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Lecture4 - Lecture Outline(Week 2 lecture 2 Amplitude...

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Lecture Outline (Week 2, lecture 2) Amplitude Modulation – Demodulation and Conventional AM Reading: Contemporary Comm. System Eng. 3.2.1, 3.2.2, 3.3.1, 3.3.3 Supplementary Reading: Comm. System Eng. 3.2.2, 3.2.3 Material covered: 1. Conventional AM Assume that m(t) is scaled so that | m ( t )|<1, then the modulated signal is ) 2 cos( )] ( 1 [ ) ( c c c t f t m A t u θ π + + = The difference with DSB-SC AM is the “large” carrier term. Fourier domain representation: Derivation: Frequency spectrum:
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Required bandwidth: 2W. Same as DSB-SC AM. Energy used to send carrier, on top of that for signal. Suitable for AM-radio broadcasting, since there are lots of receivers and only a few transmitters. Modulation index a : minimum value of ) ( t m , say a . We have a < 1. (See picture below); 2. Demodulation of DSB-SC AM - Problem: How can we recover the message ) ( t m from ) 2000 cos( ) ( ) ( t t m t u π = ? - Demodulation by envelope tracking. - Another solution: Demodulation by multiplying signal with the carrier, yet again:
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- Derivation: - Need to know phase of signal to recover signal: - By use of a Phase Lock Loop (PLL)
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