_Signal LabS2 - N ATIONAL U NIVERSITY of S INGAPORE...

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N ATIONAL U NIVERSITY of S INGAPORE DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING Course: EE2009 - SIGNALS E XPERIMENT S2 F REQUENCY M ODULATION L ABORATORY M ANUAL ______________________________________________________________________________________ IMPORTANT! READ THIS MANUAL CAREFULLY AND PREPARE FOR THE EXPERIMENT BEFORE YOU ATTEND THE LABORATORY SESSION. SUBMIT THE LABORATORY REPORT ON THE SAME DAY OF THE EXPERIMENT, BEFORE LEAVING THE LABORATORY. SOME PARTS OF THE EXPERIMENT NEED EXPLANATIONS BASED ON THEORETICAL UNDERSTANDING. STUDY THE NECESSARY THEORETICAL CONCEPTS BEFORE COMING TO DO THE EXPERIMENT. YOU MAY WISH TO TAKE A LOOK AT http://vlab.ee.nus.edu.sg/vlab/, CLICK ON “LABORATORY” TO GET ACCESS TO A SIMILAR WEB-BASED VERSION OF THIS EXPERIMENT. ______________________________________________________________________________________ 1
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1. Introduction In this experiment, you will use a spectrum analyser to study the waveforms and spectra of frequency modulated signals. After completing the experiment, you should have acquired some proficiency on the use of this important but complicated instrument. Also, you should be familiar with the characteristics and design of frequency modulation systems. To send it over great distance, a message signal has to be suitably modified or modulated to give rise to an electromagnetic wave with wavelength that is comparable to the size of the antenna used. There are many types of modulations. Two common ones are Amplitude Modulation (AM) and Frequency Modulation (FM). In all these schemes, a high frequency carrier signal is modified so that some of its features are made to change according to the input message signal. In a certain sense, the carrier signal acts as a vehicle for the message signal, while the message signal is the passenger taking the vehicle. In the case of FM, the frequency of the carrier signal is modified as a function of the message signal. In this experiment, you will investigate the characteristics of FM and see how the modulation is carried out. In particular, you will examine the spectrum of the resulting FM waveform using an important instrument called the spectrum analyser. Suppose you have a message signal ( ) t x m , say from your wireless phone, that has to be transmitted over air. Using a carrier ( ) t f a c c π 2 cos of amplitude and frequency , a FM system will produce and send the modulated signal c a c f () ( ) + = t m c c c x k t f a t x τ 2 2 cos , where c is the frequency sensitivity of the modulator. This, together with the amplitude of the message signal, determines the extent of allowable frequency variation. k The phase of the FM signal is + = t m c c d x k t f t θ 2 2 . The instantaneous frequency at time , given by the derivative of the phase, is t () () () t x k f d x k t f dt d dt t d t f m c c t m c c + = + = = 2 2 2 1 2 1 . Clearly, the instantaneous frequency ( ) t f is linearly proportional to the message signal . Figure 1 shows a sinusoidal message signal and the corresponding frequency modulated signal. As the value of t x m ( ) t x m increases, the instantaneous frequency of the carrier increases, and vice versa.
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This note was uploaded on 08/02/2009 for the course ECE EE2009 taught by Professor Prof.c.c.ko during the Fall '07 term at National University of Singapore.

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_Signal LabS2 - N ATIONAL U NIVERSITY of S INGAPORE...

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