used in radios and related communications.