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Lect-AM_6_07 - Formation of CCITT Group DEMODULATION The...

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Formation of CCITT Group
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DEMODULATION The process of extraction or recovery of the message signal from the modulated waveform 1. COHERENT 2. NON-COHERENT m(t) cosω c t m(t) cosω c t LPF 2 Cosω c t m(t)
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To demodulate a DSB-SC signal a product modulator is employed at the receiving end and the local oscillator is in phase coherence with the local oscillator of the transmitter. The product modulator output would be = 2 m(t) cos 2 ω c t = m(t)[1+ cos 2 ω c t] = m(t) after the LPF If the receiver LO is not phase coherent with the transmitter LO? Let the receiver LO be of the form cos(ω c t + Φ)
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When DSB-SC signal is received, the product modulator output will be m(t) cosω c t X 2cos(ω c t + Φ) = m(t) [ cos(2ω c t + Φ) + cosΦ ] After passing it through a LPF, = m(t) cosΦ When Φ = 0, phase coherence is there and we will be able to recover the message signal. When Φ = π/2, the output is zero and the message is lost. This effect is called the “Quadrature-null effect” Carrier in synchronism in frequency and phase is required at the receiving end
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