EE200_amradio

EE200_amradio - EE200Lab5TunedIn 1 Introduction

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Last Revised: 10/6/2008 1 EE 200 Lab 5 ‐ Tuned In 1 Introduction In this lab you will be given a raw data signal representing an AM radio communication modulating some recorded audio at a particular frequency. You will perform some analysis on the signal and then attempt to demodulate it to recover the original sound signal. 2 What you will learn You will learn to use Fourier analysis (frequency domain) methods and understand their properties to extract useful information from a signal. In addition, basic concepts of modulation (in this lab we only focus on amplitude modulation rather than FM or other methods) will be introduced. 3 Background Information and Notes Amplitude Modulation : Amplitude modulation works by taking a signal and multiplying it with a higher (usually much higher) frequency carrier signal. The result is that the original signal controls the amplitude or acts as an envelope function of the modulated frequency. Mathematically, given a signal A(t), AM modulation at a frequency, f c , is achieved by performing the operation: ܣܯሺݐሻൌܣሺݐሻכcosሺ2ߨ݂ ݐ൅ߠሻ An example is shown below where an 8Hz sinusoid is amplitude modulated at a carrier frequency of 100 Hz. The resulting modulated signal is then shown. 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -1.5 -1 -0.5 0 0.5 1 1.5 Original signal: Sinusoid of f=8 Hz 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -1.5 -1 -0.5 0 0.5 1 1.5 Modulation frequency: f=100 Hz
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EE 200 Lab 5 ‐ Tuned In 2 Last Revised: 10/6/2008 Figure 1 - a.) Original 8Hz signal. b.) Carrier Frequency. c.) Resulting AM signal. Note: Recall that Matlab uses discrete time signals. Thus we need to sample the carrier and AM signal at an even higher sampling rate (frequency), F s . Fourier Series Representation : Any periodic signal can be represented as a Fourier Series. By using Fourier analysis, we can find the frequency domain representation of a signal. Given some Fourier spectrum that results from the original signal, multiplying it by the carrier frequency is equivalent to shifting the frequency
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EE200_amradio - EE200Lab5TunedIn 1 Introduction

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