Chpt.6 Analogue Modulation-AM

# Chpt.6 Analogue Modulation-AM - Chapter 6 Analog Modulation...

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1 Chapter 6 Analog Modulation and Demodulation (Part 1. Amplitude Modulation)

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2 Analog Transmission We can obtain analog signals using various electronic devices. For example, we can use a microphone to transform sound to electronic form. Our next question is how to transmit it to the receiver. We will first concentrate on amplitude modulation method. This is the technique used in AM radio. baseband signal s ( t ) recovered signal s ( t ) radio signal s AM ( t )
3 Amplitude Modulation Amplitude modulation (AM) is a technique to transform a baseband signal to a bandpass signal (for example, a radio signal). It involves using the message signal to vary the amplitude of the carrier signal. An amplitude-modulation-double sideband-suppressed carrier (AM-DSB-SC) signal can be represented in the time domain by: where s ( t ) is the signal and f c is called the carrier frequency. s AM ( t ) is the transmitted bandpass signal. In short, the modulation process is simply to multiply the baseband signal by a cosine signal with the carrier frequency. We will learn other types of AM modulation techniques later. () ()cos (2 ) AM c s tA s t f t π = s ( t ) s AM ( t ) Acos(2 f c t )

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4 Graphic View of AM-DSB-SC s ( t ) s AM ( t ) Amplitude modulated signal s ( t )cos(2 π f c t )
5 The Envelop Property s ( t ) s AM ( t ) Original signal in blue, modulated signal in magenta. Note how original signal follows envelopes of modulated signal.

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6 The Envelop Property Again s AM ( t ) Note that we can almost see the baseband signal s ( t ) from the modulated signal s AM ( t ). However, it is only “almost”. We can only see the amplitude information. We cannot to identify the phase (i. .e., sign) of s ( t ). Original signal in blue, modulated signal in magenta. Note how original signal follows envelopes of modulated signal.
7 Time and Frequency Views Time domain Frequency domain s(t) s AM ( t ) =m ( t )cos(2 π f c t ) S ( f ) S AM ( f ) t t f f t cos(2 f c t ) f 1/2 1 1/2 Note: Time domain multiplication corresponds to frequency domain convolution.

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8 The Meaning of Sidebands Consider a signal with DSB-SC modulation, the spectrum is as follows: - f m + f m baseband spectrum S ( f ) modulated spectrum S AM ( f ) lower sideband upper sideband f c - f m f c f c + f m -f c - f m -f c - f c + f m
9 Demodulation of AM-DSB-SC Signals For simplicity, assume that there is no noise. Suppose that we received an AM- DSB-SC signal s AM ( t )= As ( t )cos(2 π f c t ). The demodulator is similar to the modulator except a bandpass filter and a lowpass filter is added. Mathematically, it is described as follows. (Note: cos 2 ( x ) = 0.5(1+cos(2 x )) ) We can recover s ( t ) by applying lowpass filtering to w ( t ). (Why and how?) Question: What are the functions of the two filters?

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Chpt.6 Analogue Modulation-AM - Chapter 6 Analog Modulation...

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